acetic acid as value-added product from pesticide-free

Dominica DM. Dacera, Jennifer P. Fronteras*, Daisic D. Bello, and Kathleen Joy L. Delos Santos

University of the Philippines Mindanao Tugbok District, Davao City 8022 Philippines

Acetic Acid as Value-added Product from Pesticide-free Banana and Pineapple Peels

Keywords: acetic acid, Acetobacter aceti, alcohol fermentation, fruit peel, pesticide contamination, Saccharomyces cerevisiae

Wastes from banana and pineapple peels pose increasing disposal and pollution problems as they represent a large fraction of the fruit. One mitigating measure to address this concern is the conversion of these wastes into high-value products. This study explored the potential of producing acetic acid from banana and pineapple peels through fermentation. Physicochemical characterization showed an initial sugar content of 12.60% for banana peel and 11.61% for pineapple peel, thus indicating their potential for conversion to acetic acid. Further, the pesticide residue analysis in the peels revealed that organochlorines, organophosphorus, and pyrethroids are way below the maximum residue limit (MRL) values set by the Bureau of Philippine Standards (BPS), Joint FAO/WHO Codex Alimentarius Commission (CAC), and the European Commission (EC) Regulation No. 396/2005, which enhanced their suitability as raw material for use in fermentation. Processing the peels to achieve various sugar concentrations of 15% (15°Brix), 20% (20°Brix), and 25% (25°Brix), and the subsequent addition of Saccharomyces cerevisiae allowed the peels to undergo anaerobic fermentation to produce ethanol. The maximum amount of ethanol obtained at a temperature of 26.7 °C and pH of 3.63 was 10.81% v/v (Day 10) from banana peels and 10.60% v/v (Day 10) from pineapple peels at 27 °C and pH of 3.27, both from 20% (20°Brix) sugar concentration. Aerobic fermentation of the extract with Acetobacter aceti converted ethanol to acetic acid. The maximum amount of acetic acid produced, which was from 20% sugar solution at 27.4 °C and pH of 3.59, was 4.56% for banana peels after 16 d while that of pineapple was obtained after 18 d of fermentation at 28 °C and pH of 3.34. For both banana and pineapple peels, no significant differences in the amount of acetic acid produced from three different sugar concentration were observed. The acetic acid produced from banana and pineapple peels can be explored further for potential industrial applications.

Philippine Journal of Science150 (2): 377-389, April 2021ISSN 0031 - 7683Date Received: 20 Jul 2020

*Corresponding Author: [email protected]

INTRODUCTIONFruit processing wastes are those end products of various fruit processing industries that have not been recycled or used for other purposes. They are the non-product flows of raw materials whose economic values are less than the cost of collection and recovery for reuse and, therefore,

discarded as wastes. These wastes could be considered valuable by-products if there were appropriate technical means and if the value of the subsequent products were to exceed the cost of reprocessing (UNIDO 2015).

For many fruit processing plants, a large fraction of the solid waste comes from the separation of the desired fruit constituents from undesired ones in the early stages

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of processing. The undesirable constituents include trimmings, peels, pits, seeds, and pulp (Ammar 2014). Peels from banana, for instance, constitute up to 30% of the ripe fruit and in Davao, about 79,000 metric tons of banana wastes are produced annually (BAS 2013). Further, considering the Philippine setting, data from the Philippine Statistics Authority (PSA 2018) show that pineapple wastes can reach up to 130,000 metric tons annually since about 75% of the fruit consists of peels, crown, and core. These wastes pose increasing disposal and potential severe pollution problems and represent a loss of valuable biomass and nutrients. Currently, in Davao, wastes from fruit processing factories are disposed of in landfills or sent to wastes processing facilities for treatment at PhP 0.50/ kg of waste, which entails an additional cost to the factories. For instance, for the fruit processing factory where pineapple and banana peels were collected, about 9,000 kg/d of wastes are produced, which would cost them PhP 4,500.00/d or about PhP 90,000.00/mo for disposal.

Disposal of solid wastes in landfills is becoming less favorable due to the generation of foul odors as communities expand and reside in proximity to fruit processing plants. Leaching of undesirable constituents such as soluble organics into the soil and groundwater is also an important concern where pollution of groundwater used by communities can occur, which can migrate into nearby streams (Ammar 2014; Hawkins 2009).

Besides their pollution and hazard aspects, in many cases, fruit processing wastes might have a potential for conversion into useful products of higher value as a by-product, raw material for other industries, or for use as animal feeds. In this study, the potential for producing acetic acid from fruit processing wastes was explored. Acetic acid is used in many industrial processes for the production of substrates; chemical compounds such as acetic anhydride, ester, vinyl acetate monomer, and vinegar; and many other polymeric materials. It is also widely used for etching metals, as a solvent in chemical laboratory analysis, fabric dyeing, production of nylon, leather tanning, additive, or flavoring in food canning and medicines. Moreover, acetic acid has also been recognized as a non-selective contact herbicide, especially for broadleaf weeds and weed grasses.

