Starch gelatinization and amylose-lipid interactions during rice parboiling investigated by temperature resolved wide angle X-ray scattering and differential scanning calorimetry

Starch gelatinization and formation of crystalline amylose-lipid complexes during the heat/moisture treatment step in rice parboiling were studied with temperature resolved wide angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC) using flour from Puntal (24% apparent amylose) and Jacinto (12% apparent amylose) rice samples [66, 40 or 25% moisture content (mc)]. Temperature resolved WAXS showed that the crystallinity index (CI, i.e. the relative amount of A-type crystals) of non-parboiled rice flours at 66% mc, monotonically decreased between ∼65 °C (Puntal) and ∼70 °C (Jacinto) and ∼90 °C (both Puntal and Jacinto). These temperatures were in agreement with the respective onset and conclusion temperatures of the M1 endotherm measured by DSC. At 40% mc, the CI decreased monotonically from ∼65 °C (Puntal) and ∼70 °C (Jacinto) until ∼105 °C. In DSC both M1 and M2 endotherms were present. The conclusion temperature of the M2 endotherm was higher than 105 °C. At 25% mc, the CI decreased very gradually and A-type crystals were no longer present at ∼145 °C. Under these conditions, no DSC endotherms were detected. No type II amylose-lipid complexes were formed during heating at 66% mc. In contrast, at 40 and 25% mc, Vh-type crystals were formed from ∼100 and ∼130 °C, respectively. Non-parboiled white rice flour had a strong A-type pattern. Mildly parboiled rice had a clear A-type, with a weak Vh-type and B-type pattern. Severe parboiling resulted in partially crystalline systems with superimposed A-type, Vh-type and B-type crystals. It was concluded that the rice variety, the combination of mc and the moisture distribution in the rice kernel and the temperature during parboiling all impact the level and the types of crystals in the parboiled rice.