Effects of Specific Volume of Air Accompanying Rice Flour on Porous Structure of Fermented and Steamed Rice Bread

Abstract

Specific volume of air accompanying rice flours with different particle sizes was measured, and its effects on porous structure of fermented bread made from rice flours of various particle sizes were investigated. Rice flour was divided in order of the particle size into 5 classes; 208-295μm, 147-208μm, 104-147μm, 61-104μm and below 61μm. The amounts of accompanying air of these 5 classes were determined by measuring apparent density; 0.8, 1.8, 1.9, 2.4 and 1.0cm³/kg-D, respectively. The amounts of accompanying air of 4 classes except the class of below 61μm were a few times larger than the volume of micropores (smaller than 30nm in diameter) inside the rice flour particles. Each fraction of sieved rice flours, mixed with water, sugar and yeast was fermented and steamed to rice breads. The loaf volume of 5 rice breads were about 1.2cm³/g. The pore size of porous structure inside rice breads of 4 classes except the class of 208-295μm, measured by microscopic observation, showed approximately logarithmic normal distribution. The amounts of accompanying air of the rice flour had a positive correlation (r=0.999) with pore number, and a negative correlation (r=-0.985) with median diameter of pores inside rice breads of 4 classes except the class of 208-295μm. Any correlation was not detected with the standard deviation of pore size distribution. It became clear that the air accompanying flour particles had an effect on the porous structure, especially on the pore number, of steamed goods prepared through the process of fermentation.