Changes In The Antioxidant Activity Of Pickled Cowpeas During The Spontaneous Fermentation

The pickled cowpeas were produced by traditional natural fermentation process, in which lactic acid fermentation predominates. Pickled cowpeas, as a kind of the traditional brined food with local characteristics, were loved by the masses in China due to their unique flavor, rich nutrition, and ready-to-eat. The research about the phenolic compositions and antioxidant activity of the pickled cowpeas during traditional spontaneous brining process is still very limited at home and abroad. It is very important to investigate the effect of natural fermentation process on phenolic substances and antioxidant capacity in pickled cowpeas, in order to comprehend their nutritional and biological value. The main contents of present paper are as follows:Firstly, the change of essential nutrients of cowpea before and after pickling process was investigated. Reducing of the nutritional value of pickled cowpea was due to some nutrients decreased after pickling. Therefore, the salt concentration of brine must be toward low-salt development.Secondly, the effect of spontaneous fermentation on the antioxidant activity of cowpea extracts was investigated by employing multiple assays including 1,1-Diphenyl-2-picrylhydrazyl(DPPH) radical scavenging activity, reducing power and total equivalent antioxidant capacity(TEAC). The total phenolic and flavonoid contents in the free and bound phenolic extracts(FP and BP) were determined by Folin-Ciocalteu and Al(NO3)3-Na NO2 colorimetric method, respectively. The results show that there were significant differences(p<0.05) in total phenolic content, flavonoid content and antioxidant activity of the pickled cowpeas after spontaneous pickling. The highest of total phenolic content, flavonoid content and antioxidant activity were determined in the pickled cowpea after 15 days of pickling(P15), and the results in the pickled cowpea after 30 days of pickling(P30) followed by P15 with respect to cowpea without pickling(Cont). The DPPH radical scavenging activity and reducing power of all the extracts tested increased as the concentration increased. The extracts of fermented cowpeas scavenged DPPH radical capacity at all concentrations and was significant higher to the raw cowpeas(p<0.05). The IC50 value of DPPH free radical and EC50 value of reducing power were in the following order: Gallic acid(GA)<Ascorbic acid(Vc)<P15 BP<P30 BP<Cont BP<P30 FP<P15 FP<Cont FP and Vc<2,6-Di-Tert-Butyl-4-Methylphenol(BHT)<P15 BP<Cont FP<Cont BP<P30 BP<P15 FP<P30 FP, respectively. The change of phenolic constituents in cowpeas during pickling process was analyzed by using high-performance liquid chromatography(HPLC). By comparison with standards, five phenolic acids in all sample extracts were identified as gallic acid, caffeic acid, p-coumaric acid, ferulic acid and salicylic acid. Significant differences in the identified phenolic compounds were observed during the cowpea brining periods.Finally, the effect of salt concentration(SC) in brine on the antioxidant activity of pickled cowpeas during spontaneous fermentation process was investigated by employing various assays including Folin-Ciocalteu colorimetric method, TEAC assay and antioxidant enzyme activities. The results show that pickling processes, both 5% SC and 10% SC, produced a significant differences in antioxidant activity of pickled cowpeas. Enzymatic activity of polyphenol oxidase in cowpea pickled with 5% SC of brine was weaker than 10% SC, the total phenolic content and TEAC value of the free and bound phenolic extract in cowpea pickled with 5% SC of brine were apparently higher than 10% SC. The analysis of phenolic compositions of cowpea extracts were performed by HPLC. The results show that the pickling processes with 5% SC of brine were superior to 10% SC for the preservation of phenolic compounds in cowpea. These results indicated that traditional spontaneous pickling with low-salt concentration of brine seems to be an adequate and effective process for preserving and increasing phenolic composition contents and antioxidant activity of cowpeas.