Preservation of Diospyros kaki with edible films

Transcription

1 Preservation of Diospyros kaki with edible films Elena Velickova a, Daniela Dimova a, Eleonora Winkelhausen a, Slobodanka Kuzmanova a a Department of Biotechnology and Food Technology, Faculty of Technology and Metallurgy, University SS.Cyril and Methodius, Rudjer Boskovic 16, 1000 Skopje, R. Macedonia; *elenavelickova@yahoo.com ABSTRACT The persimmon, Diospyros kaki, which is also known as kaki, is a very popular fruit. Its widespread in Europe is now also increasing due to the growing consumption rate. The persimmon is nutritionally important crop since it has increased sugar content and at the same time is good source of biologically active compounds such as ascorbic acid and condensed tannins. Therefore exploring the ways of its preservation is highly important. Persimmon fruits of Macedonian variety were selected according to their ripeness. The fruits before each experiment were washed, peeled and cut in form of discs with a diameter of 18 mm and 2-3 mm thickness. The discs were treated either directly with different edible films or first osmotically dehydrated and then coated, before being air-dried and packed. Textural properties for fresh and coated fruits were measured on texture analyzer using compression/penetration tests. The color parameters of persimmon such as L*, a* and b* values were accessed by colorimeter. The edible films were characterized for strength by compression test. The initial penetration force of the control samples of 15.5 N, decreased in the range between 10 and 13.5 N for the different edible films, while the osmotic treatment +coating did not cause any changes in the tissue hardness. Hardness, cohesiveness and stickiness were calculated from the compression test profile. There was a statistically significant difference between the fresh and coated fruits in all color parameters. The use of edible coatings, alone and in combination with osmotic drying, proved to be suitable methods for persimmon preservation. Keywords: persimmon; edible coatings; osmotic dehydration; INTRODUCTION The persimmon fruit, Diospyros, is of Japanese origin and is commonly grown in warm regions of the world. There are four species of Diospyros, namely Diospyros kaki, Diospyros virginiana, Diospyros oleifera and Diospyros lotus. Persimmon, Diospyros kaki, is one of the most important species from nutritional point of view (Bibi et al, 2007). Its relatively high content of dietary fibers, total phenolics, main minerals and trace elements make persimmon preferable for a healthy diet (Luo, 2006). The persimmon is also a good source of biologically active compounds such as ascorbic acid and condensed tannins which are related to various physiological functions including a protective role against oxidative stress-related diseases, and antimutagenic and anticarcinogenic capacities (Del Bubba et al, 2009). The high level of persimmon production observed in the last several years makes it necessary to provide the consumers with new persimmon products, besides the fresh fruit, that will have prolonged storage life. Minimally processed products are one of the most growing segments in the food retail chains. However, fresh-cut fruits are intensively studied because of the difficulties in preserving their fresh qualities for prolonged periods. The demand for lightly processed products with the same guarantees of safety as fruits treated by traditional methods of preservation has encouraged researchers to focus their efforts on studying new ways of extending shelf life of fresh-cut products (Igual et al, 2008). One of the possibilities is to coat the fruit with an artificial, edible barrier. The continuous consumer interest in high quality and food safety, combined with environmental

