EMULSION COATING TO EXTEND POSTHARVEST LIFE OF MANGO (MANGZFERA ZNDZCA CV. MANILA)

RAFAEL D~AZ-SOBACI.~, ALMA VAZQUEZ LUNA~, CESAR I. BERISTAI", JAVIER DE LA CRUZ' AND HUGO SERGIO GARCIA

'Instituto de Ciencias Basicas Universidad Veracruzana

Apdo. Postal 575, Xalapa, Veracruz, Mexico 2Departamento de Ingenieria Quimica y Bioquimicu

Instituto Tecnoldgico de Veracruz Apdo. Postal 1420. Veracruz, Ver. Mkxico

Accepted for Publication October 13, 1996

ABSTRACT

A coating emulsion was prepared using maltodextrins, carboxymethylcellu- lose, propiteneglycol and a mixture of sorbitan fatly acid esters with an HLB of 6. 17te emulsion was sprayed on fully mature, unripened manila mangoes, which were the stored at 15 and 25C and 80-85'31 R.H. At 25C it was observed that rate C0,production increased and lost in control fruits afrer I 2 days of storage. Coated mangoes kept their rate CO, production and suffered only 8% weight loss afrer 21 days of storage. After storage, the coating was washed out and fruits were allowed to ripen naturally, which occurred in 3-4 days. The results obtained from this study indicate that application of the hydrophobic coating extended the postharvest storage of mangoes for at least 20 days more than uncoated fruits.

INTRODUCTION

A well-known limitation of handling of tropical fruits is their short post- harvest storage life. Mangoes are climacteric fruits which have a postharvest life of 6 to 8 days at ambient temperature (25C 2C, 85 f 5% R.H.). Mangoes are also highly susceptible to damages caused by fungi, bacteria and larvae of Colleototrichum gloesporides and Anastrepha spp (fruit flies) (Jagtiani 1988).

'Author to whom all correspondence should be addressed.

Journal of Food Processing and Preservation 20 (1996) 191-202. AIL Righfs Reserved. Copyright 1996 by Food & Nutrition Press, Inc., Trumbull. CT 06611 191

192 R. DLU-SOBAC ETA.

Postharvest life of fruits is basically dependent on (1) water vapor losses, (2) ripening and senescence rates, and (3) infestation by microorganisms or insect larvae (Kader 1992). Application of physical barriers as coatings on the fruit surface regulate the selective permeability to 0,, COz and water vapor, retarding the natural physiological ripening process. In addition, insect infestation and microorganism growth are restricted. These coatings represent a viable preservation alternative, since application costs for coatings are lower than those of other technologies of postharvest handling.

A number of hydrophylic coatings have been formulated using dairy (caseins and whey) proteins because of their ability to form films that can act as barriers to oxygen and carbon dioxide diffusion (Avena-Bustillos and Krochta 1993; Maynes and Krochta 1994; McHugh ef al. 1993; MacHugh and Krochta 1994; Torres 1994). Cellulose-derived polysaccharides are considered basic structural components in film formulations, however, their brittleness led to the use of lipids and polyols to increase the flexibility of the preparations (Greener and Fennema 1989a,b; Kester and Fennema 1989). Other coatings have been formulated based on corn zein (Park ef al. 1994; Trezza and Vergano 1994) and wheat gluten (Gennadios and Weller 1990; Gennadios ef al. 1993; Gontard ef al. 1992, 1994). However, due to their inherent hydrophilic character, moisture adsorption by the coating itself produces a swelling effect with a concomitant loss of structure due to dissolution, or surface cracks and pinholes (Kester and Fennema 1989). This type of change increases the gas and water vapor permeability by the coating and prevents the use of hydrophilic coatings for storage of fruits at high relative humidities.

Because of this situation, a number of works have been published where stable emulsions were used to formulate hydrophobic coatings. Other aspects considered in these formulations were the type and structure of the lipid used, their location in the structure of the film as well as particle size and concentra- tion (Debeaufort ef al. 1993; Fennema ef al. 1994; Koelsch and Labuza 1990; McHugh and Krochta 1994; Martin Polo ef al. 1992a,b; Park ef al. 1994; Sapru and Labuza 1994). Coatings based on waxes and paraffin mixed with organic polymers, have been used to preserve citrus fruits (Erbil and Muftagil 1986). Mixtures of sucrose polyesters, carboxymethylcellulose salts and mono- and diacylglycerols (PRO-LONG@) have been applied to delay postharvest ripening of Keitt mangoes (Dalla 1988). Recently, a commercial formulation (PRIMA- FRESH) based on waxes was applied to "Bocado" mangoes, obtaining 6 additional days of shelf life as compared to uncoated fruits (Castrillo 1992).

