Improvements of technological and health functional properties of rice bran fibre by physical and enzymatic treatments

Abstract

Rice bran is a major, underutilised by-product of the rice industry and the high proportion (~90%) of insoluble fibre is the main reason limiting its applications in foods. This thesis is aimed at modifying the physicochemical properties of rice bran by physical (ultrasound and steam explosion) and enzymatic treatments, thereby improving its technological and health properties. Purified (starch and protein removed) and un-purified defatted rice brans were treated by ultrasound and steam explosion under different intensities (amplitude, time, steam pressure), followed by enzymatic hydrolysis using ShearzymePlus. Changes in physicochemical properties of the bran were determined. The physical treatments strongly impacted on the physicochemical properties, which were affected by both treatment intensity and bran purity. For purified bran, increasing treatment intensity generally led to decreases in particle size and bulk density, and increases in porosity, swelling, water and oil binding capacities, as well as the increased yield of soluble fibre. The greatest changes in these properties were generally achieved with ultrasound treatment at 60% amplitude for 20 min, where the highest yield of soluble fibre (35.2g/100g) was obtained. For un-purified rice bran, the presence of starch and proteins complicated the efficacy of the treatments. The steam explosion was found to be less effective than ultrasound in modifying the physicochemical properties of rice bran. Soluble fibre produced from both physical treatments mainly contained oligosaccharides with MW <25kDa, which showed good prebiotic potentials in promoting the growth of L. acidophilus and B. Bifidum with the highest MW fraction (~17kDa) being the most effective. Shearzyme Plus hydrolysed the insoluble fibre into the soluble fibre; however, the hydrolysis was more effective on untreated than physically treated bran. Incorporation of ultrasound and enzymatically treated rice bran to flour (15%) caused minimum deterioration in bread volume, texture and colour compared to untreated bran. The enrichment significantly improved the glucose adsorption capacity and glucose dialysis retardation, reduced glucose diffusion and glycaemic index (GI), and greatly increased the sodium cholate binding capacity of the bread. Overall, this thesis demonstrated that ultrasound and enzymatic treatment is a feasible method to modify rice bran with significantly enhanced technological and health functional properties.