effect of pectin-based coating on the kinetics of quality change

EFFECT OF PECTIN-BASED COATING ON THE KINETICS OFQUALITY CHANGE ASSOCIATED WITH STORED AVOCADOS

N. MAFTOONAZAD1 and H.S. RAMASWAMY2,3

1Department of Agricultural EngineeringResearch Center of Agriculture and Natural Resources

PO Box 73415-111Zarghan, Fars, Iran

2Department of Food ScienceMcGill University

Macdonald Campus, 21,111 LakeshoreSte-Anne-de-Bellevue, Quebec, Canada H9X 3V9

Accepted for Publication January 3, 2008

ABSTRACT

Undesirable storage conditions lead to physical and chemical qualityloss in stored avocados, which affect its consumer acceptability. Studies aimedat quantifying the extent of quality changes under different storage conditionsare valuable for minimizing the deleterious effect on product quality. Theobjective of this study was to quantify the effect of pectin-based coating on thekinetics of quality change in stored avocados. Avocados were coated with apre-evaluated pectin-based emulsion and stored at three temperatures (5, 10and 20C) for various times. At selected storage times, samples were removedand evaluated for quality parameters (physical: texture and color; chemical:ascorbic acid, pH, titrable acidity and total soluble solids; and physiological:respiration rate). Results revealed that avocados became softer and darkerwith the passage of time, and higher temperatures resulted in more rapidchanges in the different quality parameters. The rate of CO2 evolution at theclimacteric peak in control samples was 287, 253 and 186 mL/(kg·h) at 20, 15and 10C, respectively, reached after 6, 12 and 16 days, while in coatedsamples, the peak values of 232, 210 and 152 mL/(kg·h) were reached after 8,14 and 22 days, respectively. Control fruits lost 5.2, 6.8 and 11.5% of theiroriginal weight during 7 days of storage at 10, 15 and 20C, respectively, whilecoated fruits lost 3.8, 4.5 and 9.1% under the same conditions. A zero-ordermodel was used for DE, and a first-order model was used for L value. Coatingextended the shelf life to over a month at 10C. The textural softening of

3 Corresponding author. TEL: 514-398-7919; FAX: 514-398-7977; EMAIL: [email protected]

Journal of Food Processing and Preservation 32 (2008) 621–643.© 2008 The Author(s)Journal compilation © 2008 Wiley Periodicals, Inc.

621

avocados during storage followed a first-order kinetic model. Typical climac-teric behavior was observed in stored avocados under all conditions, with theappearance of respiratory peak delayed as a result of the coating. In general,most changes were well described by some form of kinetic model. Temperaturesensitivity of the rate constant was adequately described by the Arrheniusmodel.

PRACTICAL APPLICATIONS

Edible films and coatings are capable of restricting loss of moisture fromfruit during storage or of lowering the absorption of oxygen by the fruit andthereby slowing respiration. Application of coating could hinder ripening ofthe fruits as well as retard related changes in firmness, color, weight andchemical constituents. At higher temperatures, all changes, including ripening,are accelerated. Modeling of quality change kinetics during storage permitsthe best use of storage conditions for the successful marketing of the product.Validation of these models under commercial conditions can facilitate theprediction of fruit quality during storage.

INTRODUCTION

Avocado is a highly perishable fruit with a high metabolic rate resultingin a shelf life of only 3–4 weeks when stored at optimum temperature andrelative humidity (Yahia and Gonzalez-Aguilar 1998). Cold storage delaysripening and extends storage life, but serious problems can occur duringprolonged storage because of decay and chilling injury (Scott and Chaplin1978). On the other hand, higher storage temperature will result in greaterquality loss and increased respiratory activity, and hence lowered shelf life.The quality of avocado is a combination of various physicochemical andnutritional characteristics, which are greatly influenced by storage condition.High-quality avocados are firm, green and without any defects. However, thesefactors may vary according to the degree of maturity, harvest time, variety andstorage conditions. The first quality judgment made by a consumer is its visualappearance, and color is one of the most important appearance attributes,which influences consumers’ acceptability. Abnormal colors cause the productto be rejected by the consumer (Maskan 2001). In addition to color, the producttexture is of primary concern to the consumer as an indicator of producequality. Texture is usually quantified as the product’s resistance to an appliedforce. Avocados undergo considerable changes in texture during storagedepending on the temperature employed. Storage conditions can promote

622 N. MAFTOONAZAD and H.S. RAMASWAMY

extensive changes in chemical composition of fruits, which are also importantas quality attributes. Respiration is another major factor contributing to post-harvest losses which converts stored sugar/starch to energy in the presence ofan oxygen substrate, and advances senescence. Although this function isnecessary for continuing growth and development of produce even afterharvest, a high rate generally has undesirable effects on the produce quality.Minimal aerobic respiration is necessary, while anaerobic respiration is unde-sirable because it leads to the production of off-flavors and odors. Hence,control of respiration rate is very important. Lowering the respiration rateextends the shelf life and preserves the quality of products.

