2002 Volume 53 Issue 2 Pages 169-174
Steamed glutinous rice often turns brown, and its subsequent appearance or when formed into rice cake is spoiled. When and how this phenomenon occurs has been unclear. Koganemochi and hiyokumochi varieties were examined to find the relationship between the browning of rice after steaming and subsequent storage, and between the browning and standing time after washing. Browning did not occur when raw rice was stored at 4°C, but marked browning of steamed rice was observed after raw rice had been stored at 37°C for 10-30 days and washed and left at 4°C for 7-9 days. In the raw rice which had turned brown after steaming, the amounts of reducing sugars and free amino acids initially increased and then decreased after reaching the maximum levels during standing. Browning occurred in the rice while these compounds were decreasing. When the raw rice had been soaked in water, the steamed rice did not show browning. In the soaked raw rice, neither reducing sugars nor free amino acids changed their contents. These results suggest that browning was related to an amino-carbonyl reaction among the reducing sugars and amino acids in the rice : when the amount of amino-carbonyl reaction compounds reached a certain level, steamed glutinous rice turned brown.
