James Patindol a Ya-Jane Wang a Jay-lin Jane b a Department of Food Science, University of Arkansas, Fayetteville, AR, USA b Department of Food Science and Human Nutrition, Iowa State University, Ames, IA, USA Structure-Functionality Changes in Starch Following Rough Rice Storage The molecular-level features of starch in relation to the changes in rice functionality during storage are not yet fully elucidated. This work investigated the effects of rough rice storage conditions on starch fine structures and physicochemical properties. Dried rough rice samples (medium-grain Bengal and long-grain Cypress) were stored at 4, 21, and 387C in temperature-controlled chambers and then periodically removed and evaluated after 1, 3, 5, 7, and 9 months. Flour (powdered head rice) and starch (extracted from head rice by alkali steeping) samples were evaluated for pasting and thermal properties. High-performance size-exclusion chromatography and high-per- formance anion exchange chromatography were used to characterize starch molecu- lar size and amylopectin chain-length distribution, respectively. Significant changes in starch fine structure were observed primarily on the 387C lots, and to some extent on the 217C lots. The decreased amylose: amylopectin ratio, shortened amylopectin average chain length, and the shift in chain-length distribution to shorter branch chains were implicative of molecular-level starch degradation. The flour and starch samples showed inconsistent trends in pasting and thermal properties, thus suggesting the role of not only starch but also its interaction with non-starch components in rice aging. Keywords: Amylopectin; Amylose; Fine structure; Rice starch; Storage and aging 1 Introduction In the United States and other temperate rice growing countries, rice is grown once a year, harvested within a short period, but used year-round. Hence, part of the rice needs to be stored for an extended duration to provide a year-round supply before it is processed for specific end- uses. Stored rough rice is subject to a variety of physical, chemical, and biological changes, which are collectively termed as aging. Aging generally results in a higher head rice yield on milling [1–3], higher volume expansion and water absorption upon cooking [1–2, 4–7], and harder, less sticky cooked rice [4, 6, 8–12]. The changes in cooked rice texture associated with aging are enhanced by high storage temperatures [1, 4, 10–12]. The pasting and gelatinization properties of rice flour or starch pastes also change following storage. Amylograph peak and final viscosities tend to increase after a few months of storage [1, 8, 13, 14] but eventually decrease in the long term [15– 17]. These changes, that accompany rice storage, may be favorable to some processors/end-users but may be unfavorable to others, depending on the intended appli- cations and the ethnic background of the end-users. Despite the many studies on the subject, the exact mechanism of aging is still not clear and some findings have been contradictory. Moritaka and Yasumatsu [18] proposed an aging mechanism involving lipids, starch, and proteins. Lipids form free fatty acids, that can com- plex with amylose and long chains of amylopectin. Car- bonyl and hydroperoxide products of lipid oxidation can accelerate protein oxidation and condensation coupled with the accumulation of volatile carbonyl compounds [18]. Mod et al. [19] inferred that during storage, the fer- ulate esters of hemicellulose are oxidized, which leads to cross-linking and increased strength of cell walls and overall grain integrity. Chrastil [6, 7] hypothesized that many of the physicochemical and functional changes, that occur during storage, are caused by oryzenin-starch interactions. The molecular weight of oryzenin and its subunits increased substantially through disulfide bridge formation and the equilibrium binding ratio and binding constant between oryzenin and starch decrease appreci- ably, resulting in decreased rice stickiness [6, 7]. The foregoing suppositions simply reflect the complexity of the rice aging process. The functionality of rice has long been ascribed to starch because starch constitutes about 90% of milled rice on a dry weight basis. However, studies regarding the struc- tural changes of the starch molecule (amylose and amy- lopectin) following rough rice storage are limited. The fine Correspondence: Ya-Jane Wang, Department of Food Science, University of Arkansas, Fayetteville, AR 72704, USA. Phone: 1-1-479-575-3871, Fax: 11-479-575-6936, e-mail yjwang@ uark.edu. Starch/Stärke 57 (2005) 197–207 197  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.starch-journal.de Research Paper DOI 10.1002/star.200400367