A major concern in fruit processing wastes utilization, however, is pesticide contamination. Fruits are attacked by pests and diseases during production and storage, leading to damages that reduce the quality and yield. In order to reduce the loss and maintain the quality of fruits, pesticides are used to destroy pests and prevent diseases. Banana crops are pesticide-intensive since they are grown in the tropics with a warm and humid environment and are prone to infestations of insect pests and fungal diseases.

The common diseases for banana crops are the “black Sigatoka,” which is a fungal leaf spot disease and “bunchy top” disease that affects the banana fruit and foliage and is caused by a single-strand DNA virus (Banana Planters 2013). In pineapples, the common diseases and pests in pineapple are black rot, brown rot, infestation of nematodes, toy beetle, mealybug, scales, mites, weeds, and even rodents (The Pineapple Technical Committee 2010). Pesticides in fruits are classified into different families – including organophosphate, carbamate, organochlorine, and pyrethroid. These pesticides should not exceed the MRL of the pesticide residue established by the CAC at the point of entry into a country or (b) at the point of entry into trade channels within a country (FAO 2017). The use of pesticides during production often leads to the presence of pesticide residues in fruits after harvest. These residues might be present in peels, which would be of concern if utilized as a value-added food product.

The general objective of this study was to investigate the potential of banana and pineapple peels to produce a value-added product in the form of acetic acid. Specifically, the study aimed to determine the physicochemical characteristics of the fruit peels, determine possible pesticide contamination in the peels, and produce acetic acid from peel extracts with varying sugar concentrations.

MATERIALS AND METHODS

Sample Collection Banana and pineapple peels were collected from a fruit processing company located in Toril, Davao City, Philippines. The samples were immediately transported to the College of Science and Mathematics, University of the Philippines Mindanao, and stored in the chiller until processing for fermentation. Approximately 2 kg of peels was sent to a third-party laboratory for pesticide analysis. On the other hand, another 1 kg of peels was stored in an ultralow freezer at –80 °C for analysis of physicochemical properties. The rest of the peels was used for substrate preparation for acetic acid production.

Characterization of Fruit PeelsProximate composition . Determination of moisture, crude ash, crude fat, crude fiber, crude protein, and total carbohydrate content was done using the official methods of the Association of Official Analytical Chemists (AOAC 2000). All determinations were done using three replicates per fruit peel.

Volatile solids. The amount of volatile solids was determined using the method of the American Public Health Association (APHA 2005).

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Total sugar content . The total sugar content of banana and pineapple peels was determined using the anthrone method (McCready et al. 1950; Yoshida et al. 1972). About 0.05 g of sample was weighed in a test tube, after which 5 mL of hot 80% ethanol was added. The mixture was mixed through a vortex mixer then allowed to stand for 10 min with occasional mixing. The sample was then centrifuged at 3000 rpm for 5 min and the supernatant liquid was decanted into a 100-mL volumetric flask. The extraction process was repeated and the supernatant liquids were pooled. The resulting solution was diluted to the mark and mixed thoroughly. Several standard glucose solutions (10, 20, 40, and 60 μg/mL) were prepared to constitute a standard curve. In the quantification of sugar, 0.1 mL aliquot of every solution (sample and standard solutions) was pipetted into a test tube containing 0.9 mL distilled water. Three (3) mL of 0.2% anthrone reagent was then added to the mixture, after which it was mixed, and the test tube was covered with a glass marble. It was then heated in a boiling water bath for 10 min and cooled in an ice bath. The solutions were allowed to stand for at least 1 hr, then the absorbance of each solution was read at a wavelength of 630 nm using a UV-Vis spectrophotometer (Microplate Reader Stat Fax 4200). The concentration of sugar in the sample was extrapolated from the standard curve.

