Effects of Rough Rice Moisture Content at Harvest on Peak Viscosity

Abstract

During rice kernel development, kernel size and chemical composition change with maturation, and the kernel moisture content progressively decreases (Del Rosario et al 1968; Lee and Chang 1985). Therefore, kernel moisture content is an important indicator of rice kernel development. Lee and Chang (1985) found that when starch and protein synthesis were nearly completed, the rice kernel moisture content was ≈27–29% (wb). The composition of rice kernels harvested after this point reportedly did not change dramatically, although the starch content increased slightly, causing a proportional decrease in protein content in the kernel (Del Rosario et al 1968; Kim et al 2001). Rice kernel-to-kernel development on panicles is nonuniform, which in turn produces a range of kernel moisture contents within a bulk at harvest (Chau and Kunze 1982; Kocher et al 1990; Siebenmorgen et al 1992). As such, physical properties such as bulk density and milling quality change with harvest moisture content (HMC). The grade of rice typically improved and the bulk density increased as HMC decreased (Morse et al 1967; Fan et al 1998). The number of fissured kernels increased dramatically when the HMC decreased from 19 to 13% and was especially severe when kernel moisture content decreased rapidly in a short time (Jodari and Linscombe 1996). Reports of optimum HMC for maximum head rice yield (HRY) ranges from 18 to 26% based on growing location, harvest date, and cultivar (Kester et al 1963; Huey 1977; Lu et al 1988, 1992; Siebenmorgen et al 1992; Jongkaewwattana et al 1993a; Ntanos et al 1996; Bautista and Siebenmorgen 2000). Limited research has been conducted to determine the effect of the HMC on functional characteristics of rice. Kester et al (1963) reported that the peak viscosity (PV) of short-grain rice first decreased and then increased with maturation. It was explained that amylases were responsible for these changes. Kim et al (2001) also noted differences in PV of medium-grain rice with HMC, however, the trends did not follow consistent patterns. The study reported herein was designed to quantify the effects of rough rice HMC on PV of popular several rice cultivars grown in the midSouth rice producing region of the United States.