Post-harvest quality of papaya coated with polivinilic alcohol and maize starch

Qualidade pós-colheita de mamão revestido com álcool polivinílico e amido de milho

Mariana Teixeira Pigozzi Vanessa Mendes Silva Fabrícia Queiroz Mendes Isadora Rebouças Nolasco de Oliveira Allan Robledo Fialho e Moraes Everaldo Antônio Lopes About the authors

ABSTRACT

Climacteric fruits have short postharvest shelf life. Coating is an alternative to minimize fruit ripening and post-harvest losses. Maize starch (S) and polivinilic alcohol - PVOH (P), isolated or blended, can be used in the formulation of coatings. However, little is known about the potential of PVOH-containing coatings in postharvest conservation of fruits. Papaya were aftercoated with 5 coating formulations: 3% starch (S), 3% PVOH (P), 2.25% S + 0.75% P, 1.5% S + 1.5% P and 0.75% S + 2.25% P. The fruits were kept at room temperature (20 ± 5 °C and 70 ± 10% RH) and physicochemical characteristics were evaluated for up to eight days. Uncoated fruits were used as control. In general, maize starch and PVOH. In general, maize starch and PVOH coatings reduced the weight loss and did not affect total soluble solids concentration. 3% PVOH coating increased the acidity and decreased the pH of the fruits, and excessively inhibited gas exchange between fruit and the environment. In this study, 3% maize starch coating was more efficient in prolonging the postharvest life of papaya.

Index terms:
Carica papaya L.; storage; respiratory rate; fermentation; firmness.

RESUMO

Frutos climatéricos possuem reduzida vida útil após a colheita. Uma alternativa para retardar o amadurecimento e consequentemente a perda desses frutos, é a aplicação de revestimentos. Amido de milho (A) e o álcool polivinílico - PVOH (P), isolados ou em misturas, podem ser usados na formulação de revestimentos. No entanto, pouco se sabe sobre o potencial de revestimentos contendo PVOH na conservação pós-colheita de frutos. Mamões foram revestidos com 5 formulações de revestimento:: 3% de amido de milho (A), 3% de PVOH (P), 2,25% A + 0,75% P, 1,5% A + 1,5% P e 0,75% A + 2,25% P e suas características físico-químicas foram avaliadas por até oito dias após manutenção em temperatura ambiente (20 ± 5 ° C e 70 ± 10% UR). Frutos não revestidos foram usados como controle. Os revestimentos com amido e PVOH, isoladamente ou em mistura, reduziram a perda de peso e não afetaram a concentração de sólidos solúveis totais, aos oito dias de armazenamento. O revestimento do mamão contendo 3% de PVOH aumentou a acidez e diminuiu o pH da fruta, e inibiu excessivamente as trocas gasosas entre a fruta e o ambiente. Neste estudo, o revestimento com 3% de amido de milho foi mais eficiente em prolongar a vida pós-colheita do mamão.

Termos para indexação:
Carica papaya L.; armazenamento; taxa respiratória; fermentação; firmeza.

INTRODUCTION

Papaya (Carica papaya L.) is widely grown in tropical and subtropical regions. World production was 13,169,443 tons in 2017, with Brazil being the world’s second largest fruit producer (Food and Agriculture Organization of The United Nations - FAOSTAT, 2019FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS - FAO. FAOSTAT. Estatistical Databases Agriculture. 2019. Available in: <Available in: http://www.fao.org/faostat/en/#home >. Access in: June, 28, 2019.
http://www.fao.org/faostat/en/#home...
). Papaya is a climacteric fruit, so it rapidly ripens after harvesting due to ethylene production and increased respiratory rate. This rapid ripening leads to softening of fruit tissues. As a result, fruit shelf life is shortened (Cunha et al., 2018CUNHA, M. C. et al. Propolis extract from different botanical sources in postharvest conservation of papaya. Acta Scientiarum. Technology, 40(1):e31074, 2018.; Jiang et al., 2019JIANG, B. et al. Comparative proteomic analysis provides novel insights into the regulation mechanism underlying papaya (Carica papaya L.) exocarp during fruit ripening process. BMC Plant Biology, 19(1):238, 2019.; Zou et al., 2014ZOU, Y. et al. The relationship between the expression of ethylene-related genes and papaya fruit ripening disorder caused by chilling injury. PLOS ONE, 9(12):e116002, 2014.).

