Effects of Amino Acids and Fatty Acids on Rice Starch Properties: Thermal, Pasting, Resistant Starch and Structural Characterization

In this study, the effects of fatty acids and amino acids on rice starch were determined in aspects of pasting properties, thermal characteristic, starch digestibility and crystallinity structure. Results from viscosity analysis showed significantly low peak viscosity when rice starch was combined with 1.0% stearic acid and 6% lysine without any pH adjustment; within 0.6%-1.0% of stearic acid and 6% of lysine addition, there was a linear regression relationship between peak viscosity and the level of stearic acid. Similar pasting curves were found in rice starch with 1.0% stearic acid and 6% glutamine, or 6% cysteine, or 6% glycine in a starch solution of pH 10. Yet the inhibited pasting viscosity was not found in corn starch with the same additive. The thermal, retrogradation, digestibility, and X-ray diffraction of RVA treated rice starch samples were selectively assayed. When stored 10 days under refrigeration, starches with both fatty acids and lysine added were found to have lower retrogradation (13.3%) than starches with fatty acids added only and starches without additives (41.4%). With regard to RS content, no pronounced difference was found between starch with additives and without additives except cysteine; however, SDS assay observed more slowly digestible starch when fatty acids were present in the sample, due to amylose-lipid complexes with less order crystalline structure. Both X-ray diffraction and DSC scans of RVA treated starches showed elevated amounts of amylose-lipid complexes when both lysine and fatty acids were present, compared to addition of fatty acids alone. Interaction between starch, fatty acids and amino acids was confirmed, within which amino acids that are negatively charged and stearic acid were indispensible for inhibiting rice starch swelling. Because the effect of inhibiting starch swelling and pasting is similar to properties of cross-linked starch, application of fatty acids and amino acids in this study in altering starch properties offers big market potential for clean label starch as a food ingredient, such as thickening agent in a pudding, a soup or a sauce