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Heat Treatments of Milled Rice and Properties of the Flours

¹Akita Research Institute of Food and Brewing, 4-26, Sanuki, Arayamachi, Akita City, Akita, 010-1623, Japan. ²Corresponding author. Phone: +81-18-888-2004. Fax: +81-18-888-2008. E-mail: toru@arif.pref.akita.jp ³Faculty of Agriculture, Iwate University, 3-18-8, Ueda, Morioka City, Iwate, 020-8551, Japan.

The effects of autoclave and oven treatments on the gelatinization of rice flour and on the rheological characteristics of its pastes were studied by differential scanning calorimetry (DSC), rapid viscoanalysis (RVA), and rotational viscometry. Flours from autoclave-treated rice (ATR) and oven-treated rice (OTR) were prepared, respectively, by heating at 120°C for 60 min and 160°C for 60 min followed by drying (ATR sample), and grinding at 2.2–12.9% moisture content. The rice flour dispersions were adjusted between pH 6.3 and 2.8 using 0.2M citrate buffer. The retort processing of rice flour in water pastes were done at 120°C for 20 min either once or twice. The gelatinization peak temperature (PT and T_o) and the peak temperature corresponding to the amylose-lipid complexes (T_p3) of ATR increased at pH 6.3 and 2.8 compared with OTR and UTR flour. This indicates that the internal structures of the starch granules in ATR became more stable to heat and acid, even though the damaged starch content of ATR was 23% compared with 16 and 7%, respectively, for untreated rice flour (UTR) and OTR. The OTR flour pastes showed a gel-like behavior at pH 4.5 after retort processing in water at 120°C for 20 min; however, the ATR mixture behaved more like a liquid paste. Decreases in the reducing sugar content of OTR and ATR pastes suggested that enzymes in the heat-treated rice were denatured, which retarded the hydrolysis of glucose chains and the rupture of starch granules during pasting.