Effects of Rice Grain Hardness and Milling Conditions on Rice Flour Properties

Abstract

To expand rice consumption, there has been an increase in the utilization of rice flour in various food fields. Rice flour attributes are determined mainly by the properties of the raw material and the processing conditions, and have an influence on the manufactured products. For the application of rice flour as a wheat flour substitute, a small particle size and low damaged starch content are desirable; however, high starch damage can result from the milling of polished rice. Thus, in the production of fine rice flour, it is important to pulverize the grains to a particle size where the polished rice exhibits appropriate water absorption.In assessing various rice types for flour milling, detailed pulverization and processing tests are required. The production of fine rice flour is costly because optimal water absorption and drying are required. Therefore, in order to establish an efficient production method for fine rice flour, we examined the relationship between grain hardness and milling properties of rice and water absorption by modifying the structure of the raw material. The relationship between SKCS (single-kernel characterization system) hardness and the polishing property of 98 brown rice samples was determined; floury rice lines with a low polishing rate were characterized by a hardness of 40 or less. Brown rice samples with a hardness of greater than 55 were appropriate for polishing, and when milled (screen-less type of pin-mill) by limiting water absorption, exhibited a superior milling property of small particle size as the hardness value decreased. In such a case, the lower the hardness, the less the degree of starch damage of the rice flour. When the rice was completely absorbed with water or pulverized using a pin-mill that structurally passed through a mesh, the rice flour produced showed a specific range of median diameter and starch damage regardless of the hardness of the brown rice sample. In the case where the raw rice was crushed in advance and absorbed water, improved efficiency was observed; however, by limiting water absorption to about 25 %, it was possible to produce the target fine rice flour (starch damage degree 6 % or less, median diameter 75 µm or less). These results indicate that fine rice flour can be efficiently produced using a combination of rice varieties with excellent milling properties, crushing the rice grains prior to soaking and limiting the water absorption.