Total starch content. The total starch content was analyzed using a Megazyme Total Starch Assay Kit®, which is based on the amyloglucosidase/α-amylase method. Fruit peels were dried at 60 °C for 24 h and milled to pass through a 0.5-mm screen. About 100 mg of the milled sample was weighed into a glass test tube. Aqueous ethanol (0.2 mL, 80% v/v) was added to wet the sample and aid dispersion. The tube was then stirred on a vortex mixer and was immediately added with 3 mL of thermostable α-amylase (diluted with 100-mM sodium acetate buffer pH 5.0 at a ratio of 1:30). The sample was incubated in a boiling water bath for 6 min with vigorous stirring after the 2nd, 4th, and 6th min to ensure complete homogeneity. Afterward, the tube was placed in a water bath at 50 °C. Subsequently, 0.1 mL of amyloglucosidase was added to the tube then mixed and incubated at 50 °C for 30 min. The mixture was transferred into a 100-mL volumetric flask and the volume was adjusted using distilled water. The solution was then transferred to a tube and centrifuged at 3000 rpm for 10 min. Duplicate aliquots of approximately 0.1 mL of the filtrate were then transferred to glass test tubes. Three (3) mL of glucose oxidase/ peroxidase (GOPOD) reagent was added to each tube and incubated at 50 °C for 20 min. D-Glucose standard (0.1 mL) and 0.1 mL distilled water were also added with 3 mL of GOPOD and incubated as with the test samples. The absorbance for each sample and the D-glucose control was read using a UV-VIS spectrophotometer (UV-1700 Shimadzu) at

510-nm wavelength against a reagent blank. The starch content was then calculated using Megazyme Mega-CalcTM, as shown in Equation 1:

(1)

where ΔA is the absorbance (reaction) read against the reagent blank, F is 100 μg glucose/ absorbance of 100 μg glucose, FV is the final volume of the extract in mL, and W is the weight of the sample in mg.

Pesticide Analysis of Fruit PeelsThe analysis of pesticide residues present in the peels was outsourced to a third-party laboratory. A total of 2 kg of each fruit peel was sent to be analyzed for the amount of organochlorines, organophosphates, and pyrethroids. The amount of pesticide residues was determined by gas chromatography using an electron capture detector. The active compounds determined for organochlorines were aldrin, α-benzene hexachloride (BHC), β-BHC, δ-BHC, γ-BHC (lindane), α-chlordane, γ-chlordane, 4,4’-dichlorodiphenyldichloroethane (DDD), 4,4’-dichlorodiphenyldichloroethylene (DDE), 4,4’-dichlorodiphenyltrichloroethane (DDT), dieldrin, endosulfan I, endosulfan II, endosulfan sulfate, endrin, endrin aldehyde, endrin ketone, heptachlor, heptachlor epoxide (B), and methoxychlor. The active compounds determined for organophosphorus were dichlorvos, mevinphos, demeton-S, ethoprophos, phorate, demeton-O, diazinone, methyl parathion, ronnel, fenthion, chlorpyrifos, trichloronate, merphos, tetrachlorvinphos, tokuthion, fensulfothion, sulprofos, coumaphos, and tribufos. The active compounds determined for pyrethroids were tefluthrin, transfluthrin, anthraquinone, allethrin, resmethrin, tetramethrin, bifenthrin, phenothrin, λ-cyhalothrin, cis-permethrin, trans-permethrin, cyfluthrin, cypermethrin, flucythrinate, fenvalerate, tau-fluvalinate, and deltamethrin.

Laboratory-scale Production of Acetic AcidSubstrate preparation . The preparation of peel was done following the method of Elijah and Etukudo (2016) with slight modifications. For each fruit, about 5 kg of peels were washed thoroughly in water and drained for 30 min. The peels were then cut, homogenized in a Waring blender, and boiled for 20 min. The boiled mixture was cooled to room temperature and filtered using a muslin-cheese cloth. An additional amount of peels was added per fruit in order to produce three set-ups of the liquid extract with varying sugar concentrations of 15, 20, and 25%. The extracts were then sterilized using an autoclave at a pressure of 15 psi for 15 min. The residue was analyzed for total sugar, pH, and total soluble solids (TSS).



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Starter culture preparation . The yeast starter culture, Saccharomyces cerevisiae, was prepared by inoculating a loop full of isolated yeast in 20 mL of 10% sucrose solution for 1 h at 30 °C. Similarly, a loop full of Acetobacter aceti was inoculated into glucose yeast (GY) broth (D-glucose 100 gL–1; yeast extract 10 gL–1). The culture was incubated on a rotary shaker at 30 °C and 120 rpm for 24 h before adding to the fruit extract.