Coating is an alternative method to increase fruit shelf life (Cunha et al., 2018CUNHA, M. C. et al. Propolis extract from different botanical sources in postharvest conservation of papaya. Acta Scientiarum. Technology, 40(1):e31074, 2018.; Ding et al., 2019DING, J. et al. Effect of sonication duration in the performance of polyvinyl alcohol/chitosan bilayer films and their effect on strawberry preservation. Molecules, 24(7):1408, 2019.). The coatings are applied or formed directly on the fruit surface, forming thin membranes. When immersing a fruit in a film-forming dispersion, the cover is formed by the deposition of polymeric species dispersed in the medium, establishing weak and strong interactions, with the surface of the fruit. Several models have been proposed for the deposition of polymeric structures and the subsequent formation of films on solid surfaces. In these models, the characteristics of the absorbent (in our case, the peel) and the absorbate (compounds diluted in the film-forming dispersion) are how they define that the type of mechanism will be predominant in the formation of the cover. In general, five interactions may occur when dipping a fruit in dispersed polymers formulations: Hydrogen bond; Hydrophobic interaction; Interaction by dispersive frames; Polarization of π electrons and Electrostatic interaction. In practice, all types of interactions can occur simultaneously and in varying intensities, although, depending on the characteristics of the coating-forming formulation and the fruit surface, there may be a predominance of a mechanism. (Assis; Britto, 2014ASSIS, O. B. G.; BRITTO, D. de. Review: Edible protective coatings for fruits: Fundamentals and applications. Brazilian Journal of Food Technology, 17(2):87-97, 2014.). After drying, protective coating is formed, which acts as a semipermeable barrier to water vapor and gases, inhibiting the migration of oxygen, carbon dioxide, flavorings, lipids and other solutes. Senescence is then retarded, increasing fruit conservation and shelf life (Ali et al., 2013ALI, A. et al. Efficacy of propolis and cinnamon oil coating in controlling post-harvest anthracnose and quality of chilli (Capsicum annuum L.) during cold storage. Food and Bioprocess Technology, 7(9):2742-2748, 2013.; Azarakhsh et al., 2014AZARAKHSH, N. et al. Lemongrass essential oil incorporated into alginate-based edible coating for shelf-life extension and quality retention of fresh-cut pineapple. Postharvest Biology and Technology, 88(1):1-7, 2014.).

Natural or synthetic substances can be used as coatings. These coatings do not replace completely inedible synthetic packaging, but can act as adjuvants, reducing the use of non-biodegradable disposable packaging (Assis; Britto, 2014ASSIS, O. B. G.; BRITTO, D. de. Review: Edible protective coatings for fruits: Fundamentals and applications. Brazilian Journal of Food Technology, 17(2):87-97, 2014.). Various biodegradable and non-toxic polymers can be used as fruit coatings (Pilon et al., 2013PILON, L. et al. Effects of antibrowning solution and chitosan-based edible coating on the quality of fresh-cut apple. International Journal of Postharvest Technology and Innovation, 3(2):151-164, 2013.; Azarakhsh et al., 2014AZARAKHSH, N. et al. Lemongrass essential oil incorporated into alginate-based edible coating for shelf-life extension and quality retention of fresh-cut pineapple. Postharvest Biology and Technology, 88(1):1-7, 2014.; Cunha et al., 2018CUNHA, M. C. et al. Propolis extract from different botanical sources in postharvest conservation of papaya. Acta Scientiarum. Technology, 40(1):e31074, 2018.; Mendy et al., 2019MENDY, T. K. et al. Application of Aloe vera coating delas ripening and extend the shelf life of papaya fruit. Scientia Horticulturae , 246(1):769-776, 2019. ), including starch and polivinilic alcohol - PVOH (Coutinho et al, 2015COUTINHO, M. O. et al. Efeito do revestimento de amido na preservação de frutos de abacate (Persea americana) à temperatura ambiente. Journal of Fruits and Vegetables, 1 (1):49-52, 2015.; Ding et al., 2019DING, J. et al. Effect of sonication duration in the performance of polyvinyl alcohol/chitosan bilayer films and their effect on strawberry preservation. Molecules, 24(7):1408, 2019.; Lo’ay; Taha; El-Khateeb, 2019LO’AY, A. A.; TAHA, N. A.; EL-KHATEEB, Y. A. Storability of ‘Thompson Seedless’ grapes: Using biopolymer coating chitosan and polyvinyl alcohol blending with salicylic acid and antioxidant enzymes activities during cold storage. Scientia Horticulturae, 249(1):314-321, 2019.). Starch is a low-cost and highly available natural polymer, and easy to store and handle (Mali; Grossmann; Yamashita, 2010MALI, S.; GROSSMANN, M. V. E.; YAMASHITA, F. Filmes de amido: Produção, propriedades e potencial de utilização. Semina: Ciências Agrárias, 31(1):137-156, 2010.; Alcázar-Alay; Meireles, 2015ALCÁZAR-ALAY, S. C.; MEIRELES, M. A. A. Physicochemical properties, modifications and applications of starches from different botanical sources. Food Science and Technology, 35(2):215-236, 2015.). It is a polysaccharide produced by higher plants as energy storage and consists basically of amylose and amylopectin polymers. Gels are formed when starch is dispersed in water and heated, with gel-forming temperature varying, depending on pH, source and starch concentration (Mali; Grossmann; Yamashita, 2010MALI, S.; GROSSMANN, M. V. E.; YAMASHITA, F. Filmes de amido: Produção, propriedades e potencial de utilização. Semina: Ciências Agrárias, 31(1):137-156, 2010.; Alcázar-Alay; Meireles, 2015ALCÁZAR-ALAY, S. C.; MEIRELES, M. A. A. Physicochemical properties, modifications and applications of starches from different botanical sources. Food Science and Technology, 35(2):215-236, 2015.).

