Effects of amylose and amylopectin on quality of rice noodles

Abstract

Effects of amylose content, amylose solubility, and amylopectin structure of rice flour on quality of rice noodles were determined. Milled rice was ground into flour. Noodles were prepared by mixing 100 g flour, 80 g water and 1 g salt. The mixture was heated in a microwave oven for 75 sec, extruded using a pasta maker, and dried at 60⁰C using a 2-step procedure: 100% RH (2 hr) and 30% RH (2 hr). Cooking, texture, and physico-chemical characteristics of dough and noodles were determined. More amylose in flour contributed to better texture of noodles, i.e., greater firmness and less stickiness. Texture of noodles prepared from flour with 25-30% amylose was substantially improved compared to flour with less than 25% amylose. Rice flour containing amylopectin with more long chains with degree of polymerization (DP) of 70-156, improved the quality of noodles. In contrast, flour containing more intermediate chains, DP 26-69, impaired noodle quality. The starch in dough was partially modified during processing. Correlation coefficients of noodle texture with pasting properties were lower for flour compared to dough. Hence, dough pasting properties could better predict noodle quality. Heating and extrusion processes caused rice starch to partially gelatinize. The crystalline structure of amylopectin was partly disrupted and some amylose leached from the granule. The noodle drying process enhanced the mobility of amylopectin chains. Amylose and long amylopectin chains probably associated and provided a gel network that strengthened the structure of the noodles. Intermediate amylopectin chains, in contrast, inhibited these associations. Short amylopectin chains, DP 5-25, did not affect the properties of noodles. These chains were probably too short to enhance or inhibit formation of the gel network. Amylose solubility in excess water at 120⁰C did not correlate with the noodle quality. Rice flours with amylose content between 25-30% are recommended for preparing rice noodles. Using flours with amylose contents less than 25% yielded unacceptable noodle textures, i.e., too sticky, while using flours with amylose contents more than 30% yielded noodles with poor appearance and high dry matter loss.