Optimization of total phenolics and total flavonoids enrichment process of black Tartary buckwheat by solid-state fermentation and its antioxidant activity

Tartary buckwheat contains abundant polyphenols and flavonoids, which had antioxidant, liver protecting, anti-inflammatory, antitumor, antihypertensive effect and so on. However, the release of functional polyphenols in Tartary buckwheat products is difficult. Solid-state fermentation technology was an economically and environmentally friendly production technology, and had become a powerful tool for the release of phenolic substances in grains. During the fermentation process, microorganisms produced hydrolyzing enzymes such as amylase, pectinase, xylanase and cellulase. Under the action of these hydrolyzing enzymes, the plant cell wall was destroyed, resulting in the release of bound polyphenols, which significantly increased their content and antioxidant capacity. This study selected black Tartary buckwheat as raw material, and conducted solid-state fermentation with total phenolic and total flavonoid content as evaluation index. Firstly, optimal fermentation strains were screened. Then, the optimization range of fermentation parameters such as inoculation amount, loading amount and fermentation time was determined by single factor experiment, and the optimal fermentation conditions were determined by response surface experiment (three factors and three levels). Finally, the effects of solid-state fermentation on the nutritional composition and in vitro antioxidant activity of black Tartary buckwheat fermented by the optimal strains under optimal fermentation conditions were analyzed. The results showed that the optimal fermentation strain was Aspergillus oryzae, and the optimal fermentation conditions were: inoculation amount of 11.0 mL/100 g, loading amount of 28.6 g and fermentation time of 3.6 d. Under these conditions, the content of total phenolics and total flavonoids was the highest, (27.00±0.43) mg/g and (2.50±0.02) mg/g, respectively. The content of total phenols and flavonoids was 2.61, 2.72 and 2.02, 2.16 times higher than that of the blank group and the control group, respectively. The loading amount and fermentation time had significant effects on the content of total phenolics and total flavonoids, and the interaction of loading amount and fermentation time had extremely significant effects on the content of total phenolics (P<0.01). The results of nutrient composition showed that the content of starch was significantly decreased, and the content of protein, fat and ash was significantly increased (P<0.05). The in vitro antioxidant results showed that the IC₅₀ of DPPH and ABTS scavenging by fermented black Tartary buckwheat polyphenols ((33.08±2.21) μg/mL, (16.41±2.21) μg/mL) were significantly lower than those before fermentation ((50.67±3.13) μg/mL, (28.07±3.42) μg/mL). The total antioxidant capacity and total reduction capacity were significantly increased (P<0.05). In conclusion, solid-state fermentation significantly improved the content of total phenolics and flavonoids and antioxidant activities of black Tartary buckwheat, providing a theoretical basis for the development of Tartary buckwheat functional foods.