Starch coatings have been used neat or blended with others polymers to coat fruits, including avocado (Coutinho et al., 2015COUTINHO, M. O. et al. Efeito do revestimento de amido na preservação de frutos de abacate (Persea americana) à temperatura ambiente. Journal of Fruits and Vegetables, 1 (1):49-52, 2015.), papaya (Castricini; Coneglian; Deliza, 2012CASTRICINI, A.; CONEGLIAN, R. C. C.; DELIZA, R. Starch edible coating of papaya: Effect on sensory characteristics. Ciência e Tecnologia de Alimentos , 32(1):84-92, 2012.), and guava (Coelho et al., 2017COELHO, C. C. S. et al. Aplicação de revestimento filmogênico à base de amido de mandioca e de óleo de cravo-da-índia na conservação pós-colheita de goiaba ‘Pedro Sato’. Revista Engenharia na Agricultura, 25(6):479-490 2017.). Starch-based coatings are flexible, non-toxic to human health, biodegradable, flavorless, odorless and colorless, and virtually water-impermeable (Rodrigues et al., 2020RODRIGUES, A. A. M. et al. Characterization of starch from different non-traditional sources and its application as coating in ‘Palmer’ mango fruit. Ciência e Agrotecnologia, 44:e011220, 2020.). Additionally, other polymers such as PVOH, can be blended with starch to form polymeric blends and thereby making films and coating with desirable mechanical and barrier properties (Silva et al., 2016SILVA, P. L. et al. Preparation and characterization of phosphorylated starch blends with chitosan and polyvinyl alcohol. Química Nova, 39(4):450-455, 2016.).

