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Influence of xylanase treatment and cryomilling on dietary fibre and phenolic compounds of wheat bran

Wheat bran is rich in dietary fibre and bioactive compounds with proven benefits on human health. Wheat fibre is mainly insoluble, composed of arabinoxylans (pentosans) often esterified with phenolic acids, such as ferulic acid. The solubilisation of bran fibre can be achieved with xylanase treatment or cryomilling. This study aimed to investigate the combining effect of xylanase treatment and cryomilling on fibre, free phenolic compounds, and antioxidant activity of wheat bran. Coarse bran (d (50) = 503 ± 7 µm) was grinded in a CryoMill (Retsch, Germany) under liquid nitrogen cooling to obtain ‘superfine’ particle size (d (50) = 10 ± 1 µm). A suspension of 15 g bran in 100 mL of water was incubated with xylanase from Bacillus subtilis (VERON® RL, 10 UI) in a shaking water bath at 55 °C for 16 h. The content of insoluble dietary fibre (IDF), fibre soluble in water but precipitated in 78% ethanol (SDFP), and fibre soluble in water and 78% ethanol (SDFS) was determined according to the AOAC 2011.25 method, while the soluble pentosans were measured with phloroglucinol method. Total phenolic content (TPC, with the Folin-Ciocalteu) and antioxidant activity (FRAP method) was analysed spectrophotometrically, and ferulic acid was determined with HPLC-DAD method. The content of SDFP and TPC was slightly higher in cryomilled samples than in coarse bran, but xylanase treatment of superfine bran did not further enhance SDFP content. Xylanase treatment resulted with a significant decrease in IDF content (p<0.01), while it enhanced the content of soluble pentosans, SDFS, TPC, ferulic acid, and antioxidant activity depending on the interaction with cryomilling (p<0.01). Overall, after the xylanase treatment, the content of soluble fibre and phenolic compounds did not significantly differ between the coarse and superfine bran. It can be concluded that cryomilling of wheat bran before xylanase treatment did not contribute to solubilisation of fibre and xylanase activity.

Wheat bran, xylanase, dietary fibre, cryogenic grinding

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