Preservation of phenolic compounds, color and texture of Asgari grape raisin during storage using sodium alginate edible coating

Introduction: Raisin is one of the most nutritious dried fruits and rich in various vitamins, minerals, fiber and antioxidant compounds (Mehraban et al. 2015). Iran is one of the major producers and exporters of raisin in the world. During storage period, raisin turns hard and sticky due to exudates syrup and moisture loss. It has been shown that application of edible coating could be useful to overcome this problem and extend shelf life of this product (Ghasemzadeh et al. 2008). Waxes and vegetable oils have been used to coat raisin (Debiofort et al. 1998) Ayoubi et al (2015) coated raisin with carnuba wax and glycerol monostearate. Also, Ghasemzadeh et al (2008) used edible coating based on starch and pectin to coat raisin.
Sodium alginate is a water-soluble linear polysaccharide derived from alginic acid and has been considered as an edible coating due to its low cost, non-toxicity and biodegradability (Danapal and Associates 2012). In the present study, the effect of edible coating based on sodium alginate to maintain the keeping quality of raisins during storage at 25 °C for 6 months has been investigated.
Materials and methods: raisin was produced from Seedless Asgari grape variety with the soda oil method. It was coated with sodium alginate at levels of 1 and 2%. After coating, samples were packaged in Zip-kip polyethylene bags and stored at 25 °C for 180 days. Physicochemical (weight loss, brix, pH, total phenol, color and hardness), microbial (total count) and sensory (color, texture, taste and overall acceptability) properties were determined during storage in the first day and 45, 90, 135 and 180 days after starting the storage. The weight loss was calculated from the difference between raisin weight in 45, 90, 135 and 180 days with the initial weight. Brix was measured by a refractometer (Protable VBR model, Switzerland). The pH value was measured by a digital pH meter (3020, Jenway, UK). In order to determine the total phenolic contents, the Folin–Ciocalteu method (Taga et al. 1984) was employed and measurements were done at 765 nm with a spectrophotometer (UNICO 2802, China). The color measurement was done using a colorimeter (TES-135A, Taiwan). Hardness of raisins was determined in a puncture test using a QTS texture analyzer (CNS Farnelll, Essex, UK) equipped with a needle probe (stainless steel cylinder of 2 mm in diameter with a conical needle bit) and a test speed of 60 mm/ min during the test. Hardness was defined as the maximum force to puncture raisin from the top to a 2mm depth (Rolle et al. 2011). To determine the total count, pour-plating method was used in PCA medium and incubation was performed at 37 °C. The sensory evaluation was performed by 5-point hedonic scale (1 most disliked, 5 most liked) by 10 trained panelists. The sensory evaluation of all treatments was carried out during storage time (in the first day and 45, 90, 135 and 180 days after storing). A factorial test in completely randomized design with 3 replications was utilized in this study. Analysis of variance (ANOVA) was performed by using MSTAT-C software. Duncan's multiple range test was used to compare the means at the 5% significance level and the graphs were plotted by Microsoft Excel software.

Results and discussions: The results of analysis of variance showed edible coating significantly affected the weight loss. Sodium alginate coating reduced the raisin weight loss. In addition, storage had significant effect on weight loss. The weight loss was gradually increased during the storage period. The results of analysis of variance indicated significant effect of edible coating and storage time on raisin brix. During the storage time, brix increased, while the rate of this increasing in samples coated with sodium alginate was less than the control. The results also showed the effect of edible coating and storage time on the pH of raisins was not significant. During storage, the total phenolic content of raisins was gradually reduced. The edible coating reduced the intensity of this reduction. At the end of the storage period, the most total phenol content was observed in 2% sodium alginate treatment. The results of the color parameters evaluation showed that during the storage time lightness of raisin decreased. In addition, the effect of edible coating on this color parameter was significant (p<0.01). L of 1% sodium alginate treatment was significantly more than the control at 180 days after storing and this significant difference observed in 2% sodium alginate treatment at 45, 90 and180 days after storing. The effect of edible coating was not significant on redness but this color property significantly increased during storage period. It seems that browning reactions caused this color change. Also evaluation of yellowness results indicated sodium alginate coating had not significant effect on this color parameter. Storage caused significant changes in raisin yellowness (p<0.01). During storage time initially b value increased and then decreased. The most lightness and the lowest value of redness to yellowness ratio represent the best color of raisin. Analysis of variance show redness to yellowness ratio of raisin significantly increased during storage time (p<0.01). The edible coating was significantly effective in reducing rate of this change. After 180 days storage, the highest and lowest a/b value were related to the control (1.07) and 2% sodium alginate treatment (0.69), respectively. Observations showed raisin texture gradually hardened during the storage period (p<0.01). According to the results of study, the edible coating has not significant effect on raisin hardness. The data showed edible coating had no significant effect on total count, but microbial growth significantly decreased during storage (p<0.01). Acidity, water activity, inhibitor compounds such as phenolic compounds and some products of non-enzymatic browning reaction of Millard are some factors that inhibit microbial growth in raisins during storage (Zhao and Hall 2008; Bower et al. 2003). Based on the sensory evaluation results, storage darkened raisin color and reduced the color score. Edible coating was effective in preserving the color so that at the end of storage time the color score of coated samples was significantly more than the control. During storage time taste and texture scores significantly decreased. Although the edible coating had not significant effect on these sensory properties. Storing significantly reduced the overall acceptance score of raisin. At the end of the storage period, the overall acceptance score of coated samples were more than the control; However, according to the results of Duncan's multiple range tests only the difference between the overall acceptance score of the control and 2% sodium alginate treatment was significant at 135 days of storage.