Molecular Basis of the Gelatinisation and Swelling Characteristics of Waxy Rice Starches Grown in the Same Location During the Same Season

Six waxy rice samples (grown at the same site during the same season) were investigated to identify those aspects of amylopectin structure and granule composition which differentiate the starches into high (T_p 77·8 °C on average) or low (T_p 66·2 °C on average) gelatinisation temperature. All starches contained less than 2·5% amylose with negligible lipid. Granule dimensions were similar (mean diameter 4·9–5·7 μm) as was the β-amylolysis limit (59·9–63·0%). However, the structural elements of amylopectins within the two groups varied. The low gelatinisation temperature starches contained two major chain fractions upon debranching with iso-amylase, F3 (DP 16) and F1 (DP 51) on average while the high gelatinisation temperature starches contained three fractions, F3 (DP 16), F2 (DP 19) and F1 (DP 40). The proportion of F1/(F2+F3) for the low and high gelatinisation starches representing on average 20·2 and 10·5% respectively. Hence, the higher proportion of exterior chains and especially those with DP 19 appear to confer the higher gelatinisation temperature characteristic of these starches. Debranched β-limit dextrins were prepared from these starches. Regardless of their origin, (high or low gelatinisation temperature), they contained three fractions: F_3β (DP 5), F_2β (DP 13) and F_1β (DP 19). Hence, the high versus low gelatinisation characteristic was imparted by the exterior chains. Using 13-C CP-MAS/NMR it was confirmed that the number of double helices within the starches were approximately constant although small differences in crystallinity were identified by X-ray diffraction. Retrogradation of the solubilised native high and low gelatinisation temperature amylopectin molecules confirmed that the ‘high temperature gelatinisation amylopectin’ molecules form higher dissociation temperature and enthalpy crystalline domains. This confirms that the higher proportion of short and especially relatively longer short chains promotes crystallite formation.