2 concern, has induced the development of edible coatings that avoid the use of synthetic materials (Moreira et al, 2009). The edible coatings have potential to reduce weight loss, respiration rate and improve food appearance and integrity. It is one of the most effective ways to maintain the food quality. Among the most commonly used coating materials are the chitosan, due to its antifungal activity, and gelatine due to its availability and cost (Moldao-Martins et al, 2003; Park et al, 2004). The aim in this investigation was to evaluate the effect of the different coating materials on certain physico-chemical properties of the fresh-cut persimmon during one week storage at room temperature. MATERIALS & METHODS Plant material and sample preparation The persimmon variety, Diospyros kaki, was grown in an orchard in the south region of R. Macedonia. Mature/unripe fruits were harvested and divided in four lots. The first lot consisted of fresh-cut pieces of persimmon with no treatment, which were used as control samples. The second and the third lot were coated with edible coatings. The fourth lot represented the samples that were osmotically treated prior the coating. The fruits were washed and manually sliced with a specially designed sharp stainless steel blade into slices with 4-5 mm thickness. Discs with 18 mm diameter were cut out of the slices, using cork borer. Coating and storage of persimmons The following solutions were used: 1 % chitosan and 3 % gelatin for coating and 50 % sucrose for osmotic treatment. Chitosan solution was prepared by dissolving the chitosan in 1 % acetic acid. For complete dispersion of the chitosan, the solution was stirred overnight at room temperature and centrifuged to remove impurities before use. The gelatin and sucrose were prepared in distilled water. Solutions were heated to 70 C to speed up the dissolving and then cooled to 30 C before use. The fruit pieces were immersed in the coating solutions of chitosan and gelatin for 5 min and then drained. The fruit pieces osmotically treated prior coating were immersed in the sucrose solution for 30 min, then removed from the osmotic solution, blotted with tissue paper and coated with chitosan according to the previously mentioned procedure. All treated fruit pieces were dried in air for 2 h to set a coat of the film on their surface. Afterwards they were stored for a week together with the control samples at 20 C in a plastic tray vacuum sealed with plastic wrap. Measurement of color Flesh color was evaluated by Dr. Lange spectro-color colorimeter on 5 fruit pieces. After collecting the three parameters, L* (lightness), a* (greenness) and b* (yellowness) they were used to calculate the flesh color index: Measurement of texture Texture measurements were conducted by means of a texture analyzer (TA-XT2i of Stable Micro Systems, Godalming, England). To determine the texture of the fresh and treated fruit samples compression test was applied. This test was conducted using a 5 mm cylindrical probe. The probe descended at speed of 10 mm/s and compressed the sample at a speed of 1 mm/s up to a distance making 50% deformation in two repetitive cycles. The force-time curves were recorded by the instrument. Analytical methods The changes in the soluble solids, titratable acidity and vitamin C content of the fresh and coated samples were evaluated on the fruit pulp. The fruit pulp was obtained by homogenization of 25 g of fruit flesh or approximately 10 fruit pieces by using grinder and centrifugation at 3500 rpm for 20 min. The soluble solids were determined using Abbe refractometer. Titratable acidity was

3 determined by titration with 0.1 M NaOH until the ph change to 8.1 and expressed as g citric acid/ g fresh weight. Vitamin C (mg/100 g fresh weight) was determined by titrating fruit extract against standardized 2,6-dichlorophenol indophenols dye to light pink end point which persisted for 15 s. Water activity of the samples was measured with Testo-650 water activity meter. All experiments were done in 3 repetitions. Statistical analysis One-way analysis of variance (ANOVA) was performed on the physico-chemical parameters to evaluate significant differences among the samples at 95 % confidence interval according to Tukey s test using Minitab 15 statistical software. RESULTS & DISCUSSION The physico-chemical properties of the fresh cut, control and coated persimmon pieces, after preparation and one week of storage at 20 o C are presented in Table 1 and Figure 1. The data revealed that there were no significant changes in the solid content, titratable acidity, vitamin C content, color index and water activity between control and coated samples. The only treatment that showed significant changes in most of the properties was the osmotic treatment+chitosan coating. Due to the immersion in concentrated sucrose solution a solute uptake occurred which resulted in 11.3% increase of the solid content. Mass transfer during the osmotic treatment consists of simultaneous solute uptake and water removal, in and out of the plant tissue (Torreggiani, Bertolo, 2001). Besides the solute uptake, the water loss of the fruit samples was also determined and it was around 20%. The water loss was accompanied with some leaching out of the vitamin C and the citric acid present in the fresh plant tissue (Table 1). After one week of storage at 20 C, the coated samples exhibited better preservation of the nutritional values when compared to the control samples. The control fruit samples lost 45.5% of their titratable acidity and 55% of the vitamin C. The chitosan and gelatine coated fruits did not lose their titratable acidity value and had lower vitamin C loss. The samples coated with chitosan after the osmotic treatment showed 70% preservation of their vitamin C content. Persimmons, as fruits, are characterized by a bright orange-red colour and the coatings did not cause any significant changes in the color index immediately after preparation. During the one week of storage at room temperature, the chitosan and gelatine coating delayed the color evolution and therefore a slowdown of the flesh color change was observed. Table 1. The effect of the different treatments on the solid content, titratable acidity, vitamin C content, color index and water activity immediately after preparation and after one week storage at 20 C Sample Soluble solids ( o Brix) Weight loss (%) TA (g/g) Vitamin C (mg/100 g) Color index a w fresh cut 20.0 a / 0.33 a 7.04 a 1.51 a chitosan coated 20.0 a / 0.33 a 7.04 a 1.63 a gelatine coated 20.0 a / 0.33 a 7.04 a 1.48 a osmotic treated + chitosan coated 23.0 b / 0.22 b 4.81 b 1.73 a after one week storage fresh cut 17.5 c c 3.20 c 9.32 b chitosan coated 17.0 c a 3.93 c 4.05 c gelatine coated 20.0 a a 3.33 c 3.87 c osmotic treated + chitosan coated 20.4 b d 3.38 c d abcd Values with different letter in a given column are significantly different (p<0.05)

