Comparison of protein extraction solutions for rice starch isolation and effects of residual protein content on starch pasting properties

Protein extraction solutions such as aqueous solutions of sodium hydroxide (0.1 and 0.2 %), sodium lauryl sulfate (SLS, 1.2 %) containing sodium sulfi te (0.12 %), and dodecylbenzene sulfonate (DoBS, 1.2 %) containing sodium s ulfite (0.12 %) were compared in their protein removal efficiencies during isolation of starch from a rice flour (Ilpum-byo, a nonwaxy Korean rice var iety). In addition, the pasting properties of the isolated starch was compa red. More than 80 % of the flour protein was extracted in 1 h by stirring t he dispersion (1:3. w/v) at room temperature. Repeating the extractions (1 or 2 h for each step) with fresh solution significantly increased the prote in removal efficiency. When the extraction in 0.2 % NaOH was repeated four times (1 h for each step) at 25 degrees C, the residual protein content in the isolated rice starch was 0.9 % (DB), equivalent to 86 % removal of the rice protein. Raising the extraction temperature slightly increased the pro tein solubility, but starch loss also became significant. Among the solutio ns, DoBS was most effective in removing rice protein whereas SLS was least. The residual protein content had a critical role in determining the pastin g characteristics of the isolated starch, showing a negative correlation to the peak viscosity of the starch paste, but a positive correlation to the pasting temperature.