Effect Of Microwave Treatment On Sinapic Acid Derivatives In Rapeseeds And Products And Antioxidant Activities

Rapeseed is a rich source of polyphenols, and sinapic acid and sinapine are the most significant phenolic compounds in rapeseed. Sinapine is the choline ester of sinapic acid and sinapic acid can produce the canolol by thermal decarboxylation naturally during the press process or roasting of the seed. Previous investigations have shown that microwave pretreatment of rapeseed can significantly increase canolol content in rapeseed and oil from them. To improve the nutritional quality of rapeseed oil and antioxidant activity, the paper presented a research to investigate the profile and distribution of the total phenolic, sinapic acid, sinapine and canolol in three different genotypes rapeseed(Brassica napus, B.juncea, B.rapa) and products from them, including rapeseed oil, presses-cake, defatted-meal under microwave pretreatment of rapeseed and extraction of oil. The correlations were analyze between the canolol content and sinapic acid and sinapine content and the formation of canolol and the loss of sinapic acid and sinapine. In addition, the antioxidant activities of extracts obtained from rapeseed, oil, pressed-cake and defatted-meal were monitored and compared. The main conclusions obtained are as follows:(1) The microwave conditions and the extraction conditions of polyphenol were optimized. The best microwave conditions for rapeseed were as follows: rapeseed sample 150 g, microwave power 560w; the best polyphenol extraction conditions were as follows: extract time 20 min, methanol concentration was 70%, extract three times.(2) Seeds of brassica napus, B.juncea and B.rapa and products from them showed significant difference on the content of total phenolic, sinapic acid, sinapine and canolol(P<0.05). The content of total phenolic, sinapic acid and canolol in Brassica napus rapeseed were markedly higher than those of in B.juncea and B.rapa rapeseed. The content of sinapine in B.juncea rapeseed was markedly higher than that of in other two varieties. The microwave time showed significant effects(P<0.05) on the sinapic acid, sinapine and canolol in three different genotypes rapeseed. The content of sinapic acid and sinapine in three rapeseed varieties decreased with microwave time. Canolol content of Brassica napus rapeseed was first increased and then decreased with the prolonging of microwave time, whereas the canolol was not detected in B.juncea and B.rapa rapeseed for 1-4min microwave time and then increased from 5min. In rapeseed oil, the content of sinapic acid and canolol increased firstly, and then decreased; the content of sinapine increased gradually with the extension of microwave time. Both the pressed-cake and defatted-meal, the content of sinapic acid and sinapine were markedly decreased in three rapeseed varieties.(3) Most of sinapic acid and sinapine remain in the meal after pressing and remove residual oil by Subcritical fluid extraction. In total, the transfer ratio of sinapic acid was less than 3%, sinapine less than 0.2%. However, there was 23-77% of canolol transferred to oil due to its stronger fat-soluble. Along with the extension of microwave time, the transfer ratio of sinapic acid, sinapine and canolol from rapeseed to oil were increased. Therefore, sinapic acid and sinapine are the main polyphenol in rapeseed, pressed-cake and defatted-meal, canolol is the main polyphenol in rapeseed oil.(4) Correlation analysis indicated that the content of canolol were significantly negative correlation with sinapic acid(-0.977) and sinapine(-0.962) during the microwave pretreatment. And the increase of canolol was far above the loss of sinapic acid, showed the significantly positive correlated with the loss of sinapic acid(0.967) and sinapine(0.978) during microwave pretreatment.(5) The antioxidant activities in Brassica napus L. rapeseed and products from them were significantly higher than that of the other two rapeseed varieties, which is highly related with the polyphenol content. For Brassica napus L. rapeseed, the microwave pretreatment decreased the antioxidant activities of rapeseed, pressed-cake and defatted-meal, but increased the antioxidant activities of rapeseed oil. The DPPH, FRAP and CAA values of oil pressed by 7-min microwave pretreatment rapeseed were increased 472 μmolTE/100 g, 1062 μmolTE/100 g and 19.49% than that of oil from the untreated rapeseed. For Brassica juncea L. rapeseed, the antioxidant activities of rapeseed and their products were all increased under microwave pretreatment. The DPPH, FRAP and CAA values of oil pressed by 7-min microwave pretreatment rapeseed were increased 12.54 μmolTE/100 g, 190 μmolTE/100 g and 39.42% than that of oil from the untreated rapeseed. For Brassica campestris L. rapeseed, the microwave pretreatment increased the antioxidant activities of rapeseed, oil and defatted-meal, but decreased the antioxidant activities of pressed-cake. The DPPH, FRAP and CAA values of oil pressed by 7-min microwave pretreatment rapeseed were increased 14.50 μmolTE/100 g, 215 μmolTE/100 g and 45.52% than that of oil from the untreated rapeseed. The antioxidant activities of rapeseed oil pressed by 7-min microwave pretreatment rapeseed in three varieties were all higer than that of oil from the untreated rapeseed, which indicated that microwave pretreatment on rapeseed can improve the antioxidant activities in oil pressed by rapeseed.