Several techniques have been used for storage of climacteric fruits toretard the rate of ripening after harvest and thus extend shelf life. Theseinclude controlled/modified atmosphere (CA/MA) storage, modified atmo-sphere packaging (MAP) and/or application of special skin coatings. In allthese cases, the resulting atmosphere modification, i.e., lowering the O2 and/orincreasing the CO2 concentrations in the storage environment, has been shownto be helpful in extending the storage life of several perishable produce (Kader1986). The prevailing low O2 and high CO2 concentration levels reduce oxi-dative respiration. Parallel to the effect on respiration, the energy needed tosupport other metabolic processes is affected, which in turn changes thoseprocesses. As a part of normal ripening metabolism of the avocado, texture andcolor also change with time. These changes are directly influenced by O2 orCO2, or driven by the energy supplied by respiration or fermentation.

Surface coatings can create a modified atmosphere, similar to that ofMAP, their effectiveness being a function of coating permeability and fruitrespiration. Temperature control is an important step because it can affect thepermeability as well as the rate of fruit respiration. Higher temperaturesincrease fruit respiration rates. Coating can be formulated from differentmaterials including lipids, resins, polysaccharides, proteins and syntheticpolymers. Some studies have been carried out on coating of avocado.Maftoonazad and Ramaswamy (2005) used a methyl cellulose-based coatingon avocados stored at 20C and found that coated avocados demonstrated lowerrespiration rates, greener color and higher firmness as compared with theuncoated control during the entire storage. Bender et al. (1993) showed thatthe use of NatureSeal delayed ripening of avocado at 20C by about 2 days evenwhen coated fruits were treated with 100 ppm ethylene for up to 3 days;however 4 days of ethylene treatment overcame the ripening delay. Johnstonand Banks (1998) used six types of surface coatings on Hass avocado fruits tostudy the gas exchange characteristics at 20C. They reported that of sixdifferent surface coatings used, “avocado wax” provided the greatest levelof benefit (reduction in moisture loss and enhanced sheen). At the otherextreme, carboxymethyl cellulose coating (2 g/100 g) provided no benefit but

623EFFECT OF STORAGE CONDITIONS ON AVOCADOS

substantially increased risk of fermentation. Of the avocado wax concentra-tions assessed, a level of 11 g/100 g was reported to be the optimum. Higherconcentrations imposed the risk of anaerobiosis in the fruit. Salvador et al.(1999) used chitosan coating on avocado fruits and improved the avocadostorage life to 24 days at 3–10C and to 6 days at 27–29C. The effect of MAPcoupled with wax emulsion treatment on the extension of storage life ofavocado fruits was studied under different storage temperatures by Bhaskaranand Habibunnisa (2002). They reported that fruits that were given a waxemulsion (6 g/100 g) dip treatment and packed in low-density polyethylenebags remained green, firm and in good condition for about 4–5 weeks at8 � 2C. The fruits also did not develop chilling injury, while fruits withouttreatment and packaging became soft, developed internal pulp browning andbecame inedible within 3 days at 27 � 2C. At 2 � 2C, the fruits developedchilling injury, pitting of the skin and were rejected after 9 days of storage.

None of these works revealed a quantitative relationship between qualityloss and storage condition of avocado. So, the present study was conducted todetermine physicochemical and physiological changes in coated and non-coated avocados as influenced by storage condition. The coating material wasa pectin-based emulsion previously designed for different purposes. Becauseimprovement of quality factors has been made possible by the increase inknowledge of kinetics of quality deterioration (Saguy and Karel 1980), thesecondary objective of this study was to evaluate kinetic parameters (reactionrate constant and activation energy) for predicting the quality loss in storedavocados.

MATERIALS AND METHODS

Avocado Preparation and Storage

Avocado fruits (cv. Hass) were obtained from a local source. The fruitswere carefully selected to be uniform in size, color and firmness. The fruitswere surface disinfected by immersion in 0.5% commercial bleach for 3 min,washed and air-dried. The fruits were then divided into six replicate lots. Thefirst three lots constituted the control, which were stored without coating. Theother three lots were coated with a pectin-based emulsion. Both treated andcontrol avocados were stored for up to 5 weeks at three different temperatures(10, 15 and 20C), at 95% relative humidity. During storage, samples of avo-cados were removed at different intervals for quality evaluation.

Preparation of Coating Emulsion

A 3% (w/w) pectin solution was prepared by rehydrating pectin (HMRapid Set powder, TIC Gums, Belcamp, MD) in distilled water (12 h at 20C),


and 45% (pectin dry basis) of sorbitol (Sigma, Oakville, Ontario, Canada) wasadded and thoroughly mixed with magnetic stirring. Then, 40% (pectin drybasis) melted beeswax (Sigma) was added and emulsified using a homogenizer(PowerGen 700, Fisher Scientific, Pittsburg, PA) at 14,000 rpm for 4 min. Thequantities of sorbitol and beeswax were previously optimized based ontheir influence on the mechanical and barrier properties of the formed film(Maftoonazad 2006).

