Rice Starch Molecular Size and its Relationship with Amylose Content

The composition and starch molecular structure of eight rice varieties were studied. Waxy and non‐waxy (long‐, medium‐, and short‐grain) rice varieties from California and Texas were used. The amylose contents were measured using the Concanavalin A method and were found to be related to the type of rice: waxy ≈ 1.0%, short and medium grain 8.7–15.4%, and long grain 17.1–19.9%. The weight‐average molar masses (Mw) of the starches varied from 0.52 to 1.96×108 g/mol. As would be expected, a higher Mw of rice starch correlated to lower amylose content. The range of Mw of amylopectin was 0.82 to 2.50 ×108 g/mol, and there was also a negative correlation of amylopectin Mw with amylose content. Amylose Mw ranged from 2.20 to 8.31×105 g/mol. After debranching the amylopectin with isoamylase, the weight‐average degree of polymerization (DPw) for the long‐chain fraction correlated positively with a higher amylose content. California and Texas varieties were significantly different in their amylose content, starch Mw (short‐ and medium‐grain only), and amylopectin Mw (p < 0.05).