PVOH is an inert, stable, non-toxic, water-soluble synthetic petroleum-derived polymer. PVOH-based films form moisture and oxygen barriers and have high tensile and abrasion resistance (Bellelli et al., 2018BELLELLI, M. et al. Properties of poly (vinyl alcohol) films as determined by thermal curing and addition of polyfunctional organic acids. Food Packaging and Shelf Life, 18(1):95-100, 2018.; Ding et al., 2019DING, J. et al. Effect of sonication duration in the performance of polyvinyl alcohol/chitosan bilayer films and their effect on strawberry preservation. Molecules, 24(7):1408, 2019.; Muppalaneni; Omidian, 2013MUPPALANENI, S.; OMIDIAN, H. Polyvinyl alcohol in medicine and pharmacy: A perspective. Journal of Developing Drug, 2(3):1-5, 2013.). Polymeric blends containing PVOH and natural polymers, such as chitosan and carboxymethyl cellulose, increase fruit shelf life of strawberries (Ding et al., 2019DING, J. et al. Effect of sonication duration in the performance of polyvinyl alcohol/chitosan bilayer films and their effect on strawberry preservation. Molecules, 24(7):1408, 2019.), grapes (Lo’ay; Taha; El-Khateeb, 2019LO’AY, A. A.; TAHA, N. A.; EL-KHATEEB, Y. A. Storability of ‘Thompson Seedless’ grapes: Using biopolymer coating chitosan and polyvinyl alcohol blending with salicylic acid and antioxidant enzymes activities during cold storage. Scientia Horticulturae, 249(1):314-321, 2019.), and bananas (Senna; Al-Shamrani; Al-Arifi, 2014SENNA, M. M. H.; AL-SHAMRANI, K. M.; AL-ARIFI, A. S. Edible coating for shelf-life extension of fresh banana fruit based on gamma irradiated plasticized poly (vinyl alcohol) / carboxymethyl cellulose / tannin composites. Materials Sciences and Applications, 5(6):395-415, 2014.; Lo’ay; Dawood, 2017LO’AY, A. A.; DAWOOD, H. D. Minimize browning incidence of banana by postharvest active chitosan/PVA combines with oxalic acid treatment to during shelf-life. Scientia Horticulturae , 226(1):208-215, 2017.).

However, little is known about the potential of polymeric blends based on starch and PVOH as a coating of papaya. Assuming that coatings containing starch and PVOH can increase fruit shelf life, the objective of this work was to evaluate whether these polymers individually or in blends enhance physicochemical characteristics and conservation of papaya for up to eight days after harvesting.

MATERIAL AND METHODS

Freshly harvested papaya fruits (Carica papaya L. variety Solo) selected considering uniformity, color and degree of ripeness, based on skin color with 15-25% of the yellow peel surface area, were previously washed in running water and superficially disinfected with 200 μL L-1 sodium hypochlorite solution for 10 min. After cleaning, they were submitted to treatments (containing maize starch and, or PVOH and, or control). Maize starch and PVOH were dispersed in distilled water, gradually heated to 70 ° C and kept under constant stirring until maize starch gelatinization or PVOH completely dispersion. Glycerol was added to the maize starch dispersion at a rate of 30% (30 g of glycerol per 100 g of maize starch). Then, maize starch and PVOH dispersions were mixed to prepare coating-forming formulations dispersions with a final polymers concentration of 3%, taking into account the total of maize starch and PVOH summed, according to experimental design of simplex-centroid for binary mixture: 3% corn maize starch (S) solution; 3% PVOH (P) solution; 2.25% S and 0.75% P solution; 1.5% S and 1.5% P solution; 0.75% S and 2.25% P solution. These concentrations were determined based on studies with starch coatings (Oliveira; Cruz; Alves; 2016OLIVEIRA, B. F.; CRUZ, A. F.; ALVES, E. Cassava starch coatings for postharvest control of papaya anthracnose. Phytopathologia Mediterranea, 55(2):276-284, 2016.; Castricini; Coneglian; Deliza, 2012CASTRICINI, A.; CONEGLIAN, R. C. C.; DELIZA, R. Starch edible coating of papaya: Effect on sensory characteristics. Ciência e Tecnologia de Alimentos , 32(1):84-92, 2012.) and other biodegradable compounds (Ali et. al, 2011ALI, A. et al. Effect of chitosan coatings on the physicochemical characteristics of eksotika II papaya (Carica papaya L.) fruit during cold storage. Food Chemistry, 124(2):620-626, 2011.; Ali; Cheong; Zahid, 2014ALI, A.; CHEONG, C. K.; ZAHID, N. Composite effect of propolis and gum arabic to control postharvest anthracnose and maintain quality of papaya during storage. International Journal of Agriculture & Biology, 16:1117-1122, 2014.), besides in positive results with climacterics fruits (Pigozzi et al., 2020PIGOZZI, M. T. et al. Qualidade pós-colheita de banana revestida com álcool polivinílico e amido. Brazilian Journal of Development, 6(10):74637-74648, 2020.), there are few studies with coatings using PVOH. After mixing and dispersing coating-forming constituents, they were kept under slow stirring until cooling to room temperature.