Avocados were immersed in the coating solution for 1 min at 20C andthen were drained. The treated fruits were dried in a cold-air draft for 3 h to seta coat of the film on their surface. They were then stored along with controlsamples at 10, 15 and 20C.

The moisture loss, firmness, respiration rate, changes in color, totalsoluble solids, titrable acidity and pH of coated and control samples wereevaluated until the overall acceptability was considered poor for each lot ofsamples.

Analytical Methods

The following analyses were carried out to evaluate quality changes inavocado samples every 2 days:

(1) Moisture loss: The moisture loss occurred as a result of transfer of watervapor from the samples to air. This was determined by weighing thesamples on a digital balance (model TS4KD, OHAUS, Florham Park, NJ)and was reported as percentage loss in moisture based on the originalmass.

(2) Firmness: Texture measurements were made using a computer-controlledLRX material testing machine (Lloyd Instrument Limited, Fareham, U.K.)equipped with a 50 N load cell. Samples were subjected to a puncture testat a constant speed of 50 mm/min, using a 5-mm diameter round-tippedpuncture probe. Force–deformation curves were recorded, and firmness(as represented by the slope (N/mm) of the linear section of the force–deformation curve) was used as the indicator of textural property. At leastsix measurements were made on each fruit at different locations, and fourfruits were tested for each storage time; the results were averaged. Themeasurements were carried out on fruits with their skin peeled.

(3) Color: The color characteristics were assessed using a tristimulus MinoltaChroma Meter (Minolta Corp., Ramsey, NJ) to determine L (lightness orbrightness), a* (redness or greenness) and b* (yellowness or blueness)values of the avocado samples. The colorimeter was warmed up for 20 minand was calibrated with a white standard. Measurements were taken forfour samples, and the average of L, a* and b* values were obtained. In


addition to the L, a* and b* values, the total color difference was com-puted as the root mean square of the differences in individual L, a and bvalues using the following equation:

Δ Δ Δ ΔE L a b= + +( )2 2 2 1 2* * (1)

DL, Da* and Db* were obtained as differences in L, a* and b* values of testsamples on any given day from those which existed on the first day, thusrepresenting the time-related changes. Also, chroma (Eq. 2) and hue angle(Eq. 3) were calculated from L, a* and b* values as follows:

Chroma = +( )a b2 2 1 2(2)

Hue angle = ( )−tan 1 b a (3)

The color of avocado was determined on six different locations on thesurface of the fruit. The instrument was calibrated with a white standardtile: L = 95.87, a = –0.86 and b = 2.47.

(4) Respiration rate: A known quantity of avocados (about 1 kg) was placed ina plexiglass chamber (18 ¥ 12 ¥ 27 cm). A CO2 sensor (ACR SystemsInc., St-Laurent, Quebec, Canada) connected to a data logger (SmartReader plus 7, Data Logger Analysis Software, version 1.0 for Windows,ACR Systems Inc.) was installed in the chamber to monitor CO2 concen-tration. The data logger was programmed to collect online data of CO2

concentration at 1-min intervals over a 2-h period. Respiration rate wasobtained from the regression slope of CO2 concentration versus time dataand was evaluated as milliliter CO2 per kilogram-hour (Maftoonazad andRamaswamy 2005).

(5) pH, titrable acidity and total soluble solids: Avocados were peeled anddiced. One hundred grams of diced avocados were weighed into a 250-mLbeaker containing 100 g distilled water. The sample was homogenized ina blender for 1 min. The homogenate was filtered through a cheesecloth,and the filtrate was centrifuged at 2,000 rpm for 1 min using a tabletopcentrifuge (Sorval GLC-2B general laboratory centrifuge, Montreal,Quebec, Canada). The resulting supernatant subsequently was used todetermine the pH using a pH meter (Metrohm Ltd. Herisau, Switzerland)and total soluble solid contents using ATAGO hand refractometer(ATAGO N1, ATAGO Inc., Kirkland, DC). To measure the tritrableacidity, 10 mL of homogenate was pipetted to a 50-mL beaker, and 0.3 mLof 1% phenolphthalein in 95% ethanol used as an indicator was added andtitrated with 0.1 N sodium hydroxide to a permanent pink color (pH 8.1).


Titrable acidity was calculated as number of milliliters of 0.1 N sodiumhydroxide multiplied by an appropriate conversion factor. The conversionfactor of 0.28 was chosen based on linoleic acid, a predominant acid inavocado.

