Sanitary Effect of Heating Process of Bean Curd Packed in Hi-Zex Bags

Abstract

Bacterial contamination occurs very frequently during the conventional manufacturing process of tofu, or soy bean curd, a favorite food for the Japanese. Recently, tofu has come to be sold in package prepared with Hi-Zex. Since packaged tofu is heated at the final step of the manufacturing process, the sanitary quality of this food can be greatly improved.
At the end of boiling at 90°C for 40 minutes, it was found that the central part of a packed piece of tofu had been maintained at 73°C, or a safetyindicating temperature, for 20 minutes, and that 1ess than 300 1iving bacteria, including no coli-form ones, were contained in 1g of tofu in a package.
When a specimen of tofu was allowed to stand at room temperature (30°C at the highest) for 24 hours, it showed a living bacterial count of 10³, containing no coli-form organisms in it. When another specimen was allowed to stand at room temperature for 15 minutes immediately after the process of heating, it remained at a level higher than 73°C for more than 30 minutes. Accordingly, the process of heating is satisfactory from a sanitary point of view.
Packaged tofu had a heat-conducting index of 0.083. Then the temperature at the central part of a piece of tofu could be calculated from the temperature and time of heating by using the following formula: T=T∞(1-e^-kt).