59D-5 |
Evaluation of protein denaturation in rice subjected to varying drying and tempering conditions |
Z. Y. JU, N. S. Hettiarachchy, T. J. Siebenmorgen, and A. Cnossen. Dept. of Food Science, Univ. of Arkansas, College of Agriculture, Fayetteville, AR 72704
Environmental conditions during drying, milling and storage affect rice quality by influencing functionality of rice components. Rice proteins contain albumin, globulin, glutelin and prolamin. Glutelin (83%) is predominant in rice protein and its structure and functionality plays a key role in rice quality. Rice drying may alter protein structure by denaturation. Protein denaturation increases protein hydrophobicity. Protein hydrophobicity can be used to evaluate rice protein denaturation. Research objective was to evaluate effects of drying and tempering conditions on denaturation of rice proteins. Harvested rough rice (Cypress variety) containing 18% moisture were dried at 60°C (RH=50%) to remove 4% and 5% moisture, respectively. One half of the samples were tempered for 3 h and another half were not tempered. Control samples were dried at 21°C and not tempered. The whole rice was milled to obtain bran and flour. Glutelin was extracted from each sample with 0.02-M NaOH (pH 9.5), precipitated at pH 4.8 and freeze-dried. Surface hydrophobicity (S0) of glutelin was determined using a spectrofluorometer with anilino-8-naphthalene sulfonate as fluorescence probe. Results showed that hydrophobicities of control flour (S0=857.2) and bran (S0=716.2) samples were much lower than high-temperature dried flour (S0=1561.6-2160.6) and bran (S0=2084.0-4206.9) samples. Hydrophobicities of 4% tempered (S0=2044.5) and tempered 5% (S0=2160.6) flour samples were higher than 4% (S0=1561.6) and 5% (S0=1695.7) not tempered flour samples. 4% and 5% tempered and not tempered bran samples had similar effect. High-temperature drying and tempering did not influence total protein contents of flour (8.4%) and rice bran (13.5%), but slightly lowed moisture contents of flour (8.5%) and bran (4.5%) than their controls (8.8% and 4.9%). The hydrophobicity data suggests that high drying temperature (60°C, RH=60%) and tempering (3 h) can denature rice and rice bran proteins. Therefore, gentle drying is recommended to maintain rice quality.
Session 59D, Food Chemistry: Proteins and Physicochemical Properties
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