Kinetic Data Analysis

Loss of various quality characteristics of avocados as a result of storageconditions is expressed in terms of a rate constant (k) and dependence of rateconstant on temperature as activation energy (Ea), or the corresponding termsemployed by bacteriologists, decimal reduction time (D) and z, respectively.The change of quality index A with time (dA/dt) can be represented by thefollowing kinetic equation:

dA dt k A n= − ( ) (4)

where k is the rate constant; n is the order of reaction; A is the concentrationof a quality factor C at time t. For zero-order rate, n = 0, and for first-order rate,n = 1, the negative sign in Eq. (4) indicates that, when held at a constanttemperature, the magnitude of the quality parameter decreases with time. Butin some cases, the opposite is true as in the case of formation of chemicalsrather than disappearance. For these cases, Eq. (1) can be written without thenegative sign and the rate designated as activation or formation rate rather thaninactivation or destruction rate. On the basis of a first-order rate of reaction, thequality changes during storage can be expressed as

ln A A kt kt0 = − ( )or (5)

where A0 = initial value (time zero); A = value at time t, and k is the rateconstant.

The time dependence kinetic parameter can also be expressed as a Dvalue (or decimal reduction time) which is the time in minutes for a one logcycle change in the property value at a constant temperature. Again, withrespect to activation or formation, the D value has to be defined as a decimalactivation or formation time. The D value and the first-order reaction rateconstant are related by the following equation:

K D= 2 303. (6)

The Arrhenius relationship is used to describe the temperature depen-dence of deterioration rate:

Ln k k E RTa0 = − (7)


where k0 is a pre-exponential factor; Ea is an activation energy in kilojoule permole; R is the gas constant (8.314 J/mol·K), and T is an absolute temperature(K). Thus, a plot of the rate constant on semilogarithmic scale as a function ofreciprocal absolute temperature (1/T) gives a straight line. The activationenergy is determined from the slope of the line (multiplied by the gas constantR). A steeper slope means the reaction is more temperature sensitive (i.e., asmall change in T produces a large change in rate). Again, when the ratesdecrease with temperature, a negative activation energy can arise from Eq. (7);in such cases, the negative sign in Eq. (4) is replaced with a positive sign, andthe associated activation energy is likewise differentiated as representing anactivation or formation scenario rather than destruction or disappearance.

In this study, the order reaction was determined based on superior fittingof data either to a zero- or first-order model (rate of change in concentrationeither constant or a function of concentration).

Statistical Analysis

Experiments were designed according to a factorial design. A statisticalanalysis system (SAS Institute Inc., Cary, NC, version 8.0, 2000) was used toconduct an analysis of variance using PROC GLM, to find out if the effect ofdifferent storage variables (presence or absence of coating, temperature andtime) of avocado was significant. The significance levels used were P �0.05(*) and P � 0.01 (**). Duncan’s multiple range test was used to compare themean values in different storage days.

RESULTS AND DISCUSSION

Loss of Overall Acceptability

Overall acceptability of avocado fruits was determined subjectively basedon visual observation with the retention of the characteristic green color andtexture. Loss of green color (changing toward dark) and/or undue softening oftissue was considered unacceptable. An exponential decrease in shelf life ofboth coated and control avocados was observed as a function of storagetemperature (Fig. 1). Overall acceptability was limited to 6, 14 and 26 days forcontrol samples and 10, 19 and 33 days for coated avocados stored at 20, 15and 10C, respectively. In an article on methyl cellulose coating of avocados,Maftoonazad and Ramaswamy (2005) reported somewhat similar results,except that the pectin-based formulation performed considerably superior bymore effectively suppressing respiration of avocados. In the previous studies,the peak respiration rate for control avocados was ~150 mL/(kg·h), while in


the present studies, it was almost double. Still a 10-day shelf life at 20C waspossible, which could be extended to over a month at 10C.

The time taken for loss of acceptability in avocado was related tostorage temperature by the following models (shown by the solid lines inFig. 1):

Control: ln . . .83 7 0 118 0 9992y T R( ) = ( ) = (8)

Coated: ln . .110 0 119 0 9982y T R( ) = ( ) = (9)

Respiration Rate

Figure 2 shows the respiration rate of stored avocados as a function ofstorage time and temperature. Both coated and control avocados showedrespiratory patterns characteristic of climacteric fruits under each storagecondition. With all samples, a sharp increase in respiration was observed, andas expected, avocados had a higher rate of respiration at higher temperatures.Statistical analysis showed a highly significant difference (P < 0.01) betweencontrol and coated samples at all temperatures. The start-up value graduallyincreased at each temperature; the trend was reversed, however, after passingthe respiratory climacteric peak. While rate of CO2 evolution at the climactericpeak was 287, 253 and 186 mL/(kg·h) in control samples, reached after 6, 12and 16 days, in coated samples, it reached the peak values of 232, 210 and152 mL/(kg·h) after 8, 14 and 22 days, respectively, at 20, 15 and 10C. Thus,

