Ultrasound and Steam Explosion Treatments on the Quantity and Molecular Size of Soluble Fibre Obtained from Un-purified and Purified Rice Bran

Abstract

Rice bran (RB) is a major by-product of the rice industry, and the high proportion (~90%) of insoluble fibre (IF) is the main reason limiting its applications in foods. Thus, the objective of this research is to enhance the solubility of rice bran fibre and decrease the molecular weight (MW) of the soluble fibre (SF) fraction through ultrasound (US) and steam explosion (SE) treatments. The main sugars in the RB fibre were xylose and arabinose, with glucose, galactose, and mannose present in the side chains. The ratio of Ara/Xyl was 0.92 for the un-purified and 1.02 for the purified RB, reflecting the high degree of substitution of the xylan backbone. The highest amount of SF was obtained from RB treated at 60% US amplitude, 20 min treatment, where 7.8% (un-purified) and 35.2% (purified), respectively. For SE treatments, the amount of SF in un-purified RB increased as the pressure increased from 0.3 and 0.6 MPa, which were 6.10±0.34 and 8.83±0.56%, respectively. Meanwhile, the highest SF fraction (35.2%) of purified RB was obtained from the SE treatment at 0.6 MPa. The SF produced from both treatments mainly contained oligosaccharides with MW <1 kDa, with those produced by the SE treatment generally smaller than those by the US treatment. Purification of RB significantly enhanced the efficiency of the US and SE treatments in breaking down the IF into the SF.