effects of freeze drying, refractance window drying …
TRANSCRIPT
EFFECTS OF FREEZE DRYING, REFRACTANCE WINDOW DRYING AND HOT-AIR DRYING ON THE QUALITY PARAMETERS OF AÇAÍ
BY
MARIANA A. PAVAN
THESIS
Submitted in partial fulfillment of the requirements for the degree of Master of Science in Food Science and Human Nutrition
in the Graduate College of the University of Illinois at Urbana-Champaign, 2010
Urbana, Illinois
Adviser:
Associate Professor Hao Feng
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Abstract
Açaí is a dark purple berry native to the Amazonian region in Brazil and widely
consumed in South America. The fruit is a major component of the diet and significant to the
local economy. Recently, more attention has been paid to açaí due to its high antioxidant
properties. However, açaí is a highly perishable fruit and should be processed quickly after
harvesting in order to preserve its nutritional and sensory properties as well as its bioactive
compounds.
Drying is one of the processing techniques used to preserve the fruit. Dehydrated
products present high stability and are easy to transport and store. Dried açaí is, currently,
mainly produced by freeze drying. Even though this method results in high quality products, the
high cost associated with capital investment and operation is its major disadvantage. Thus, the
search for alternative and more economic drying methods is of interest. Refractance Window
drying is a novel and promising drying technique, and it has been demonstrated to retain food
quality. Hot-air convective drying is an ancient and inexpensive process. It is the most widely
used drying method in the food industry, although care should be taken to ensure product
quality. The objective of this research was to evaluate the impact of three drying methods -
freeze drying, refractance window drying and hot-air drying - on the quality parameters of açaí
after drying and over a 3 month storage period at 25°C. To examine the potential quality
changes, the following analyses were performed: moisture content, water activity, moisture
working isotherm, glass transition temperature, color, antioxidant capacity, anthocyanin
quantification, and flavor analysis.
The moisture contents of the dried samples were all below the monolayer values. The
water activity values of the açaí immediately after drying and during storage were also low,
indicating relatively good product stability. The Brunauer-Emmett-Teller (BET) and Guggenhein-
Anderson-de Boer (GAB) models showed a good fit to the isotherm data. Differential Scanning
Calorimetry (DSC) thermograms suggested that there exists a subtle glass transition between
50-60°C. DSC analysis also revealed that the lipids present in açaí are liquid at room
temperature. Drying affected both the color and flavor of açaí juice. Dried samples were
featured by a lighter color and lower hue angle and chroma when compared to the juice.
Storage was marked by a decrease in lightness, most likely because of the formation of brown
pigments by Maillard reaction and polymerization of oxidized phenolic compounds. For flavor
analysis, the compounds (E,Z) 2,6-nonadienal, α-ionone and 2-phenylethanol/β-ionone were the
most important odorants identified in açaí samples. The dried açaí had a higher concentration of
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Strecker aldehydes and compounds derived from lipid oxidation, which indicates the occurrence
of those reactions during drying. During storage, lipid oxidation was the most important reaction
that took place in the dried samples. Freeze drying proved to be superior to the other drying
methods, as shown by a best retention of the anthocyanins and antioxidant capacity (ORAC) of
the fruit. Refractance Window dried açaí also exhibited a good retention of anthocyanin content
similar to that of freeze-dried samples and an ORAC value higher than that of the hot-air dried
powders. Anthocyanin content and antioxidant capacity, within each treatment, did not change
over the 3 months of study.
Keywords: açaí, refractance window drying, freeze drying, ORAC, anthocyanins, glass
transition
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With love to my parents, Nizar and Marina, and my brother Felipe
for all their love and for being my motivation to succeed.
And to Marcelo for his unconditional love and support.
Com amor aos meus pais, Nizar e Marina, e ao meu irmão Felipe
por todo o seu amor e por serem minha motivação para triunfar.
E ao Marcelo por seu amor incondicional e todo seu apoio.
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Acknowledgements
First, I would like to thank God for all the blessings I have received. Special thanks to my
adviser, Dr. Hao Feng, for his support, kindness and trust. I would like to express my gratitude
to Dr. Keith Cadwallader, Dr. Shelly Schmidt and Dr. Nicki Engeseth for their time, guidance and
willingness to be part of my committee. I sincerely thank John Jerrel and Dr. Hun Kim for all
their assistance and suggestions on my project. Also, thanks to all my labmates, especially to
Atilio and Bin Zhou for their friendship and encouragement. I must as well thank the members of
Dr. De Mejia, Dr. Cadwallader and Dr. Schmidt’s laboratory, for allowing me to use their
equipment and resources and for their time. I want to thank Gad Yousef, Ning Zhu, Yang Gao,
Joo Won Lee and Li Xu for their help on my research.
I am eternally thankful to my parents Nizar and Marina for always being there for me and
for being an example in my life. I want to thank my brother Felipe for all his love and support. All
my gratitude goes to my boyfriend Marcelo for his unconditional love and understanding and for
all his help in times when I needed him the most. I sincerely thank all my family and friends for
their endless support.
This project was supported by the USDA 1890 Institution Capacity Building Grants
(#2006-05897-00-01,A4873).
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Table of contents
List of Tables ............................................................................................................................vii
List of Figures .......................................................................................................................... viii
Chapter 1: Introduction .............................................................................................................. 1
References ................................................................................................................................ 2
Chapter 2: Literature Review ..................................................................................................... 4
Açaí ........................................................................................................................................... 4
Dehydration Process ................................................................................................................. 9
Water Activity, Moisture Sorption Isotherms, and Glass Transition Temperature ................... 14
Anthocyanins ........................................................................................................................... 21
Flavor Analysis ........................................................................................................................ 24
References .............................................................................................................................. 26
Chapter 3: Effects of freeze drying, Refractance Window drying, and hot-air drying on the quality parameters of açaí ........................................................................................................ 35
Materials and Methods ............................................................................................................ 35
Results and Discussion ........................................................................................................... 44
References .............................................................................................................................. 71
Chapter 4: Conclusions ............................................................................................................ 78
Appendix A: HPLC Chromatograms ....................................................................................... 80
Appendix B: DSC Thermograms ............................................................................................. 83
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List of Tables
Table 1. Nutrient analysis of açaí pulp .......................................................................................... 6
Table 2. Types of processed açaí ................................................................................................. 8
Table 3. Color measurements of açaí juice and powders on day 0 ............................................ 48
Table 4. Estimated GAB and BET parameters ........................................................................... 55
Table 5. Identification of anthocyanins ........................................................................................ 56
Table 6. Odor-active compounds of açaí juice identified by AEDA ............................................. 64
Table 7. Concentration of volatile compounds in açaí juice and powders on day 0 (expressed in
μg/L) .................................................................................................................................... 65
Table 8. Concentration of volatile compounds in freeze, RW and hot-air dried açaí on day 0 and
after 3 months of storage (expressed in μg/L) ..................................................................... 66
Table 9. Concentration of volatile compounds in açaí powders after a 3-month storage
(expressed in μg/L). ............................................................................................................. 67
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List of Figures
Figure 1. Bunch of açaí ................................................................................................................. 4
Figure 2. Açaí berries .................................................................................................................... 5
Figure 3. Processing of açaí fruits ................................................................................................ 7
Figure 4. Freeze dryer configuration ........................................................................................... 10
Figure 5. Refractance Window dryer .......................................................................................... 12
Figure 6. Hot air dryer ................................................................................................................. 13
Figure 7. “Stability map” .............................................................................................................. 15
Figure 8. Classification of van der Waals adsorption isotherms ................................................. 17
Figure 9. Sorption isotherm ......................................................................................................... 17
Figure 10. General anthocyanin structure (the flavilium cation) .................................................. 21
Figure 11. Anthocyanin structures in aqueous solutions ............................................................ 23
Figure 12. Effect of different drying methods on moisture content ............................................. 45
Figure 13. Effect of storage on moisture content ........................................................................ 46
Figure 14. Effect of different drying methods on water activity ................................................... 47
Figure 15. Effect of storage on water activity .............................................................................. 47
Figure 16. Representation of sample’s hue ................................................................................ 49
Figure 17. Effect of different drying methods on Lightness ......................................................... 50
Figure 18. Effect of different drying methods on hue angle ........................................................ 50
Figure 19. Effect of different drying methods on chroma ............................................................ 51
Figure 20. Effect of storage on Lightness ................................................................................... 52
Figure 21. Effect of storage on hue angle ................................................................................... 52
Figure 22. Effect of storage on chroma ....................................................................................... 53
Figure 23. Moisture sorption working isotherms of the freeze-dried, RW-dried and hot-air dried
açaí ...................................................................................................................................... 55
Figure 24. HPLC chromatogram for açaí juice on day 0 ............................................................. 56
Figure 25. Effect of different drying methods on anthocyanin content ........................................ 57
Figure 26. Effect of storage on anthocyanin content .................................................................. 58
Figure 27. Effect of different drying methods on antioxidant capacity ........................................ 60
Figure 28. Effect of storage on antioxidant capacity ................................................................... 60
Figure 29. DSC thermogram for freeze-dried açaí ...................................................................... 68
Figure 30. DSC thermogram for freeze-dried açaí oil ................................................................. 69
Figure 31. DSC thermogram for freeze-dried açaí solids ........................................................... 71
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Figure 32. HPLC chromatogram for freeze dried açaí on day 0 ................................................. 80
Figure 33. HPLC chromatogram for RW-dried açaí on day 0 ..................................................... 81
Figure 34. HPLC chromatogram for hot-air dried açaí on day 0 ................................................. 82
Figure 35. DSC thermogram for RW-dried açaí .......................................................................... 83
Figure 36. DSC thermogram for RW-dried açaí oil ..................................................................... 84
Figure 37. DSC thermogram for RW-dried açaí solids ............................................................... 85
Figure 38. DSC thermogram for hot-air dried açaí ...................................................................... 86
Figure 39. DSC thermogram for hot-air dried açaí oil ................................................................. 87
Figure 40. DSC thermogram for hot-air dried açaí solids ........................................................... 88
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Chapter 1: Introduction
It has been suggested that consumption of fruits and vegetables contributes to the
prevention of many chronic diseases, such as cancer and coronary heart disease. The health
benefits associated with consumption of those foods are mainly attributed to their
phytochemicals, especially polyphenolic compounds (Dauchet et al. 2006; Franceschi et al.
1998; Joshipura et al. 2001). Trends towards healthier lifestyles have resulted in an increase in
consumer’s demand for health-promoting foods and food products, such as fresh fruits and fruit
juices. A report from the United States Department of Agriculture (USDA) showed that the
consumption of fruits and vegetables in U.S. increased 25% from 1977-79 to 1997-99 (Pollack
2001). However, most fruits with potential health benefits, such as wild blueberries, are very
perishable. To find appropriate food preservation methods that retain the bioactive compounds
with relatively low cost is challenging.
Açaí, a fruit of Euterpe oleracea Mart palm tree, has recently received considerable
attention, mainly due to its high antioxidant capacity and potential health benefits. The fruit, a
dark purple berry native to the Amazonian region, has a high nutritional value and is rich in
polyphenolic compounds (Lichtenthaler et al. 2005; Menezes et al. 2008b; Schauss et al. 2006).
However, açaí is highly perishable, being degraded due to enzyme activity, high microbial load
and elevated temperatures and relative humidity in the harvesting/production area (Menezes et
al. 2008a). Therefore, the fruit must be pulped within 24 h after harvest. Fresh pulp or juice,
which has a shelf life of 12 h, should be either frozen or dried in order to increase its shelf life
and preserve its bioactive compounds (Tonon et al. 2009).
Drying is one of the most efficient ways to preserve foods. It reduces product’s water
activity, which inhibits microbial growth and decreases degradative reactions, resulting in higher
stability. Besides, drying results in substantial volume reduction, which facilitates transport and
storage (Marques et al. 2009; Maskan 2001). Dehydrated açaí is currently produced primarily by
freeze drying. Freeze dried products are known to have high quality, since the technique
preserves bioactive compounds, color, texture and flavor. However, due to the use of vacuum
and very low temperatures, freeze drying is the most expensive drying technology (Hammami et
al. 1997; Khalloufi et al. 2003). Therefore, the search for less expensive drying methods, which
are able to result in high quality products, is of interest.
Refractance Window drying is a relatively new drying method introduced by MCD
Technologies (Tacoma, Washington, USA). The technique involves applying the product, as a
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thin layer, to the top surface of a transparent plastic conveyor belt. Under the plastic sheet, hot
water circulates and it is used to carry thermal energy to the product. The method uses
moderate temperatures and short drying times, which has been reported to result in high quality
products. The technique is especially suitable for liquid and semi-liquid products, such as puree
and juices with particulates (Nindo et al. 2007).
Hot-air drying is the most extensively used drying method. The process consists of
blowing a stream of hot air on top of the product, which results in moisture evaporation.
Although been very common, the method involves high temperatures and long processing
times, which are known to damage nutritional and sensory properties of foods (Ratti 2001). For
comparison purposes hot-air drying is frequently used in drying studies.
The objective of this project was to examine how freeze drying, refractance window
drying and hot-air drying affect the quality of açaí juice. It was also of interest to evaluate the
effects of storage on the quality of the powders and, therefore, dried açaí was stored for 3
months at room temperature, with samples being taken monthly for analysis. Antioxidant
capacity and anthocyanin content were evaluated in order to further understand potential health
benefits of the product. Moisture content, water activity, glass transition temperature and
moisture sorption isotherms were analyzed with the aim of of understanding and predicting
product stability. Color and flavor analyses were performed as possible indicators of consumer’s
acceptability.
The hypothesis of this project was that the quality of the refractance window dried açaí
juice would be comparable to that of the freeze-dried product.
References Dauchet, L., Amouyel, P., Hercberg, S. and Dallongeville, J. 2006. Fruit and vegetable consumption and risk of coronary heart disease: A meta-analysis of cohort studies. J. Nutr. 136, 2588-2593. Franceschi, S., Parpinel, M., La Vecchia, C., Favero, A., Talamini, R. and Negri, E. 1998. Role of different types of vegetables and fruit in the prevention of cancer the colon, rectum, and breast. Epidemiology. 9, 338-341. Hammami, C. and René, F. 1997. Determination of Freeze-drying Process Variables for Strawberries. J. Food Eng. 32, 133-154.
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Joshipura, K.J., Hu, F.B., Manson, J.E., Stampfer, M.J., Rimm, E.B., Speizer, F.E., Cloditz, G., Ascherio, A., Rosner, B., Spiegelman, D. and Willett, W.C. 2001. The effect of fruit and vegetable intake on risk for coronary heart disease. Ann. Intern. Med. 134, 1106-1114+I. Khalloufi, S. and Ratti, C. 2003. Quality deterioration of freeze-dried foods as explained by their glass transition temperature and internal structure. J. Food Sci. 68, 892-903. Lichtenthaler, R., Rodrigues, R.B., Maia, J.G.S., Papagiannopoulos, M., Fabricius, H. and Marx, F. 2005. Total oxidant scavenging capacities of Euterpe oleracea Mart. (Acai) fruits. Int. J. Food Sci. Nutr. 56, 53-64. Marques, L.G., Prado, M.M. and Freire, J.T. 2009. Rehydration characteristics of freeze-dried tropical fruits. LWT - Food Sci. Technol. 42, 1232-1237. Maskan, M. 2001. Kinetics of colour change of kiwifruits during hot air and microwave drying. J. Food Eng. 48, 169-175. Menezes, E.M.S., Rosenthal, A., Sabaa-Srur, A., Camargo, L., Calado, V. and Santos, A. 2008a. High hydrostatic pressure effect on enzyme activity of aç pulp. Cienc. Tecnol. Aliment. 28, 14-19. Menezes, E.M.D.S., Torres, A.T. and Srur, A.U.S. 2008b. Lyophilized acai pulp (Euterpe oleracea, Mart.) nutritional value. Acta Amazon. 38, 311-316. Nindo, C.I. and Tang, J. 2007. Refractance window dehydration technology: A novel contact drying method. Dry Technol. 25, 37-48. Pollack, S.L. 2001. Changing structure of global food consumption and trade. Regmi, A. editor. http://www.ers.usda.gov/publications/wrs011/wrs011h.pdf. Accessed 2010 April 1st. Ratti, C. 2001. Hot air and freeze-drying of high-value foods: A review. J. Food Eng. 49, 311-319. Schauss, A.G., Wu, X., Prior, R.L., Ou, B., Huang, D., Owens, J., Agarwal, A., Jensen, G.S., Hart, A.N. and Shanbrom, E. 2006. Antioxidant capacity and other bioactivities of the freeze-dried Amazonian palm berry, Euterpe oleraceae Mart. (Acai). J. Agric. Food Chem. 54, 8604-8610. Tonon, R.V., Alexandre, D., Hubinger, M.D. and Cunha, R.L. 2009. Steady and dynamic shear rheological properties of açai pulp (Euterpe oleraceae Mart.). J. Food Eng. 92, 425-431.