0

5

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0 5 10 15 20 25

Temperature (ºC)

Stor

age

tim

e (d

ays)

Control

Coated

FIG. 1. TIME TAKEN FOR LOSS OF OVERALL ACCEPTABILITY IN COATED ANDNONCOATED AVOCADOS AT DIFFERENT TEMPERATURES


the pectin-based coating was effective in not only delaying the occurrence ofthe respiratory climacteric peak but also in reducing the intensity of therespiratory peak; both of these help to extend the shelf life and improve thestorage quality of avocados. Surface coating can delay ripening of fruits bymodifying their internal atmospheres, achieving similar beneficial effects tothat of CA/MA storage. Generally, the effects of reduced O2 and/or elevatedCO2 on reducing respiration rate have been assumed to be the primary reasonsfor the beneficial effects of CA/MA on fruits and vegetables (Kader 1989). Themajority of literature concerning optimization of CA/MA storage has focusedon the effects of low O2 levels, because smaller and less consistent effects ofhigh CO2 have been reported on respiratory metabolism of fruits and veg-etables in comparison with those observed for low O2 (Dadzie et al. 1996;Amarante and Banks 2001). Increasing CO2 levels can reduce respiration rate,depending on the crop and partial pressure of CO2. An inhibition of oxidativerespiration by increased CO2 concentration has been reported for asparagus,broccoli and cut chicory (Peppelenbos et al. 1996). Evidence that elevated CO2

has little or no effect in reducing respiration rate has been reported for banana(Young et al. 1962) and mushroom (Peppelenbos et al. 1993). Joles et al.(1994) reported that partial pressures of CO2 < 17 kPa did not affect O2 uptakefor raspberry. The combination of low O2 and elevated CO2 can have a syner-gistic effect in suppressing C2H4 biosynthesis (Gorney and Kader 1996).Temperature also affects the rate of physiological processes in harvested crops.Increases in the respiration of fruits and vegetables associated with tempera-tures are often described as power (Dadzie et al. 1993) or exponential(Cameron et al. 1994) functions of temperature, though there is some indica-

0

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350

0 5 10 15 20 25 30 35Time (days)

Res

pira

tion

rat

e (m

l CO

2/kg

.hr)

Control-20ºCCoated-20ºCControl-15ºCCoated-15ºCControl-10ºCCoated-10ºC

FIG. 2. RESPIRATION RATE OF COATED AND NONCOATED AVOCADOSAT DIFFERENT TEMPERATURES

Error bars show �1 SD in data.


tion that the relationship flattens off toward high temperatures (Banks et al.1997). Typical reduction of the respiration rate as a result of coating withedible films has been reported for other fruits and emulsions: avocado withmethyl cellulose coating (Maftoonazad and Ramaswamy 2005), banana(Banks 1984), pear (Meheriuk and Lau 1988) and tomato (Nisperos andBaldwin 1988) with other coatings.

Weight Loss

During storage period, fruits lost about 8.6–13.8% of their initial weight,depending on the presence of coating and storage temperature. Weight lossmainly consisted of moisture loss through transpiration (and to a minor extentcarbon loss through gas exchange) (Maguire et al. 2001). Figure 3 shows thechanges in produce weight as a function of storage time and temperature incoated and noncoated samples. For the entire duration of the experiment, thepercentage of moisture loss in the control sample was higher than in the coatedsamples (P < 0.01) at each temperature. Control fruits lost weight at about 5.2,6.8 and 11.5% of their original weight during 7 days of storage at 10, 15 and20C, respectively, while coated fruits lost 3.8, 4.5 and 9.1% under the sameconditions; the longer the storage duration, the larger the difference in weightloss between the control and coated samples. Rapid ripening of the fruit,moisture loss and the incidence of spoilage, however, did not permit thestorage of control to go beyond 7 days.

The primary mechanism of moisture loss from fresh fruits and vegetablesis by vapor-phase diffusion driven by a gradient of water vapor pressurebetween inside and outside the fruit. The coating helps to reduce this becauseit forms a film on the top of the skin. The thickness of the barrier and moisture

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Time (days)

Wei

ght

loss

(%

)


FIG. 3. CHANGES IN WEIGHT LOSS OF COATED AND NONCOATED AVOCADOSDURING STORAGE AT DIFFERENT TEMPERATURES



permeability of the coatings are both important factors from the viewpoint ofmass transfer rate. Temperature and relative humidity of the environment areimportant factors because they provide the driving force for moisture loss inthe form of water vapor pressure difference between fruit and atmosphere.Produce respiration can also cause some weight loss because of the degrada-tion of sugars (Pan and Bhowmik 1992). Slower rates of moisture loss incoated fruits can be attributed to the barrier properties for gas diffusion ofstomata, the organelles that regulate the transpiration process and gasexchange between the fruit and the environment.

