39B-26 |
Interactions between phytate and protein coagulation on tofu quality as affected by storage of soybean |
H. J. D. HOU and K. C. S. CHANG. Food and Nutrition, North Dakota State University, P.O. Box 5057, Fargo, ND 58105 Phytate is a naturally occurring compound, and is the major form of phosphorus (70%-80%) in soybean. It comprises 1.0-1.5% of seed by dry weight basis. Phytate has strong chelating ability with di- and trivalent cations to form complex. Phytate can be degraded by phytase, which is found in most phytate-containing seeds. Tofu quality decreases gradually as soybean stored in adverse conditions. However, the mechanisms of how stored soybeans affect tofu quality have not been completely understood. We hypothesize that phytate may be hydrolyzed by phytase during storage and lost its ability to chelate cations, and subsequently influenced protein coagulation in tofu processing. Our objective was to investigate the effect of interactions between phytate and protein coagulation on tofu quality during storage of soybeans. Four conditions, 84% RH 30 C, 57% RH 20 °C, 4 °C cool, and warehouse, were applied. Soybeans in 84% RH were stored for up to 9 months and sampled at 30 days intervals. Soybeans in other conditions were stored for up to 18 months and sampled at 3 months intervals. Control soybeans were stored in -18 °C. Phytate was extracted and analyzed by anion exchange column chromatography. Tofu was made by small-scale method developed in our lab. Yield, texture, and color of tofu were evaluated. The quality of tofu made from soybeans in 84% RH, 30 °C decreased significantly (p<0.01). Phytate content of soybeans in all conditions showed a decreasing tendency with storage time. There were significant relationships between phytate content and yield (r=0.90) and between phytate content and tofu firmness (r=-0.79). The result showed that hydrolysis of phytate occurred during storage and subsequently affected Ca++-induced protein coagulation due to the loss of chelating ability. This study has increased our understanding of the mechanisms of how stored soybeans gradually lost their qualities for tofu making.
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