4
Chapter 2: Literature Review
Açaí General Aspects
Euterpe oleracea Martius is a palm tree native to the Amazonian region, where it can be
abundantly found in the Amazon River estuary (Muñiz-Miret et al. 1996). In Brazil, the state of
Pará is the main producer of this palm, but it can be also found in other Brazilian states,
including Amapá, Maranhão, Mato Grosso, and Tocantins, as well as in other Central and South
American countries, such as Venezuela, Colombia, Ecuador, Suriname, and Panama (De
Sousa 2000).
The tree is a slender, multi-stemmed, monoecious palm that can reach over 30 meters in
height. It is mainly found on estuarine floodplains, slightly less so on perennially flooded areas
and not very common on upland areas (Lewis 2008). It is often known as açaí palm and it is
famous for its hearts of palm and for its fruit, called açaí. Each palm produces 3 or 4 bunches of
açaí per year (Figure 1), each one containing 3 to 6 kg of fruit (Del Pozo-Insfran et al. 2004).
The palm is of great importance in the Amazonian region, since its products are major
components of the diet and of great significance to the local economy (Brondizio et al. 1994).
Figure 1. Bunch of açaí (www.liquidhealth.com.au)
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The heart of palm is the inner core of the tree and it is treated as a delicacy, because of
the intensive labor task required for its harvesting (Bovi et al. 1993). The product is often eaten
in salads or as a filling. Nowadays, 95% of the hearts of palm produced in Brazil come from açaí
palms, since the native trees were devastated without preservation (Tonon 2009).
Açaí is a round-shaped fruit with a diameter ranging from 1.1 to 1.5 cm (Figure 2). Most
of the açaí varieties are green when young and turn into a dark purple color when ripe.
However, some varieties remain green in their mature stage, being called “açaí branco” or white
açaí (Bovi et al. 1993). The seed corresponds to 85% of the fruit total weight and is used for oil
extraction, animal feed or as a fertilizer, among others. The seed is covered by fibrous fibers on
top of which is the edible part (pulp), accounting for 15% of the fruit (Nogueira 2006).
Figure 2. Açaí berries (www.truestarhealth.com)
After being processed into juice, açaí is widely used in smoothies, ice cream, jelly and a
variety of other products. It is estimated that approximately 120,000 liters per day of açaí juice
are consumed only in the city of Belém, Pará state, where the product is part of the staple food
of the population (Carneiro 2000). Until a couple years ago, production of the juice was almost
exclusively intended for the local market. However, recently, the product is becoming more and
more popular in other Brazilian states and internationally. The production of açaí is growing
around 15% per year. In 2008, 540,000 tons of açaí juice were produced in Brazil (Sampaio
2009).
Açaí is considered a fruit of high nutritional value, representing a good source of
proteins, fiber and minerals, mainly potassium and calcium. Interestingly, it also has a high lipid
content, being able to provide approximately 65% of the recommended daily value (Tonon
2009). The oil extracted from açaí juice is rich in monounsaturated (60%) and polyunsaturated
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(10%) fatty acids, mainly oleic and linoleic acids. Unsaturated fatty acids are said to be
important for cardiovascular diseases prevention (Do Nascimento et al. 2008). Açaí oil has also
a high phenolic content, mainly phenolic acids and procyanidins (Pacheco-Palencia et al. 2008).
Table 1 shows the nutrient analysis for açaí pulp.
Table 1. Nutrient analysis of açaí pulp
Source: Tonon 2009
The fruit’s purple color is due to the presence of anthocyanins, natural pigments that
belong to the flavonoid group. Anthocyanins are powerful antioxidants and may have other
potential benefits for humans, like antiinflammatory and anticarcinogenic properties (Del Pozo-
Insfran et al. 2004).
Açaí fruits are produced throughout the year, with production peaks varying from state to
state. However, fruits harvested during August to December (“dry months”) usually have shown
higher yields and better organoleptic quality (Lichtenthaler et al. 2005) Harvesting is typically
done manually and is an arduous task, since the bunches of fruits may reach 10 – 15 m in
height. The harvester climbs the palm tree and cuts the bunches with a knife. After that, the
bunches are placed on the floor and the fruits are picked and selected by hand. Rotten fruits or
those attacked by insects are removed (De Vasconcelos et al. 2006).
Açaí fruits are perishable and should be processed in 24 hours, maximum, after
harvesting (Tonon et al. 2009b). Their degradation is due to high microbial load, enzymes
(mainly peroxidase and polyphenol oxidase), and elevated temperatures and relative humidity in
the harvesting/production area (Menezes et al. 2008).
A typical processing of açaí fruits (Figure 3) involves, mainly, the following steps (Cohen
et al. 2006):
Analytes AmountDry Weight (%) 15.26Proteins (g/100g DW) 10.05Total Lipids (g/100g DW) 52.64Total Sugars (g/100g DW) 2.96Reducing Sugars (g/100g DW) 2.91Sucrose (g/100g DW) 0.05Fiber (g/100g DW) 25.22Ash (g/100g DW) 3.09
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- Reception of fruits: after harvesting, the fruits are placed in plastic boxes or baskets, which are
then transported, usually in boats, to processing plants.
- Selection of fruits: during selection, unripe and damaged fruits are removed.
- Pre-washing, Softening, and Washing:
- Pre-washing: fruits are immersed into water to remove dirt;
- Softening: fruits are immersed into water with temperature ranging from room
temperature to 60°C for 10 to 60 minutes. This step is used to soften the fruits,
facilitating pulping, and
- Washing: fruits are then washed with chlorinated water, followed by potable water.
- Blanching: an optional step used to inactivate enzymes, reduce microbial load and stabilize
color. Blanching is done at 80°C for 10 seconds.
- Pulping: açaí pulp is removed. Water is added during pulping to facilitate the process.
- Homogenization: after pulping, pulp is sieved, to remove any dirt remaining, and homogenized.
- Pasteurization: the time/temperature used is 80-85°C / 10sec. After pasteurization, product
can be frozen or dried.
Figure 3. Processing of açaí fruits
Reception of fruits
Selection
Pre-washing, Softening, Washing
Pulping
Homogenizing
Pasteurization
Freezing Drying
Blanching
Addition of water
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According to the Brazilian Regulation there are four types of processed açaí (Ministerio
da Agricultura, Pecuaria e Abastecimento 2000) and they are presented in Table 2:
Table 2. Types of processed açaí
Functional Properties
Recently, much attention has being paid to açaí due to potential health benefits
associated with its high antioxidant capacity and phytochemical composition. The fruit is
abundant in polyphenolic compounds, which are thought to prevent many chronic diseases,
including cancer, hiperlipidemia, cardiovascular, and neurodegenerative diseases (Hogan et al.
2010). Some polyphenolics already identified in açaí include anthocyanins, ferulic acid, gallic
acid, ellagic acid, (-)-epicatechin, (+)-catechin, orientin and procyanidin polymers, among others
(Del Pozo-Insfran et al. 2004; Pacheco-Palencia et al. 2009).
Schauss et al. (2006a) evaluated the antioxidant capacity and other bioactivities of
freeze-dried açaí and reported a high scavenging capacity for superoxide and peroxyl radical
(the highest of any fruit or vegetable reported to date). Mild activity was detected against the
peroxynitrite and hydroxyl radical. Freeze-dried açaí also seemed to inhibit the formation of
reactive oxygen species in freshly purified human neutrophils. The extract appeared to be
effective at very low doses (5μg/mL). In addition, the product was found to be a potential
cyclooxygenase (COX)-1 and COX-2 inhibitor, indicating anti-inflamatory and pain-relieving
properties.
Oxidative damage of brain tissue leads to an increased susceptibility to
neurodegenerative diseases, such as Parkinson’s and Alzheimer’s. The effects of frozen açaí
juice in reducing oxidative stress induced by H2O2 in cerebellum, cerebral cortex, and
hippocampus from rats was studied by Spada et al. (2009). H2O2 caused an increase in lipid
and protein damage in brain tissue from rats. However, when tissues were pretreated with açaí
a significant decrease in lipid and protein damage and in superoxide dismutase and catalase
activities were observed. The reduction of lipid damage was around 48% in the cerebral cortex,
64% in the hippocampus, and 72% in the cerebellum. Protein damage reduction was 55% in the
Type Addition of water % SolidsAçaí Pulp NoThick or special (type A) Yes >14Medium or regular (type B) Yes 11-14Thin or popular (type C) Yes 8-11
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cerebral cortex, 36% in the hippocampus, and 42% in the cerebellum. The results suggest
possible use of açaí to help prevent the development of age-related neurodegenerative
diseases.
Oliveira de Souza et al. (2009) studied the antioxidant and hypocholesterolemic effects
of açaí pulp in control and in hypercholesterolemic fed rats. The addition of açaí pulp to control
and hypercholesterolemic diets significantly improved antioxidant capacity by decreasing the
level of carbonyl protein (marker of oxidative stress) and increasing the sulfhydryl groups (free
radical scavengers) in the sera of animals fed those diets. Rats fed the hypercholesterolemic –
açaí diet also showed significant reduction of SOD (antioxidant enzyme) activity, compared with
the control rats. This suggested that açaí supplementation reduced oxidative stress, which
diminished the need for protective responses. Also, addition of açaí juice in the
hypercholesterolemic diet reduced cholesterol levels (total and non-HDL) of rats fed that diet.
Dehydration Process
As previously mentioned, açaí is a highly perishable fruit and processing is essential to
preserve its quality and functional properties. Unless the fresh fruit is consumed quickly, it must
be preserved by drying or freezing.
The dehydration process, as defined by Fellows (2000), constitutes “the application of
heat under controlled conditions to remove the majority of the water normally present in a food”.
Removal of water can be done by evaporation or sublimation (in case of freeze drying) (Fellows
2000).
The main goal of dehydrating food products is to extend their shelf life (Vega-Mercado et
al. 2001). Extension of shelf life is achieved by reducing the product’s water activity, which
inhibits or retards microbial growth as well as the occurrence of chemical reactions (Zhang et al.
2006; Mayor et al. 2004). Drying also decreases the weight and bulk of foods, which reduces
packaging, shipping and storage costs (Chou et al. 2001).
An efficient drying process must consider the following (Hawlader et al. 2006):
- Avoid undesirable changes, preserving food quality,
- Operate at optimum conditions, lowering cost,
- Reduce environmental impact (low energy consumption and waste generation).
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Currently, dried açaí is produced mainly by freeze drying. Freeze drying consists of a
low temperature drying process, in which most of the water is removed by sublimation. The
technique involves three main stages (Koroishi et al. 2009):
- Freezing: quickly lowers the product below its freezing temperature to maximize ice content;
- Primary drying: sublimation of ice, at a temperature below freezing and under reduced
pressure (below triple point);
- Secondary drying: removal of unfrozen, bound water by evaporation at a temperature above
0oC.
The water vapor, removed from the product chamber by a vacuum pump, is directed to a
condenser, where it is retained frozen. Figure 4 shows the basic configuration of a freeze dryer.
Ice Condenser
Figure 4. Freeze dryer configuration (Adapted from http://www.rpi.edu/dept/chem-eng/Biotech-Environ/DOWNSTREAM/fig7.htm)
During the freeze drying process, heat is provided to the product in order to sublime the
ice and remove the water. It is important to ensure that the product temperature is below its
glass translation temperature to avoid collapse (Okos et al. 2007). Sublimation starts at the
surface of the product and then moves toward the bottom. During the process, the dry layer acts
as an insulation material and the drying rate slows as the layer thickens (Vega-Mercado et al.
2001).
It is well known that freeze-dried foods have high quality. When water is removed by
sublimation a porous structure with minimal shrinkage remains. Also, the use of low
11
temperatures and vacuum prevents degradative reactions, such as lipid oxidation and
nonenzymatic browning, and preserves nutrients and bioactive compounds. In addition, there is
little or no flavor loss during the process (Marques et al. 2009).
There are a large number of published data showing the superior quality of freeze-dried
products. Yurdugül (2008) compared the quality characteristics of fresh and freeze-dried alpine
strawberries and found no difference between the two products. The researcher evaluated
sugar content, pH, color, vitamin C and anthocyanins, among others, and all the results were
very similar between the fresh and dried samples, indicating that the freeze drying process
preserved the quality of the fruit.
The effect of different drying methods - freeze drying, hot-air drying, vacuum microwave
drying, and combination drying - on phytochemical content and antioxidant activity of Saskatoon
berries was studied by Kwok et al. (2004). Freeze-dried berries showed the greatest retention of
phenolics, anthocyanins and reducing power and the highest antioxidant activity compared to
berries processed by other methods. Results were attributed to the reduced exposure of berries
to heat and oxygen during freeze drying.
Krokida et al. (2006) evaluated the retention of aroma during air and freeze drying of
apples. Three representative compounds of apple flavor were evaluated before, during and after
the drying processes. The researchers found a higher retention of flavor in freeze-dried
samples, attributed to the lower temperatures used in the process.
On the other hand, because of the use of vacuum, sub-zero temperatures, and thermal
energy for sublimation and evaporation (phase change), freeze drying is the most expensive
drying method. Low temperature drying results in slow drying rates and long drying times. In
addition, the low pressure required for sublimation and therefore the use of a vacuum pump, as
well as the refrigeration system for maintaining a low temperature to collect moisture in the form
of ice on the condenser all add cost to the process. As a result, both the capital investment for
purchasing a freeze drying system and the production cost, mainly the high energy
consumption, are high compared to other drying technologies. The process is therefore mainly
employed for very sensitive foods or high value products (Sablani et al. 2007b). Consequently, the search for an alternative drying method, which would result in a
product with quality parameters comparable to those of freeze-dried material, is of great
interest. Refractance Window® (RW) drying is a novel drying process developed by MCD
Technologies (Tacoma, Washington, USA). In this method hot water is used to carry thermal
energy to the product to be dehydrated (Nindo et al. 2003a).
12
The temperature of the water is usually a few degrees below boiling point (96-98°C) and
is kept at that range to avoid formation of bubbles, which would reduce heat transfer. During the
process the temperature of the food is usually around 71°C, far below the circulating water
temperature. Also, the residence time of the food on the equipment is around 3 to 5 min (Nindo
et al. 2004). This relatively low temperature and short residence time reduce product
degradation (Vega-Mercado et al. 2001).
Figure 5 shows the layout of a Refractance Window® dryer. The product is spread as a
thin film on top of a transparent plastic conveyor belt, under which hot water circulates. The food
flows co-currently with the hot water. Heat is transferred through the plastic sheet directly into
the product for drying. At the end of drying the product is cooled down, by moving over cold
water, and separated from the belt by a scraper device (Nindo et al. 2007).