The purpose of the present work was to prepare emulsions based on a mixture of maltodextrins with a 10 DE and carboxymethylcellulose as structural components to prepare coating films; sorbitan esters were added to provide a hydrophobic character and propyleneglycol was included as plastifier. The emulsions were used to form films which were employed to coat manila

POSTHARVEST HANDLING OF MANGO 193

mangoes in order to study their postharvest life under different conditions, and to evaluate the effects of the coatings on the respiration patterns as well as on the chemical composition of the fruits.

MATERIALS AND METHODS

Coating Formulation

A 50 Bx solution of Maltodextrin (10 DE) was prepared (Amidex 10, Arancia, Co., Mexico City), to which 3 % of carboxymethylcellulose- sodium salt of low viscosity (Sigma C-5678) and 10% of a mixture of esterified fatty acids with an HLB of 6 (Sorbac-60, Polisorbac-80, Arancia) were added. The mixture was homogenized with a magnetic stirrer for 15 min at room tempera- ture. The mixture was allowed to stand for 2 h and then kept refrigerated after the addition of 0.1 sodium benzoate as preserving agent.

Coating Application

Emulsions prepared as described above were applied by first warming at 40C, and then they were sprayed on physiologically mature but unripened mangoes from the Actopan producing fields in the state of Veracruz (Mexico). Batches of coated and uncoated fruits were formed and stored at either 15 or 25C and both at 80-85% R.H.

Analytical Methods

Four fruits were withdrawn at random from each lot every four days and analyzed. The following compositional and physical attributes were evaluated: weight loss was determined by direct weighing. Production of CO, was determined by the gravimetric method of Denny (Miller 1981). Determination of pH was made by direct measurement (AOAC 1980). Soluble solids were measured at 20C with a temperature-controlled refractometer (ATAGO) and reported as percent weight. Insoluble solids in alcohol were measured according to the method reported by Roe and Brummer (1981). Electrolyte leakage was evaluated following the method described by Kuo and Parkin (1989). Titratable acidity expressed as citric acid, was determined by direct titration with NaOH 0.1 N (AOAC 1980). Changes in color peel were evaluated according to the scale used by Medlicott (1992). Coating thickness was measured using a Mitutoyo micrometer.

194 R. DfAZ-SOBAC ETAL..

Statistical Analysis

Experiments were carried out in a complete randomized fashion, with four replications. The statistical software package Systat v. 5.01 was used for performing ANOVA and Duncan multiple range test.

RESULTS AND DISCUSSION

The emulsion applied on the mangoes formed a smooth and shiny film after it was allowed to dry for 20 min at ambient conditions (25 f 2C and 80 f 5% R.H.). When applied on the fruits, visual inspection of the film showed no evidence of superficial breakages or pinholes, and it had a thickness of 120 f 10 pm that can be easily removed by washing. Upon commercial operation, drying times can be considerably shortened by hot air drying (ca. 50C for 5 min) and can be easily removed by washing. The film was formed only a few minutes after application on the fruit surface at ambient temperature and relative humidity of 8045%. This characteristic is of practical importance because mangoes and other fruits have to be handled at relative humidities above 80%. Reduction of the drying time could be due to a tensoactive effect originated by the hydrophylic/lipophylic balance of the emulsifier mix, which expels water readily from the structure of the applied film. We consider it to be necessary to pursue additional research on the subject, as well as on the use of hot air, to further accelerate drying rates of the films. However, this practice is not recommended since important structural modifications of the film could occur, mainly those changes associated with the physical state of membrane lipids. These alterations could bring about variations in the permeability of the membranes.

Figure 1 shows that control and coated mangoes lost weight at different rates when stored at both temperatures. After 15 days of storage, control mangoes lost almost twice the weight as the coated fruits. Coated mangoes suffered weight losses of 8-9% after the 24 days of storage at 25C. Slower rates of weight loss of coated fruits can be attributed to the barrier properties for gas diffusion of stomata, the organelles that regulate the transpiration process and gas exchange between the fruit and the environment (Kays 1991). Coated mangoes stored at 15C showed slight skin wrinklage, which suggests the possibility of damage to the coating integrity; however no evidence was found to confirm such harm. The barrier effect reduces oxygen availability as well, which, in turn controls the rate of respiration, as measured by CO, production. Figure 2 shows that production of C02 in coated fruits was reduced from 54 to 49 mg CO,/kg-h, at 15C. Coated fruits stored at 25C showed a small increase

POSTHARVEST HANDLING OF MANGO 195

-A- Coated 25 C

0 5 10 15 20 25

Time (days) FIG. 1 . WEIGHT LOSS OF EXPERIMENTAL MANGOES DURING RIPENING

AT 15 AND 25C AND 85% R.H.