4 Figure 1. Hardness, cohesiveness and springiness of control and coated persimmon pieces a) immediately after preparation and b) after one week storage at 20 C. Bars with different letter for a specific attribute are significantly different (p<0.05). The preservation of the turgor pressure and therefore the maintenance of the structural integrity of the tissue is represented by fruits hardness, while the springiness gives the cells ability to regain their original form (Khin et al., 2007). From the texture analysis it was evident that the coated samples exhibited lower hardness than the control, but they preserved well the cohesiveness and springiness of the tissue (Figure 1). The most severe loss of the fruit texture immediately after coating was noticed in the samples osmotically treated prior the chitosan coating. In persimmon fruit, the maintenance of fruit quality during storage mainly depends on the rate of flesh softening. Therefore delaying softening during storage is an important aim in the postharvest persimmon research. The positive effect on delaying fruit softening was proved by the textural properties of the samples. The samples coated with edible films preserved well their hardness, cohesiveness and

5 springiness which might be due to decreased activity of the cell wall degrading enzymes that are responsible for the fruit softening. CONCLUSION In this study three different coating treatments for minimal processing of fresh cut fruits were tested for better preservation of persimmon during storage at room temperature. The chitosan and gelatine coated fruits had the better preservation of the nutritional properties, color and texture, when compared to the control samples or samples osmotically dehydrated prior coating. Edible coatings could be considered as useful alternative for storage of fruits with fresh cut characteristics. REFERENCES Bibi N., Khattak A.B., Mehmood Z., 2007, Quality improvement and shelf life extension of persimmon fruit (Diospyros kaki), Journal of Food Engineering, 79, Del Bubba M., Giordani E., Pippucci L., Cincinelli A., Checchini L., Galvan P., 2009, Changes in tannins, ascorbic acid and sugar content in astringent persimmons during on-tree growth and ripening and in response to different postharvest treatments, Journal of Food Composition and Analysis, 22, Igual M., Castello M.L., Ortola M.D., Andres A., 2008, Influence of vacuum impregnation on respiration rate, mechanical and optical properties of cut persimmon, Journal of Food Engineering, 86, Khin M.M., Zhou W., Yeo S.Y., 2007, Mass transfer in the osmotic dehydration of coated apple cubes by using maltodextrin as the coating material and their textural properties, Journal of Food Engineering, 81, Luo Z., 2006, Extending shelf-life of persimmon (Diospyros kaki L.) fruit by hot air treatment, European Food Research and Technology, 222, Moldao-Martins M., Beirao-da-Costa S.M., Beirao-da-Costa M.L., 2003, The effects of edible coatings on postharvest quality of the Bravo de Esmolfe apple, European Food Research and Technology, 217, Moreira M.D.R., Ponce A., Del Valle C.E., Roura S.I., 2009, Edible coatings on fresh squash slices: effect of film drying temperature on the nutritional and microbiological quality, Journal of Food Processing and Preservation, 33, Park S., Stan S., Daeschel M., Zhao Y., 2005, Antifungal coatings on fresh strawberries (Fragaria x ananassa) to control mold growth during cold storage, Journal of Food Science, 70, Torreggiani D., Bertolo G., 2001, Osmotic pre-treatments in fruit processing: chemical, physical and structural effects, Journal of Food Engineering, 49,