Figure 5. Refractance Window dryer (Abonyi et al. 2002)
Refractance Window drying has shown to be a promising technique. Abonyi et al. (2002)
studied the effect of Refractance Window drying on quality retention of strawberry and carrot
purees. The retention of vitamin C in strawberry purees dried with RW drying was 94% and was
comparable to 93.6% in freeze-dried samples. Also, the color of RW dried strawberry purees
was similar to freeze-dried products. The overall flavor of strawberry purees was altered by RW
drying, with a decrease in fruity and green notes. For carrot purees, the color of RW dried
product was similar to fresh puree. The carotene losses for RW dried puree were 8.7% (total
carotene), 7.4% (α-carotene) and 9.9% (β-carotene) and were similar to losses for the freeze-
dried product.
13
Nindo et al. (2003b) analyzed the effects of different drying methods on the retention of
physical quality and antioxidants in asparagus. RW-dried pureed asparagus was closest in
greenness to freeze-dried product. RW and freeze drying enhanced the total antioxidant activity
of asparagus. RW drying retained the highest amount of ascorbic acid compared to other drying
methods.
The effect of different drying methods and storage on color characteristics of paprika
was studied by Topuz et al. (2009). The processes used were RW drying, freeze drying, hot-air
oven drying and natural convective drying. For reflected color, RW and freeze drying resulted in
products with better characteristics. The less color alterations were attributed to the mild drying
conditions of those methods. RW and freeze drying also resulted in samples with the lowest
browning index, indicating the occurrence of less non-enzymatic browning reaction during those
processes. During storage, a gradual discoloration in all paprika samples was noted.
Based on the results reported by other researchers, it is of interest to analyze how RW
drying would affect the quality of açaí juice and, therefore, this technique was selected for this
research.
Hot-air convective drying is the most widely used process for production of dehydrated
foods (Krokida et al. 2003). This technique consists of exposing the food product to a
continuously circulating stream of hot air, which supplies the thermal energy for vaporization
and also carries away moisture (Ratti 2001). The cabinet dryer consists of a closed cabinet
fitted with metal trays, which can be perforated or not. The product is placed on the trays as a
thin layer, usually 2 – 6 cm deep. Air is first heated and then blown over and/or through each
tray at 0.5 – 5 ms-1 (Fellows 2009). Figure 6 shows the basic layout of a convective cabinet
dryer.
Figure 6. Hot air dryer (Heldman et al. 1997)
14
Despite being very popular, hot-air convective drying uses high temperatures and long
drying times, which can cause serious damage to food color, flavor, nutrients, and texture,
negatively affecting the quality of the final product (Marfil et al. 2008). Nevertheless, for being an
ancient process and extensively used in the food industry, hot air drying is often used in drying
studies for comparison purposes. It was therefore selected for the present research.
Water Activity, Moisture Sorption Isotherms, and Glass Transition Temperature
Water Activity
It is well known that a relationship exists between the water content of foods and their
perishability (Franks 1991). However, different foods with the same moisture content may show
different perishability, implying that the water content, alone, is not a reliable indicator of product
deterioration. This is, partially, due to differences in intensity of association of water molecules
with nonaqueous constituents. In order to reflect the intensity of water association with other
compounds, the “water activity” (aw) concept was developed (Reid et al. 2008).
Water activity can be described as:
(1)
where p is the partial vapor pressure of water above the food, p0 is the vapor pressure of pure
water at the same temperature, and ERH is the equilibrium relative humidity (%) (Labuza et al.
1998).
The water activity concept can be used to assess moisture sorption isotherms, develop
new products, and aid in process design and control, among others (Schmidt 2004). However,
probably, the most important use of the water activity concept is to determine product stability
and shelf life. It has been extensively reported that water activity influences the rate of chemical
reactions, physical changes, and microbial growth/resistance in food products. The “stability
map” (Figure 7), introduced by Labuza et al. (1970), shows the general relationship between
water activity and several food stability phenomena (Nelson et al. 1994).
15
The map illustrates that lipid oxidation occurs rapidly at low water activities, which
means dehydrated foods are susceptible to this reaction. The rate of non-enzymatic browning is
very low at low water activities (Labuza et al. 1970). There is also a critical aw below which no
microorganisms can grow. For most foods, this value is around 0.6 – 0.7 aw (Vos et al. 1974).
Figure 7. “Stability map” (Labuza et al. 1972)
Domínguez et al. (2007) studied the effect of water activity on the stability of macadamia
nuts. For lipid oxidation evaluation, the aw values used were 0.215, 0.436 and 0.628. A higher
peroxide value was observed for samples stored at the lowest and highest aw. The total
oxidation index (TOTOX) was also higher for nuts stored at 0.215 and 0.628 aw. This is in
agreement with the stability map, which shows that lipid oxidation occurs faster at low and high
water activities
Acevedo et al. (2008) analyzed the kinetics of non-enzymatic browning in dried potatoes
through water-solids interactions and water mobility. The non-enzymatic browning rate,
measured at 70°C, showed a maximum value at aw equal to 0.84 and decreased markedly
above this value. At low water activities, an increase in aw accelerated the browning reaction,
probably due to increased molecular mobility. At high water activities water inhibited the reaction
and/or diluted the reactants, causing a decrease in non-enzymatic browning.
The effects of water activity and temperature on growth of Aspergillus niger, A. awamori
and A. carbonarius isolated from different substrates were investigated by Astoreca et al.
(2007). Mycelial growth and lag phase prior to growth were significantly influenced by aw,
temperature and their interactions. For most of the isolates the optimum aw for growth was 0.97,
with optimum temperature of 30°C. Overall growth was reduced up to 50% at 0.93 aw. Minimal
16
aw for growth was 0.85 at 30°C. Lag times at the optimal temperature varied a lot depending on
the water activity. Lag time for A. niger, for example, varied from 2 h at 0.97 aw to 260 h at 0.85
aw.
Sauvageot et al. (1991) studied the effect of water activity on crispness of different
breakfast cereals. Samples were equilibrated over saturated salt slurries with aw varying from 0
to 0.843. Sensory tests and mechanical analysis were used to investigate crispness. All the
products analyzed showed very similar results. In the sensory evaluation, a high crispness
intensity was observed until 0.53 aw. From 0.53 to 0.71 aw, however, there was a rapid decrease
in crispness intensity, followed by a slight decrease of that sensation as aw increased above
0.71. The results of the mechanical analysis were in agreement with the sensory test.
Moisture Sorption Isotherms
Moisture sorption isotherms describe the equilibrium relationship between moisture
content of a food and water activity, at constant temperature (Moreira et al. 2008). Isotherms
provide valuable information for food processing operations, such as drying and packaging.
They are also important for predicting final quality and product stability during storage (Moraga
et al. 2004).
Sorption isotherms can be obtained by rehydrating a dried sample (adsorption isotherm),
drying a wetted sample (desorption isotherm) or by the combination of the two methods
(working isotherm). Brunauer et al. (1940) classified adsorption isotherms into five types, as
showed in Figure 8. Most foods show a sigmoidal shape (type II) isotherm, while foods
containing soluble crystalline molecules, such as crystalline sugars, show a J-shape isotherm
(type III) (Al-Muhtaseb et al. 2002). Several factors influence the position and shape of an
isotherm, for example, sample composition, crystalline or amorphous sample structure, sample
preparation, temperature, and method of determination.
17
Figure 8. Classification of van der Waals adsorption isotherms (Brunauer et al. 1940)
Figure 9. Sorption isotherm (Okos et al. 2007)
According to Figure 9, type II sorption isotherms may be divided into three main regions
(Al-Muhtaseb et al. 2002):
- Region A: strongly bound and least mobile water (unfreezable at -40°C, not available for
chemical reactions, no plasticizing effect);
- Region B: less tightly bound water (also unfreezable at -40°C);
- Region C: exhibits properties of bulk water (available as solvent for reactions, supports the
growth of microorganisms).
18
The boundary of zones A and B represents the monolayer moisture content, which can
be thought as single layer of water molecules bound only to highly polar groups of the solid
matrix. Most dried foods show moisture content comparable to the monolayer value. At that
moisture content dry products exhibit their highest stability (Sablani et al. 2007a).
Mathematical models are usually used to describe sorption isotherms. The most
commonly ones are Brunauer-Emmett-Teller (BET) and Guggenhein-Anderson-de Boer (GAB)
models. BET is only linear at lower aw and fails above 0.5. The BET is a two-parameter equation
and is expressed as:
(2)
where M is the equilibrium moisture content (dry basis, db), M0 is the monolayer moisture
content (db), and C is a constant related to the heat of sorption of monolayer.
The GAB model is an extension of the Langmuir and BET theory, having three
parameters. The model is successful up to high water activities, but underestimates moisture
content values at aw > 0.93. The GAB equation is expressed as:
(3)
where M is the equilibrium moisture content (db), M0 is the monolayer moisture content (db), C
is a constant related to the heat of sorption of monolayer, and K is a constant related to the heat
of sorption of multilayer region (Basu et al. 2006; Brunauer et al. 1938)
Several methods are available for determining sorption isotherms. These methods can
be classified into the following categories: gravimetric (involves the measurement of weight
changes); manometric (measures the vapor pressure of water in the vapor space surrounding
the food); and hygrometric (measures the equilibrium relative humidity of air in contact with a
food product, at a specific moisture content) (Gal 1981).
Research about sorption isotherms of fruit powders is vast. Syamaladevi et al. (2009)
studied the water adsorption isotherm of freeze-dried raspberry at room temperature. The
isotherm exhibited a sigmoidal shape and was modeled using BET and GAB models. The
19
monolayer water content was 0.059 kg water/kg solids for the BET model and 0.074 kg water/
kg solids for the GAB model.
Sorption isotherms of vacuum dried mulberry at different temperatures (10, 20 and 30°C)
were analyzed by Maskan et al. (1998). The isotherms obtained for the three temperatures were
very similar and showed a sigmoidal shape. Although isotherms followed a typical type II
behavior, a sharp increase in moisture content at high water activities was observed, probably
due to the elevated sugar content of the product.
A J-shape isotherm was characteristic of freeze-dried acerola, a high sugar fruit. The
samples sorbed relatively small amounts of water at low aw, but an increase of sorption capacity
was observed at high water activities. Sorption isotherms were practically not affected by the
different freezing techniques analyzed (cryogenic freezing with liquid nitrogen, with vapor of
nitrogen, and by putting the samples in a freezer) (Marques et al. 2007).
Information about moisture sorption characteristics of dried açaí is important for
selection of appropriate packaging materials and prediction of its stability during storage.
Previous work of Tonon et al. (2009a) with spray dried açaí at 25°C showed that the product
presented a type III isotherm, which is probably due to the presence of carrier agents
(crystalline molecules). Da Silva et al. (2008) also studied the water sorption of spray dried açaí
and reported a type III isotherm for the product, at all temperatures used (15, 25 and 35°C).
Up to now, there is no information about the water sorption characteristics of freeze
dried, RW-dried or hot-air dried açaí.
Glass Transition Temperature
The moisture sorption isotherm of a food product is influenced, among others, by the
structure of the solid. Solids can exist as a crystal (ordered molecular arrangement) or as an
amorphous product (random molecular arrangement) (Bell et al. 2000).
The amorphous state can be further divided into glassy and rubbery states. Products in
the glassy state show low internal mobility and high internal viscosity, and are described as
solids. Rubbery materials show viscoelastic behavior and are described as supercooled liquids
(Le Meste et al. 2002).
The conversion between the glassy and the rubbery states is known as glass transition
and it occurs over a temperature range, which is frequently referred to as single temperature,
called glass transition temperature (Tg). Glass transition is a second-order transition and it is
characterized by a change in physical properties of the material, mainly heat capacity, free
20
volume, thermal expansion coefficient, and viscoelastic properties. The main factors affecting
glass transition are moisture content and temperature (Slade et al. 1995b; Roudaut et al. 2004;
Kasapis 2008).
The concept of glass transition has been used to predict food stability, especially of low
moisture products. Changes from the glassy to the rubbery state have shown to affect food
structure and have been suggested to influence molecular mobility and, consequently, the rates
of chemical reactions. Products in the glassy state (lower molecular mobility) have a higher
stability compared to products in the rubbery state (Roos 2003). At temperatures above glass
transition, the solid state is converted to a rubbery material and undesirable changes will occur,
like caking, stickiness, crystallization, collapse and loss of crispiness. As a result, the final
product might be unattractive and unfit for consumption (Schmidt 2004).
According to studies in the field of polymer science, the glass transition temperature of a
mixture is a non-linear function of the glass transition temperature of the individual components.
Due to the complexity of food systems and the occurrence of several thermal changes and
chemical reactions in the same temperature range, it is usually difficult to determine Tg of food
products (Bhandari et al. 2000).
Glass transition temperature is highly dependent on the molecular weight of the material.
In foods, Tg decreases with increase in low molecular weight compounds, mainly water. It is well
known that water acts as a plasticizer, increasing mobility due to increased free volume and
decreased local viscosity (Slade et al. 1995a; Slade et al. 1991). Also, due to the non-
equilibrium nature of the amorphous phase, glass transition is time-dependent. Therefore,
different glass transition temperatures may be found for the same product depending on the
time scale of the method used (Roos 2003). In addition, sample preparation, sample history and
the analytical procedure used will also influence glass transition temperature.
Many experimental methods can be used to measure glass transition temperature.
Among them, differential scanning calorimetry (DSC) is one of the most commonly used. DSC
measures changes in heat flow during heating or cooling of the test material. The glass
transition is manifested as a step change in the heat flow, due to different heat capacities of the
glassy (lower specific heat capacity) and rubbery states (higher specific heat capacity) (Schmidt
2004; Liu et al. 2006).
Methods used to produce materials in the amorphous state involve mainly two events:
rapid decrease in water content or rapid cooling (avoiding the formation of the crystalline state
due to lack of time). Dehydration processes result in foods in the amorphous state. For higher
stability, it is important to assure that dried products are in the glassy state. Any change in that
21
state may result in alteration of physico-sensorial attributes, structural damage and increase in
the rate of biochemical reactions, affecting the quality of the product (Bhandari et al. 1999).
Knowledge about glass transition temperature of açaí is still lacking. Tonon et al.
(2009a) analyzed the glass transition temperature of spray-dried açaí juice produced with
different carrier agents (maltodextrin with 10DE, maltodextrin with 20DE, gum Arabic and
tapioca starch). Temperatures varying from, approximately, -62 to 74°C were reported,
depending on the water activity at which the product was stored and the carrier agent used. The
researchers stated that spray dried pure açaí juice might have a much lower glass transition
temperature than the values obtained for the materials produced with additives. No information
about glass transition temperature of freeze dried, RW-dried or hot-air dried açaí is currently
available.
Anthocyanins
Anthocyanins are pigments that belong to the flavonoid group. They are responsible for
a wide variety of colors in plants, including purple, blue, red and orange. Anthocyanins are
water-soluble and have been widely used as food colorants (Castañeda-Ovando et al. 2009).
Anthocyanidins are the basic structure of anthocyanins. When anthocyanidins are
bonded to a sugar moiety (glycoside form), they are known as anthocyanins (Mazza et al.
1987). Figure 10 shows the general structure, also called flavilium cation, of anthocyanins.
Figure 10. General anthocyanin structure (the flavilium cation) (Mazza et al. 1987) R1 and R2 are H, OH or OCH3, R3 is a glycosyl or H and R4 is OH or a glycosyl
Differences in anthocyanins are due to: 1) number of hydroxyl/methoxyl groups present;
2) nature, number and sites of sugar attachment; and 3) types and number of aliphatic or
aromatic acids attached to the sugars. The most commonly encountered sugars are glucose,
galactose, rhamnose and arabinose. Caffeic, p-coumaric, ferulic and sinapic acids are the most
22
common aromatic acids, while acetic, malonic, malic, oxalic and succinic acids are examples of
aliphatic acids involved in acylation of sugars (Clifford 2000).