Lastly, the fruits were immersed for 5 s in the coating-forming formulationsand maintained for 5 min in a horizontal position on a sieve to drain the excess of liquid. Uncoated fruits were used as control. The fruits were kept at room temperature (20 ± 5 °C and 70 ± 10% RH), most common way of marketing papaya in Brazil, for 8 days.

The analysis of the weight loss rate was performed in a completely randomized design, with six post-harvest treatments and four replications.

Fruits were weighted every 2 days in a semi-analytical electronic scale (BL-320H model; Splabor - Presidente Prudente, Brazil). Weight loss rate was calculated by the angular coefficient of the curve of weight x time (days).

The respiratory rate was determined indirectly by the adapted method of Alef and Nannipieri (1995ALEF, K.; NANNIPIERI, P. Methods in applied soil microbiology and biochemistry. London: Academic Press, 1995, 576p.). Fruits and flasks containing CO2 capture solution (NaOH) were placed in hermetically sealed containers (respirometers) and incubated for one hour. The CO2 released by the fruits during respiration reacts with the capture solution. After incubation, 1 mL BaCl2 was added to the capture solution for precipitation of carbonate and phenolphthalein ions as an indicator for HCl titration of the remaining NaOH in the capture solution. The same procedure was done without the fruit in the containers (control). The amount of CO2 emitted, and the respiratory rate were calculated according to Equations 1 and 2, respectively.

CO 2 = B A × HCl × F × 22 × v 1 v 2 (1)

(B - A): mean HCl volume spent on control titration - volume spent on sample titration (in mL)

[HCl]: HCl concentration (in mol L-1)

F: HCl concentration correction factor

22: CO2 molar mass ÷ 2 (since each CO2 molecule reacts with 2 of NaOH)

(v1/v2): ratio between capture NaOH volume and titrated NaOH volume (in mL)

RR = CO 2 W × T (2)

RR: Respiratory rate (mg kg-1 h-1)

W: Fruit weight (kg)

T: time (h)

Fruit firmness was determined using a digital penetrometer (PTR-300 model; Instrutherm - Brazil), coupled with a 5 mm diameter conical probe. Firmness was measured at two opposite points, on the equatorial belt of the fruit, after peeling with a sharp blade. The results were expressed in Newtons (N). Soluble solids (SS) were directly measured with analog refractometer (model ATC 103; Biobrix- Brazil). Results were given in percentage. The titratable acidity (TA) was determined using the titration method. Pulp tissues (5.0 g) were homogenized with 50 mL of distilled water, added with two to four drops of phenolphthalein 1% (indicator) and then titrated using 0.01 mol L-1 NaOH until it reached an pink endpoint (pH 8.1). The results were expressed as the percentage of citric acid, the predominant acid in papaya. Fruits pH was ascertained using a digital pHmeter (MPA-210 model; Tecnopon - Piracicaba, Brazil) by immersing the glass electrode directly into the papaya pulp. Results were expressed by the absolute value found.

Fruit firmness, soluble solids, titratable acidity, pH and respiratory rate were performed in a completely randomized design in the factorial split plot scheme 6 x 4 + 1 (treatments x storage period + additional control - time 0), with postharvest treatments as plots and storage period as subplots, and four replicates.

The data were submitted to the tests of homogeneity of variances (Hartley test) and normality of residuals (Jarque-Bera test). Transformations were performed in the weight loss analysis, titratable acidity, firmness and soluble solids to allow the ANOVA (Analysis of variance) assumptions. The weight loss, firmness and soluble solids data were adjusted by log x transformation. For titratable acidity, data were adjusted by transformation Factors significance (postharvest treatments x storage period) and their interactions were subjected to factor analysis of the split plot. Means of the postharvest treatments were compared usingStudent-Newman-Keuls test (SNK, p < 0.05). When appropriated, means of the treatments over time were submitted for regression analysis. Adjustment of the data to models was sought with up to two dependent factors. The models described that the equations are significant at 5% and the F test showed no significant lack of fit. For statistical analysis, SPEED Stat software was used (Carvalho et al., 2020CARVALHO, A. M. X. et al. SPEED Stat: A free, intuitive, and minimalist spreadsheet program for statistical analyses of experiments. Crop Breeding and Applied Biotechnology, 20(3):e327420312, 2020.).