Alcohol fermentation . Twenty (20) mL of yeast (S. cerevisiae) culture was inoculated into 2.5 L of fruit extract contained in a 12-L sterile glass container. The glass container consisted of two holes – one for sample collection and the other for aeration purposes. The fermentation broth was initially aerated for 2 d. During this period, the hole was covered with a small screen to prevent the entry of contaminants. After the 2-d aeration, the glass container was properly sealed and corked to prevent the entry of both contaminants and air. Fermentation was allowed to proceed at ambient temperature until the maximum alcohol content was obtained. Parameters like alcohol content, temperature, pH, and TSS of the fermented fruit extract were monitored daily. The alcohol content was estimated using Equation 2 that involved the conversion of °Brix to specific gravity, then to percent alcohol using measurements obtained before and after the fermentation process (Lee 2015):

(2)% Alcohol (v/v) = (Original Specific Gravity − Final Specific Gravity x 131.25

Acetic acid production. In a 15-L sterile glass container, Acetobacter aceti was added to the fermented extract at a ratio of 2.5 L: 5 mL (clarified fermented extract: acetic acid bacterial suspension). The mixture was then incubated at ambient temperature until the maximum acetic acid content was obtained. The set-up for acetic acid fermentation consisted of glass containers with stainless-steel stirring propeller and precision air pump with an air output of 8,500 cm3/min connected to a stainless-steel nipple pipe

to increase the available oxygen. Changes in temperature, pH, and percent acetic acid were monitored daily.

Analysis of Monitoring ParametersThe pH of the mixture was determined using a pH meter while the TSS in the extract and fermented liquid were measured as °Brix with a handheld refractometer. The measurements were made in triplicate for each sample. The amount of acetic acid expressed in terms of percentage by mass was determined by titrating 5 mL of the sample with a standard 0.1N NaOH solution and calculated using Equation 3:

(3)

Statistical Analysis The results of the analyses were reported as mean ± standard deviation of the triplicates. The data for banana and pineapple peel characterization were analyzed for significant differences using a one-sample t-test. On the other hand, the data for acetic acid fermentation were subjected to analysis of variance and the means were compared using Duncan’s multiple range test at a 5% level of significance.

RESULTS AND DISCUSSION

Physicochemical Characteristics of Banana PeelsBased on the Banana Ripening Stage Chart (Munasinghe 2013), the peels collected were considered to be in the third stage of ripening as characterized by the presence of more yellow than green color in the peel.

Table 1 shows the physicochemical properties of banana peels. The analysis revealed that the peels contained a relatively high amount of moisture (12.75%), which is comparable to the result of Abubakar et al. (2016). The

Table 1. Physicochemical characteristics of banana peels.

Composition (%) This study Abubakar et al . (2016) Happi Emaga et al . (2006)

Moisture 12.745 ± 0.830 13.49 ± 0.17 8.7 ± 0.1

Ash 7.365 ± 0.128 9.83 ± 0.06 9.6 ± 0.2

Fat 11.950 ± 0.601 23.93 ± 0.68 3.8 ± 0.1

Fiber 7.228 ± 0.817 14.83 ± 0.28 43.2± 0.5

Protein 0.664 ± 0.108 5.53 ± 0.11 6.3 ± 0.1

Volatile solids 84.216 ± 0.243 ND ND

Sugar content 12.60 ± 1.58 ND ND

Starch content 14.02 ± 0.22 ND 11.1 ± 0.1

ND – not determined



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ash content was also high (7.365%) for a plant-based material, and this signifies the possible presence of high amounts of minerals in the peels. Anhwange (2008) stated that banana peels can be a good source of minerals such as potassium, manganese, and calcium. The fat content of banana peels as determined in this study was just half of that obtained by Abubakar et al. (2016). This may be due to differences in the variety of the samples used. The fiber content was significantly lower at 7.23% than that obtained by Happi Emaga et al. (2006). The protein content of banana peels was found to be 0.664%, an amount lower than those obtained by Abubakar et al. (2016) and Happi Emaga et al. (2006). This, however, is comparable to the 0.9% protein content obtained by Anhwange (2008). The difference in values obtained may be due to the difference in the variety of samples used. The amount of volatile solids in banana peel was very high at 84.216%. Scott and Ma (2004) stated that food wastes typically have high ratios of volatile solids. The latter gives an approximation of the amount of organic matter present, which is suitable for biological treatments such as anaerobic digestion (Peces et al. 2014).

The average concentration of sugar in banana peels determined to be 12.60 ± 1.58% was brought about by the conversion of starch in the peel to sugar during maturation of the fruit. The Cavendish banana peels used in this study were classified under stage three of ripening, thus implying the onset of starch conversion to sugar. The presence of sugar in the peel makes it a suitable raw material for fermentation to other high-value products like alcohol and organic acids.

The average starch content of banana peels was 14.02 ± 0.22%, which is comparable to that obtained by Happi Emaga et al. (2006). Banana such as Cavendish tends to have low starch content and, as the fruit matures/ripens, starch decreases further as it is broken down to simple sugars (Happi Emaga et al. 2007).

