YAMAMOTO CO., LTD.

Heating

Heating and Gelatinization of Rice Starch

Starch extracted from rice kernel turns easily into a paste by heating with water for only a short time. However, starch inside the rice kernel requires a lot of energy to gelatinize. As Table 3 shows, rice must be kept at 98℃ for more than 20 minutes. The cells on the surface of cooked rice are destroyed, but other cells are sound and they just swell. Perfect gelatinization of starch inside the cell is desirable.

Three Steps of Heating

The heating process for cooking rice can be divided into three steps: Pre-boiling period, boiling period, and steaming period.

Pre-boiling period:
At the beginning of pre-boiling period, rice continues absorbing water. Gelatinization begins at about 65℃. If the temperature rises too fast to the boiling point, gelatinization is retarded because of the lack of water absorbed.
On the other hand, if it takes too much time to reach the boiling point, rice absorbs too much water and there is little water left when boiling starts. This condition is not desirable because it causes inadequate heating under the boiling period. Heating control is best if it takes 10~15 minutes to reach boiling. During this period, the taste of the rice might be improved by the action of an enzyme such as amylase.

Boiling period:
During boiling, it is necessary to raise heating and promote gelatinization of rice starch in violently boiling water.
To do this, it is necessary to have enough water, to enable the free movement of rice kernels in the cooker. Rice changes into cooked rice during this process. It is desirable to maintain the boiling condition for about five minutes.
It is said that grains of well-cooked rice are erect in the pot. This is because the steam vigorously moving up from the bottom of the pot to the surface seems to make rice grains become erect.

Steaming point:
In the end, as the rest of the water is absorbed by rice or evaporates, heating should be lowered and the rice kept in steaming condition. The temperature in the cooker is kept above 98℃ even during the steaming period.
It is necessary to give more than 15 minutes for steaming from the beginning of boiling before turning off the heat.
Thus, if the temperature in the cooker is kept above 98℃ for several minutes after removing the heat, the condition of 98℃ for more than 20 minutes mentioned before can be satisfied.