Kong et al. (2003) estimated that more than 400 different anthocyanins exist in nature.
The researchers also report the existence of 17 anthocyanidins, with pelargonidin, peonidin,
cyanidin, malvidin, petunidin and delphinidin being the most common in plants.
Anthocyanins are soluble in polar solvents and are generally extracted by using acidified
methanol. Identification and quantification of anthocyanins are done, primarily, by HPLC and
HPLC-MS. Several studies about anthocyanins in açaí juice have been published, but with
contradictory results. Lichtenthaler et al. (2005), Gallori et al. (2004) and Vera de Rosso et al.
(2008) detected cyaniding-3-rutinoside and cyaniding-3-glucoside as the main anthocyanins in
açaí whereas Del Pozo-Insfran et al. (2004) reported cyaniding-3-glucoside and pelargonidin-3-
glucoside as the main compounds. Schauss et al. (2006b) analyzed anthocyanins in freeze-
dried açaí and concluded that the cyaniding-3-rutinoside and cyaniding-3-glucoside were the
most abundant ones.
One of the most significant properties of anthocyanins is their antioxidant capacity. By
donating phenolic hydrogen atoms, these compounds are able to capture free radicals and
prevent oxidative damage. It is being suggested that anthocyanins contribute to the protective
effect of fruits and vegetables against degenerative and chronic diseases. These compounds
also exhibit therapeutic benefits, including cardioprotective, antiinflammatory and
anticarcinogenic properties (Heinonen et al. 1998; Stoner et al. 2007; Zafra-Stone et al. 2007).
Many assays have been used to measure the antioxidant capacity of anthocyanins and
other phenolic compounds. Among them, Oxygen Radical Absorbance Capacity (ORAC) has
been extensively used. ORAC provides a measure of antioxidant capacity against peroxyl
radicals. The test uses the following components: (a) an azo radical initiator, usually 2,2’-
azobis(2-methylpropionamidine) dihydrochloride (AAPH); (b) a molecular probe, normally
fluorescein (FL), to monitor the reaction progress; (c) the antioxidant compound of interest; and
(d) reaction kinetic parameter to quantify the antioxidant capacity. The method, basically,
involves the mixture of samples, controls (phosphate buffer) or standard (6-hydroxy-2,5,7,8-
tetramethylchroman-2-carboxylic acid - Trolox) with fluorescein and incubation at 37°C, followed
by addition of AAPH to initiate the reaction. The fluorescence intensity is measured periodically
and, as the reaction progresses, fluorescein is consumed and its intensity decreases.
Fluorescein decay is inhibited / limited in the presence of antioxidants (Huang et al. 2005).
Anthocyanins are fairly unstable and quite susceptible to degradation. Their greatest
stability occurs under acidic conditions. The main factors affecting anthocyanins stability are pH,
23
chemical structure, temperature, and oxygen concentration. Secondary factors include light,
metal ions, and presence of degradative enzymes (Markakis 1982).
In an aqueous medium, depending on the pH of the solution, anthocyanins can exist in
different chemical forms (Figure 11): red flavylium cation (AH+), blue quinonoidal base (A),
colorless carbinol pseudobase (B), and colorless chalcone (C). Usually, as the pH increases
anthocyanins are converted into colorless forms (Brouillard 1982).
Figure 11. Anthocyanin structures in aqueous solutions (Brouillard 1982)
Anthocyanins are susceptible to molecular oxygen due to their unsaturated nature and
reaction with oxygen causes degradation of their color. Anthocyanins are also readily destroyed
by heat during processing and storage of foods (Delgado-Vargas et al. 2000).
Mejia-Meza et al. (2010) studied the effect of drying on the anthocyanin content and
antioxidant capacity of raspberries. Dehydration resulted in a significant decrease in
anthocyanin content. Losses ranged from, approximately, 30-80% for aglycones and 75-95% for
glycosides, depending on the drying method used. Considerable loss of antioxidant capacity
was also observed.
The effect of different drying methods – convective drying, vacuum-microwave drying,
vacuum drying and freeze drying - on bioactive compounds of strawberry fruits was studied by
Wojdyło et al. (2009). The researchers found that drying destroyed anthocyanins and flavanols.
Also, all dried products showed a significantly lower antioxidant capacity.
24
Lohachoompol et al. (2004), by analyzing anthocyanin degradation during drying of
blueberries, reported losses of 41% for samples dried in a cabinet drier and 49% for fruits dried
using osmotic pretreatment followed by cabinet drying.
Meschter (1953) examined the effect of temperature on the stability of anthocyanins in
strawberry preserves. At a storage temperature of 20°C, the half-life of the pigment was 54 days
whereas at 38°C the half-life was 10 days. Processing strawberry preserve at 100°C would
result in a half-life of 1 hour.
Flavor Analysis
Flavor is a sensory impression of a food and is established by the chemical senses of
taste, smell and “trigeminal” (d'Acampora Zellner et al. 2008). Among those senses, smell is,
probably, the most important flavor determinant. Smell can be divided into aroma - smell of a
food before it is put in the mouth - and odor - retronasal smell of a food in the mouth (Acree
1993).
The study of flavor is of great importance for food, since, together with color, texture and
other parameters, it determines food quality (Acree 1993). Flavor is also used to define
characteristics and identity of products, such as in wines (Flamini 2005)
The knowledge and identification of odor active compounds, also called flavor
compounds, has improved with the advent of gas chromatography (GC) and mass spectrometry
(MS) (d'Acampora Zellner et al. 2008). Gas chromatography is a separation technique, not an
identification technique. However, when coupled with a mass spectrometer the instruments
provide a great identification method (Mussinan 1993). Currently, approximately 7000 flavor
compounds have been identified (Rowe 2005).
It has been reported that just a small portion of the large number of volatiles occurring in
foods contributes to their perceived odor (Grosch 1994). Also, human perception of volatile
compounds depends on the extent of release from the matrix and their odor properties (Van
Ruth 2001). Further, the contribution of those molecules to the flavor profile of a product is not
equal and the peak area generated by a “chemical” detector is not directly translated into
intensity (d'Acampora Zellner et al. 2008).
Therefore, the grouping of olfactometry and GC (GC-O) results in another useful tool for
discrimination of odor active compounds. As the human nose is more sensitive than “chemical”
detectors for many flavor compounds, GC-O uses the human olfactory system as one of the
25
detectors. In this method, the human subject sniffs the gas chromatographic effluent of a food
extract, determining which volatiles are odor active and providing descriptors. With the use of
GC-O it is possible to list and order the odorants present in a food (Van Ruth 2001).
The detection of flavor compounds by GC-O depends on several factors, such as odor
threshold (main factor), amount of food sampled, dilution of the volatile fraction, and sample size
analyzed. Therefore, a screening method is necessary to identify the most important odorants
(Grosch 1993). Aroma extract dilution analysis (AEDA) is probably the most frequently used
method for that purpose. In this technique, the flavor extract is serially diluted (usually 1:2, 1:3,
1:5 or 1:10) and each dilution is analyzed by GC-O. The result is expressed as a flavor dilution
(FD) factor, which corresponds to the highest dilution at which the odorant can be perceived
(Ferreira et al. 2002).
The flavor profile of a food product is closely related to the extraction procedure used,
which should yield an extract representative of the sample. For that reason, the choice of an
adequate isolation procedure is also very important. Two very popular techniques used for
sample preparation are solvent-assisted flavor evaporation (SAFE) and solid-phase
microextraction (SPME) (d'Acampora Zellner et al. 2008).
SAFE is a technique that extracts volatiles under high vacuum and low temperature
conditions. The method involves adding, into the apparatus, small droplets of samples, which
will be immediately transformed into a vapor spray. As a result, volatiles and solvent are
transferred into a flask, which is cryogenically cooled with liquid nitrogen. The method is suitable
for a solvent extract or a food matrix (aqueous foods or matrices with high oil content) (Engel et
al. 1999).
SPME uses a fused silica fiber coated with a specific stationary phase to adsorb the
flavor compounds. The fiber is first submerged in a liquid phase or exposed to a gaseous
phase, allowing the analytes to be directly extracted. Next, absorbed analytes are thermally
desorbed in the GC injection port. The method involves short preparation time and saves
solvent and disposal costs (Kataoka et al. 2000; Deibler et al. 1999).
Many factors can affect the flavor of a food product and processing is, probably, one of
the most important. Thermal processing of foods usually results in loss of volatile compounds,
or reduction of their concentration, and development of new odors due to a series of chemical
reactions, such as Maillard and lipid oxidation. These factors will change the flavor profile of the
product, which can affect its acceptability (Ruiz Perez-Cacho et al. 2008).
The characterization of the flavor compounds of açaí juice is of great interest to food
companies, since a large number of new products containing the fruit are being launched. It is
26
also important to evaluate how drying affects the flavor of açaí. However, up to date, information
about the flavor profile of açaí juice or dried açaí is still not available.
The effect of heat treatment on the flavor profile of fruits has been widely studied. Feng
et al. (1999) analyzed the effect of different drying methods on flavor retention of blueberries.
Heating caused some odor compounds to disappear while new volatiles were created or had
their concentration increased, which changed the flavor profile of the fruit.
Changes in volatile compounds during dehydration of d’Agen prunes was evaluated by
Sabarez et al. (2000). Four major components were identified in fresh plums. After drying, three
of the four main compounds completely disappeared and three new volatiles were generated.
Among the new ones, the researchers highlighted the presence of a furan derivative, which was
probably derived from caramelization or Maillard reactions.
Wang et al. (2007) compared the volatiles of banana powder dehydrated by air drying
(AD), vacuum belt drying (VBD), and freeze drying (FD). The researchers found that the
characteristic aroma of fresh banana was present in the dried samples, but the level of flavor in
those products was different depending on which drying technique had been used. As the
drying temperatures used in AD and VBD were higher than that of FD, some of the original
esters disappeared in the AD and VBD samples while some new compounds were formed.
Luning et al. (1995) evaluated the effect of hot air drying on flavor compounds of bell
peppers and found that many “fresh” odor notes decreased or disappeared after drying,
whereas several compounds derived from lipid oxidation were formed or had their
concentrations increased during the drying process.
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Chapter 3: Effects of freeze drying, Refractance Window drying, and hot-air drying on the quality
parameters of açaí
Materials and Methods
Chemicals
- Moisture Working Isotherm: NaCl, KI, Mg(NO3)2·6H2O, K2CO3, MgCl2, and LiCl salts were
obtained from Fisher (Fair Lawn, NJ), and KC2H3O2 was obtained from Sigma-Aldrich (St. Louis,
MO).
- HPLC: Cyanidin-3-glucoside and cyaniding-3-rutinoside were purchased from Polyphenols
Laboratories (Sandnes, Norway).
- ORAC assay: 2,2’-Azobis-(2-methylpropionamidine) dihydrochloride (AAPH) and 6-Hydroxy-
2,5,7,8- tetramethylchroman-2-carboxylic acid (Trolox) were obtained from Sigma-Aldrich (St.
Louis, MO). Fluorescein was purchased from Fisher (Hanover, IL). Potassium phosphate
monobasic and potassium phosphate dibasic were purchased from Fisher (Fair Lawn, NJ).
- Flavor Standards: All standard compounds were purchased from Sigma-Aldrich (St. Louis,
MO), except for (Z) 3-hexen-1-ol and α-ionone, which were obtained from Bedoukian Research
(Danbury, CT); isoeugenol and 1-octen-3-one were purchased from Alfa-Aesar (Ward Hill, MA);
(E,E,Z) 2,4,6 nonatrienal was synthesized following the method described by Schuh et al.
(2005); (Z) 2-nonenal was synthesized from (Z) 2-nonen-1-ol (Bedoukian Research, Danbury,
CT) by oxidation with Dess-Martin periodinane (0.3M in dichloromethane) based on the
procedure described by Meyer et al. (1994); β-damascenone was provided by Firmenich
(Princeton, NJ), and (Z) 1,5-octadien-3-one was synthesized as previously described by Ullrich
et al. (1988a).
Açaí Juice
Açaí juice (36lb) was purchased from Earthfruits (South Jordan, UT) and shipped frozen
overnight to the Department of Food Science and Human Nutrition at University of Illinois at
Urbana-Champaign. The juice was produced with fruits of Euterpe oleracea Martius species and
contained 11-13% total solids, 2 – 5 °Brix, pH < 4.6 and 0.4 – 0.45% acidity (data from
specification sheet). Juice production involved a pasteurization step.
36
Upon arrival, the juice was thawed overnight and then divided into 3 buckets containing
14 lb (to be used for hot-air drying), 14 lb (to be used for Refractance Window drying), and 8 lb
(to be used for analysis of the fresh juice).
Drying Methods
Freeze drying
Freeze-dried açaí was kindly donated by Liotécnica (Embu, São Paulo, Brazil). The
product was vacuum packed and was kept in the original package at -20°C until beginning of
storage test. The fruits used to produce the freeze-dried product were from the same species
and were harvested at the same location as the fruits used to produce the açaí juice from
Earthfruits.
Although freeze-dried açaí was from a different lot of fruit, this product was used for
comparison with RW-dried and hot-air dried samples in order to evaluate the effect of each
drying method.
Hot-air drying
For hot-air drying, açaí juice was thawed overnight and then placed in metal trays, 1-2
cm deep. Product was dried at 65°C for approximately 20 hours. Drying was performed on a
convection cabinet dryer V-33 (Despatch Oven Co, Minneapolis, MN). The procedure was
based on the work of Krokida et al. (2003).
Refractance Window drying
Frozen açaí juice was shipped overnight to Tacoma, WA. MCD Technologies performed
the RW drying of the product. The dwell time of the juice on the dryer’s heated surface varied
between 1min 15 sec to 1min 29 sec. The circulating water temperature was approximately
94°C and the product temperature was around 62 to 71°C.
Storage Test
After drying, products were placed in amber glass bottles, flushed with nitrogen for 4 min
and stored at room temperature (25°C) for up to 3 months. Samples of dried açaí were taken
from storage every month and placed at -20°C until analysis (Piga et al. 2003). Açaí juice was
analyzed only on day zero and the results were used as a reference for comparison with dried
samples.
37
Moisture Content and Water Activity
The moisture content of the açaí powders (2 g) was determined using a vacuum oven
(Equatherm, Curtin Matheson Scientific, Inc., Houston, TX) at 60°C and 30 inches Hg for 24
hours (Yu et al. 2008). The moisture content of the açaí juice was determined in two steps: first,
16 g of juice were dried in a convection oven (Precision Scientific, Inc.) at 80°C for 2.5 hours;
then, samples were dried in a vacuum oven (Equatherm, Curtin Matheson Scientific, Inc.,
Houston, TX) at 60°C and 30 inches Hg for 24 hours.
The water activity of açaí powders was measured using the AquaLab aw meter
(Decagon Devices, Inc., Pullman, WA) at 25°C.
Color
Instrumental color was determined using a HunterLab spectrocolorimeter (LabScan II,
Hunter Associates Laboratory Inc., Reston, VA), calibrated with white and black standards. The
spectral curve was determined over the 400 to 700 nm range. Color values were expressed as
L (whitness/darkness), a (redness/greenness) and b (yellowness/blueness). Also the hue angle
(Eq. (4)) and chroma (Eq. (5)) were used to characterize the products.
(4)
(5)
Moisture Sorption Working Isotherm
To obtain the sorption isotherms, duplicate samples from each drying method (1.5 g)
were placed in previously weighed plastic cups. Samples were then placed in “Lock & Lock”
(Heritage Mint, Ltd., Scottsdale, AZ) containers and equilibrated to seven saturated salt slurries
at 25°C. The relative humidity values of NaCl (75%), KI (69%), K2CO3 (43%), MgCl2 (33%),
KC2H3O2 (23%), and LiCl (11%) were obtained from Greenspan (1977), and Mg(NO3)2·6H2O
(55%) was obtained from Hirose et al. (2006).
During the equilibration period, samples were weighed every week until the observed
weight change was within criteria (2 mg/g). After that, equilibrium moisture content (db) of the
dried açaí was calculated based on the weight change and initial moisture content. The water
activity of samples and salt slurries were also determined using the AquaLab. The moisture
content of the samples and the relative humidity values from the references were used to
38
generate the working isotherms, which were then fit to GAB and BET models. BET model was
applied to water activity values lower than 0.5.
The root mean square error (RMSE) was calculated and used to represent the fitting
ability of the models in relation to the number of data points.
Glass Transition Temperature
Glass transition temperature was determined by differential scanning calorimetry in a
DSC Q2000 (TA Instruments, Newcastle, DE) using Tzero hermetically sealed aluminum pans
and lids. DSC was equipped with a refrigerated cooling system.
Açaí powders (~10 mg) were placed into the DSC pans and analyzed. Samples were
first cooled to -65°C, at 50°C/min, then equilibrated to -65°C, and finally heated to 250°C, at
10°C/min. An empty pan was used as a reference. Dry nitrogen, 50 mL/min, was used as a
purge gas. Analysis of data was done using the software Universal Analysis 2000.
Preliminary studies suggested that the high fat content of açaí was interfering with the
DSC results. Therefore, it was decided to also perform an oil extraction and run DSC with both
the oil and solid phases in order to try to identify the glass transition temperature of the product
and to estimate the influence of the fat on the thermogram. For oil extraction 1g of dried sample
plus 5 mL of pentane:ether (50:50, v/v) were placed in test tubes. Tubes were shaken 30min
and centrifuged. Supernatant containing the oil phase was collected and the above extraction
procedure was repeated 2 more times. Solvent was removed under a stream of nitrogen in
order to recover the oil and solids.
Anthocyanin Analysis
Extraction Procedure
Extraction of anthocyanins was based on the method of Wu et al. (2004b) with
modifications. For açaí juice, 5 g of samples were placed in 125 mL screw cap erlenmeyers.
Next, 25 mL of methanol and 150 μL of acetic acid (resulting in 85:15:0.5 methanol/water/acetic
acid; v/v) were added and samples were extracted on an orbital shaker (250 rpm) at room
temperature for 2 h. Product was then centrifuged at 4550 g for 10 min, at 4°C. The supernatant
was filtered using a 0.2 μm Teflon syringe filter prior to HPLC analysis.
For açaí powders, 1 g of freeze-dried, 1g of RW-dried or 3 g of hot-air dried samples
were placed in 125 mL screw cap erlenmeyers. After that, 25 mL of methanol/water/acetic acid
(85:15:0.5; v/v) were added and samples were extracted on an orbital shaker (250rpm) at room
39
temperature for 2 h. Product was then centrifuged at 4550 g for 10 min, at 4°C. The supernatant
was filtered using a 0.2 μm Teflon syringe filter prior to HPLC analysis.
Analysis by HPLC and HPLC-MS
Anthocyanin separation was based on the method of Grace et al. (2009) with
modifications.
Analysis was performed on a Waters Alliance 2695 HPLC (Waters, Milford, MA)
equipped with a photodiode array detector (HP Series 1050). Separation was carried out on a
reversed phase Phenomenex Prodigy 5u ODS (3) 100A column, 250 x 4.6mm x 5 micron, with
a Phenomenex Prodigy 5u ODS (3) 100A guard column, 30 x 4.6mm x 5 micron. Mobile phases
consisted of 5% formic acid aqueous solution (A) and 100% methanol (B). The flow rate was
1ml/min and detection was at 520 nm. The gradient system used was 10%, 15%, 20%, 25%,
30%, 45%, 10%, and 10% of solvent B at 0, 5, 15, 20, 25, 35, 37, and 50 min, respectively. The
volume of samples injected was 15 μL.
The HPLC-MS analysis was made with an LCQ Deca XP mass spectrometer (Thermo
Finnigan Corp., San Jose, CA), electrospray ionization (ESI) in the positive ion mode (m/z 200-
2000) and a photodiode array (PDA) detector (200-600 nm). Separation was carried out on a
150 x 2.1mm x 5 micron C18 reversed-phase column (Vydac, Western Analytical, Murrieta, CA).
Mobile phases consisted of 0.1% formic acid in water (A) and 100% methanol (B). A step
gradient of 10%, 15%, 20%, 25%, 30%, 60%, 10%, and 10% of solvent B at 0, 5, 15, 20, 25, 45,
47, and 60 min, respectively, a flow rate of 200 μL/min, and an injection volume of 15 μL were
employed.
For quantification and identification purposes cyaniding-3-glucoside (C-3-G) and
cyaniding-3-rutinoside (C-3-R) were used as external standards. Results were expressed as mg
anthocyanin (C-3-G + C-3-R) / g dry weight (DW).
Oxygen Radical Absorbance Capacity (ORAC)
Extraction Procedure
Extracts were prepared according to the method of Ou et al. (2001) with modifications.
For açaí juice, 4.17 g of sample were placed in 125 mL screw cap erlenmeyers. Then,
6.3 mL of water and 10 mL of acetone (resulting in 50:50 acetone/water; v/v) were added and
samples were extracted on an orbital shaker (250 rpm) at room temperature for 2 h. Product
40
was centrifuged at 16300 g for 15 min, at 4°C. Supernatant was diluted (1:200) with phosphate
buffer before analysis.
For dried açaí, 0.5 g of powders were placed in 125 mL screw cap erlenmeyers.
Samples were extracted with 20 mL of acetone/water (50:50; v/v) on an orbital shaker (250 rpm)
at room temperature for 2 h. Product was then centrifuged at 16300 g for 15min, at 4°C.
Supernatant was diluted (1:200) with phosphate buffer before analysis.
Solutions
- Phosphate buffer: 0.075 M phosphate buffer, pH = 7.4, was used for all dilutions and as a
blank in the assay.
- Fluorescein (FL): A stock fluorescein solution (Stock #1) was prepared by dissolving 0.0188 g
FL in 50 mL of phosphate buffer. A second stock solution (Stock #2) was prepared by diluting
516 μL of Stock #1 in 25 mL of phosphate buffer. Then, 141 μL of Stock #2 were diluted in 25
mL of phosphate buffer and 120 μL of this solution was added to each well.
- AAPH: The AAPH solution was prepared by dissolving 0.1085 g AAPH in 10 mL of phosphate
buffer. Sixty microliters of that solution were added to each well.
- Trolox standards: A Trolox stock solution (1mM) was prepared by adding 0.0129 g of Trolox in
50 mL of phosphate buffer. The stock solution was further diluted with phosphate buffer to give
the following concentrations: 150, 100, 80, 60, 40 and 20 μM. The Trolox solutions were used
as standards to generate the calibration curve.
Microplate Assay
For ORAC assay, the method of Dávalos et al. (2004) was used. The assay was carried
out in black-walled 96-well plates (Fisher Scientific, Hanover Park, IL). Aliquots of 20 μL of
phosphate buffer (blank); either 20 μL of Trolox standards or 20 μL of samples; were added into
appropriate wells. Then, 120 μL of fluorescein was added to each well and the plate was
incubated at 37°C for 15 min. After incubation, 60 μL of AAPH were added. The plate was read
in a fluorescent plate reader, FLx800tbi (Bio-Tek, Winooski, VT) at 37°C, sensitivity 60,
excitation 485 nm and emission 582 nm. Readings were done every 2 min for 120 min.
The final ORAC values were calculated by using a regression equation between the
Trolox concentrations and the net area under the FL decay curve. Data are expressed as
micromoles of Trolox equivalents (TE) per gram dry weight (μmol TE/g DW).
41
Flavor Analysis
Açaí juice
Internal Standards
An internal standard solution, containing 2-methyl-3-heptanone and 2-ethylbutyric acid,
was prepared at concentration of 1 mg/mL each.
Sample Preparation
Açaí juice, 500 g, was first thawed and then placed in a beaker. Under stirring, 5 μL of
internal standard solution was added to the product. Juice was then mixed for 2 min and sieved
prior to extraction of volatiles.
Isolation of Volatiles
Solvent-assisted flavor evaporation (SAFE) was used to isolate the volatile compounds
and was based on the procedure of Lozano et al. (2007b), with slight modifications. Over a
period of 1h the sample was fed in dropping aliquots into the SAFE system (distillation head at
40°C, sample flask at 40°C and vacuum at less than 1 x 10-4 Torr). SAFE unit was then kept at
10-5 Torr for 2 h prior to removal of the product from the system. Product was kept outside for 30
min and then 50 mL of ether was added. Mixture was thawed overnight before extraction.
Extraction was done according to the procedure of Lozano et al. (2007a), with some
modifications. First, 2 M NaOH was added to the SAFE extract until pH=9 (approximately 0.5
mL). Product was then transferred to a separatory funnel and 40 g of NaCl were added. The
mixture was shaken for 3 min and let stand for 3 min. The ether layer was placed in a 500 mL
round-bottom flask and the aqueous layer was extracted two more times with 30 mL of ether,
following the same procedure. After that, the aqueous layer was acidified with 4N HCl to a pH=2
(around 0.7 mL). Aqueous layer was extracted three times with 30 mL of ether.
The combined ether layers (in 500 mL round-bottom flask) were concentrated to 30 mL
by distillation at 42°C using a Vigreaux column. The concentrate was fractionated into
neutral/basic (NB) and acidic (AC) fractions, according to the procedure of Karagül-Yüceer et al.
(2003) with modifications. Extract was first washed with sodium carbonate (3 x 20 mL), followed
by separation of the ether (NB fraction) and aqueous (AC fraction) layers. The ether layer was
washed twice with 10 mL of saturated NaCl and the upper ether phase was collected and
concentrated to 10 mL by distillation at 42°C using a Vigreaux column. It was next dried over
42
anhydrous Na2SO4 and concentrated to 1 mL, again by distillation. Extract containing NB
volatiles was transferred to 2 mL vial and concentrated to 200 μL under N2. The aqueous layer
was washed twice with 15 mL of ether (which was discarded) and then acidified with 4N HCl
saturated with NaCl to pH ~2. Acidic volatiles were extracted with ether (3 x 20 mL). Ether layer
was concentrated to 10 mL by distillation at 42°C using a Vigreaux column, dried over
anhydrous Na2SO4 and concentrated to 1 mL again by distillation. AC extract was transferred to
2 mL vial and concentrated to 200 μL under N2. Both fractions were kept at -70°C until analyses.
In order to determine the predominant odorants in açaí juice, each of the fractions was diluted
sequentially with diethyl ether in a ratio of 1:3 according to the aroma extract dilution analysis
technique. Each dilution (1 μL) was later injected in GC-O.
Identification of volatiles
GC-MS and GC-O were used to identify the main volatile compounds of açaí juice. GC-
MS was performed on an Agilent 6890N GC/5973N mass selective detector (MSD) system.
Each extract (1 μL) was injected in the cold on-column injection mode. Two columns with the
same dimensions (30 m length x 0.25 mm i.d x 0.25 µm film thickness) but different polarities
(Stabilwax and SAC-5) (Restek, Bellefont, PA and Supelco, Bellefont, PA, respectively) were
used. The oven was programmed from 35 to 225°C at a rate of 4°C/min with initial and final hold
times of 5 and 25 min, respectively. Helium was used as carrier gas at a constant rate of 1.0
mL/min. MSD conditions were as follows: MS transfer line heater, 280 °C; ionization energy, 70
eV; mass range, 33–350 amu; and scan rate, 5 scans/s. For better sensitivity the electron
multiplier voltage was 200 V above the autotune setting.
GC-O was conducted on an Agilent 6890 GC (Palo Alto, CA) equipped with an FID and
a sniffing port (DATU, Geneva, NY). Two different polarity columns (Stabilwax and RTX-
5SILMS) (Restek, Bellefont, PA) with the same dimensions (15 m length x 0.32 mm i.d x 0.5 µm
film thickness) were used to obtain odor description and retention indices. Each extract (1 μL)
was injected in the cold on-column injection mode. The oven was programmed from 35 to
225°C at a rate of 10°C/min for NB fraction and 6°C/min for AC fraction in Stabilwax column and
6°C/min for both fractions in RTX-5SILMS column. Initial and final hold times were 5 and 25
min, respectively. Helium was used as carrier gas at a constant rate of 1.0 mL/min.
Temperature of FID and transfer line were held at 250°C. The end of the capillary column was
split 1:5 between the FID and sniffing port.
Compounds were, first, tentatively identified based on comparison with published
retention indices and odor quality (Rychlik et al. 1998; Acree et al. 2004). Positive identification
43
of odorants was based on matching retention indices, odor description (both on two different
polarity columns), and mass spectra of unknown compounds with those of authentic standards.
An n-alkane series was used for the determination of retention indices.
Dried Açaí
Headspace volatiles were extracted and concentrated using SPME headspace
sampling. Açaí juice (used as control) and dried samples from day 0 and after 3 months of
storage were used for the analysis.
Sample Preparation
For açaí juice, 5 mL of samples, 1 g of NaCl and a micro stirring bar were placed into 20
mL screw cap glass vials with PTFE/Silicone-coated septa (SPME vials). For açaí powders,
samples were first rehydrated with deodorized water to a final moisture content of 87.86%
(same moisture content of juice). Then, 5 mL of samples were added to SPME vials as
previously described. An internal standard (IS) solution containing 2-methyl-3-heptanone, was
prepared at concentration of 0.0232 mg/mL and 2 μL of IS were added to each vial.
Headspace sampling
Headspace sampling was based on the procedure of Mahattanatawee et al. (2007), with
modifications. SPME vials were, first, incubated for 15 min at 40°C in a single magnetic mixer,
with agitation set to 250 rpm. After that, a SPME fiber (50/30μm DVB/Carboxen/PDMS on a
1cm StableFlex fiber, Supelco, Bellefont, PA) was inserted into the headspace of the sample
vial and exposed for 45 min at 40°C. Fiber was, then, thermally desorbed in the GC-MS injector
port for 14 min at 260°C (vent after 4 min). The fiber was preconditioned prior to analysis at
270°C for 1 h.
CG-MS
GC-MS was performed on an Agilent 6890N GC/5973N mass selective detector (MSD)
system equipped with an RTX-Wax (30 m length x 0.25 mm i.d x 0.25 µm film thickness) column
(Restek, Bellefont, PA). The oven was programmed from 35 to 225°C at a rate of 4°C/min with
initial and final hold times of 5 and 15 min, respectively. Helium was used as carrier gas at a
constant flow rate of 1.0 mL/min. MSD conditions were as follows: MS transfer line heater, 280
44
°C; ionization energy, 70 eV; mass range, 33–350 amu; and scan rate, 5 scans/s. For better
sensitivity the electron multiplier voltage was 200 V above the autotune setting.
Compound Quantification
For quantification purposes, standard curves of selected flavor compounds were used.
Known amounts of authentic standards plus internal standard were added into a deodorized
açaí juice matrix and SPME was performed in the same way as described above. The ratios of
authentic standards and IS concentrations were: 1:1, 1:10, 1:50 and 1:100. Deodorized juice
matrix was achieved by running the product through SAFE twice and collecting the solids left
after water evaporation.
Statistical Analysis
Significant differences between treatments were determined using the one-way ANOVA
followed by Tukey’s test at α = 0.05.
Results and Discussion Moisture Content
The moisture content of the juice was 87.86 ± 0.04%, confirming that the product used
was a Type B açaí (11-14% solids) (Table 2). The moisture content of the dried products over
time is presented in Figure 12. The RW-dried samples presented the highest moisture content
(2.19% wb), followed by freeze-dried (1.55% wb) and hot-air dried samples (1.36% wb)
(reported moisture content values on day 0). Similar results were obtained during the entire
study.
