On the Formation of Cracks in Rice Kernels during Wetting and Drying of Paddies

This study was made to research the cause of crack formation in rice kernels during wetting and drying. 1. Moisture content of rice kernels during maturation. Changes of moisture content of rice kernel during maturation are divided into three periods. In the first period, moisture content decreases physiologically with the increase of dry weight of kernel and reaches about 28% when dry weight becomes the maximum value. In the second period, moisture content of approximately 28% lasts about 10 days without any appreciable change. In the third period, moisure content decreases physically under the influence of weather. (Fig. 1) Cracks do not occur in the first and the second periods because moisture content is not affected by weather condition in these periods, whereas cracks may occur in the field during third period by weather condition. 2. Crack formation during wetting. When paddies are soaked in water, cracks occur increasingly with the progress of soaking time until moisture content of kernel reaches about 20%, then cracks gradually decrease wiht the lapse of time and almost all cracks disappear until moisture content reaches 30%. (Table 2, 3) Because, endosperms moistened over about 20% become very flexible and cracks are cemented by the pressure resulted from enforced swelling of kernels in the chaffs. The moist kernels, above about 20% in moisture contents, do not crack by wetting, for they are too flexible to be checked by the stress and the immature kernels having the soft cells which are filled insufficiently with starch do not crack by the buffer action of such cells. It is, therefore, thought that the essential conditions of crack formation are: (1) the stress set up in the endosperm by the unequal swelling as a result of uneven absorption of water; (2) endosperm hardness which is hard enough to respond to the stress. When the stress is set up in the endosperm which is hard enough, crack is formed in parallel with the short axis of the kernel alons the cell arrangement, because this direction is most irresistible against the stress. 3. Crack formation during drying. During one-pass drying, decrease of moisture is very rapid in chaffs but is very slow in kernels during the first 60 minutes, and as the time elapses, kermels are enforced to dry and inclination of moisture contents occurs within kernel as indicated by the difference between the upper halves and the lower halves of kernels as shown in Fig. 5, hence cracks are increasingly formed. On the other hand, on five-pass drying, moisture moves slowly from kernels into chaffs during tempering and moistened chaffs dry rapidly but kernels are not enforced to dry during the succeeding drying. Accordingly, disturbances in moisture distribution do not occur in kernels, and crack formation is far than one-pass drying. The cause of crack formation during drying is the stress set up by the unequal shrinking of endosperm as the result of uneven dehydration. (Fig. 4, 5) 4. Processes of hydration and dehydration of kernels. The distribution of moisture within kernels was investigated during wetting and drying to study the mode of hydration and dehydration. Hardness of any specified point within endosperm increases or decreases linearly according to decrease or increase of its moisture content, consequently, the moisture content of some points in endosperm can be calculated by measuring the hardness of that point. (Fig. 7) Moisture distribution in endosperm during wetting and drying of paddy was measured by this method. (Fig. 8, 9) Water penetrates mainly from the region around the germ into the endosperm and advances to the middle and apical regions through the peripheral region of endosporm and penetrates slowly, at the same time, from the peripheral region to the central region. Water penetration through the attachment region as well as the testa is not remarkable. Then the ventral region that lies close to the germ is moistened more rapidly than dorsal region. The mode of