RESULTS AND DISCUSSION

Postharvest coatings and storage time influenced fruit mass loss (p<0.05), with no interaction between factors. Mass loss was lower in coated fruits compared to control, regardless of coating type (Figure 1). Besides, there was a gradual increase in mass loss in all postharvest treatments during the storage period, being less pronounced on the coated fruits. Fruit coating is not intended to replace the use of conventional packaging materials or even permanently eliminate the use of cooling, but rather to present a functional and supporting role, contributing to the preservation of texture and nutritional properties, by reducing surface gases exchange and excessive water loss (Assis; Britto, 2014ASSIS, O. B. G.; BRITTO, D. de. Review: Edible protective coatings for fruits: Fundamentals and applications. Brazilian Journal of Food Technology, 17(2):87-97, 2014.), increasing the post-harvest period.

Peel is an important factor for fruit mass loss. During fruit ripening, latex rupture occurs, and the integrity of the peel is reduced (Chitarra; Chitarra, 2005CHITARRA, M. I. F.; CHITARRA, A. B. Pós-colheita de frutas e hortaliças. Lavras: Editora UFLA, 2005. 785p.). Fruits that lose 10% of their initial weight are considered unfit for human consumption (Kader, 2002KADER, A. A. Postharvest technology of horticultural crops. California: University of California, 2002, 535p.). In this study, we observed this condition in uncoated fruits stored for 8 days (Figure 1). On the other hand, coated fruits would present this % mass loss after 10 days. This shows that the coating provided an increase of 25% in the storage time of papayas.

Coating with 3% maize starch did not reduce mass loss compared to the others coatings (Figure 1), although it reduced the respiratory rate of the fruits (Table 1). This demonstrates that moisture loss from fruits through perspiration or evaporation is more significant than respiration for final mass loss over storage time (Hazarika; Lalthanpuii; Mandal, 2017HAZARIKA, T. K.; LALTHANPUII; MANDAL, D. Influence of edible coatings on physico-chemical characteristics and shelf-life of papaya (Carica papaya) fruits during ambient storage. Indian Journal of Agricultural Science, 87(8):1077-1083, 2017.).

Table 1:
Respiratory rate of papaya fruits (mg CO2 Kg-1 h-1) coated with solutions containing maize starch (S) and polivinilic alcohol - PVOH (P) at 0, 2, 4, 6 and 8 days under room conditions (20 ± 5 °C and 70 ± 10% RH).

The respiratory rate of papaya fruits was influenced only by the postharvest coatings, regardless of fruit storage time (Table 1). The 3% maize starch treatment reduced the respiration of papaya fruits, with average values of 67% compared to the control (Table 1).

The coating of papaya with 3% maize starch reduced gas exchange between fruit and environment. The reduction of gas exchange creates a modified atmosphere in fruit cells, increasing CO2 and reducing O2 concentrations on the fruit cell surface, which leads to lower respiration rate and consequently reduces the release of CO2 by the fruit (Chitarra; Chitarra, 2005CHITARRA, M. I. F.; CHITARRA, A. B. Pós-colheita de frutas e hortaliças. Lavras: Editora UFLA, 2005. 785p.). Since respiratory processes are of a degradative nature, the higher the respiratory rate, the shorter the storage time (Vieites; Daiuto; Fumes, 2012VIEITES, R. L.; DAIUTO, E. R; FUMES, J. G. F. Capacidade antioxidante e qualidade pós-colheita de abacate ‘Fuerte’. Revista Brasileira de Fruticultura , 34(2):336-348, 2012.). PVOH films were not efficient in reducing CO2 production.

Fruit acidity was influenced by the interaction between coating formulation and storage time (Table 2). From the sixth day of storage higher concentrations of PVOH (3%) resulted in higher fruit acidity at the end of the storage period while opposite behavior was observed for maize starch coating or uncoated fruits.

Table 2:
Papaya acidity (g citric acid 100 g-1 pulp) coated with solutions containing maize starch (S) and polivinilic alcohol - PVOH (P) at 0, 2, 4, 6 and 8 days under room conditions (20 ± 5 °C and 70 ± 10% RH).

