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Cereal Chem. 82(4):468-474The role of proteins in the pasting and cooking properties of nonparboiled (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 experiment. 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 stickiness 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. Rice (Oryzasativa L.) is one of the most important food crops. Starch is the major constituent of the rice kernel. The most abundant minor constituents in milled rice are proteins (4.5-10%) and lipids (0.5%). Initially, amylose content (Juliano et al 1965), and later insoluble amylose content and amylopectin fine structure (Radhika Reddy et al 1993; Ramesh et al 1999) were identified as the most important factors determining the properties of cooked nonparboiled rice (npb) rice. Rheological studies showed that starch granules of rice with high apparent amylose content are strong and rigid, while those of low-amylose rice are weak and fragile (Sandhya Rani and Bhattacharya 1985, 1989). This led to the conclusion that rice cultivars with strong and rigid starch granules result in hard and nonsticky cooked rice, while cultivars with weak granules result in soft and sticky cooked rice (Radhika Reddy et al 1994). However, the difference in amylograph viscosity between rice flour and rice starch (Singh et al 2000) suggests that rice components other than amylose (such as proteins and lipids) also affect starch granule swelling and rigidity. The Brabender Visco Amylograph and the Rapid Visco Analyser (RVA) were used earlier to study the effect of proteins on the pasting properties of rice flour (Hamaker and Griffin 1990; Martin and Fitzgerald 2002; Meadows 2002). Viscosity decreased along all the points of the curve when rice flour was treated with a reducing agent to break disulfide bridges in the proteins. Adding a protease during the viscosity measurements (Hamaker and Griffin 1990) and removing proteins with a protease before the analysis (Martin and Fitzgerald 2002) had the same effect...

The effects of the soaking and steaming steps in rice parboiling on color changes and the levels of reducing sugars in rice were studied. Brown rice was soaked to different moisture contents (MC, 15, 20, 25, and 30%). The L*, a*, b* color parameters of the Commission Internationale de L'Eclairage (CIE 1976) indicated that during soaking, red and yellow bran pigments diffused from the bran into the endosperm. The increase in brightness brought about by soaking rice was attributed to migration of rice compounds (e.g., lipids) from the inner to the outer bran layers (rice surface). The levels of reducing sugars in brown and milled soaked rice samples increased with increasing brown rice MC after soaking. The total color difference (ΔE) between parboiled and nonparboiled rice increased with increasing MC after soaking and depended on the intensity of the steaming conditions as reflected in the degree of starch gelatinization. Parboiling affected yellowness more than redness in mildly steamed brown rice and most in intermediately steamed brown rice. Severe steaming of brown rice affected redness more than yellowness. All three parboiling conditions equally affected the yellow color more than the red color in milled rice. Linear regression analyses indicated that parboiling had a larger effect on ΔE of milled parboiled rice than of brown parboiled rice. Furthermore, the linear relationship between the level of gelatinized starch and ΔE of the milled parboiled rice samples showed that both parameters are indicators for the degree of parboiling. Reducing sugars were formed and lost during steaming, suggesting Maillard reactions during steaming.

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