Compared to HRW, HRS had lower PPO activity in the groups treated with the same number of fine grinding. It is mainly due to the diverse genotype between the two wheat classes. For both classes, it was observed that WWF with finer millfeed particles exhibited significantly higher PPO activity than that with coarser millfeed particles (P < 0.05). PPO and phenolic compounds are primarily located in the cell plasmid of wheat bran layer, so the results are probably attributed to the fact that fine grinding of millfeeds promotes the exposure of PPO active sites by fracturing cell walls in bran and provides more
opportunity for PPO to access enzyme substrates.