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The equilibrium vapor pressure, the heat of vaporization, the dielectric increment, and the NMR spectra of partially dried cells were studied in Saccharomyces cerevisiae with water contents varying in the range from 25 to 0.8%. The comparative study of those physical properties suggests that physical states of the microbe can be classified into four regions in accordance with the states of the cell water: the solution region, the gel region, the mobile adsorption region, and the localized water region. Much difference in the physiological properties is found between the cells in the solution region and those in the gel region, whereas the pattern changes in physical properties take place when the cells in the gel region are dried to a further extent into the mobile or the localized region. The various modes in the molecular motion of the cell water reflected in those physical properties of the cell seem to give some insight into the biological functions of the molecule in the native as well as the dried states of the cell.

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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