Alcohol Fermentation and Acetic Acid Production of Banana PeelsFigure 1 shows the changes in the TSS and alcohol content of the mixture as the fermentation period progressed. Although TSS also includes other soluble solids present, it is a widely accepted estimate of the amount of sugar present in the fermentation medium. As can be seen in Figure 1, TSS showed a gradual decrease until reaching the final values of 6.07, 8.37, and 13.40°Brix at the end of fermentation for 15, 20, and 25% sugar concentration, respectively. The decrease in TSS can be explained by the fact that sugars are continuously being consumed as a substrate during fermentation. Conversely, the alcohol content showed an increasing trend where the optimal amount reached 10.12% (Day 11) at a temperature of 27 °C and pH of 3.83, 10.81% (Day 10) at 26.7 °C and pH of 3.64, and 9.06% (Day 10) at 27.9 °C and pH of 3.34 for 15, 20, and 25% sugar concentrations, respectively. The decrease in TSS and increase in alcohol content indicate that sugar was being converted into alcohol, specifically ethanol. This conversion was initiated by the yeast added to the medium. Carbon dioxide gas was liberated along the process as a by-product (Benazir and Mishra 2015), as shown in Equation 4:

Figure 1. Alcoholic fermentation of banana peels at different sugar concentrations (B1 = 15%, B2 = 20%, B3 = 25%).



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(4)

After obtaining the maximum alcohol concentration, Acetobacter aceti was then added to the fermentation medium. The latter facilitates the conversion of alcohol to acetic acid. Shown in Figure 2 are the changes in the acetic acid content of the medium as the fermentation progressed. Results showed a gradual increase in the amount of acetic acid ranging from 0.66–4.52% for 15% sugar concentration, 0.98–4.56% for 20% sugar concentration, and 0.72–4.50% for 25% sugar concentration. The increase in acetic acid content indicates the conversion of ethanol to acetic acid (Cheryan 2009), as shown in Equation 5:

(5)

The maximum amounts of acetic acid obtained were 4.52% (Day 17) at 28.5 °C and pH of 3.53, 4.56% (Day 16) at 27.4 °C and pH of 3.59, and 4.50% (Day 18) at 27.8 °C and pH of 3.27 for 15, 20, and 25% sugar concentrations, respectively. In terms of yield, these values are equivalent to 12.48, 10.85, and 10.43 g acetic acid/ kg peels for 15, 20, and 25% sugar concentrations, respectively. Statistical analysis revealed that these amounts did not differ significantly from each other. Hence, it can be said that for economic reasons, fermentation can be carried out for 16 d. Furthermore, the concentration of sugar in the fermentation medium did not significantly affect

the amount of acetic acid produced. Hence, in future fermentation experiments, a 15% sugar solution can be used as a substrate for fermentation. The amount of acetic acid obtained from the fermentation of banana peels conforms to the 4–6% v/v range reported by Morales et al (2002). This indicates that the fermentation conditions used in this study represent a suitable condition for A. aceti to be capable of oxidizing ethanol to acetic acid.

Physicochemical Characteristics of Pineapple PeelsThe pineapple samples used in this study were observed to be completely ripe. This level of ripeness is ideal as the fruit contains high levels of fermentable sugar (Joy and Rajuva 2016).

Table 2 shows that pineapple peels contain 85.64% moisture, which is comparable to the values obtained by Morais et al. (2017) and Abdullah and Mat (2008). The slight differences could be accorded to varying ripening stages of pineapples used in the studies. On the other hand, the ash content was found to be 3.58%, indicating a significant presence of minerals in the peels – which can be presumed to be calcium, magnesium, potassium, sodium, copper, iron, manganese, and zinc (Hossain et al. 2015). The crude fat content of pineapple peels was found to be only 1.21%. The low-fat content of the peel makes it suitable as a possible ingredient to simulate and replace fat in some food products. Selani et al. (2016) used pineapple peels as a fat replacer in low-fat beef burgers and observed that this resulted in a healthier

Figure 2. Acetic acid production of banana peel at different sugar concentrations (B1 = 15%, B2 = 20%, B3 = 25%). Per concentration, columns with common letters are not significantly different at P < 0.05.



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product with reduced cholesterol content and improved nutritional quality. The crude fiber content of pineapple peel was found to be 8.94%, which suggests that the peel can be a great source of fiber when incorporated into food products. The protein content was found to be 0.48%, which is compared to the results of Morais et al. (2017) and Abdullah and Mat (2008). Fruits in general are not potential sources of protein. However, the peels can still be utilized as feed despite low amounts of crude protein. Pineapple peel was combined with pineapple pulp residue and was processed using submerged liquid fermentation in the study of, which resulted in higher protein content in the pulp and peel mixture. This supports the statement of Adrizal et al. (2017) that pineapple peels must be processed to enhance nutritional quality before using as animal feed. The volatile solids for pineapple peels were found to be very high at 96.52%. This implies the high potential of pineapple peel as a suitable substrate for metabolic processes such as fermentation that requires sugar as raw material.

