Calorimetric studies of microbial activities in relation to the water content of food.

Abstract

The water content of foods influences the reproduction, metabolic activity and the ability of the micro-organisms present to survive and it is closely related to food preservation. For practical analyses, the effects of the water on microbial activity are studied by modifying the water effect by adding salts or sugars to the foods. But with such usual methods it is difficult to separate the effects on micro-organisms of salts, sugars, moisture and osmotic pressure. In this study, the water content was controlled by varying the ambient humidity.Many calorimetric studies of microbial activity have been reported from various view points. But so far there are no reports of the relationship between the metabolic activity of micro-organisms and the water content of foods. This paper introduces a method using calorimetry for studying the effect of water content on microbial activity.The test organisms used were Aspergillus oryzae IAM 2630 as one of the popular moulds and Bacillus cereus IFO 3131 as a typical spore-forming bacterium. Spores of both organisms were used as the starting microbes because they are in a dry, powdered state, so the water content of the food is not influenced by the inoculation. Active dry yeast (ADY), commercially available from Oriental Kobo Co., Ltd., Tokyo, was also used to study the effect of the water content on its endogenous metabolism. The test material used was skimmed-milk powder (Meiji Nyugyo Co., Ltd.). A study of a starchy sample took much time and showed small heat effect, so it is not described in this paper.A series of samples with selected water contents were prepared by placing

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