Rice Starch Isolation by Alkaline Protease Digestion of Wet-milled Rice Flour

Abstract

Alkaline protease digestion with a food-grade enzyme was used to produce rice starch from wet-milled rice flour (WMRF). In a 3×3 factorial modelling experiment, recoveries of starch and levels of protein contamination were determined at pH 8·5–10·0, protease levels of 0·5–1·5% (based on WMRF), and digestion times of 5·0–30·0 h. The following digestion conditions were kept constant; 55 °C with mild agitation, 34–37% (w/v) flour solids, and alkalinity to within ±0·2 pH units. Regression equations with the three variables explained 92% and 98%, respectively, of the variances in starch recovery and protein contamination. Upon digestion with 1·1% protease at pH 10·0 and 18·0 h, starch recovery was 95% and protein contamination was 0·5%. Most hydrolysis of rice protein occurred in the first 3–4 h of digestion as determined by the consumption of sodium hydroxide (NaOH). Rice starch also was isolated by extraction of WMRF with c. 2·5 parts of 0·05 m NaOH at c. pH 12. The recovery of starch was c. 10% higher with the protease method than with the NaOH method, and the effluents contained mostly amino acid salts as opposed to protein mixed with alkali. The rice starch isolated by protease digestion was lighter in appearance, contained more lipid, and gave a somewhat lower consistency after pasting. The raw materials used to isolate rice starch by the protease method were approximately twice as costly in 1996 as those in the NaOH method, principally because of the cost of the protease (55% of total).