The total sugar content of pineapple peels was 11.91 ± 0.90%. This is higher than the value obtained by Saraswaty et al. (2017). The high sugar content implies that pineapple peel is a good substrate for fermentation.

On the other hand, the starch content of pineapple peels was only 1.28 ± 0.06%. This value is anticipated as the pineapple fruit was already mature and ripe. This implies that starch has broken down into smaller carbohydrates such as sugar by amylase enzymes (Happi Emaga et al. 2007).

Alcohol Fermentation and Acetic Acid Production of Pineapple PeelsFigure 3 shows the changes in the TSS and alcohol content of the pineapple peel extract as the fermentation period progressed. TSS showed a gradual decrease until reaching a final value of 6.03, 8.17, and 13.37°Brix at the end of fermentation for 15, 20, and 25% sugar concentrations, respectively. Conversely, the alcohol content showed an

increasing trend where the maximum amounts of 10.57% (Day 10) at 27 °C and pH of 3.98, 10.60% (Day 10) at 27 °C and pH of 3, and 9.77% (Day 10) at 27.9 °C and pH of 3.53 were achieved for 15, 20, and 25% sugar concentrations, respectively. The decrease in TSS and the corresponding increase in alcohol content indicate that sugar is being converted to ethanol. This conversion was initiated by the yeast, S. cerevisiae added in the fermentation medium.

After obtaining the maximum alcohol concentration, A. aceti was then added to the fermentation medium. Changes in acetic acid content are presented in Figure 4. Results showed a gradual increase in the amount of acetic acid ranging from 0.64–4.52 % for the setup with 15% sugar concentration, 0.92–4.56% acetic acid for 20% sugar, and 0.84–4.56% for 25% sugar concentration. This increase in acetic acid content indicates the conversion of ethanol to acetic acid by A. aceti. The maximum amounts of acetic acid obtained were 4.52% (Day 19) at 28 °C and pH of 3.83, 4.56% (Day 18) at 28 °C and pH of 3.34, and 4.56% (Day 19) at 27 °C and pH of 3.53 for 15, 20, and 25% sugar concentrations, respectively. In terms of yield, these values are equivalent to 18.18, 13.94, and 11.62 g acetic acid/ kg peels for 15, 20, and 25% sugar concentrations, respectively. However, statistical analysis revealed that no significant differences exist among these values. Hence, future fermentations of pineapple peel can be conveniently carried out for 18 d at 20% sugar solution. The maximum acetic acid obtained conforms to the 4–6% v/v range reported by Morales et al. (2002).

Pesticide Residue Determination for Banana and Pineapple Peels Appendix Tables I and II show the amount of pesticide residues in banana and pineapple peels as determined by a third-party laboratory. The MRL values are also shown as obtained from the BPS (2015), Joint FAO/WHO CAC (2020), and EC Regulation No. 396/2005. Results

Table 2. Physicochemical characteristics of pineapple peels.

Composition (%) This study Abdullah and Mat (2008) Morais et al . (2017)

Moisture 85.64 ± 0.69 87.50 82.7

Ash 3.58 ± 0.48 04.05 05.1

Fat 1.21 ± 0.02 00.15 07.3

Fiber 8.94 ± 0.41 10.57 15.9

Protein 0.481 ± 0.105 5.18 7.3

Volatile solids 96.52 ± 0.50 ND ND

Sugar content 11.91 ± 0.90 ND ND

Starch content 1.28 ± 0.06 ND ND

ND – not determined



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Figure 3. Alcohol fermentation of pineapple peels at different sugar concentrations (P1 = 15%, P2 = 20%, P3 = 25%).

Figure 4. Acetic acid fermentation of pineapple peel at different sugar concentrations (P1 = 15%, P2 = 20%, P3 = 25%). Per concentration, columns with common letters are not significantly different at P < 0.05.

showed that the amount of pesticides in banana and pineapple peels were below the limit of detection of the gas chromatography instrument used. Accordingly, values were way below the MRL. These results imply that the utilization of pineapple and banana peels for conversion to acetic acid does not pose a threat of possible pesticide contamination. Information like this is valuable as the presence of pesticides in the raw material could depreciate the versatility of the latter for conversion to other high-value products due to the inherent hazards and tendency for the accumulation of these pesticides both to humans and the environment.