The loss of weight in avocados during storage followed a fairly linearmodel (zero order). The rate constant, k, calculated from the regression ofweight loss versus storage time plots, is summarized in Table 1, indicating agood fit in all cases. k values increased by increasing the temperature ofstorage. The associated activation energies (Arrhenius model) are also sum-marized in Table 1 indicating high R2 values.

Firmness

Firmness of avocados decreased with the passage of time at differentstorage temperatures both for coated and control fruits (Fig. 4); however, thepectin-based coating showed a beneficial effect on firmness retention. Statis-tical analysis showed a significant difference (P < 0.05) between coated andcontrol samples at 20C, while highly significant differences (P < 0.01) wereobserved between coated and noncoated fruits at 10 and 15C for texturalchanges. No significant difference was observed between firmness of coatedsamples at 15C and that of control fruits at 10C. Thus, the coating effect wasequivalent to decreasing the storage temperature by 5C. The lower storage

TABLE 1.KINETIC PARAMETERS FOR WEIGHT LOSS OF COATED

AND NONCOATED AVOCADOS STORED ATDIFFERENT TEMPERATURES

Storage temperatures (C) k value (day-1) R2

10 Control 0.770 0.998Coated 0.577 0.996

15 Control 0.932 0.995Coated 0.750 0.993

20 Control 1.641 0.999Coated 1.201 0.993

Ea = 52.0 kJ/mol (control) Ea = 50.5 kJ/mol (coated)

Ea, activation energy.


temperatures delayed the associated changes. Responses of avocadosto the storage time were consistent, but were dependent on the differenttemperatures.

Jeong et al. (2003) also found that uncoated fruits softened rapidly andcompleted ripening within 7 days of storage at 20C. In contrast, fruits coatedwith either wax or 1-methylcyclopropene exhibited roughly half to one-thirdretention of firmness after 7 days at 20C. Retention of firmness can beexplained by retarded degradation of insoluble protopectins to the moresoluble pectic acid and pectin. During fruit ripening, depolymerization orshortening of the chain length of pectin substances occurs with an increase inpectin-esterase and polygalactronase activities. Low oxygen and high carbondioxide concentrations reduce the activities of these enzymes and allow reten-tion of the firmness of fruits and vegetables during storage (Salunkhe et al.1991). In our previous studies on avocados (Maftoonazad and Ramaswamy2005), we found a similar effect of lowering of rate of texture softening withmethyl cellulose coating, but that was less effective than the pectin-basedemulsion coating.

The textural softening of avocados during storage followed a first-orderkinetic model (R2 > 0.940) (Fig. 5a). Kinetic parameters D and k, calculatedfrom the regression of log firmness versus storage time plots, are summarizedin Table 2, indicating a good fit in all cases. D values decreased and k valuesincreased by increasing the temperature of storage. An Arrhenius plot is shownin Fig. 5b relating the rate constant k to the reciprocal absolute temperature.The associated activation energies (Ea) are summarized in Table 2.

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Time (Days)

Fir

mne

ss (

N/m

m)


FIG. 4. FIRMNESS CHANGES IN COATED AND NONCOATED AVOCADOS DURINGSTORAGE AT DIFFERENT TEMPERATURES



Color Changes

Figure 6a,b indicates the results of L value and total color differencechanges in avocado skin at different temperatures as influenced by storagetime. These changes were manifested by a decrease in L values and an increasein total color difference values. Figure 6a indicates that the L value reductionin the control samples was higher than in the coated samples. Statisticalanalysis showed highly significant effects for coating on L values (P < 0.01) ateach temperature. No significant difference was observed between the coated

a

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3.4 3.42 3.44 3.46 3.48 3.5 3.52 3.54

Reciprocal absolute temperature (1/T) o(K-1)*1000

ln k

control

Coated

FIG. 5. (a) FIRST-ORDER KINETIC PLOT OF CHANGES IN FIRMNESS OF COATED ANDNONCOATED AVOCADOS STORED AT DIFFERENT TEMPERATURES AND

(b) ARRHENIUS PLOT OF RATE CONSTANT FOR FIRMNESS CHANGES IN AVOCADODURING STORAGE


TABLE 2.KINETIC PARAMETERS FOR FIRMNESS CHANGES IN COATED AND NONCOATED

AVOCADOS STORED AT DIFFERENT TEMPERATURES

Storage temperatures (C) D value (day) k value (day-1) R2

10 Control 23.8 0.0966 0.961Coated 30.6 0.0752 0.970

15 Control 14.1 0.163 0.943Coated 18.1 0.127 0.984

20 Control 4.96 0.464 0.975Coated 7.13 0.323 0.965

Ea = 108 kJ/mol (control) Ea = 99.8 kJ/mol (coated)

Ea, activation energy.

