Distribution of phenolic compounds and antioxidant activities in layered grinding wheat flour and the regulation effect of nitrogen fertilizer application

Abstract

<p id="C3">Clarifying the distribution of phenolic compounds in layered grinding wheat flour and its response to nitrogen fertilizer application would provide useful information for wheat quality improvement and high-quality cultivation. Two wheat cultivars, purple wheat (Jizi 439) and white wheat (Xinhuamai 818), were planted with two nitrogen application rate (HN, N 210 kg hm^-2; LN, N 105 kg hm^-2) in Zhengzhou and Yuanyang experimental sites during growing period in 2019 and 2020. The mature grains were ground into five milling fractionations (LY1, LY2 LY3, LY4, and LY5) from the bran layer to the endospermic layer by layered grinding, and the total phenolic content (TPC), the total flavonoids content (TFC), and the anthocyanin content (AC), and their antioxidant activity were determined. The results showed that the TPC, TFC, AC, and antioxidant activity (TEAC, FRAP) in free phenols and conjugated phenols extracts decreased from the outer layer flour fractionation to the inner layer flour fractionation. The purple wheat, named as Jizi 439, had a higher antioxidant content and antioxidant activity than white wheat (Xinhuamai 818), but the difference between the two cultivars displayed a decreasing trend from the bran layer flour fractionation to endosperm layer flour fractionation. The TPC, TFC, and AC of LY1 to LY3 flour fractionation increased with the increase of nitrogen fertilizer application except for TPC of LY1 in Yuanyang. However, the contents of total phenolics and total flavonoids of LY4 to LY5 flour fractionation had a weak response to nitrogen fertilizer application. The content of ferulic acid accounted for more than 93% of the TPC in wheat grains and had a higher value under low nitrogen condition. In conclusion, purple wheat had higher antioxidant substances such as phenolics than white wheat, and the difference between purple wheat and white wheat decreased with the deepening of grinding degree. The content and activity of antioxidants in the outer layer flour fractionation were significantly responsive to nitrogen regulation, and the content increased with the increase of nitrogen.