Effect of wheat germ oil extracted with microwave pretreatment on the oxidative stability of common Kilka (Clupeonella cultriventris caspia) oil

Wheat germ is a by-product of wheat milling industry. It contains about 11% oil. Wheat germ oil is well known as a tocopherol rich food lipid. It also contains more than 55% polyunsaturated fatty acids, mainly linoleic and alpha-linolenic acid (Simopoulos 1999; Schwartz et al. 2008). Wheat germ processing presents challenges due to its high content of bioactive compounds. Microwave-assisted extraction is a new extraction technology used for the extraction of bioactive compounds, which is based on combination of microwave and conventional solvent extraction. This technique which is used has many advantages such as short time, less solvent usage, and higher extraction yield (Hao et al. 2002).Common Kilka (Clupeonellacultriventris caspia) oil is considered as one of the most healthy and functional oils. It is highly rich in polyunsaturated ω-3 fatty acids such as eicosapentaenoic acid and docosahexaenoic acid. However, Kilka oil is highly vulnerable to oxidation due to its high content of poly unsaturated fatty acids. Oxidations of poly unsaturated fatty acids such as eicosapentaenoic acid and docosahexaenoic acid result in a number of oxidation products that have negative impacts on the flavor and odor of Kilka oil, and also can affect the amount of these fatty acids that are made available to the body (Lin and Lin 2004; Fazli et al. 2009; Pazhouhanmehr et al. 2015; Yu et al. 2002). In order to preserve polyunsaturated fatty acids of Kilka oil from oxidative degradation, the use of novel and effective antioxidants can offer methods to maintain the health of consumers.The objective of this study was to investigate the effect of microwave-assisted extraction method on extraction yield and some chemical characteristics of wheat germ oil in comparison with conventional Soxhlet method. Also, wheat germ oil was investigated as a natural antioxidant for improving oxidative stability of Kilka oil. Wheat germ used in this research was supplied from Sepidan Flour Mill (Shiraz, Iran). Crude Kilka oil with no added antioxidants was supplied by a local fishery factory (Rasht, Iran).Wheat germ samples were pretreated with microwave at 200 W for 5 min. Thereafter, the samples were extracted with Soxhlet method. Samples were analyzed at 2, 4, 6, 8, and 10 h of extraction process. Extraction yield, saponification value, acid value, iodine value, and fatty acid profile of wheat germ oil extracted with microwave-assisted method were compared with those extracted with conventional Soxhlet method. Fatty acid composition of wheat germ oil was determined according to the method described by Golmakani et al. (2012) with some modifications. Saponification, acid, and iodine values of wheat germ oil were determined by using the AOAC official methods (AOAC 2000). Wheat germ oil was added to Kilka oil at a concentration of 1000 ppm. For the control, Kilka oil without any added antioxidant was used. Peroxide, anisidine, and Totox values of wheat germ oil were measured during 15 days storage at 60 °C. Peroxide, anisidine, and Totox values of wheat germ oil were determined using the AOCS official methods (AOCS 2000). Induction period was considered as the number of days required for a sample to reach a PV of 20 meq O2/kg (Keramat et al. 2016). The microwave-assisted extraction method increased the extraction yield of wheat germ oil by 15-27%. Increase in extraction yield is due to cell membrane rupture by microwave which results in greater porosity, enabling the passage of oil from the cell membrane (Uquiche et al. 2008). The amounts of saturated fatty acids, monounsaturated fatty acids, and polyunsaturated fatty acids in samples extracted by microwave-assisted extraction method were similar to those extracted by conventional Soxhlet method. Acid value of samples extracted by microwave-assisted extraction method was slightly higher than those extracted by conventional Soxhlet method. This result is in agreement with the previous studies (Kiralan et al. 2014; Uquiche et al. 2008). The saponification value of wheat germ oil sample extracted by microwave-assisted extraction method was 9.65% higher than those extracted by conventional Soxhlet method. Thus, wheat germ oil sample extracted by microwave-assisted extraction method contained higher short chain fatty acids than those extracted by conventional Soxhlet method. The iodine value of wheat germ oil sample extracted by microwave-assisted extraction method was lower than those extracted by conventional Soxhlet method. Accordingly, microwave-assisted extraction method has a positive effect on the oxidative stability of wheat germ oil. Wheat germ oil significantly decreased the peroxide, anisidine, and Totox values of Kilka oil by 61.59%, 65.01%, and 61.97%, respectively, compared to the control. The induction period and protection factor of Kilka oil sample containing wheat germ oil (120.20 h and 1.42, respectively) was significantly higher than those of control sample (84.40 h and 1.00, respectively). The inhibitory effect of wheat germ oil against Kilka oil oxidation can be attributed to the presence of high amounts of biological active compounds. Based on the results of this study, microwave extracted wheat germ oil can be proposed as a natural antioxidant for improving oxidative stability of Kilka oil.