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 (me)]. Temperature resolved WAXS showed that the crystallinity index (Cl, i.e. the relative amount of A-type crystals) of non-parboiled rice flours at 66% me, monotonically decreased between similar to 65 degrees C (Puntal) and similar to 70 degrees C (Jacinto) and similar to 90 degrees C (both Puntal and Jacinto). These temperatures were in agreement with the respective onset and conclusion temperatures of the M-1 endotherm measured by DSC. At 40% me, the CI decreased monotonically from similar to 65 degrees C (Puntal) and similar to 70 degrees C (Jacinto) until similar to 105 degrees C. In DSC both M-1 and M-2 endotherms were present. The conclusion temperature of the M-2 endotherm was higher than 105 degrees C. At 25% me, the CI decreased very gradually and A-type crystals were no longer present at similar to 145 degrees C. Under these conditions, no DSC endotherms were detected. No type 11 amylose-lipid complexes were formed during heating at 66% me. In contrast, at 40 and 25% Me, V-h-type crystals were formed from similar to 100 and similar to 130 degrees C, respectively. Non-parboiled white rice flour had a strong A-type pattern. Mildly parboiled rice had a clear A-type, with a weak V-h-type and B-type pattern. Severe parboiling resulted in partially crystalline systems with superimposed A-type, V-h-type and B-type crystals. It was concluded that the rice variety, the combination of me 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. (c) 2005 Elsevier Ltd. All rights reserved.