Effect of lipid edible coatings on physicochemical and microbial properties of raisin

Raisin is a principal traditional export product of Iran and has a special position in the foreign trade of the country.During storage, the product turns sticky and hard due to exudates syrup and moisture loss. To solve this problem, the application of edible coating would be an appropriatesolution. Edible coating may enhance the boundary layer resistance resulting to the enhanced shelf life of the product. Edible coatings are thin layers of edible materials applied to the product surface in addition to or as a replacement for natural protective waxy coatings and provide a barrier to oxygen, water and solute movement. They are also applied in foods to minimize the migration of components within the food systems or between the foods and their surrounding environment. They are used directly on the food surface by spraying, dipping or brushing to create a modified atmosphere. There are different kinds of films which are used such as polysaccharide, protein, lipid and composites. Lipid based coatings are primarily used for their hydrophobic properties which make them good barriers to water loss. In addition to preventing water loss, lipid coatings have been used to improve appearance by generating a shiny coverin fruits and vegetables, and to decrease respiration, thereby extending shelf life. The moisture barrier properties of hydrophilic coatings can sometimes be improved by addition of lipid materials The purpose of this research was to study the effect of lipid based edible coating (glycerylmonoestearate and carnauba wax) on physicochemical and microbial properties of raisin.
Material and Raisin samples (cvAskari)were dried usingsoda oil method and treated with coating materials based on lipid with essential oil of thyme (thymus vulgaris).Glycerylmonoestearate, carnauba wax and tween 80 were purchased from Sciencelab, Sigma-Aldrich and Merck Co. respectively.
Raisinswerecoated with the following coating solutions: -Aqueous solution of tween 80 (0.5% w/w)
-Aqueous solution of glycerylmonoestearate (0.5% w/w), tween 80 (0.5% w/w)
-Aqueous solution of glycerylmonoestearate (0.5% w/w), tween 80 (0.5% w/w), 150 ppm essential oil of thyme
-Aqueous solution of carnauba wax (0.5% w/w), tween 80 (0.5% w/w)
-Aqueous solution of carnauba wax (0.5% w/w), tween 80 (0.5% w/w), 150 ppm essential oil of thyme
Control sample was treated with aqueous solution of tween 80 (0.5% w/w).Distilled water was used for preparing all of coating solutions. Coating solutions were stirred and heated to 75oC (melting point of coating materials) for 30 min on a hotplate with a magnetic stirred to obtain uniform solutions. Dipping technique was used for coating of raisins. The coated raisins were placed on a basket to drip off residual coating solution and were dried in 25oC. The raisin smples were packaged in a 3 laminatedlayer bags (PE/PA/PE) with thickness of 80µ by Henkelman packaging machine and were stored at20, 35 and 50oC for 12 weeks. During the storage period, moisture content, water activity, hardness, color parameters (L*(lightness), a*(redness) and b*(yellowness)), percent of pestilence and microbial properties (total count, mold and yeast) were evaluated. Hardness of raisins was determined in apuncture test using a QTS texture analyzer (CNS Farnelll, Essex, UK) equipped with a needle probe (stainless steel cylinder of 2 mm of diameter with a conical needle bit) and a test speed of 60 mm. min−1 during the test. Hardness was defined as maximum force to puncture raisin from the top to a 2mm depth (Rolleet al. 2011).For colormeasurement, raisins were placed in to a black wooden box (interior size of 0.5 (width) × 0.5 (length) × 0.8 (height) m3 and were illuminated using 3 fluorescent tube lights (10W, 6500K; 40cm in length). A digital camera (Canoon EOS 1000D, Powershot, Taiwan) was located vertically at a distance of 20cm from the raisin surface. The images were preprocessed by Adobe Photoshop software, version 5.5. The Color properties were analyzed using ImageJ software, version 1.44o. The experiments were factorial with a completely randomized design. The coating material, temperature and storage time were the independent variables. Analysis of variance (ANOVA) was conducted for data using MSTAT-C software, version 1.42 (Michigan State University). Differences among the mean values were also determined using Duncan’s Multiple Range test. A significant level was defined as a probability of 0.05. The experiments were carried out on 4, 8 and 12 weeks after storage with three replications.
Results showed that coating, temperature and storage time would have considerable effect on the physicochemical and microbial properties of raisin. Coating caused decreasein moisture loss, hardness, lightness and redness. Increasing temperature and storage time increasedmoisture loss and hardness. Minimum hardness values were measured for 20 oC and 4 weeks storage and maximum hardness values were related to 50 oC and 12 weeks storage. Also it seems that increasing temperature and storage time influences thehardness by increasing moisture loss. Increasing temperature and storage time significantly decreased L* (p≤0.01). Increased enzymatic browning in raisins with increasing temperature was accompanied by an increase in a* value, and a decrease in b* value and caused to presenting dull colors. Total count, mold and yeast of coated samples decreased during storage period. Carnauba wax was more effective than glycerylmonoestearate on stability of raisin. Essential oil of thyme also caused improving color and microbial properties. Least values of redness (-2), total count (2.06 log cfu/g) and mold and yeast (1.32 log cfu/g) were related to carnauba wax coating with essential oil of thyme.