Effects of Different Pretreatments on Wheat Bran and Its Arabinoxylan Obtained by Sequential Extraction with Dilute Alkali and Alkali–Urea Mixture

Arabinoxylan (AX), an abundant and highly valuable component in wheat bran, has its structure and function influenced by the extraction method. A two-step extraction method, involving sequential extraction with a dilute alkali followed by a concentrated alkali-urea mixture, was employed to extract AX from wheat bran. This approach aimed to obtain AX with a high phenolic acid content while achieving a relatively high extraction yield. The dilute alkali extraction could effectively retain the phenolic acid content in the AX extract (≤89 μg/g). However, its yield and sugar content were relatively low. In contrast, the alkali-urea extraction could achieve a relatively high yield (≤55%) and sugar content (≤75%). Different pretreatments (defatting, deproteinization, and delignification) were performed before extraction, causing significant changes to the chemical composition and cell wall structural characteristics of destarched wheat bran, which, in turn, affected the yield and composition of the AX extracts. Deproteinization effectively increased the sugar content, phenolic acid content, and overall yield of the extracts. Different pretreatment and extraction methods significantly affected the DPPH radical scavenging rate and Fe²⁺ chelating rate of the AX extracts but had little impact on the ABTS radical scavenging rate. The antioxidant activity of AX extracted using alkali-urea was unexpectedly higher than that extracted using a dilute alkali. This suggests that the antioxidant activity of AX does not entirely depend on its phenolic acid content but is influenced by various other factors.