Sake Brewing Science - vol. 3 - Steaming Methods

I realized that it is important to study sake brewing thoroughly, so I have planned to decipher sake from the perspective of brewing science.

Let’s start with the raw material: rice.

vol.3 - Steaming Methods

After being selected based on whether it is hard or soft, the rice is polished to the necessary amount. Next, it is allowed to absorb water and then steamed. Today, I’ll discuss how it’s steamed and the results that this process brings about.

The Difference Between Steaming and Cooking

Rice is meant to be cooked. Anything else is like sautéing and then boiling, as you would with pilaf. I believed this for a long time. I was also often told that rice is steamed in sake brewing because it becomes too mushy if cooked. I’ve also often heard that we steam it to achieve a texture that is "hard on the outside, soft on the inside."

I felt like I understood it somewhat, but wondered if I really grasped it. Are you perhaps thinking that if you just cook it with less water, it won't get mushy?

Steaming: Steam hits the rice, and heat enters from the outside in. Since the exterior cools down quickly due to latent heat of vaporization, it is difficult for heat to penetrate, and the moisture content on the outside is also low. Inside, heat becomes trapped, and gelatinization (α-transformation) progresses.

Cooking: Since the rice is constantly in contact with moisture, gelatinization also progresses quickly on the outside, resulting in rice that is soft throughout. Consequently, the moisture content is also higher.

That is the difference. The greatest advantage of steaming over cooking is that it prevents the exterior of the rice from containing too much heat and moisture. This affects the relationship with the koji mold and, naturally, influences the final quality of the sake.

What is Gelatinization (α-transformation)?

Grains cannot be consumed without adding a certain amount of moisture and heat. To be precise, you can eat them, but you can't derive calories from them. Why? Because the microorganisms in the intestines cannot break down the starch. More accurately, it is because the enzymes possessed by the microorganisms cannot access the starch.

The initial process of making the starch in grains accessible to the enzymes held by microorganisms is called gelatinization [α-transformation, as converting it from here on is tedious].

α-transformation is a reaction that requires moisture and heat.
The temperature required for α-transformation varies by grain, and rice is relatively high. There is a technique in beer brewing called "cereal mashing," where ingredients like rice, which have a higher α-transformation temperature than the mashing temperature (around 70°C), are first boiled or cooked before being mashed. This is because mashing cannot occur at a temperature lower than the α-transformation temperature.

I digressed.
The point is that the temperature at which each grain undergoes α-transformation is very important.

Why can't enzymes access starch without α-transformation?
In fact, the starch in grains is packed very tightly with few gaps. When you pour hot water on it, gaps form in the starch, changing its water-retention capacity. Enzymes can enter through these gaps where the moisture has gone.

You can't get chopsticks into a cup of instant noodles before the hot water, but after you pour it in, the chopsticks go right through, right? (Wait, what?)

Why "Hard Outside, Soft Inside" is Best

Instead of cooking, we intentionally steam it to suppress the moisture and heat content on the exterior. What is the reason for avoiding excessive moisture?

Easier for Koji Mold to Attach

Koji mold is aerobic, so it seeks oxygen. If there is too much moisture, it is like telling us humans to live in the ocean—it is difficult. Also, because koji mold is sprinkled as a powder, if there is high moisture on the outside, the rice grains will stick together, reducing the surface area and limiting the spread. Ultimately, it seems ill-suited for growing a large amount of koji mold. When considering koji, it appears crucial to create a state where the exterior moisture evaporates easily and dries out.
Koji Mold Doesn't Over-Penetrate the Shinpaku / Sake Becomes Complex and Savory

Koji mold extends its roots into the center of the rice, produces enzymes, and breaks down starch and proteins. At first glance, this seems suitable for brewing, but if the outside of the rice is soft and the mold reaches the center easily, the enzymatic reaction progresses "only on the shinpaku" (the starchy white core), which is composed almost entirely of starch. As a result, the abundant proteins on the outside of the rice are not easily broken down, and amino acids are reduced. Since amino acids are the source of umami, rice with a soft exterior tends to result in a flavor that is somewhat bland.
Fermentation Progresses Slowly

Similar to the reason above, because the rice is hard on the outside, it takes time for the koji mold to reach the shinpaku, so the speed at which sugar is supplied to the yeast is also slow. Consequently, fermentation is more gradual. As to whether slower fermentation is better, there are "yes" and "no" answers from various angles, but in terms of the quality of the sake, there are many benefits. It is known that fermentation that proceeds soundly in a low-temperature (which it tends to be) environment with few off-flavors results in fewer higher alcohols. Higher alcohols create a "sharp" aroma, which is a cause of that "sake smell."

I Felt That Steaming Skillfully is Very Important

Much like mashing in beer brewing, steaming is a critical step in itself, and I found it fascinating that there are so many scientific reasons hidden within it.

It seems that the results also change depending on the water quality used for steaming. While I think this is likely correct, I couldn't cover it fully because there has been little research and there are too many parameters, leaving me with a few "???" moments.

More on water next time.