Microscopic observation showed that a group of small air cells entrained during the early stage of mixing is the original cause of cell structure of bread. At the beginning of fermentation, about 3 × 108/m2 gas cells with diameters between 3 × 10−6 and 8 × 10−4 m were entrained in the dough. The distribution curve of cell size was approximately normal on a logarithmic scale. During fermentation and proofing, a great portion of carbon dioxide was released into cells larger than about 10−4 m in diameter that was equivalent to a few percentages of total number of gas cells. After baking, gas cells smaller than 10−4 m in diameter were not observed and the total number of cells in baked bread reduced to about 106/m2 with diameters between 10−4 and about 5 × 10−3 m. The critical cell size to expand generally agreed with the calculated value using an equation, rc’= 3s/E (re': critical radius to expand, s: surface tension, E: elasticity), and cited value of s and E.

Changes in electrical and rheological properties in freezing-thawing were measured and compared for various food materials, which could be classified into three groups. In the first group, plant tissue (vegetables and fruits) showed large and frequency-dependent impedance and clear characteristic Cole-Cole arc in their fresh state. These electrical properties drastically changed after freezing-thawing: impedance greatly decreased and the Cole-Cole arc disappeared, suggesting the serious change in the cell structure. Correspondingly, the dynamic elasticity and viscosity substantially decreased. In the second group, animal and fish meats showed the lower impedance and unclear Cole-Cole arc as compared with plant tissue, but these properties scarcely changed after freezing-thawing. Rheological properties were also unchanged reflecting the large freezing-tolerance for food materials in this group. In the third group, konnyaku (alimentary yam paste) and tofu (soyprotein curd) showed increase both in impedance and viscoelasticity after freezing-thawing reflecting the freezedenaturation of the major components.

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The volume of air accompanying flour particles was determined through the measurement of the apparent density and compared with the volume of micropores, smaller than 30 nm in diameter, of the flour particles, which was measured by the Cranston-Inkley method. Weak wheat flour had the largest amount of accompanying air, strong wheat flour an intermediate amount, and rice flour the least. Most of the air accompanying the wheat flours was held in pores larger than 30 nm in diameter, while most of the air accompanying the rice flour was held in pores smaller than 30 nm in diameter. From the viewpoint of the expansion of dough, the condition of the wheat flours was much better than that of the rice flour.