Water transport in Thai glutinous rice during soaking: investigation using 1H-NMR relaxation

Nattawoot Maleelai

Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen, Thailand

Wiwat Youngdee

Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen, Thailand

Nath Saowadee

Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen, Thailand

Keywords: Rice soaking, Time-domain NMR, NMR relaxation, Glutinous rice, Khao Dawk Mali 105, Chai Nat 1

Abstract

Glutinous rice requires a lo[nger soaking time than other rice cultivars. In this study, water transport in Thai glutinous rice RD6 was compared with that in the non-glutinous cultivars Khao Dawk Mali 105 (KDML) and Chai Nat 1 (CN1) using time-domain nuclear magnetic resonance. The amylose and amylopectin percentages of the three rice cultivars were measured to study their potential links to the water absorption of the rice, which was measured after soaking for 0, 0.5, 1, 2, 3, 4, 5, and 6 h. CN1 became saturated with water within 1 h, whereas RD6 and KDML became saturated after about 2 h. Glutinous rice RD6 absorbed the greatest amount of water, whereas CN1 absorbed the least. The amount of water absorbed was inversely related to the amylose percentage of the three cultivars. A smaller amount of amylose in the amylopectin structure may provide more space to absorb water. The rate of water absorption of RD6 and CN1 was considered to have been greatest in the first half hour of soaking, where their free-water peaks dominated the T2 spectra. Most of the absorbed water of the three rice cultivars was in the loosely bound state, corresponding to the water in the amorphous growth shells of granules. The physically bound water peak of RD6 gradually shifted to a higher T2 value with increasing soaking time, indicating that the molecular mobility of water increases with soaking time.

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