| Walnut (Carya cathayensis Sarg.) is very popular with consumers for its appealing flavor and abundant proteins, unsaturated fatty acids, variety of essential amino acids and minerals. Walnut kernel contains about70%lipids, of which about91%percent are unsaturated fatty acids. Therefore, rancidity, which leads to quality losses, probable happens during storage and processes. Currently, walnut can be preserved for only3-5months on the market.which will tasted bad and of which the price will also decrease after Spring Festival. Therefore, it is crucial to study the fat oxidation and quality deterioration of walnut during storage and processes. In this paper, the effect of storage conditions and processes on lipid oxidation of walnut and the environmental factors, intrinsic factors and critical manufacturing processes involved were investigated. The results were as follows:(1) Studies on effect of different temperatures, different types of packaging and light treatment on raw walnut during storage showed that:Storage in low temperature, general packaging or dark could maintain the activity of SOD, POD and CAT; inhibit the rising of the formation rate of O2--and the content of MDA; suppress the rupture or fusion of lipid droplets within cells; reduce the extent of the damage of cell structure; delay the aging process on walnut during storage. Storage in low temperature, general packaging or dark could inhibit the increase of LOX activity, acid value, peroxide value,and anisidine value; decrease iodine value; delay the oxidation rate of fatty acid, especially the polyunsaturated fatty acid. Through dynamic changes of fatty acids content test, the results showed that every fatty acid oxidated during storage, of which the content declined as a whole. Percentage content of SFA and MUFA raised and Percentage content of PUFA and UFA decreased. The higher level the unsaturated fatty acids was, the faster the oxidation rate was, and the relative percentage content of fatty acids, of which oxidation rate is lower than the average rate of fatty acid oxidation, raised. Therefore, the oxidation of unsaturated fatty acids, especially the one of polyunsaturated fatty acid was the main reason of lipid oxidation in raw walnut. Temperature, type of packaging and light were three important environmental factors which affected the aging and lipid oxidation of raw walnut; and the composition of fatty acids itself was the main intrinsic factor.(2) It was found from the study on effect of traditional processing methods on lipid oxidation of walnut that the acid value, peroxide value, anisidine value and contents of MDA increased. In the two processes of braising and soaking, the acid value, peroxide value and anisidine value increased remarkably. The content of fatty acids in the braising process changed greatly, indicating that braising and soaking were the key processes in the traditional processes. By analyzing the changes of LOX activity and the contents of linoleic acid and linolenic acid, it was found that enzymatic oxidation was occurred in braising process. The main reason for oxidative rancidity in walnut was not enzymatic oxidation, but the autooxidation after braising. From assay on dynamic changes of contents of fatty acids during processes, we found that Percentage content of SFA and MUFA raised and Percentage content of PUFA and UFA decreased. The main reason for the decrease of unsaturated level was the rapid oxidation of linoleic acid and linolenic acid.(3) It was shown by the study on effect of different temperatures, different types of packaging and light treatments on walnut during storage that:storage on the conditions of low temperature, vacuum packaging or in dark could inhibit the increase of acid value, peroxide value, anisidine value and the content of MDA, suppress the decrease of iodine value, delay the oxidation rate of unsaturated fatty acid, especially the polyunsaturated fatty acid. From the study on dynamic changes of the content of fatty acids during storage, we found that every fatty acid oxidated during storage, of which the content declined. Percentage content of SFA and MUFA raised and Percentage content of PUFA and UFA decreased. The higher level the unsaturated fatty acid was, the faster the oxidation rate was. The relative percentage content of fatty acids of which the oxidation rate is lower than the average rate of fatty acid oxidation rose. Therefore, the oxidation of unsaturated fatty acids, especially the one of polyunsaturated fatty acid was the main reason of lipid oxidation in walnut products. Temperature, type of packaging and light were three important environmental factors which affected the lipid oxidation of walnut products, and the composition of fatty acids itself was the main intrinsic factor. |
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