Thermal and physicochemical properties of rice grain, flour and starch.

National Food Research Institute, Kannondai 2-1-2, Tsukuba, 305-8642 Japan.
Journal of Agricultural and Food Chemistry (Impact Factor: 3.11). 08/2000; 48(7):2639-47. DOI: 10.1021/jf990374f
Source: PubMed

ABSTRACT Three types of rices, namely, Thailand rice (Indica), Nipponbare (Japonica), and Himenomochi (Japonica waxy), in grain, flour, and starch forms have been studied for their thermal and physicochemical properties. In grain form, Indica was slender and Japonica rices were bold and thick. Indica had the highest protein and amylose equivalent. Protein contents in isolated starches varied from 0.2 to 0.9%. Cooked Indica grain was hardest and waxy rice was softest; stickiness was highest in Japonica rice. Glass transition temperature (T(g)) was highest in Indica rice flour (approximately 222 degrees C) and almost the same in Japonica rice flours. Melting point was highest for Japonica (approximately 264 degrees C) and almost the same for Japonica waxy and Indica rice flours. T(g) values of starches were almost the same in Indica and Japonica waxy (approximately 237 degrees C); defatting caused reduction in this property in all of the starches. Highest melting point was shown by Indica starch (approximately 276 degrees C) and was almost the same for the other two starches. Protein and fats play a critical role in glass transition and melting points of rice flours and their respective starches. Viscosities of the cooked pastes of flour and starch during cooking in an RVA instrument and their gel and other properties have been discussed.

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    ABSTRACT: Cereal Chem. 82(4):468-474 The role of proteins in the pasting and cooking properties of non- parboiled (npb) and parboiled (pb) rice was tested by means of a reducing agent dithiothreitol (DTT) and a protease (trypsin). DTT increased the swelling power and carbohydrate leaching of flour from npb rice flour but decreased its amylose leaching. Although DTT slightly increased the Rapid Visco Analyser (RVA) viscosity at the initial stages of the pasting process, it decreased RVA viscosity in the further phases of the experi- ment. Preincubation of flour with a trypsin decreased RVA viscosity along the whole temperature profile. Addition of DTT to the cooking water decreased water absorption and rice hardness and increased leaching of solids during cooking and stickiness of the cooked npb rice. Addition of DTT to the cooking water of flour from pb rice increased swelling power, carbohydrate leaching, and amylose leaching. Addition of DTT also increased RVA viscosity. Preincubation with trypsin had a similar effect but the changes were less pronounced. Addition of DTT increased sticki- ness of cooked pb rice and increased water absorption and leaching of solids during cooking. Taken together, the results provide evidence for the existence of a protein barrier affecting starch swelling, rheological, and cooking properties of both npb and pb rice. Chrastil (1992) stated that the stickiness of cooked rice is correlated with the molecular weight of the oryzenin protein fraction as determined by the viscosity of isolated oryzenin. A considerable fraction of the world production of rice is parboiled. Parboiling is a process of steeping, heating, and drying the rice. It has a pronounced effect on the pasting properties of rice flour: the temperature of initial viscosity rise increases, the setback decreases, and the concentration needed to obtain an obser- vable breakdown increases (Ali and Bhattacharya 1980). Sticki- ness of cooked rice, the levels of water absorbed, and the levels of amylose leached during cooking in excess water are reduced by parboiling (Damir 1985). These changes in pasting and cooking properties can be interpreted as the result of reduced swelling and increased rigidity of the starch granules remnants of the pb rice. It was suggested that degradation of the structure of the starch granules during the heat treatment and retrogradation of the starch during the drying stage of the parboiling process are responsible for reduced starch swelling in pb rice (Ali and Bhattacharya 1980; Damir 1985). During parboiling, lipids form complexes with amy- lose molecules (Priestley 1976; Biliaderis et al 1993). These com- plexes also restrict the swelling and the solubilization of the starch during cooking (Gray and Schoch 1962). No research has been reported on the effect of proteins on the pasting properties of flour and on the cooking properties of kernels of pb rice. Parboiling drastically reduces the solubility of the rice protein (Dimopoulos and Muller 1972). Therefore, it is not unlikely that proteins in pb rice influence the pasting and cooking properties in a different way than in npb rice. The purpose of this study was to determine the role of proteins on the pasting and cooking properties of npb and pb rice by study- ing the effects of the reducing agent DTT and preincubation with protease (trypsin).
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