a

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L v

alue


b

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otal

col

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iffe

rent

(D

elta

E) Control-20ºC

Coated-20ºCControl-15ºCCoated-15ºCControl-10ºCCoated-10ºC

FIG. 6. CHANGES IN L VALUE AND TOTAL COLOR DIFFERENCE OF COATED ANDNONCOATED AVOCADOS DURING STORAGE AT DIFFERENT TEMPERATURES



samples at 20C and the noncoated samples at 15C. The difference between thecoated samples at 15C and the control samples at 10C was significant(P < 0.05). Because the L value is a measure of the color in the light–dark axis,this falling value indicates that the avocado samples demonstrated a loss inbrightness as storage temperature and duration increased (tissue taking adarker shade); again, the rate of decrease was dependent on the storage tem-perature. The magnitude of the decrease was higher for avocados stored athigher temperatures.

The a* value was more negative in the coated samples indicating theavocado skin to be greener, and statistical analysis showed highly significantdifferences in a* value between the test samples during storage (P < 0.01). Thetime-related color shift toward positive a* value indicates more redness incolor that is the result of ripening. Fruits stored at 20C exhibited the mostchange in a* value, and those stored at 10C exhibited the least change in a*value. Also b* value decreased, with highly significant differences betweencoated and uncoated samples (P < 0.01) at 15 and 20C and a significantdifference at 10C. This decrease in b* value indicates reduction in yellownessof samples and an increase toward darker chroma. No significant differencewas observed between the coated samples at 20C and the control samples at15C for a* and b* values. When changes in a* and b* values are small, thetotal color difference, chroma and hue are employed for interpreting colorchange.

Figure 6b shows an increase in DE for all samples during storage time atdifferent temperatures. The total color difference DE, which is a combinationof L, a* and b* values, is a colorimetric parameter extensively used to char-acterize the variation in color perception. Again statistical results showedhighly significant differences (P < 0.01) between coated and control fruits.The total color difference in the coated samples changed at a much lower ratethan in the uncoated samples, and thus it can be recognized that coating has abeneficial effect on the reduction of color changes in avocado. No significantdifference was observed between coated samples at 15C and control fruits at10C. Jeong et al. (2003) showed that at the full-ripe stage, fruits treated withwax and/or 1-methylcyclopropene had more green color than control; thisconfirms the effect of coating on green color in the present work. But they alsoreported that treated fruits had a significantly lower L* value than control,which is in contrast with the results obtained for the L value in present study.The lower color changes in coated fruits may be related to the effect of coatingin creating modified atmospheres within the fruit. The presence of CO2 in thestorage atmosphere is an important factor in preventing chlorophyll degrada-tion. Chlorophyll retention is increased in broccoli by a progressive increase inCO2 and decrease in O2. Higher concentrations of CO2 (2.5–10.0 mL/kg) slowdown the degreening processes in apricots and peaches. The degradation of


chlorophyll in asparagus can be delayed by CA storage (Salunkhe et al. 1991).Maftoonazad and Ramaswamy (2005) also found reduction in color changesof fruits coated with methylcellulose.

Chroma and hue (Eqs. 2 and 3) are shown in Fig. 7a and b, respectively.Chroma values decreased during storage time and closely followed the b*values. The chroma value indicates the degree of saturation of color and isproportional to the strength of the color. Little change was found in chromabetween coated and noncoated avocados, and except at 20C, the changes werenot different (P > 0.05) among storing techniques. Several studies (Barreiroet al. 1997; Lee and Coates 1999; Palou et al. 1999) have reported similarobservations. The hue angle values also decreased during storage at differenttemperatures. This suggests a reduction from a more green to a red color in

a

05

101520253035

0 10 20 30 40Time (days)

Chr

oma


b

0

20

40

60

80

100

0 10 20 30 40Time (days)

Hue

ang

le


FIG. 7. CHANGES IN CHROMA AND HUE ANGLE IN COATED AND NONCOATEDAVOCADOS STORED AT DIFFERENT TEMPERATURES



avocados (Waliszewski et al. 1999). The changes in hue angle values in coatedand noncoated fruits were not significant (P > 0.05) in all conditions.

In an attempt to characterize the color change kinetics, a zero-ordermodel was used for DE and a first-order model for L values. Table 3 summa-rizes the derived kinetic first-order parameters. Several authors have stated thatthe first-order kinetic model is better for L and b* values (concentrated tomatopaste [Barreiro et al. 1997] and peach puree [Avila and Silva 1999; Garza et al.1999]). On the other hand, Maskan (2001) reported that only zero-ordermodels fitted well with the data of a* values and DE in kiwi fruit. Similarresults for a* values and DE have been reported by Rhim et al. (1989).