CONCLUSIONPhysicochemical characterization of banana and pineapple peels showed a relatively high sugar content, thus indicating their potential for conversion to other high-value products such as acetic acid. Further, analysis for possible pesticide contamination in the peels revealed that organochlorines, organophosphorus, and pyrethroids are way below the MRL values set by the BPS (2015), Joint FAO/WHO CAC (2020), and EC Regulation No. 396/2005, thereby reinforcing their suitability for use as raw material for fermentation. The peels were



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processed accordingly in order to achieve various sugar concentrations of 15, 20, and 25%. Anaerobic and aerobic fermentation was then done by the sequential addition of S. cerevisiae and Acetobacter aceti. The maximum concentrations of alcohol from banana peels amounted to 10.12% (Day 11) at a temperature of 27 °C and pH of 3.83 from 15% sugar concentration, 10.81% (Day 10) at 26.7 °C and pH of 3.64 from 20% sugar concentration, and 9.06% (Day 10) at 27.9 °C and pH of 3.34 from 25% sugar concentration. This led to the production of acetic acid at concentrations of 4.52% (Day 17) at 28.5 °C and pH of 3.53, 4.56% (Day 16) at 27.4 °C and pH of 3.59, and 4.50% (Day 18) at 27.8 °C and pH of 3.27 for the three sugar concentrations, respectively. For pineapple peels, the maximum alcohol concentrations obtained were 10.57% (Day 10) at 27 °C and pH of 3.98 from 15% sugar concentration, 10.60% (Day 10) at 27 °C and pH of 3 from 20% sugar concentration, and 9.77% (Day 10) at 27.9 °C and pH of 3.53 from 25% sugar concentration. This eventually resulted in conversion to acetic acid at concentrations of 4.52% (Day 19) at 28 °C and pH of 3.83, 4.56% (Day 18) at 28 °C and pH of 3.34, and 4.56% (Day 19) at 27 °C and pH of 3.53 for the three sugar concentrations, respectively. For both banana and pineapple peels, no significant differences were observed in terms of the amount of acetic acid produced from the fruit peel extracts with 15, 20, and 25% sugar solutions.

ACKNOWLEDGMENTSThis study was funded by the Department of Science and Technology–Philippine Council for Industry, Energy, and Emerging Technology Research and Development. We are also grateful to the University of the Philippines Mindanao for the invaluable support to this research.

NOTES ON APPENDICESThe complete appendices section of the study is accessible at http://philjournsci.dost.gov.ph

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386

Type of pesticide Amount, mg/kg Limit of detection, mg/kg MRL, mg/kg

Organochlorine

Aldrin < 0.0006 0.0006 0.01***

alpha-BHC < 0.0007 0.0007 0.01***

beta-BHC < 0.0006 0.0006 0.01***

delta-BHC < 0.0008 0.0008 0.01***

gamma-BHC (lindane) < 0.0007 0.0007 0.01***

alpha-chlordane < 0.0006 0.0006 0.01***

gamma-chlordane < 0.0006 0.0006 0.01***

4,4’-DDD < 0.0007 0.0007 0.05***

4,4’-DDE < 0.0006 0.0006 0.05***

4,4’-DDT < 0.0008 0.0008 0.05***

Dieldrin < 0.0006 0.0006 0.01***

Endosulfan I < 0.0006 0.0006 0.05***

Endosulfan II < 0.0006 0.0006 0.05***

Endosulfan sulfate < 0.001 0.001 0.05***

Endrin < 0.0006 0.0006 0.01***

Endrin aldehyde < 0.0006 0.0006 0.01***

Endrin ketone < 0.0007 0.0007 0.01***

Heptachlor < 0.0006 0.0006 0.01*

Heptachlor epoxide (B) < 0.0006 0.0006 0.01***

Methoxychlor < 0.001 0.001 0.01***

Organophosphorus

Dichlorvos < 0.03 0.03 0.01***

Mevinphos < 0.03 0.03 0.01***

Demeton-S < 0.03 0.03 0.01***

Ethoprophos < 0.02 0.02 0.02**

Phorate < 0.03 0.03 0.01***

Demeton-O < 0.03 0.03 0.01***

Diazinone < 0.02 0.02 0.01***

Methyl parathion < 0.03 0.03 0.01***

Ronnel < 0.02 0.02 0.5***

Fenthion < 0.03 0.03 0.01***

Chlorpyrifos < 0.02 0.02 4.0***

Trichloronate < 0.003 0.003 No ADI***

Merphos < 0.03 0.03 0.01***

Tetrachlorvinphos < 0.03 0.03 For animal products only***

Tokuthion < 0.003 0.003 0.02**

Fensulfothion < 0.03 0.03 0.01***

Sulprofos < 0.02 0.02 –***

Coumaphos < 0.03 0.03 –***

Table I. Pesticide analysis of banana peels.

