Properties and Structures of Flours and Starches from Whole, Broken, and Yellowed Rice Kernels in a Model Study

Cereal Chem. 79(3):383–386 The objective of this study was to compare the structure and properties of flours and starches from whole, broken, and yellowed rice kernels that were broken or discolored in the laboratory. Physicochemical properties including pasting, gelling, thermal properties, and X-ray diffraction patterns were determined. Structure was elucidated using high-performance sizeexclusion chromatography (HPSEC) and high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD). The yellowed rice kernels contained a slightly higher protein content and produced a significantly lower starch yield than did the whole or broken rice kernels. Flour from the yellowed rice kernels had a significantly higher pasting temperature, higher Brabender viscosities, increased damaged starch content, reduced amylose content, and increased gelatinization temperature and enthalpy compared with flours from the whole or the broken rice kernels. However, all starches showed similar pasting, gelling, thermal properties, and X-ray diffraction patterns, and no structural differences could be detected among different starches by HPSEC and HPAEC-PAD. α-Amylase may be responsible for the decreased amylopectin fraction, decreased apparent amylose content, and increased amounts of low molecular weight saccharides in the yellowed rice flour. The increased amount of reducing sugars from starch hydrolysis promoted the interaction between starch and protein. The alkaline-soluble fraction during starch isolation is presumed to contribute to the difference in pasting, gelling, and thermal properties among whole, broken, and yellowed rice flours. The quality of rough rice (Oryza sativa L.) is determined by factors such as cracked grain, immature grain, discolored grain, damaged grain, red rice, and varietal purity (van Ruiten 1979). Rice is generally consumed as whole grains; thus head rice yield is a very important grain quality indicator. Head rice is defined as unbroken kernels of rice and broken kernels of rice that are at least three-fourths of an unbroken kernel (USDA 1983). Broken kernels are considered a result of stresses and strains developing within the kernel from moisture sorption, resulting in the formation of fissures and cracks and eventually breakage during milling (Kunze and Hall 1965; Kunze and Choudhary 1972, Siebenmorgen and Jindal 1986, Siebenmorgen et al 1992, 1998ab; Lloyd and Siebenmorgen 1999). Siebenmorgen et al (1998b, 1999) have demonstrated that the amount of breakage of milled rice kernels is influenced by relative humidity (RH), temperature of the exposure air, and kernel moisture content (MC) during the drying process. Milled rice is susceptible to rapid moisture transfer. Increasing air temperature produced a greater amount of broken kernels across the RH range and milled rice at the midrange RH conditions (50–70%) experienced minimal breakage. High MC rice was more susceptible to damage at low RH and low MC rice was more susceptible at high RH. Varietal effects did not play an important role in affecting the milled rice breakage. Postharvest discoloration of rice, commonly referred to as yellowing, is another major factor affecting rice quality. Rice yellowing is a serious problem in rice-producing countries with the introduction of high-yielding varieties and wet season harvests (de Padua 1985). The cause of yellowing has been identified as a combination of microbiological and chemical activities triggered by delaying in drying of harvested rough rice in the field and extremely humid environmental conditions that result in overheating of the grain before it is dried (Sahay and Gangopadhyay 1985; van Ruiten 1985). Factors responsible for yellowing include fungi or mold growth (Philips et al 1988, 1989), grain water activity, surrounding air temperature, oxygen, and carbon dioxide content (Bason et al 1990), and nonenzymatic browning reaction (Gras et al 1989). The underlying mechanism of rice yellowing, however, is yet to be uncovered. This study was conducted to compare the chemical structure and physicochemical properties of flours and starches of whole, broken, and yellowed rice kernels from a medium-grain cultivar. Because of the difficulty in collecting enough field samples under controlled environments for characterization, this study was designed to use controlled conditions that intentionally produced broken and yellowed rice kernels in the laboratory. MATERIALS AND METHODS Materials Bengal rough rice was obtained from the 1999 crop of the University of Arkansas Rice Research and Extension Center farm in Stuttgart, AR. Whole rice kernels were prepared by drying rice sample in a conditioning chamber controlled at 21°C and 50% RH and equilibrated until reaching the target moisture content (MC) of 12% (wb). Only head rice was used from the whole rice kernels for characterization after milling. Broken rice kernels were produced by drying rough rice at high temperature (60°C) and low RH (17%) until reaching 12% MC. Then the dried sample was immediately placed in a cold room (5°C) to induce breakage of rice kernels before milling. Yellowed rice kernels were produced by adjusting the MC of rough rice to 20% (wb) by spraying with the calculated amount of water and then covering the rice sample with aluminum foil before heating in a convection oven at 60°C for 72 hr. The conditions were chosen based on the results from Dillahunty et al (2001). Thereafter, the sample was dried to ≈12% MC in a 40°C convection oven. Each rice sample was prepared in duplicate. Samples were dehulled with a dehusker (THU-35, Satake, Tokyo, Japan) and polished for 30 sec in a friction mill (McGill Miller #2, Rapsco, Brookshire, TX). The conditions used to produce broken and yellowed rice samples resulted in every grain breaking during milling. Milled rice samples were ground into flours using a Udy cyclone sample mill with a 100-mesh sieve. Samples (10 g) were used for MC determination using an infrared moisture balance (model MB200, Ohaus, Florham Park, NJ). Crude protein of rice flour was measured according to Approved Method 46-13 (AACC 2000) and a factor of 5.75 was used to convert nitrogen values to protein values. Starch was isolated from milled rice flour according to a modified alkali steeping method (Yang et al 1984). Total starch and damaged starch were determined according to Approved Methods 76-13 and 76-31. Starch yield was calculated by dividing isolated starch with total starch on a dry weight basis. Apparent amylose content was determined by iodine colorimetry (Juliano et al 1981). 1 Department of Food Science, University of Arkansas, Fayetteville, AR 72704. 2 Corresponding author. E-mail: [email protected] Phone: 479-575-3871. Fax: