Alkali treatment of maize bran affects human gut microbiota composition

The fermentation of dietary fibers by gut microbes has beneficial impacts on host health. Unfortunately, maize bran dietary fibers resist microbial fermentation due to complex cross-linking of arabinoxylan (AX) by phenolic compounds. Alkali treatment can break the cross-linking and potentially increase microbial fiber utilization; however, it is not well understood how varying degrees of alkali treatment affect fiber fermentation. Therefore, maize bran was treated with alkali at six NaOH concentrations ranging from 0 to 1.25M. Alkali-treated maize bran was neutralized, digested, dialyzed, and then subjected to in vitro fermentation using the fecal microbiotas from three healthy subjects. Increasing alkali concentration resulted in the release of soluble fibers, especially AX, with a significant increase in microbial fermentation of AX with increasing alkali concentrations (p<0.001). However, there was a decrease in fermentation of insoluble fiber (p<0.001), such that total fiber utilization decreased (p<0.001). Additionally, more acetate (p=0.004) and propionate (p=0.018), but less butyrate (p<0.001), were produced as alkali concentration increased. Alkali treatment decreased the abundance of Lachnospiraceae (q=0.033), Butyticicoccaceae (q=0.002), and Veillonellaceae (q=0.045), while Bifidobacteriaceae (q=0.045) and Bacteroidaceae (q=0.045) increased. We hypothesized that residual free phenolic compounds released during the alkali treatment led to the decrease in polysaccharide fermentation. After extraction of free phenolic compounds, soluble polysaccharide utilization further increased (p=0.005), but total fiber fermentation was not improved (p=0.39). This study revealed that although alkali treatment breaks cross-links and releases soluble polysaccharides from maize bran, it has a negative impact on insoluble and overall fiber utilization, which, in turn, reduces butyrate production by the microbiome. Our research also suggested a negative impact of free phenolic compounds on soluble, but not insoluble, fiber fermentation.