Drying time and air relative humidity are very important in drying operations since they
affect the final moisture content of the product (Kaya et al. 2008; Krokida et al. 1998). During
RW drying, ambient air is in contact with the product and, therefore, air relative humidity is not
45
controlled. The higher moisture content of RW-dried açaí may have been caused by insuficient
drying time or air with high relative humidity.
Figure 13 shows the effect of storage on the moisture content of açaí powders. The
moisture content of all dried products on day 0 was significantly lower than the values obtained
for samples after one month of storage. Changes in moisture might have been caused by
changes in the material, resulting in more volatiles being produced during vacuum oven
measurement. It could also be due to a measurement error on day 0. Water could also have
been produced during the first month of storage, resulting in higher moisture content, although
this fact is not compatible with the water activity results. Further research is necessary in order
to explain why moisture content was significantly lower on day 0.
Figure 12. Effect of different drying methods on moisture content * Within each segment, bars with different letters are significantly different
46
Figure 13. Effect of storage on moisture content * Within each segment, bars with different letters are significantly different
Water Activity
All dried samples exhibited low water activity values, suggesting good product stability.
When comparing different treatments (Figure 14) at day 0, the RW-dried açaí showed a
significantly higher water activity (~0.240), followed by the freeze-dried (~0.196) and hot-air
dried samples (~0.119), respectively. For samples having a relatively high moisture content, the
water activitiy was also higher. The water activity for samples from the three drying methods is
thus in the sequence of RW > FD > HAD.
The effect of storage time on water activity is presented in Figure 15. Water activity
values, within each individual treatment, did not change over the studied period of time,
suggesting that glass is a good packaging material to avoid moisture uptake from the
surrondings during storage. Similar results were reported by Mahayothee et al. (2009), working
with dried lychees stored in polyethylene film bags, and Yongsawatdigul et al. (1996), working
with dehydrated cranberries (packaging material was not described).
47
Figure 14. Effect of different drying methods on water activity * Within each segment, bars with different letters are significantly different
Figure 15. Effect of storage on water activity * Within each segment, bars with different letters are significantly different
Color
For evaluation of color, L, a and b values were measured using a Hunter colorimeter.
The values of a and b were used to calculate hue angle and chroma, measurements that are
most significantly related to visual color scores than the individual a and b values (Shishehgarha
et al. 2002). A comparison of the color readings of açaí juice with those in samples dried by
48
different methods at day 0 is given in Table 3. The L values increased significantly in all dried
samples compared with the juice, with the L for the RW-dried being significantly higher or
"lighter" than the hot-air and freeze-dried. The changes in a and b values after drying seemed to
follow similar trend. A significant decrease in a and b values can be observed in all the samples.
Obviously, the dried açaí had a color totally different from that of the juice.
In the analysis of color, plotting a and b values results in a color wheel, which subtends
360°, with red-purple placed at an angle 0°, yellow at 90°, bluish-green at 180° and blue at 270°,
counterclockwise. The hue angle determines in which quadrant the sample will be placed and
gives information about the actual hue of the product (McGuire 1992). Drying caused a
significant decrease in hue angles for all dried samples (Table 3). However, all the samples
were still in the same quadrant (Figure 16) and the changes in the hue angle, and consequently
the hue, although significant, were relatively small. The higher hue angle of the juice represents
a less redish and, consequently, higher yellowish color, which is probably due to its oily
appearance. On the other hand, the lower hue angles of the dried samples stand for a more
redish color. A red color reinforcement in dried samples has been attributed to pigment
concentration due to water reduction (Hammami et al. 1997). In dried samples, the chroma
values, which indicate color saturation or intensity, decreased compared to the açaí juice. A less
vivid and less intense color may be expected in the dried products. Overall changes in color due
to drying were most likely caused by anthocyanin degradation and formation of brown pigments
by enzymatic browning and Maillard reaction.
Table 3. Color measurements of açaí juice and powders on day 0
* Within each column, bars with different letters are significantly different
L a b Hue angle ChromaJuice 12.42c 16.61a 4.94a 0.29a 17.33aFreeze-dried 14.56b 4.94c 0.35b 0.075b 4.97cHot-air dried 15.26b 3.66d 0.48b 0.13ab 3.69dRW-dried 19.05a 6.44b 0.59b 0.092b 6.47b
49
Figure 16. Representation of sample’s hue
The color changes as affected by different drying methods are presented in Figures 17-
19, using lightness, hue angle and chroma. Figure 17 shows the effect of the three drying
methods on lightness. No significant difference was observed between freeze and hot-air dried,
samples. However, the RW-dried samples presented consistently higher L values compared to
the other dried products. This higher lightness might have been caused by the açaí oil, which
may have coated the surface of the thin layer of product used in this technology, giving it a shiny
appearance and the impression of a lighter color. Figure 18 presents the effects of the drying
techniques on hue angle. No significant difference in this parameter among the treatments was
observed for up to two months of storage. However, on the third month, the hot-air dried
samples showed a significantly higher hue angle, which represents a decrease in its red color.
When considering chroma (Figure 19), the RW-dried açaí showed the highest chroma value,
followed by the freeze and hot-air dried samples. A more saturated color in RW-dried samples
might have been influenced by the oil coating, which gives it a more vivid appearance.
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0 2 4 6 8 10 12 14 16 18
a
b
Freeze-dried Hot-air dried RW-dried Juice
Red-Purple
Yellow
50
Figure 17. Effect of different drying methods on Lightness * Within each segment, bars with different letters are significantly different
Figure 18. Effect of different drying methods on hue angle * Within each segment, bars with different letters are significantly different
51
Figure 19. Effect of different drying methods on chroma * Within each segment, bars with different letters are significantly different
When considering the color differences within each drying method as a function of
storage time, values of the lightness, hue angle, and chroma are plotted against each drying
method and are shown in Figures 20-22. For all the drying technologies, lightness decreased
during storage, which means that the products were turning darker. This was most likely due to
the formation of brown pigments as a result of Maillard reaction and/or polymerization of
oxidized phenolic compounds (Perera 2005). At the water activity values of the dried samples,
occurrence of chemical reactions was not expected. However, it has been reported that even at
low water activities and below glass transition temperature, Maillard reaction can occur (Karmas
1992; Telis et al. 2009). All dried samples presented an increase in hue angle over storage. A
significant increase in hue angle indicates a reduction in red color. Storage also caused a
significant increase in chroma for all dried samples. The occurrence of non-enzymatic browning
reactions may account for part of the changes observed in hue angle and chroma during
storage.
52
Figure 20. Effect of storage on Lightness * Within each segment, bars with different letters are significantly different
Figure 21. Effect of storage on hue angle * Within each segment, bars with different letters are significantly different
53
Figure 22. Effect of storage on chroma * Within each segment, bars with different letters are significantly different
Moisture Sorption Working Isotherm
Moisture working isotherms of freeze-dried, RW-dried and hot-air dried açaí are
presented in Figure 23. All the isotherms were sigmoidal in shape, as characteristic of most
foods, and presented similar water sorption behavior. The hot-air dried sample exhibited a
slightly higher water adsorption capacity than the RW and freeze-dried materials at all aw
values. A type II isotherm was also observed by Maskan et al. (1998) for vacuum dried
mulberries and Syamaladevi et al. (2009) for freeze-dried raspberry.
The water sorption data of the dried samples were fitted to the Guggenheim-Anderson-
de Boer (GAB) and Brunauer-Emmett-Teller (BET) models. The estimated GAB and BET
parameters are presented in Table 4. Both the BET and GAB models showed a good fit to the
experimental data, as shown by low root mean square errors (RMSE). The GAB model is an
extension of the BET model by introducing the constant K. The GAB model can be used to
predict the isotherm parameters at a water activity of up to 0.9, whereas BET allows isotherm
modeling for aw up to 0.5. The constants in the GAB equation are temperature dependent. In
this experiment the values of C fell between 1 and 20 and K between 0.7 and 1.0, which is in
the range of most food products reported in the literature (Rahman 1995).
Both the BET and GAB models are based on the monolayer moisture concept and
provide the monolayer moisture content of the product, an important parameter for assuring
food stability. As it can be seen from Table 3, the monolayer moisture content values of the
54
three dried açaí samples calculated using the BET model were lower than those obtained from
the GAB model, a phenomenon that has been documented in published papers. Timmermann
et al. (2001) stated that GAB monolayer moisture content is always higher than BET monolayer.
The moisture content of all dried samples was below their monolayer moisture content
value, indicating good product stability over time. Bell et al. (2000) reported that, for most dried
foods, the monolayer value is usually at water activity of 0.2-0.4 and, in general, the results of
the current project fall between that range (except for hot-air dried samples, which had an
average aw of 0.119). The same authors showed that, for most chemical reactions in aqueous
phase, the rate of quality loss begins to increase above aw 0.2-0.3. Beyond that range, water
adsorbed is enough to behave as a solvent, increasing the mobility of the reactants and the rate
of degradative reactions. An exception of that is lipid oxidation, for which the rate of reaction
increases as the water activity decreases below the monolayer. The occurrence of lipid
oxidation will be discussed later.
As previously mentioned, freeze-dried, RW-dried and hot-air dried samples presented a
type II isotherm. The monolayer moisture content values obtained for those samples varied from
2.43% to 2.53% according to the BET model and from 2.5% to 2.62% according to the GAB
model, depending on the drying method used (Table 4). The monolayer values obtained in this
research were lower than the ones previously reported. Tonon et al. (2009) reported a type III
isotherm for spray dried açaí at 25°C, probably due to the presence of carrier agents (crystalline
molecules). The monolayer moisture content reported by Tonon et al. (2009) varied from 3.1%
to 5.8% according to the BET model and from 3.2% to 6.3% when obtained from the GAB
model, depending on the carrier agent used. Da Silva et al. (2008) also worked with spray dried
açaí and reported a type III isotherm for the product, at all temperatures used (15, 25 and 35°C).
The reported monolayer moisture content, according to the BET model, was 7.7% (25°C).
55
Figure 23. Moisture sorption working isotherms of the freeze-dried, RW-dried and hot-air dried açaí
Table 4. Estimated GAB and BET parameters
Anthoycanins
Cyaniding-3-glucoside (peak 1) and cyaniding-3-rutinoside (peak 2) were found to be the
major anthocyanins (ACNs) in all açaí samples, which is in agreement with previous reports of
Lichtenthaler et al. (2005), Schauss et al. (2006b), and Vera de Rosso et al. (2008). Of them,
cyaniding-3-rutinoside was found to be the predominant ACN, having a concentration of 5 to
30% above the concentration of cyaniding-3-glucoside depending on the drying method. The
MS spectral data of these compounds are presented in Table 5 and a typical HPLC
0
1
2
3
4
5
6
7
8
9
0 0.2 0.4 0.6 0.8
Water activity
Moi
stur
e C
onte
nt (%
db)
Hot-air dried Freeze-dried RW-dried
Model Parameters Freeze-dried RW-dried Hot-air driedC 7.13 6.82 11.82M0 2.5 2.56 2.62K 0.92 0.93 0.92RMSE 0.08 0.07 0.08
C 6.38 6.23 10.91M0 2.43 2.49 2.53RMSE 0.02 0.03 0.03
GAB
BET
56
chromatogram for açaí juice is shown in Figure 24. HPLC chromatograms for freeze, RW and
hot-air dried samples are presented in Figures 32-34 in Appendix A.
Table 5. Identification of anthocyanins
Figure 24. HPLC chromatogram for açaí juice on day 0
The effect of different drying methods on anthocyanin content (reported as sum of
cyaniding-3-glucoside and cyaniding-3-rutinoside) is shown in Figure 25. On day 0, the freeze-
dried samples presented an anthocyanin content (5.46 mg/g DW) comparable to that from the
açaí juice (5.74 mg/g DW), followed by the RW-dried (4.87 mg/g DW) and hot-air dried (2.79
mg/g DW). During storage, the anthocyanin content of freeze dried samples was always
significantly higher than that of the RW dried açaí, which was significantly higher than that of
hot-air dried samples. The anthocyanin concentration of hot-air dried açaí was approximately
40-50% lower than that of other açaí samples. Freeze drying resulted in the greatest retention of
anthocyanins compared to other drying methods, which confirms the superiority of this
technique. A better retention of anthocyanins by freeze drying compared to other drying
methods was also reported by Nsonzi et al. (1998) for blueberries and Michalczyk et al. (2009)
Peak no. tR (min) MS (m/z) Compound
1 24.4 449 cyanidin-3-glucoside
2 26.3 595 cyanidin-3-rutinoside
57
for raspberry, strawberry and bilberry. Although significantly different, the RW-dried samples
showed an anthocyanin content similar to the freeze-dried, indicating that this technique is
superior to hot-air drying, with respect to preserving ACNs. Anthocyanins are very unstable and
can be easily degraded during hot-air drying due to the use of high temperatures for long time
and presence of oxygen (Lohachoompol et al. 2004; Piga et al. 2003).
Figure 25. Effect of different drying methods on anthocyanin content * Within each segment, bars with different letters are significantly different
The contents of anthocyanins found in this study, 5.7 mg/g DW for açaí juice and 2.8 to
5.5 mg/g DW for dried samples, are different from previously reported data. For açaí juice Vera
de Rosso et al. (2008) reported around 3.0 mg/g DW, whereas Pacheco-Palencia et al. (2009)
reported approximately 13 mg/g DW. Anthocyanin contents of 0.5 mg/g DW and 3.19 mg/g DW
have been reported by Gallori et al. (2004) and Schauss et al. (2006b), respectively, for freeze
dried açaí. Differences in pigment content are to be expected since the level of anthocyanins is
affected by factors like climatic conditions, harvesting period, maturity stage and processing
techniques, which, in case of açaí juice, normally include a thermal pasteurization. Lichtenthaler
et al. (2005) reported anthocyanin contents varying between 13 and 463mg/L for different açaí
juice samples. In fact, the authors also analyzed the juice of fruits collected from the same trees
in different years and noticed that anthocyanin levels ranged from 88 to 211mg/L.
Cyaniding-3-glucoside and cyaniding-3-rutinoside account for more than 95% of the total
anthocyanins in açaí (Pacheco-Palencia et al. 2009; Schauss et al. 2006b). Interestingly, the
content of anthocyanins in this fruit is much lower than that of other berries. For instance, wild
58
blueberries were reported to have around 487 mg/100g FW (~45 mg/g DW), while blackberries
and black raspberries were said to have approximately 245 and 687 mg/100g FW (~19 and 46
mg/g DW, respectively) (Wu et al. 2006).
The effect of storage on anthocyanin content of dried samples is presented in Figure 26.
The content of ACNs, within each drying method, did not change over the storage period,
indicating good stability of those compounds under the experimental conditions of the project.
It has been reported that water activity plays an important role on anthocyanin
degradation (Amr et al. 2007; Garzón et al. 2001). High water contents result in high molecular
mobility, which facilitates chemical reactions. According to Erlandson et al. (1972) high water
activity values results in either hydrolysis of the glycosidic bond to form aglycones, which are
unstable, or opening of the pyrilium ring to form a substituted chalcone. The low water activity
values, as well as the low moisture contents of below monolayer values, of the açaí powders
used in this study may contribute to a relative good retention of ACNs during storage.
Brønnum-Hansen et al. (1985) evaluated the effect of different water activity values and
storage on stability of freeze-dried elderberry anthocyanins. They reported that samples stored
at or below the monolayer moisture content showed the highest stability, with essentially no loss
of anthocyanins after 1 year of storage.
