Effect Of Solid-state Fermentation Of Aspergillus Niger On Antioxidant Activity And Chemical Composition Of Tartary Buckwheat Leaves

As one of important economic crops,Tartary Buckwheat has been widely developed to manufacture many kinds of products,most of which are mostly made from its seeds.As an annual herb,remaining parts,which are mostly comprised of Tartary Buckwheat leaves and stems,are usually not well utilized after maturation.It is very meaningful and of great importance to find an effective way for the development of the old stems and leaves of Tartary Buckwheat.Based on its extensive production of enzymes and efficient catalysis,Aspergillus niger are widely applied to conduct biotransformation and has made good achievements in chemical industry,organic synthesis,transformation of traditional Chinese medicine,drug development and other areas.Meanwhile,it is an important strain during Pu-er tea process.Therefore,A.niger was applied to conduct solid state fermentation on Tartary Buckwheat leaves.Firstly,antioxidant activity of Tartary Buckwheat leaves was measured based on DPPH free radical scavenging and ferricyanide reduction.And then,changes on chemical constituents of Tartary Buckwheat leaves were investigated by HPLC,combining with Pearson bivariate correlation analysis of the relationship between the chemical composition and antioxidant activity.The results were showed as follows:(1)Change law of DPPH free radical scavenging showed similarity with that of reducing power in Tartary Buckwheat leaves during A.niger fermentation,that is increasing firstly and then decreasing.The reducing power of Tartary Buckwheat leaves increased and reached the maximum value(A700=(0.263 + 0.036)),and the DPPH free radical scavenging ability increased vigorously to maximum value at(59.27+ 0.33)%.(2)Based on HPLC technology and heatmap analysis,fermentation process was divided into three stages: the first stage(ESF),for 10 days of fermentation,the second stage(MSF),for eleventh days to twentieth days of fermentation,and the third stage(LSF),for twenty-first to twenty-fourth days of fermentation.Content changes of total polyphenols,total flavonoids,rutin and quercetin in Tartary Buckwheat leaves showed a same trend when compared with that of DPPH free radical scavenging and the total reducing power.(3)Correlation analysis showed close relationships between the active ingredient content of total polyphenols,total flavonoids,rutin and quercetin and DPPH free radical scavenging and total reducing power,which might be a main reason why A.niger fermentation could be a good way to enhance antioxidant activity of Tartary Buckwheat leaves.(4)Experimental results of combination of rutin and quercetin showed that content of total flavonoids decreased but DPPH radical scavenging rate increased with the reduction of rutin proportion and rise of quercetin proportion.This results indicated that the proportion of rutin showed more conducive to the content of total flavonoids while the proportion of quercetin is more conducive to the promotion of DPPH free radical scavenging capacity.The results of this study showed that A.niger fermentation could be used to improve the antioxidant activity of Tartary Buckwheat leaves based on the content changes of its bioactive components,and A.niger fermentation could be used for the development and utilization of Tartary Buckwheat deeply.But long-term fermentation will lead to reduced components and antioxidant activity of Tartary Buckwheat leaves,indicating that the long-term fermentation is not conducive to improve the activity value of buckwheat leaves,fermentation process should be controlled effectively.