APPENDICES



387

Type of pesticide Amount, mg/kg Limit of detection, mg/kg MRL, mg/kg

Tribufos < 0.02 0.02 –***

Pyrethroids

Tefluthrin < 0.002 0.002 0.05***

Transfluthrin < 0.002 0.002 0.01***

Anthraquinone < 0.01 0.01 0.01***

Allethrin < 0.005 0.005 0.01***

Resmethrin < 0.01 0.01 0.01***

Tetramethrin < 0.05 0.05 –***

Bifenthrin < 0.003 0.003 0.1***

Phenothrin < 0.01 0.01 0.02***

lambda-Cyhalothrin < 0.002 0.002 0.15***

cis-Permethrin < 0.005 0.005 5.00**

trans-Permethrin < 0.005 0.005 5.00**

Cyfluthrin < 0.003 0.003 0.02*

Cypermethrin < 0.005 0.005 0.05***

Flucythrinate < 0.005 0.005 0.01***

Fenvalerate < 0.003 0.003 0.02***

tau-Fluvalinate < 0.005 0.005 0.01***

Deltamethrin < 0.002 0.002 0.01***

ADI – acceptable daily intakeValues obtained from: *PNS (2015)**CAC (2020)***EC Regulation No. 396/2005

Type of pesticide Amount, mg/kg Limit of Detection, mg/kg MRL, mg/kg

Organochlorine

Aldrin < 0.0006 0.0006 0.01***

alpha-BHC < 0.0007 0.0007 0.01***

beta-BHC < 0.0006 0.0006 0.01***

delta-BHC < 0.0008 0.0008 0.01***

gamma-BHC (lindane) < 0.0007 0.0007 0.01***

alpha-chlordane < 0.0006 0.0006 0.01***

gamma-chlordane < 0.0006 0.0006 0.01***

4,4’-DDD < 0.0007 0.0007 0.05***

4,4’-DDE < 0.0006 0.0006 0.05***

4,4’-DDT < 0.0008 0.0008 0.05***

Dieldrin < 0.0006 0.0006 0.01***

Endosulfan I < 0.0006 0.0006 0.05***

Endosulfan II < 0.0006 0.0006 0.05***

Endosulfan sulfate < 0.001 0.001 0.05***

Endrin < 0.0006 0.0006 0.01***

Endrin aldehyde < 0.0006 0.0006 0.01***

Endrin ketone < 0.0007 0.0007 0.01***

Heptachlor < 0.0006 0.0006 0.01*

Table II. Pesticide analysis of pineapple peels.



388

Type of pesticide Amount, mg/kg Limit of Detection, mg/kg MRL, mg/kg

Heptachlor epoxide (B) < 0.0006 0.0006 0.01***

Methoxychlor < 0.001 0.001 0.01***

Organophosphorus

Dichlorvos < 0.03 0.03 0.01***

Mevinphos < 0.03 0.03 0.01***

Demeton-S < 0.03 0.03 0.01***

Ethoprophos < 0.02 0.02 0.02***

Phorate < 0.03 0.03 0.01***

Demeton-O < 0.03 0.03 0.01***

Diazinone < 0.02 0.02 0.3***

Methyl parathion < 0.03 0.03 0.01***

Ronnel < 0.02 0.02 0.5***

Fenthion < 0.03 0.03 0.01***

Chlorpyrifos < 0.02 0.02 0.01***

Trichloronate < 0.003 0.003 No ADI***

Merphos < 0.03 0.03 0.01***

Tetrachlorvinphos < 0.03 0.03 For animal products only***

Tokuthion < 0.003 0.003 0.02***

Fensulfothion < 0.03 0.03 0.01***

Sulprofos < 0.02 0.02 –***

Coumaphos < 0.03 0.03 –***

Tribufos < 0.02 0.02 –***

Pyrethroids

Tefluthrin < 0.002 0.002 0.01***

Transfluthrin < 0.002 0.002 0.01***

Anthraquinone < 0.01 0.01 0.01***

Allethrin < 0.005 0.005 0.01***

Resmethrin < 0.01 0.01 0.01***

Tetramethrin < 0.05 0.05 –***

Bifenthrin < 0.003 0.003 0.01***

Phenothrin < 0.01 0.01 0.02***

lambda-Cyhalothrin < 0.002 0.002 0.01***

cis-Permethrin < 0.005 0.005 5.00**

trans-Permethrin < 0.005 0.005 5.00**

Cyfluthrin < 0.003 0.003 0.02*

Cypermethrin < 0.005 0.005 0.05***

Flucythrinate < 0.005 0.005 0.01***

Fenvalerate < 0.003 0.003 0.02***

tau-Fluvalinate < 0.005 0.005 0.01***

Deltamethrin < 0.002 0.002 0.01***

ADI – acceptable daily intakeValues obtained from: *PNS (2015)**CAC (2020)***EC Regulation No. 396/2005



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acetic acid as value-added product from pesticide-free

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