[6-1015-A] Functional/Wellness Foods & Nutrition (2)

[6-1015-A-02] Impact of Crystallinity Change During In Vitro Digestion on Starch Digestibility of Microwave- and Steam-Cooked Black Rice

*Sukanya Thuengtung¹, Yoshitaka Matsushita², Yukiharu Ogawa¹ (1. Graduate School of Horticulture, Chiba University(Japan), 2. Research Network and Facility Services Division, National Institute for Materials Science (NIMS)(Japan))

Keywords:Black rice, Crystallinity, Microwave-cooking, Steam-cooking, Starch digestibility, X-ray diffraction

Rice is a major cereal crop that is consumed as staple food by around half of the world’s population. Rice can be distinguished by the color of grain’s pericarp layer, as pigmented and non-pigmented rice. Black pigmented rice has been long cultivated in Southeast Asian countries, and has received much attention due to its nutritional values. Carbohydrate is known as a major macronutrient of rice that could be accounted as starch content for 90%. Starch digestion is a complex process that occurs mainly in the small intestine. Several factors could influence the starch digestibility such as starch granule structure, particle size, and food processing. Consequently, the effect of microwave and steam-cooking on change of crystallinity during in vitro digestion, including their impact on starch digestibility of black rice were examined. Influence of rice attribute on starch digestibility was also investigated. In this study, unpolished Thai black rice (cv. Hom Nin) was soaked at 10 ºC for 19 h before cooked by microwave and steam methods for 12 and 25 min, respectively. No white core inside cooked rice grain indicated full gelatinization. A portion of cooked black rice grain was ground to obtain homogenized slurry sample. Intact rice grain and homogenized slurry samples were then transferred into each in vitro reactor to start the simulated digestion. There were two digestion states included simulated gastric and simulated small intestinal digestion, which the supernatant was collected during these digestive states to analyze hydrolysis of starch. In addition, rice grain samples of uncooked rice, and cooked rice before and during in vitro digestion, were collected and prepared as the rice flour for analysis of X-ray diffraction (XRD) pattern. The results revealed that XRD pattern of uncooked black rice exhibited the diffraction peak at 15, 17, 18, and 23º (2θ), indicating A-type crystalline structure. However, XRD pattern was changed after cooking which diffraction peak was found at 13 and 20º (2θ), indicating V_h-type crystalline structure. Moreover, XRD pattern and the degree of crystallinity of steam-cooked rice were outstandingly changed after 360 and 480 min of simulated small intestinal digestion, comparing to microwave-cooked rice. The equilibrium starch hydrolysis (C_∞) percentage of steam-cooked rice showed lower than that of microwave-cooked rice as well. Besides, structure-less of homogenized slurry sample influenced faster starch hydrolysis rate when compared with intact grain sample. This study implied that rice attribute has an impact on starch hydrolysis rate of black rice, which different C_∞ between two cooking methods could be involved with change of the crystallinity degree during in vitro digestion.