Type of Japanese Rice and Science of Starch


While researching the classification and use of rice flour, I realized that the type of rice is actually very important. In this article, I carefully explain the scientific knowledge of rice needed to classify rice flour.


Non-Glutinous and Glutinous Rice

There are two types of rice used in rice flour: non-glutious rice and glutinous rice. Non-glutious rice is called as Uruchi rice also. The texture of the rice is quite different because of the different types of starch contained in the rice.

Non-glutinous rice, Uruchi rice

粳米 in Kanji read as Uruchimai. Uruchi rice is the most common type of rice eaten in Japan.
The ratio of starch is amylose to amylopectin = 2:8.

Glutinous rice

糯米 in Kanji and read as Mochigome. Glutinous rice is a sticky rice used for making mochi, rice cakes.
It contains only amylopectin.

Let’s take a closer look at amylose and amylopectin.

How Starch is Formed

Starch is made up of many α-glucose molecules. Alpha glucose is a sweet-tasting substance.
Plants use solar energy to produce α-glucose from water and carbon dioxide. This process is called photosynthesis.
Alpha glucose is an indispensable energy source for plants. For this reason, it is stored in the form of starch, which is made up of many connected α-glucose molecules. These starches are stored in potatoes, beans, and rice.
When we chew starchy foods well, they taste sweet. This is because an enzyme called amylase, which is found in saliva, breaks the bonds between the α-glucose in the starch so that the sweetness of the α-glucose itself can be tasted.

α-Starch and β-Starch

In plants, starch exists in the form of β-starch. This is where the starch particles are tightly stuck together. When we eat it as it is, it is difficult to digest and does not taste good.
When you add water to β-starch and heat it, it becomes α-starch. α-starch is in a state where the starch particles have absorbed water and loosened up. It can be digested by humans and tastes good.
This change from β-starch to α-starch is called α-formation or gelatinization.

Let’s think about it concretely.
When rice is cooked after thoroughly absorbing water, it becomes chewy. This is because the β-starch in the rice has been gelatinized and turned into α-starch.
One of the reasons why the core of the rice remains after cooking is because the water has not been absorbed long enough to gelatinize the center of the rice. In this case, adding a small amount of sake and steaming the rice thoroughly will help the rice to gelatinize to the center.
Even if the rice is cooked well, when it cools down, it will be dry and not as sweet as it should be. This is because the α-starch has reverted to β-starch due to the drop in temperature and the passage of time. By reheating it again, it will revert to alpha starch and become tasty.

Amylose and Amylopectin

There are two types of starch, amylose and amylopectin. Amylose and amylopectin differ in the way the α-glucose is connected: amylose is a straight chain of α-glucose, and amylopectin is a branching chain.

image figure of Amylose)left and Amylopectin(right)

Amylose and amylopectin differ not only in shape but also in properties.
In relation to cooking
The more amylopectin, the more glutinous the rice.
Amylopectin absorbs water more easily.
The more amylopectin there is, the more the rice becomes glutinous.


Copied title and URL