Chemical Changes

Figure 8a,b illustrates changes in chemical parameters as influenced bystorage conditions. Figure 8a depicts pH variations for all samples. Asexpected, coated fruits showed less variation in pH at each temperature. ThepH of avocados decreased with time, and the changes were more rapid athigher temperatures. Consistent with the other results presented, control avo-cados had greater pH decrease associated with the utilization of excess organicacids stored in the vacuoles as respiratory substrate (Medlicotte et al. 1987).As seen in Fig. 8a, the rate of decrease of pH in the control samples is higher

TABLE 3.KINETIC PARAMETERS FOR COLOR CHANGES IN AVOCADOS

AT DIFFERENT TEMPERATURES

Storagetemperature (C)

Parameter Zero-order model First-order model

k0 (day-1) R2 k1 (day-1) D (day) R2

10 Control L -0.009 256 0.905DE 1.44 0.919 – – –

Coated L – – -0.007 329 0.969DE 1.09 0.916 – – –

15 Control L – – -0.0128 179 0.977DE 1.89 0.946 – – –

Coated L – – -0.0109 211 0.990DE 1.59 0.944 – – –

20 Control L – – -0.0277 83.1 0.971DE 3.96 0.948 – – –

Coated L – – -0.0229 100 0.930DE 3.19 0.959 0.0968 23.8 0.908

Activation energy (kJ/mol)Control L: 77.3 DE: 69.5Coated L: 81.6 DE: 73.9


than in the coated fruits. Titrable acidity and °Brix increased during storagetime at all temperatures for coated and noncoated samples, and the rate ofincrease of these parameters was greater at higher temperatures. Although°Brix and titrable acidity were increasing, the ratio of °Brix to acid had adecreasing trend in all samples (Fig. 8b). These changes would indicate anapparent increase in values, thereby somewhat amplifying the increasing effectand depressing the decreasing effect for acidity and total soluble solids. Thesevalues represent the existing concentration; however, they may also show thepossible influence of weight loss on these components.

a

6/6

6/7

6/8

6/9

7

7/1

7/2

7/3

7/4

7/5

0 10 20 30 40

Time (days)

pHControl-20ºCCoated-20ºCControl-15ºCCoated-15ºCControl-10ºCCoated-10ºC

b

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35

Time (days)

Bri

x/A

cidi

ty

Control-20ºCCoated-20ºCControl-15ºCCoated-15ºCControl-10CCoated-10ºC

FIG. 8. CHANGES IN (a) pH AND (b) °Brix/ACID RATIO IN COATED AND NONCOATEDAVOCADOS STORED AT DIFFERENT TEMPERATURES



Both pH and °Brix/acid showed significant differences (P < 0.05)between coated and noncoated fruits at 10 and 20C. No significant differencewas observed between coated and control avocados at 15C. The changes in pHand °Brix/acid ratio were modeled based on the zero-order rate model(R2 > 0.900) for the rate constant k, and Arrhenius results are summarized inTable 4. It should be noted that the k values for titrable acidity and total solublesolids are for accumulation. Because there was an increase in acidity, it wasobvious that pH would show a decreasing trend.

CONCLUSIONS

This study revealed that several physicochemical quality changes ofstored avocados were dependent on the presence or absence of coating andstorage conditions. Coating could hinder ripening of the fruits and all relevantparameters including loss of firmness, color changes, weight loss and chemicalchanges. Elevated temperature had an adverse influence on quality attributes ofavocados, especially with longer times. Fruits became softer and darker withthe passage of storage time. All changes were accelerated at higher tempera-tures, and hence, presence of coating in lower temperatures offered betterchoices for storage. As a result, in any storage study, a combination of storage

TABLE 4.KINETIC PARAMETERS FOR CHANGES IN pH AND °Brix/ACID RATIO IN COATED AND

NONCOATED AVOCADOS STORED AT DIFFERENT TEMPERATURES

Storagetemperature (C)

Parameter Zero-order model

k0 (day-1) R2

10 Control pH 0.0228 0.957°Brix/acid 0.289 0.933

Coated pH 0.011 0.900°Brix/acid 0.223 0.959





Activation energy (kJ/mol)Control pH 73.3 (R2 = 0.952) °Brix/acid: 59.9 (R2 = 0.952)Coated pH 92.7 (R2 = 0.997) °Brix/acid: 67.3 (R2 = 0.973)


time and temperature has to be considered with regard to the end use of theproducts. Some storage-associated changes were successfully modeled byfirst-order models, while zero-order models had a better fit with other changes.

The derived rate constants were linked to temperature using an Arrheniusrelationship. Kinetic parameters and temperature dependence indicators (acti-vation energy) of the reaction rate constant were reported to facilitate the useof these models. Validation of these models under commercial conditions canfacilitate the prediction of fruit behavior during storage.

ACKNOWLEDGMENT

This research was funded by a grant from the Natural Sciences andEngineering Research Council of Canada.

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effect of pectin-based coating on the kinetics of quality change

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