.c

0 0

ui 5 u

F

- -+- - Control 15 C

0 5 10 15 20 25 30 Time (days)

FIG. 2. CHANGES IN CO, PRODUCTION OF MANGOES DURING STORAGE

196 R. DfAZ-SOBAC E T A .

FIG. 3. CHANGES IN pH (A) AND TITRATABLE ACIDITY @) DURING POSTHARVEST STORAGE

in the respiration rate, from 54 to 62 mg C02/kg-h during the first 12 days. Control mangoes exhibited increased respiration rates from 54 to 78 mg COJkg-h during the same storage period.

Figure 3a depicts pH variations for all samples. As expected, coated fruits showed less variation in acidity, related and expressed also as titratable acidity (Fig. 3b). Consistent with the other results presented, control mangoes had greater pH increases associated with the utilization of excess organic acids stored in the vacuoles as respiratory substrate (Medlicott 1987).

Reduction of alcohol-soluble solids is presented in Fig. 4a. Again, coated mangoes showed a smaller reduction in values than controls at both tempera- tures. This is a direct consequence of the hydrolysis of starch and pectic material during normal ripening. This is also observed as pulp softening and is followed as the increase in soluble solids as presented in Fig. 4b, where it is evident that

POSTHARVEST HANDLING OF MANGO 197

18

16

14

12

10

8

6

4

20

18

16

14

12

10

8

6

+ConM25 C +-Ma c ....... - - O - - ~ W l S c - -+- ~ Contrd 15 C 8

........... ............... - . ' *- - . ; I $ .................... I - ............. 1.1 I ..-.-I........-....- I

0 5 10 15 20 25 Time (days)

FIG. 4. EVOLUTION OF ALCOHOL-INSOLUBLE SOLIDS (A) AND SOLUBLE SOLIDS (B) IN THE FRUITS DURING STORAGE

coated mangoes have lesser amounts of soluble solids produced during storage than uncoated fruits. The loss of structural rigidity associated with starch, cellulosic and pectic material hydrolysis, affected the integrity of cell walls, which is measured as electrolyte leakage and presented in Fig. 5. Loss of cell wall integrity is related to spoilage due to decompartmentalization of enzymes and substrates, thus increasing enzymatic reaction rates. Coated fruits showed lower values of electrolyte leakage at both temperatures than control fruits. Between coated fruits, mangoes kept at 25C presented slightly but significantly higher values than mangoes at 15C.

Changes color in control fruits from green to yellow was apparent between 7 and 9 days of storage. In the same time period, coated mangoes showed only a minor yellowing. These color changes are presented in Fig. 6. Reducing the 0, in the atmosphere surrounding the fruits reduced ethylene production and its

198 R. D~AZ-SOBAC ma. 100

90

80

70

60

50 0 5 1 0 15 20 25

Time (days) FIG. 5. ELECTROLYTE LEAKAGE OF MANGOES DURING POSTHARVEST STORAGE

FIG. 6. CHANGES IN PEEL COLOR OF MANGOES BY RIPENING DURING STORAGE

POSTHARVEST HANDLING OF MANGO 199

activity as a ripening hormone (Medlicott 1977). In addition the internal concentration ethylene levels and the rate of chlorophyll degradation were decreased. The reduction in the synthesis of carotene (yellow coloration) is directly correlated to decreasing chlorophyll breakdown.

The coating limited transpiration and consequently diminished weight losses by water evaporation, due to reduced respiration rates. These are considered to be two key factors for post harvest fruit handling. After 24 days, the coating was removed by washing. Neither spoilage, anthracosis or changes in the color and chemical composition of the fruits compared to controls were evident after mangoes reached a fully ripe stage, which occurred in 3-4 days at 25C. Treated fruits did not show the incidence of dark spots or discolorations on the skin or pulp. On the other hand, some of the control mangoes evidenced some of these defects after six days of ripening at either 15 or 25C. Statistical analysis showed a significant effect (p <0.05) by the application of the coating to reduce weight losses, respiration rate and changes in the chemical composition. The effect of coating was more important than the effect of temperature and the interaction coating : temperature.

CONCLUSIONS

Application of the hydrophobic coating on pre-climacteric mangoes acts as a physical barrier which reduces the gas exchange between the fruit and the environment. It lowered both the rate of substrate catabolism and the ability to generate the energy required to drive the biochemical reactions associated to fruit ripening. The coating also prevented penetration and proliferation of spoilage microorganisms and did not pose any adverse impact on fruit wholesomeness or its chemical composition. This treatment extends the post harvest storage ability of manila mangoes at least 20 days more than uncoated fruits without the need of refrigerated storage.

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