Tonon et al. (2010) studied the effects of water activity (0.328 and 0.529) and storage
temperature (25 and 35°C) on anthocyanin stability of spray dried açaí. A higher temperature
and water activity led to faster ACN degradation. Since all samples were stored at aw values
below the critical water activity, degradation was not expected. According to the authors,
diffusion of oxygen may have caused anthocyanin degradation.
Figure 26. Effect of storage on anthocyanin content * Within each segment, bars with different letters are significantly different
59
Antioxidant Capacity
The antioxidant capacity of açaí samples against peroxyl radicals was assessed by
ORAC assay. The effect of different drying methods on the antioxidant capacity of açaí is shown
in Figure 27. On day 0, the freeze-dried samples presented an ORAC value (993.12 μmol TE/g
DW) comparable to the açaí juice (1125.33 μmol TE/g DW), suggesting that this drying method
retained most of the bioactive compounds responsible for the antioxidant properties of açaí. The
RW-dried samples showed an intermediate antioxidant capacity (710.39 μmol TE/g DW) while
hot-air dried samples exhibited the lowest ORAC value (528.35 μmol TE/g DW). The ranking for
the ORAC among the three drying methods remained the same over the 3-month storage, with
the freeze-dried having the highest while the hot-air dried being with the lowest ORAC values.
Schauss et al. (2006a) studied the antioxidant capacity of freeze-dried açaí and reported
an ORAC value of 996.9 μmol TE/g DW. In the present research, a similar ORAC value, 993.12
μmol TE/g DW, was obtained for the freeze-dried product on day 0. Different results for the
antioxidant capacity of açaí juice have been reported in the literature. Pacheco-Palencia et al.
(2007) recorded an ORAC value of 54.4 μmol TE/mL juice (~453 μmol TE/g DW), while Del
Pozo-Insfran et al. (2004) and Pacheco-Palencia et al. (2009) reported 48.6 μmol TE/mL juice
(~405 μmol TE/g DW) and 87.4 μmol TE/g juice (~728 μmol TE/g DW), respectively. In this
research, the açaí juice was found to have an ORAC value of 1125.3 μmol TE/g DW. The
differences in antioxidant capacity of açaí might be due to differences in polyphenolic
composition, which is affected by geographical and seasonal differences as well as by post-
harvest practices and the processing method used.
It has been stated that açaí has the highest antioxidant capacity, of any fruit reported to
date, against peroxyl radicals (Schauss et al. 2006a). In this research, the açaí juice (1125.3
μmol TE/g DW) and freeze-dried açaí powders (~838 - 993 μmol TE/g DW) were found to have
very high antioxidant capacity, being superior to other berries with high ORAC values, such as
fresh lowbush blueberries (92.09 μmol TE/g fruit or ~837 μmol TE/g DW) and cranberries (92.56
μmol TE/g fruit or ~718 μmol TE/g DW) (Wu et al. 2004a). Despite of being lower than the
freeze-dried product, RW-dried and hot-air dried açaí showed ORAC values higher than other
fruits, such as fresh blackberries (52.45 μmol TE/g fruit or ~400 μmol TE/g DW), raspberries
(47.65 μmol TE/g fruit or ~336 μmol TE/g DW), and strawberries (35.41 μmol TE/g fruit or ~398
μmol TE/g DW) (Wu et al. 2004a).
The effect of storage on the antioxidant capacity of dried products is presented in Figure
28. For all the three dried samples, no discernable changes in antioxidant capacity were
60
observed over time. These results agree with our previous discussion that anthocyanin content
did not change during storage. Anthocyanins have been identified as important antioxidants in
açaí, together with other compounds, some of which are yet to be identified.
Figure 27. Effect of different drying methods on antioxidant capacity * Within each segment, bars with different letters are significantly different
Figure 28. Effect of storage on antioxidant capacity * Within each segment, bars with different letters are significantly different
61
Flavor Analysis
Açaí Juice
Predominant odorants of açaí juice were identified by means of AEDA and are
presented in Table 6. Twenty-six neutral/basic and 4 acidic compounds were identified, with
flavor dilution (FD) factors ranging from 9 to 2187. All compounds were positively identified by
RI, MS and odor.
Esters are important flavor compounds of many fruits and fruit juices. Interestingly, only
one ester, ethyl butyrate, was identified in açaí juice. Ethyl butyrate is an important odor-active
compound found in many foods, being significant in fruit juices such as orange, apple and
strawberry. It is actually described as the most important odorant in orange juice, being
responsible for fruity, pineapple notes (Nisperos-Carriedo et al. 1990; Perez-Cacho et al. 2008).
Similar to açaí juice, watermelon is another fruit that do not have many esters in its volatile
composition (Beaulieu et al. 2006).
The most important flavor compounds identified in açaí juice were (E,Z) 2,6-nonadienal,
α-ionone, and 2-phenylethanol and/or β-ionone, with FD factors of 2187. (E,Z) 2,6-nonadienal
was most likely derived from lipoxygenase activity on linolenic acid (Tressl et al. 1981). It could
also have been produced by autoxidation of methyl linolenate (Ullrich et al. 1988b). This
compound is the main volatile in cucumber, being also important in melon and watermelon
(Buescher et al. 2001; Perry et al. 2009; Pino et al. 2003). Alfa-ionone, a norisoprenoid, is a
product of degradation of carotenoids. Mahattanatawee et al. (2005), working with orange juice,
reported that this compound is formed by oxidative cleavage of double bonds in α-carotene and
α-cryptoxanthin. This odorant is responsible for a floral note and is an important constituent in
other fruits, such as raspberries (Hampel et al. 2007) and blackberries (Qian et al. 2005). 2-
phenylethanol and β-ionone were probably co-eluted and it was not possible to identify which
compound was the major responsible for the additional floral notes found in açaí juice. Since β -
ionone has a much lower threshold compared to 2-phenylethanol, this volatile may be the most
important one. 2-phenylethanol is derived from phenylalanine and the pathway for its
biosynthesis in plants is not well understood yet (Tieman et al. 2007). The compound is the
major aroma volatile in roses (Sakai et al. 2007) and it is also of great importance for the flavor
of lychee and apple cider (Ong et al. 1998; Xu et al. 2007). Beta-ionone, like α-ionone, is a
norisoprenoid and has been reported as an important volatile in other fruits, such as
blackberries and raspberries (Du et al. 2010; Klesk et al. 2004).
62
Many aldehydes derived from lipid oxidation were identified in the açaí juice. It is well
known that the disruption of plant tissues induces lipoxygenase activity, leading to the formation
of hydroperoxydes. By action of hydroperoxyde lyases, hydroperoxydes are then converted into
volatile aldehydes. Thermal oxidation, autoxidation and photo-oxidation are also responsible for
the formation of fatty acid derived aldehydes (Tressl et al. 1981). As previously mentioned, açaí
has a high lipid content and, probably, both pathways were responsible for the formation of
those compounds in açaí juice. Volatile production most likely occurred during post-harvest,
processing (mainly) and storage. Among the aldehydes identified, (E,Z) 2,6-nonadienal (already
discussed above), (E,E,Z) 2,4,6-nonatrienal, (E) 2-nonenal, (E,E) 2,4-nonadienal, and (E,E) 2,4-
decadienal showed the highest FD factors. (E,E,Z) 2,4,6-nonatrienal can be formed by
enzymatic degradation and autoxidation of linolenic acid (Schuh et al. 2005); (E) 2-nonenal has
been reported as a product of thermal decomposition of hydroperoxides from linoleic and
linolenic acids (Lozano et al. 2007a); and (E,E) 2,4-nonadienal and (E,E) 2,4-decadienal can be
formed by oxidation of linoleic acid (Frankel 1983).
Lipid oxidation-derived aldehydes may be further reduced to alcohols by alcohol
dehydrogenases. This may be the mechanism behind the formation of (Z) 3-hexen-1-ol. This
volatile has been reported as a product of the metabolism of linolenic acid hydroperoxydes by
hydroperoxyde lyases and alcohol dehydrogenases (Angerosa et al. 1999). (Z) 3-hexen-1-ol is
responsible for fresh green and grassy odors. The compound is one of the main contributors to
the characteristic odor of green leaves. It has also been detected in elderberry juice and grapes
(Fan et al. 2010; Hatanaka 1993; Jensen et al. 2001).
The ketones 1-octen-3-one and (Z) 1,5-octadien-3-one may also have been derived from
lipid oxidation, probably of linoleic acid (Tressl et al. 1981). (Z) 1,5-octadien-3-one imparted a
plastic, styrene flavor, whereas 1-octen-3-one was characterized as mushroom-like or metallic.
These compounds constitute important odorants in mushrooms. 1-octen-3-one has been also
identified in lychees and elderberry juice (Mahattanatawee et al. 2007; Jensen et al. 2001).
Some thermally generated volatiles were as well identified. Methional is formed by
Strecker degradation reaction involving methionine. The compound is responsible for a potato-
like note (Di et al. 2003). Phenylacetaldehyde is a Strecker aldehyde of phenylalanine, with a
characteristic floral note (Hofmann et al. 2000). Vanillin (4-hydroxy-3-methoxybenzaldehyde)
has been reported to be a thermally induced decomposition product of ferulic acid and
contributes with a pleasant vanilla, sweet note (Peleg et al. 1992). Those compounds might
have been generated, or had their concentration increased, during pasteurization of the juice.
63
In addition, two phenolic compounds, guaiacol and isoeugenol, were present in the açaí
juice, showing relatively high FD factors. Guaiacol is responsible for smoky, medicinal and/or
phenolic notes, and is usually characterized as an off-flavor in fruit juices. The presence of this
compound in juices has been related to product spoilage, mainly by Alicyclobacillus
acidoterrestris. The spores of this microorganism are resistant to pasteurization and can grow
over a wide range of temperatures, producing off-flavors (Eisele et al. 2005; Perez-Cacho et al.
2007). In the case of açaí, spoilage might have occurred in the juice or in the fruit, before
processing, since the elevated temperatures and relative humidity of the harvesting/production
area may favor microbial growth. On the other hand, isoeugenol provides a pleasant floral
scent. The compound is one of the main volatiles of the petunia flower and was also detected in
blueberries (Dexter et al. 2007; Su et al. 2010). Moreover, an unknown compound (no. 4), with
high FD factor (729) was detected in açaí juice. This odorant was responsible for green, apple
notes.
Several acids were also identified. Acetic acid contributed to vinegar, sour notes, while
butanoic and 3-methyl butanoic acids were responsible for cheesy, sweaty notes. Butanoic and
3-methyl butanoic acids are volatile short-chain fatty acids and provide important notes for the
characteristic flavor of cheeses (Innocente et al. 2000).
64
Table 6. Odor-active compounds of açaí juice identified by AEDA
All compounds identified by RI, MS and odor * Compounds were co-eluted Dried Açaí
To evaluate the effect of drying and storage on the flavor of açaí the SPME technique
was employed and three groups of odor-active compounds were selected for analysis. 2-methyl
butanal, 3-methyl butanal, benzaldehyde and phenylacetaldehyde were chosen as indicators of
Maillard reaction; pentanal, hexanal, heptanal, (E) 2-hexenal, (E,E) 2,4-heptadienal and (E,E)
2,4-decadienal were used as indicators of lipid oxidation; and 2-phenylethanol, (Z) 3-hexen-1-ol
and octanal were selected as characteristic flavor compounds identified in açaí juice by AEDA
(named as “target compounds”).
Table 7 presents the concentration of the analyzed volatile compounds in each product
on day 0. It can be seen that dehydration changed the flavor of the açaí juice. In general, the
no. compound aroma Stabilwax RTX-5 FD factor fraction1 2,3-butanedione buttery 969 <700 9 NB2 ethyl butyrate fruity, bubble gum 1036 801 27 NB3 hexanal green, cut-leaf 1078 796 27 NB4 unknown green, apple 1135 — 729 NB5 octanal orange, pungent, citrus 1287 1003 9 NB6 1-octen-3-one mushroom, metallic 1295 976 243 NB7 unknown green, floral 1348 — 9 NB8 (Z )1,5-octadien-3-one plastic, styrene 1367 981 27 NB9 (Z ) 3-hexen-1-ol green leaf 1381 859 729 NB10 acetic acid vinegar 1429 <700 27 AC11 methional potato 1443 906 27 NB12 (Z ) 2-nonenal hay, stale 1495 1147 27 NB13 (E ) 2-nonenal hay, stale 1524 1160 729 NB14 (E,Z ) 2,6-nonadienal fatty, cucumber 1574 1152 2187 NB15 butanoic acid cheesy, fecal 1615 820 27 AC16 phenylacetaldehyde rosy, floral 1626 1045 27 NB17 3-methyl butanoic acid cheesy, sweaty 1652 862 27 AC18 (E,E ) 2,4-nonadienal fatty, fried 1687 1217 243 NB19 unknown stale, hay, fatty 1715 — 27 NB20 (E,Z ) 2,4-decadienal cucumber, fatty 1756 1291 27 NB21 (E,E ) 2,4-decadienal fried, fatty 1796 1320 243 NB22 β-damascenone apple sauce, honey 1807 1380 9 NB23 α-ionone floral, wine-like 1835 — 2187 NB24 2-methoxy phenol (guaiacol) smoky, hospital 1841 1087 243 NB/AC25 (E,E,Z ) 2,4,6-nonatrienal oats, fatty 1858 1238 729 NB26 2-phenylethanol rosy, wine-like 1892 1115 NB27 β-ionone rosy 1901 — NB28 eugenol cloves, cinnamon 2146 1359 81 NB29 isoeugenol cloves 2319 1455 729 NB30 vanillin vanilla 2531 1421 27 AC
RI
2187 *
65
concentration of Strecker aldehydes was higher in dried samples, indicating the occurrence of
Maillard reaction during drying. The majority of lipid oxidation compounds also had their
concentration increased with drying. This was expected since açaí juice is rich in lipids and the
application of heat during processing would most likely induce lipid oxidation and the release of
volatile aldehydes. All the Maillard reaction indicators and three indicators of lipid oxidation were
detected in açaí juice. These compounds might be either originally present in açaí, being
characteristic of the fruit flavor, or might have been produced during juice pasteurization and
storage. Lower concentrations of (Z) 3-hexen-1-ol were found in dried products, suggesting that
this volatile was lost during drying. It has been reported that the small size of the molecule and
its polarity are factors that cause C6 alcohols to evaporate together with water (De Torres et al.
2010). The concentration of octanal did not change due to processing whereas 2-phenylethanol
presented contradictory results (significantly higher in freeze-dried samples, lower in RW-dried
samples and equal to açaí juice for hot-air dried samples).
Table 7. Concentration of volatile compounds in açaí juice and powders on day 0 (expressed in μg/L)
* Within each row, bars with different letters are significantly different
Comparison among the three dried products on day 0 revealed that, in general, RW and
hot-air dried samples had a lower volatile concentration compared to freeze-dried samples. This
may not mean that RW and hot-air drying resulted in less Maillard reaction and/or lipid
Juice FD RW HADStrecker Aldehydes2-methyl butanal 85 ± 18 b 212 ± 66 a 91 ± 27 b 175 ± 36 ab3-methyl butanal 147 ± 32 b 278 ± 65 ab 234 ± 66 ab 404 ± 94 abenzaldehyde 23 ± 4 bc 133 ± 18 a 14 ± 3.4 c 43 ± 11 bphenylacetaldehyde 235 ± 123 bc 456 ± 77 ab 119 ± 35 c 669 ± 155 a
Lipid Oxidationpentanal 0 b 126 ± 8.3 ab 204 ± 99 a 151 ± 105 abhexanal 233 ± 93 a 200 ± 67 ab 49 ± 5 c 55 ± 12 bcheptanal 3.4 ± 1.4 b 7.3 ± 0.5 a 2.8 ± 0.8 b 9.8 ± 2.2 a(E) 2-hexenal 118 ± 48 a 95 ± 14 a 52 ± 5.5 ab 24 ± 11 b(E,E ) 2,4-heptadienal 0 b 50 ± 7.3 a 4.7 ± 0.4 b 6.4 ± 1 b(E,E ) 2,4-decadienal 0 b 161 ± 29 a 0 b 0 b
Target compoundsoctanal 3.9 ± 1.1 a 5.6 ± 1.4 a 4.5 ± 0.6 a 5.4 ± 2.8 a(Z ) 3-hexen-1-ol 1172 ± 373 a 106 ± 18 b 22 ± 5.8 b 43 ± 15 b2-phenylethanol 276 ± 81 b 613 ± 145 a 41 ± 8.7 c 158 ± 35 bc
66
oxidation. As previously described, both techniques expose the product to oxygen and relatively
high temperatures. In case of hot-air drying, the exposure occurs during a long time. Therefore,
the lower concentration of odorants might have been caused by evaporation / loss of volatiles.
