E: Food Engineering & Physical Properties Physical Changes in White and Brown Rice during Simulated Gastric Digestion Fanbin Kong, Mecit Halil Oztop, R. Paul Singh, and Michael J. McCarthy Abstract: Previous studies have shown that brown rice generates lower glycemic responses than white rice, a trait that may be beneficial in the dietary management of chronic diseases such as diabetes and hyperlipidemia. The objective of this study was to investigate influence of rice digestion on the physical properties of the gastric digesta that may further impact intestinal absorption. A dynamic stomach model, human gastric simulator, was used to simulate the gastric digestion of white and brown rice. The pH, solids content, and rheological properties of the gastric digesta, as well as the size distribution of particles were studied. Static soaking was conducted to reveal the changes in moisture absorption and texture in rice kernels during simulated gastric digestion, as affected by shaking and the acid in gastric juice. Magnetic resonance imaging (MRI) was used to image the diffusion of gastric juice into the rice kernels. The results indicate that the bran layer on brown rice had a profound effect in digestion, as it inhibited the absorption of moisture and acid leading to decreased texture degradation, thus delaying the rice disintegration as well as dissolution and slowing emptying of solids. MRI is effective in exhibiting the diffusion of gastric juice as affected by gastric acid and the influence of bran. This study provided quantitative evidence regarding the manner in which structural differences between white and brown rice affect their gastric digestion. Keywords: diffusion, emptying, gastric digestion, magnetic resonance imaging (MRI), pH Practical Application: The study presented in this paper focuses on how the structural differences in white and brown rice affect their gastric digestion. This information may help consumers to better understand the health benefits associated with eating brown rice. Introduction Rice is the most important staple food for over half of the world’s population, providing the primary dietary source for carbohydrate. Brown rice is produced when only the outermost layer (hull) of the rice kernel is removed, while white rice is obtained after com- plete milling and polishing that removes the natural bran covering the rice. Brown rice not only provides 4 times the amount of insol- uble fiber found in white rice, it also contains a substantial amount of fat, vitamins, phytic acid, minerals, high-quality protein, and phenolic compounds (Tian and others 2004). Previous studies have shown that eating brown rice generates lower glycemic response than white rice (Snow and O’Dea 1981; Leonora and others 2006; Sun and others 2010), a trait that may be beneficial in the dietary management of chronic diseases such as diabetes and hyperlipidemia. For example, an in vivo study has shown that replacing one-third serving per day of white rice with the same amount of brown rice could lead to 16% lower risk of type 2 diabetes (Sun and others 2010). This effect was partly attributed to the higher amounts of polyphenols, phytic acid, and MS 20110071 Submitted 1/18/2011, Accepted 5/12/2011. Authors are with Dept. of Biological and Agricultural Engineering, Univ. of California, Davis, CA 95616, U.S.A. Author Singh is also with Riddet Inst., Massey Univ., Palmerston North, New Zealand. Author McCarthy is also with Dept. of Food Science and Technology, Univ. of California, Davis, CA 95616, U.S.A. Direct inquiries to author Singh (E-mail: rpsingh@ucdavis.edu). oil in brown rice compared to white rice. More importantly, dietary fiber-rich bran fraction of brown rice may serve as a barrier to digestive enzyme action, and further leads to reduced rates of stomach emptying and starch hydrolysis (Leonora and others 2006) contributing to a reduced glucose response. Additionally, when brown rice was ground, it hydrolyzed at the same rate as ground white rice despite the presence of the same amount of fiber (Snow and O’Dea 1981), indicating the physical form of bran may play an important role in regulating postprandial glycaemia. Extensive research on rice digestion has been reported in lit- erature using both in vivo and in vitro trials. In vivo studies have focused on the effect of rice on the glucose and insulin response as well as chronic disease such as diabetes (Leonora and others 2006; Sun and others 2010). In vitro trials investigated the starch digestion rate using pancreatic enzymes, emphasizing the role of dietary fiber (Han and others 2008). For example, Leonora and others (2006) tested the starch hydrolysis in the rice and found that the total sugar released by brown rice in vitro was 23.7% lower than in white rice. The study presented in this paper focuses on the physical changes of white and brown rice in human stom- ach and the effect on gastric fluid properties. This information is currently limited in the literature. Specifically, we investigated gastric pH, solids content, viscosity, and particle size distribution in the gastric digesta resulting from rice digestion using the human gastric simulator (HGS), a stomach model we have recently de- veloped (Kong and Singh 2010). The moisture absorption and texture of rice during digestion were studied by soaking rice samples in gastric juice, under different pH and mixing settings. C  2011 Institute of Food Technologists R  E450 Journal of Food Science  Vol. 76, Nr. 6, 2011 doi: 10.1111/j.1750-3841.2011.02271.x Further reproduction without permission is prohibited