The increase in acidity due to the increase of PVOH concentration and storage time may be due to fruit fermentation. Probably, at higher concentrations, PVOH chains interactions are stronger, creating more compact molecular structure which difficults gas exchange between fruit and environment. Under low concentrations or absence of oxygen, anaerobic respiration occurs. In this situation, pyruvate is converted to CO2 and acetaldehyde, which is converted to ethanol (Nelson; Cox, 2014NELSON, D. L.; COX, M. M. Lehninger - Princípios de bioquímica. Porto Alegre: Artmed Editora, 2014, 1328p.). Even in anaerobiosis, part of the tricarboxylic acid cycle happens, generating accumulation of succinic acid, increasing fruit acidity (Coulter; Godden; Pretorius 2004COULTER, A. D.; GODDEN, P. W.; PRETORIUS, I. S. How is it formed, what is its effect on titratable acidity, and what factors influence its concentration in wine? Wine Industry Journal, 19(6):16-25, 2004.; Zamora, 2009ZAMORA, F. Biochemistry of alcoholic fermentation. In: POLO, M. C.; MORENO-ARRIBAS, M. V. (Eds.). Wine Chemistry and Biochemistry. New York: Springer, p.3-26, 2009. ). Moreover, fermentation produces less energy per mol of glucose, requiring more molecules to produce energy (Nelson; Cox, 2014). For these reasons, an increase in fruit acidity was observed (Table 2), even with no reduction in CO2 production (Table 1).

Fruit pH was altered due to the interaction between storage time and coating formulation. pH decreases during storage, and on the sixth day there were changes in pH by fruits coated with PVOH. At 8 days of storage, the pH of the fruits was lower with increasing concentration of PVOH (Table 3).

Table 3:
pH of papaya fruits coated with solutions containing maize starch (S) and polivinilic alcohol - PVOH (P) at 0, 2, 4, 6 and 8 days under room conditions (20 ± 5 °C and 70 ± 10% RH).

Papaya pulp present low levels of organic acids, which are weak acids with low dissociation constants. For this reason, the pH of this fruit is usually higher than 5 (Fagundes; Yamanishi, 2001FAGUNDES, G. R.; YAMANISHI, O. K. Características físicas e químicas de frutos de mamoeiro do grupo solo comercializados em 4 estabelecimentos de Brasília-DF. Revista Brasileira de Fruticultura, 23(3):541-545, 2001.; Almeida et al., 2006ALMEIDA, R. F. et al. Influência da temperatura de refrigeração sobre as características químicas do mamão cv. “Golden”. Ciência e Tecnologia de Alimentos, 26(3):577-581, 2006.). In this study, the pH values of the fruits ranged from 4.12 to 5.90 (Table 3), similar to those found in the literature (Mendy et al., 2019MENDY, T. K. et al. Application of Aloe vera coating delas ripening and extend the shelf life of papaya fruit. Scientia Horticulturae , 246(1):769-776, 2019. ). The decrease in pH over the storage time of PVOH coated fruits was possibly due to acid production during fermentation, as observed in Table 2. The data showed a negative correlation between pH and acidity (r = - 0.85), showing that the pH reduction is related to the increase in the acid content.

Fruit firmness was reduced as the storage time increased, regardless of coating formulation (Table 4). Hazarika, Lalthanpuii and Mandal (2017HAZARIKA, T. K.; LALTHANPUII; MANDAL, D. Influence of edible coatings on physico-chemical characteristics and shelf-life of papaya (Carica papaya) fruits during ambient storage. Indian Journal of Agricultural Science, 87(8):1077-1083, 2017.) also reported reduced firmness of coated papaya fruits with increased shelf life. Escamilla-García et al. (2018ESCAMILLA-GARCÍA, M. et al. Effect of an edible coating based on chitosan and oxidized starch on shelf Life of Carica papaya L., and its physicochemical and antimicrobial properties. Coatings, 8(9):318, 2018.) reported that uncoated papaya fruits had a firmness loss of 92% during storage, while those coated with chitosan and oxidized starch had a firmness reduction of only 47%. The valor similar values to the present work, where at the end of 8 days the fruits presented 43% less firmness.

Table 4:
Firmness of papaya fruits coated with solutions containing maize starch (S) and polivinilic alcohol - PVOH (P) at 0, 2, 4, 6 and 8 days under room conditions (20 ± 5 °C and 70 ± 10% RH).

Chitarra and Chitarra (2005CHITARRA, M. I. F.; CHITARRA, A. B. Pós-colheita de frutas e hortaliças. Lavras: Editora UFLA, 2005. 785p.) and Vieites, Russian and Daiuto, (2014VIEITES, R. L.; RUSSO, V. C.; DAIUTO, E. R. Qualidade do abacate ‘Hass’ frigoarmazenado submetido a atmosferas modificadas ativas. Revista Brasileira de Fruticultura , 36(2):329-338, 2014.) cite that the fruit’s firmness is due to the action of hydrolytic enzymes, including pectinamethylesterase (PME) and polygalacturonase (PG), which degrade structural carbohydrates. PG contributes to the softening of fruit pulp, as its activity is greater in the early stage of fruit ripening (Bonnin; Garnier; Ralet, 2014BONNIN, E.; GARNIER, C.; RALET, M. C. Pectin-modifying enzymes and pectin-derived materials: Applications and impacts. Applied Microbiology and Biotechnology, 98(2):519-532, 2014.). The expressive initial activity of PME would provide substrate for PG’s performance (Pinto et al., 2011PINTO, L. K. A. et al Atividade da pectina metilesterase e da β-galactosidase durante o amadurecimento do mamão cv. Golden. Revista Brasileira de Fruticultura , 33(3):713-722, 2011.). PME promotes the partial demethylation of methyl esters of pectin polygalacturonic acids, facilitating the access of PG, which determines the depolymerization and solubilization of pectic substances (Bonnin; Garnier; Ralet, 2014).

Fruit coating and storage time did not influence the concentration of total soluble solids (Table 5). The Papaya Technical Regulation establishes a minimum value of 11 ºBrix for papaya harvesting (Brazil, 2010BRAZIL. MINISTÉRIO DA AGRICULTURA, PECUÁRIA E ABASTECIMENTO - MAPA. Instrução Normativa nº 4, de 22 de janeiro de 2010. Estabelece o regulamento técnico do mamão. Diário Oficial da República Federativa do Brasil, Brasília, 25 de janeiro, 2010.). In this study, the total soluble solids values observed were below this value, indicating that the fruits were harvested before the correct time.

Table 5:
Total soluble solids (° BRIX) of papaya fruits coated with solutions containing maize starch (S) and polivinilic alcohol - PVOH (P) at 2, 4, 6 and 8 days under room conditions (20 ± 5 °C and 70 ± 10% RH).

The absence of variation in total soluble solids concentration during the papaya ripening period is possibly justified by the low starch accumulation (less than 1%) during fruit development, which did not allow significant variation in the soluble sugar content during maturation (Gómez; Lajolo; Cordenunsi, 2002GÓMEZ, M.; LAJOLO, F.; CORDENUNSI, B. Evolution of solubles sugars during ripening of papaya fruit and its relation to sweet taste. Journal of Food Science, 67(1):442-447, 2002.). Soluble sugars are mostly accumulated when the fruit is still connected to the plant, mainly due to photosynthesis (Chitarra; Chitarra, 2005CHITARRA, M. I. F.; CHITARRA, A. B. Pós-colheita de frutas e hortaliças. Lavras: Editora UFLA, 2005. 785p.). At the beginning of papaya fruit development, glucose is the predominant sugar. In later stages, sucrose becomes the sugar found in higher concentration, reaching higher levels than fructose and glucose (Kader, 2002KADER, A. A. Postharvest technology of horticultural crops. California: University of California, 2002, 535p.).

CONCLUSIONS

Coating papaya fruits with maize starch and polivinilic alcohol, neat or in blends, reduces mass loss after eight days of storage. Fruits coated with 3% starch reduces gas exchange between papaya fruits and environment. 3% PVOH formulation coating increases the acidity and decreases the pH of the fruit, and excessively inhibited gas exchange between the fruit and the environment, leading to fruit fermentation. It probably occurs due to the strong PVOH chains interactions at this concentration. Thus, it is suggested to perform a new experiment reducing the concentration of PVOH in the coatings, in order to observe whether there will be a reduction in the fermentation of the fruits. In this study, the coating with 3% maize starch was more efficient in prolonging the postharvest life of papaya.

ACKNOWLEDGEMENT

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.

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Publication Dates

  • Publication in this collection
    27 Nov 2020
  • Date of issue
    2021

History

  • Received
    01 July 2020
  • Accepted
    21 Oct 2020
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