Pasting and paste properties of waxy rice starch as affected by hydroxypropyl methylcellulose and its viscosity.

The objective of this study was to investigate the impacts of hydroxypropyl methylcellulose (HPMC) and its viscosity on the pasting and paste properties of waxy rice starch (WRS). HPMC with different viscosities (low, BN50; medium, BN40M; high, BN40H) but similar substituted group contents was used. Although the total solid contents of WRS-HPMC mixtures varied depending on measured attributes, the mixing ratio of WRS to HPMC was 19:1 (w/w). WRS-HPMC mixtures exhibited lower swelling factors than WRS alone, and these decreased with HPMC viscosities. Relative to WRS, gelatinization onset and peak temperatures were shifted to higher temperatures in WRS-HPMC mixtures, and gelatinization enthalpies were lowered. Delayed pasting temperatures, lower peak and breakdown viscosities, and higher trough viscosities were observed in WRS-HPMC mixtures. Although final viscosities increased with HPMC viscosities, WRS-BN40H exceeded WRS, whereas the reversed pattern was found in WRS-BN40M and WRS-BN50. Thermal gelation of HPMC occurred in WRS-HPMC mixtures. With increasing HPMC viscosities, G', G", and tan δ of their pastes increased, and their syneresis decreased. Overall, these results may arise from restricting the availability of water to WRS granules and suppressing their swelling due to the greater water holding capacity and thermal gelation tendency of HPMC.