Introduction: Pistachio is one of the dried and native products of Iran, which has a special economic and export significance due to its excellent quality. Pistachio kernel is rich in unsaturated fatty acids; therefore, its unsaturated fatty acids can tolerate oxidation and, as a result, change the flavor of the product. Various factors, such as temperature, relative humidity, light, etc., affect the storage of many nuts. During pistachio storage, the lowest rate of oxidation and hydrolysis occurs in low humidity and under the carbon dioxide atmospheric. Therefore, packaging with low-permeability plastic films, in a dry product with a moisture content of less than seven percent, can be a good way to maintain product quality. Pistachio packaging depends on several reasons include physical protection of the product, preventing product spoilage, attracting customers, preventing aflatoxin contamination, and ensuring a high-quality, healthy consumer product. Considering the high nutritional value of pistachios and its importance for exports, research is needed to improve the storage and the availability of a healthy, quality product. Therefore, in this research, the role of plastic coatings with different materials and layers on the shelf life and maintaining the nutritional value of dry pistachios was investigated.
Materials and methods: In this experiment, dry commercial Ahmad Aghaei pistachio cultivar was used. 200 grams of intact fruit was placed in plastic bags of different materials and then sealed with thermal sewing. Finally, the packages were placed at 20 ± 3 ° C. Polymeric coatings used include uncoated (control), single layer polyethylene plastics (PE), dual layer plastics with metallized polyamide composition, and extruded polyethylene (PA- ONM / PE ex), three layer plastics by combining polyester, polyurethane and polyethylene (PEs / Pu / PE), three layers of plastics with cast polypropylene, polyurethane and polyester composition (CPP / Pu / PEs), five layers plastics with polyamide, Ethylene vinyl Alcohol, two layers of extrusion and polyethylene plastics (PA / EVOH / Tie / Tie / PE), seven layers of plastics with polyethylene, two layers ethylene vinyl alcohol, two layers of extrusion bonded polymer, polyamide and extruded polyethylene (PE / EVOH / EVOH / Tie / Tie / PA / PE ex). The desired traits (weight loss, kernel hardness, peroxide value, free fatty acid, soluble sugars and sensory evaluation) was measured before the treatments and after 2, 4, 6, 8, 10 and 12 months of the storage period. The experiment was carried out in a factorial arrangement in a completely randomized design with three replications. Statistical analysis of the data was performed using SAS software version 9.1.3 and comparisons of the meanings by Duncan's multiple range test at a probability level of 1 percent. Drawing of the diagrams was done using Excel software.
Results and discussion: Pistachios packed with seven-layer and double-layered coatings showed the least weight loss and kernel hardness in the packaging due to the polymeric coating. Plastic coatings play an important role in preventing weight loss by creating a saturated micro-atmosphere around the fruit, and probably due to less moisture loss (less weight loss) in these packaging coatings, the hardness of the kernel was also lower than other materials. Also, during the storage period, the amount of peroxide value and free fatty acid in the control and the various types of packaging coatings increased, but this increase in the package with two and seven layers of slope was slower than the control and other materials. Double-layered and seven-layered treatments in comparison with control and other coatings had significantly lower peroxide value and free fatty acid content. In the tenth and twelfth months, the treat of (1.6) and seven layers (1.93) compared with control (10.4) and other coatings had significantly lower peroxide value. Also, at the end of the storage period (12 months), polyethylene (1.54), uncoated treatment (1.22) and tree layer packaging (CPP / PU / PEs (1.08) showed more free fatty acid content compared to other types of packaging. The most important degradation reaction that results in reduced product quality during the storage period is the radical oxidation mechanism and the formation of hydroperoxides. The main reason of kernel rancidity in nut packagings is inappropriate sewing of packages and the penetration of oxygen into the package. In the present study, two-layer and seven-layer packaging films showed less free fatty acid due to their low permeability to oxygen and moisture compared to other coatings. According to the results of other reports, it is likely that the higher peroxide value in unpackaged or packaged fruits in this study will be related to the presence of oxygen in the pack, which leads to an exacerbation of fat oxidation and the release of hydroperoxides. The influence of gases and light in the package, causing adverse changes in appearance and accelerating chemical reactions, as well as the presence of oxygen inside the package, also accelerates the oxidation reaction. Therefore, so the packaging type and the atmosphere inside it are two factors determining the shelf life of pistachios and the decrease score in texture, tastes, flavors of nuts during storage can be due to the absorption of moisture by nuts and the oxidation of fatty acids which affect the texture, taste and flavor. The coating used in this study showed better sensory parameters than the control.