The volatile composition of each dried product was monitored during storage and results
are presented in Table 8. All dried samples showed a significant increase in the concentration of
lipid oxidation-derived compounds after 3 months of storage. These results indicate that lipid
oxidation was, probably, the most important reaction that took place in açaí powders. It is well
known that oxidation rate at low water activity, which is the case of dried açaí samples, is high,
because no enough water is available to reduce free radical concentration (Angelo 1992).
Storage of dried samples also resulted in an increase in the concentration of all Strecker
aldehydes, suggesting the occurrence of Maillard reaction. However, this increase in volatile
concentration was significant only for RW-dried products. RW-dried açaí showed the highest
water activity values. This may help explain the higher extent of Maillard reaction in those
samples. With respect to the target compounds, in general, the concentration of (Z) 3-hexen-1-
ol and 2-phenylethanol did not change during storage. However, the concentration of octanal
was significantly higher after 3 months. Octanal is also a lipid oxidation derivative, which might
explain the increase in its concentration during storage.
Table 8. Concentration of volatile compounds in freeze, RW and hot-air dried açaí on day 0 and after 3 months of storage (expressed in μg/L)
* Within each segment, bars with different letters are significantly different
day 0 3M day 0 3M day 0 3MStrecker Aldehydes2-methyl butanal 212 ± 66 a 247 ± 95 a 91 ± 27 b 221 ± 47 a 175 ± 36a 241 ± 56 a3-methyl butanal 278 ± 65 a 468 ± 206 a 234 ± 66 b 561 ± 103 a 404 ± 94 a 552 ± 108 abenzaldehyde 133 ± 18 a 206 ± 78 a 14 ± 3.4 b 82 ± 25 a 43 ± 11 a 73 ± 15 aphenylacetaldehyde 456 ± 77 a 592 ± 236 a 119 ± 35 b 461 ± 105 a 669 ± 155 a 719 ± 77 a
Lipid Oxidationpentanal 126 ± 8.3 b 466 ± 170 a 204 ± 99 a 335 ± 39 a 151 ± 105 b 510 ± 130 ahexanal 200 ± 67 b 1363 ± 516 a 49 ± 5 b 522 ± 82 a 55 ± 12 b 912 ± 194 aheptanal 7.3 ± 0.5 b 37 ± 11 a 2.8 ± 0.8 b 12 ± 1.4 a 9.8 ± 2.2 b 17 ± 4 a(E) 2-hexenal 95 ± 14 a 108 ± 40 a 52 ± 5.5 b 110 ± 18 a 24 ± 11 b 60 ± 10 a(E,E ) 2,4-heptadienal 50 ± 7.3 b 135 ± 41 a 4.7 ± 0.4 b 83 ± 17 a 6.4 ± 1 b 96 ± 20 a(E,E ) 2,4-decadienal 161 ± 29 b 212 ± 14 a 0 0 0 0
Target compoundsoctanal 5.6 ± 1.4 b 52 ± 21 a 4.5 ± 0.6 b 11 ± 1.4 a 5.4 ± 2.8 b 16 ± 4.2 a(Z ) 3-hexen-1-ol 106 ± 18 a 112 ± 32 a 22 ± 5.8 a 29 ± 4.7 a 43 ± 15 a 51 ± 5.1 a2-phenylethanol 613 ± 145 a 771 ± 285 a 41 ± 8.7 b 76 ± 11 a 158 ± 35 a 145 ± 7.5 a
RW HADFD
67
At the end of storage, the concentrations of the selected flavor compounds in all dried
samples were compared (Table 9). In general, all açaí powders showed similar concentration of
Strecker aldehydes. Freeze-dried samples showed significantly higher concentration of three
lipid oxidation indicators (hexanal, heptanal and (E,E) 2,4-decadienal) and all target
compounds. Additional research is necessary in order to better understand the occurrence of
Maillard and lipid oxidation during storage. Also a consumer test should be performed to find out
consumer preferences. Some of the results reported for flavor could as well be due to matrix
effects.
Table 9. Concentration of volatile compounds in açaí powders after a 3-month storage (expressed in μg/L).
* Within each row, bars with different letters are significantly different
Glass Transition Temperature (Tg)
Knowledge of glass transition temperature is important because this parameter has been
related to food stability. Glass transition of amorphous food systems has been found to cause
structural and textural changes, which increases molecular mobility and decreases product
stability. In general, amorphous foods are more stable in the glassy state, in which mobility and,
therefore, degradative reactions are limited (Slade et al. 1991).
FD RW HADStrecker Aldehydes2-methyl butanal 247 ± 95 a 221 ± 47 a 241 ± 56 a3-methyl butanal 468 ± 206 a 561 ± 103 a 552 ± 108 abenzaldehyde 206 ± 78 a 82 ± 25 b 73 ± 15 bphenylacetaldehyde 592 ± 236 a 461 ± 105 a 719 ± 77 a
Lipid Oxidationpentanal 466 ± 170 a 335 ± 39 a 510 ± 130 ahexanal 1363 ± 516 a 522 ± 82 b 912 ± 194 abheptanal 37 ± 11 a 12 ± 1.4 b 17 ± 4 b(E) 2-hexenal 108 ± 40 a 110 ± 18 a 60 ± 10 a(E,E ) 2,4-heptadienal 135 ± 41 a 83 ± 17 a 96 ± 20 a(E,E ) 2,4-decadienal 212 ± 14 a 0 b 0 b
Target compoundsoctanal 52 ± 21 a 11 ± 1.4 b 16 ± 4.2 b(Z ) 3-hexen-1-ol 112 ± 32 a 29 ± 4.7 b 51 ± 5.1 b2-phenylethanol 771 ± 285 a 76 ± 11 b 145 ± 7.5 b
68
In this project, the glass transition temperature was studied using differencial scanning
calorimetry (DSC). A typical DSC thermogram is presented in Figure 29 for the freeze-dried
sample. Thermograms for RW-dried and hot-air dried samples can be found in Figures 35 and
38 (Appendix B). Similar curves were obtained for all dried samples. All DSC thermograms
showed three major endothermic peaks: the first one between -5 and -3°C, the second between
10 and 12°C and the third between 150 and 200°C. The last endothermic peak was probably
due to sample decomposition. We hypothesized that the first two endothermic peaks were
caused by melting of lipids. There seemed to be glass transition between 50 and 60°C.
Figure 29. DSC thermogram for freeze-dried açaí
In order to confirm our hypothesis and due to the uncertainty of finding the product’s
glass transition temperature, samples were subjected to an oil extraction and then both the oil
and solids fractions were used for a second DSC analysis. Figure 30 shows the thermogram for
the oil of freeze-dried açaí. Thermograms for the oil of RW-dried and hot-air dried açaí are
shown in Figures 36 and 39, respectively (Appendix B). Once again, the curves for the oil of all
dried samples were similar. Interestingly, the thermograms for the oil fractions were
69
characterized by the same first two endothermic peaks found in the dried samples, validating
our hypothesis.
Figure 30. DSC thermogram for freeze-dried açaí oil
Since the lipids present in dried açaí melt into two endothermic peaks at relatively low
temperatures, it is possible to conclude that they are liquid at room temperature. Being liquid
gives them higher mobility to participate in degradative reactions, such as lipid oxidation. This
agrees with the results discussed previously for flavor, which indicated the occurrence of lipid
oxidation during storage of açaí powders. For higher stability against this reaction, addition of
carrier agents, such as maltodextrin, prior to drying would be very important. Those compounds
would form a “more glassy” matrix, encapsulating the oil and protecting it from degradation.
Schauss et al. (2006b) studied the fatty acid composition of freeze-dried açaí and found
that the predominant fatty acid was oleic acid (56.2%), followed by palmitic acid (24.1%) and
linoleic acid (12.5%). The melting points of those individual fatty acids are around 13°C, 63°C
and -5°C, respectively. The same authors reported that ~74% of the total fatty acids present in
freeze-dried açaí is composed by unsaturated fatty acids. This helps explain the low melting
temperature of the endotherms.
70
Fatty acids are, probably, present in açaí powders in the form of triacylglycerol (TAG).
TAG polymorphism may have influenced the shape and position of the melting peaks. In most
thermograms, for both the whole sample (Figures 29, 35 and 38) and the oil (Figures 30, 36 and
39), it is possible to notice that if a line is drawn between the baseline of the first melting peak
and the baseline of the second melting peak, an exothermic peak appears between the two
endothermic peaks. This might be due to TAG conversion to a more stable stable polymorph via
recrystallization (exothermic peak). These more stable polymorphs will later melt together with
other forms to give the second endothermic peak. Different TAG composition, due to different
drying methods (Gutiérrez et al. 2008), and TAG polymorphs may result in slightly different
melting temperatures and bigger or smaller exothermic peaks. It would be interesting to further
investigate the lipid fraction of açaí powders in order to better describe their melting behavior.
In preliminary studies, crimped pans were used in DSC analysis. Comparing the
thermograms obtained when using crimped pans with those obtained when using hermetically
sealed pans revealed the same first two endothermic peaks. However, since crimped pans
permit moisture evaporation, an evaporation peak was found and no decomposition peak was
observed up to 250°C. When using hermetically sealed pans, the decomposition peak was
present between 150 and 200°C. This indicates that the presence of water allows
decomposition to begin at lower temperatures.
Figure 31 shows the thermogram for freeze-dried açaí solids. Thermograms for RW and
hot-air dried solids are presented in Figures 37 and 40 (Appendix B). Interestingly, the
thermograms for the solid fractions showed a very subtle step change in the heat flow between
50 and 60°C, which may indicate the occurrence of a glass transition around that temperature.
This step change can be more easily visualized in the thermogram of RW-dried solids. The
magnitude of change in heat capacity is proportional to the amount of amorphous material
undergoing glass transition. The subtle step change observed in the thermograms might be due
to the small amount of amorphous material in the samples undergoing glass transition. In
addition, due to the complexity of food products, it is usually difficult to determine their Tg. Glass
transition usually occurs over a range of temperatures. Further studies are required in order to
better identify the Tg of the product.
71
Figure 31. DSC thermogram for freeze-dried açaí solids
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Chapter 4: Conclusions
This project evaluated the impact of three different drying methods (freeze drying, RW
drying, and hot-air drying) and subsequent storage on the quality of açaí. To accomplish the
objective, anthocyanin content, antioxidant capacity, moisture content, water activity, moisture
sorption isotherm, glass transition temperature, flavor and color were analyzed.
All three methods produced dried samples with low moisture contents, in the range of
monolayer values, thus having a low water activity and relatively good product stability during
storage. The dried powders showed a sigmoidal shape isotherm, with hot-air dried açaí sorbing
slightly more water than the other samples at all aw values. The GAB and BET models both
produced a good fit to the isotherm data.
DSC analysis showed a very subtle step change in heat flow for the solid fraction of all
dried samples, suggesting the occurrence of a glass transition for the oil-free solid between 50
and 60°C. Thermograms for the oil fractions revealed that the lipids present in açaí powders
are liquid at room temperature, which will increase their mobility, making them more prone to
degradative reations, such as lipid oxidation.
Drying definitely affected the color of açaí. All dried samples exhibited higher lightness
and lower hue angle and chroma compared to açaí juice. Comparison among the treatments
revealed that RW-dried açaí had the highest lightness and chroma values. In general, no
significant difference was observed for hue angle. The color of the RW-dried samples was
probably influenced by the açaí oil, which made comparison with other dried samples difficult.
Color changes during storage were marked by a decrease in lightness and an increase in hue
angle and chroma for all dried samples. The color changes during storage might have been
caused by the formation of brown pigments due to Maillard reaction and/or polymerization of
oxidized phenolic compounds. A consumer test would have to be performed in order to
determine which dried sample has the most representative color of açaí.
Analysis of the flavor of açaí juice identified (E,Z) 2,6-nonadienal, α-ionone and β-ionone
and/or 2-phenylethanol as the most important odorants. A number of thermally generated
compounds and aldehydes derived from lipid oxidation were also identified. These compounds
may either be characteristic of the fruit or might have been formed during thermal
pasteurization, drying, and storage. The effects of drying and storage on the flavor of acai were
also remarkable. The concentrations of Strecker aldehydes and lipid oxidation compounds in
dried samples were higher in dried powders compared to that in açaí juice, indicating the
79
occurrence of Maillard reaction and lipid oxidation during drying. In general, RW and hot-air
dried samples showed a lower concentration of Strecker aldehydes and lipid oxidation
compounds compared to freeze-dried açaí. It is possible that loss of volatiles occurred during
RW and hot-air drying, resulting in lower concentrations of those odorants. During storage, all
dried samples showed an increase in lipid oxidation compounds, indicating the occurrence of
that reaction. Significant increase in Mailard reaction products was observed only in RW-dried
samples. At the end of the storage period, all three dried powders showed similar volatile
concentration. It would be important to perform a consumer test in order to verify which dried
sample has the most representative flavor of açaí juice and to find out if consumers can
perceive the changes that occurred during storage.
The anthocyanin concentrations in the freeze-dried açaí were comparable to those in the
açaí juice, suggesting that this drying method preserved most of the pigments. The RW-dried
açaí showed intermediate anthocyanin content whereas hot-air dried samples recorded the
lowest pigment concentration. During storage, anthocyanin content did not change within each
treatment.
For antioxidant capacity, the highest ORAC value (993.12 μmol TE/g DW) was obtained
in the freeze-dried samples, being again comparable to açaí juice. The RW-dried açaí had a
medium ORAC value while the hot-air dried samples presented the lowest antioxidant capacity.
The ranking for antioxidant capacity is in the same sequence as that for the anthocyanin content
(FD>RW>HAD), indicating that freeze drying preserved the majority of the bioactive compounds
present in açaí. All dried samples maintained their antioxidant capacity during storage.
In general, the results presented in this work indicate that the freeze-dried açaí retained
the antioxidant properties of the raw material to a higher degree compared to other dried
powders. Therefore, lyophilisates can probably satisfy the need for a more shelf stable product
with a better retention of bioactive compounds. The RW-dried samples showed superior quality
compared to the hot-air dried samples. Moreover, an economic analysis taking into
consideration of capital investement or operation costs of each drying method is needed when
doing drying method selection for açaí preservation.
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Appendix A: HPLC Chromatograms
Figure 32. HPLC chromatogram for freeze dried açaí on day 0
81
Figure 33. HPLC chromatogram for RW-dried açaí on day 0
1
2
82
Figure 34. HPLC chromatogram for hot-air dried açaí on day 0
1
2
83
Appendix B: DSC Thermograms
Figure 35. DSC thermogram for RW-dried açaí
84
Figure 36. DSC thermogram for RW-dried açaí oil
85
Figure 37. DSC thermogram for RW-dried açaí solids
86
Figure 38. DSC thermogram for hot-air dried açaí
87
Figure 39. DSC thermogram for hot-air dried açaí oil
88
Figure 40. DSC thermogram for hot-air dried açaí solids