Optimization of Extraction Process, Composition and Antioxidant Activity Analysis of Conjugated Phenols from the By-product Residue of Pueraria thomsonii

To improve the comprehensive utilization value of the by-product residue of Pueraria thomsonii (kudzu), the extraction of conjugated phenolics from two common by-product residues (kudzu peel and kudzu dregs) was optimized. Infrared spectroscopy and scanning electron microscopy were used to compare the microstructure of the by-products of kudzu before and after extraction. The composition of the conjugated phenolics was characterized by LC-MS and the antioxidant activity of the two by-product residues was determined. The results showed that the optimal extraction conditions for the conjugated phenolics in the by-product kudzu dregs were as follows: Liquid to material ratio of 30:1 mL/g, hydrolysis time of NaOH solution of 4 h, and concentration of NaOH solution of 3 mol/L, and the yield of conjugated phenolics under these conditions was 2.54±0.070 mg/g. Accordingly, the optimal extraction conditions for conjugated phenolics in the by-product kudzu bark were 40:1 mL/g liquid/feed ratio, 5 h hydrolysis time in NaOH solution, and 3 mol/L concentration of NaOH solution, and the yield of conjugated phenolics under these conditions was 5.087±0.155 mg/g. The ester bond between phenolics and cellulose in the by-products, and the overall fiber structure was broken, which effectively facilitated the release of conjugated phenolics. The composition and content of conjugated phenolics in the two by-product residues differed greatly, with phenolic acids and flavonoids dominating the conjugated phenolics in kudzu peel, while flavonoids dominated the conjugated phenolics in kudzu dregs. The conjugated phenolics in both kudzu peel and kudzu dregs had strong antioxidant activity, and the antioxidant capacity of conjugated phenolics in kudzu peel was found better than that of kudzu dregs. In conclusion, the by-product residues of kudzu are rich in conjugated phenolics and have significant antioxidant activity, which is of good value for development and utilization.