Why Use Steamed Rice to Make Sake? - Learn About the Reasons, Tools, and History

While the rice we eat daily is typically boiled, sake brewing calls for steamed rice—a process known in Japanese as jokyo. This steaming step plays a vital role in the production of sake, affecting not only the texture and enzymatic readiness of the rice, but also the overall quality of the final brew.

Steamed rice is essential at multiple stages of sake production. It serves as the foundation for koji (rice inoculated with mold spores), is added to the yeast starter (shubo), and later forms part of the main fermentation mash (moromi). The quality of the steamed rice directly influences each of these steps, making jokyo a critical point of craftsmanship in sake brewing.

In this article, we will examine the purpose of rice steaming, explore the tools and techniques involved, and trace the historical development of jokyo in the evolution of sake production.

Purpose of steaming

Why is steaming rice—rather than boiling it—essential in the production of sake? The answer lies in the critical role of microorganisms, particularly koji mold, which drive the fermentation process.

The primary objective of steaming is to create a physical and chemical environment that allows koji mold to propagate efficiently and for the enzymes it produces to act effectively on the rice. Raw rice starch is composed of tightly packed glucose chains arranged in a crystalline structure. In this state, the starch is resistant to enzymatic breakdown because the enzymes cannot penetrate the dense matrix of glucose molecules.

To resolve this, the rice is first soaked in water and then steamed. The heat and moisture disrupt the crystalline structure, loosening the glucose chains and creating spaces that enzymes can access—an essential transformation known as the alpha conversion (α-conversion) of starch. This process renders the starch more digestible for enzymatic activity, making it possible to convert it into fermentable sugars during sake production.

In sake brewing, ideal steamed rice is characterized by complete alpha conversion while maintaining structural integrity and avoiding surface stickiness. The grains should be evenly separated, with a firm outer texture that holds their shape and a soft, pliable interior that allows for enzymatic penetration—an ideal balance for both koji development and fermentation.

Unlike cooked rice, which typically contains 60–70% moisture, steamed rice used in sake production is adjusted to a lower moisture content—usually around 30–40%. This range is considered optimal for the propagation of koji mold, which requires a carefully balanced environment to thrive.

The hardness and moisture content of the steamed rice play a crucial role in its solubility during both koji propagation and the moromi (main mash) fermentation stage. If the rice contains too much moisture, it dissolves too quickly, making it difficult to control the fermentation process. Managing the moisture content of steamed rice is therefore critical, and is closely linked to the soaking process that precedes steaming.

In addition to enabling enzymatic saccharification, steaming rice plays a crucial role in protein control, which directly influences the flavor and quality of sake. Amino acids—responsible for umami, body, bitterness, and occasionally off-flavors—are generated when enzymes from koji mold break down rice proteins. Rice primarily contains two types of proteins: glutelin and prolamin. Glutelin, the dominant protein in rice, becomes less susceptible to enzymatic degradation once it is denatured by heat.

When proteins are moderately denatured during steaming, enzymatic hydrolysis produces an optimal amount of amino acids, contributing to a sake with richness and balance, and minimal off-flavors. However, excessive heat treatment reduces the availability of protein for enzymatic action, leading to fewer amino acids and a flatter, more one-dimensional flavor profile.

Steaming also serves a vital sterilization function. It eliminates microorganisms present on raw rice, reducing the risk of contamination by undesirable molds and bacteria. Additionally, steamed rice supports stable temperature control during fermentation, limits the intrusion of wild yeast during yeast starter preparation, and helps prevent spoilage by lactic acid bacteria during the fermentation process.

Tools for steaming

Traditionally, rice was steamed using a large wooden or metal vessel known as a koshiki. In contemporary brewing, however, this has largely been replaced by the use of continuous rice steamers, which allow for greater efficiency, consistency, and control in large-scale production settings.

Koshiki

A koshiki is a large, barrel-shaped steamer—resembling a traditional steaming basket—with perforations at the base to allow for even steam distribution. It is typically made of wood or metal, with cedar being the traditional material of choice due to its insulating properties and subtle aromatic contribution. The heat source is generally a burner or boiler, although electric models have become more common in recent years.

Historically, steam was produced using a wagama, a traditional Japanese iron kettle placed over an open flame. However, the number of artisans capable of crafting these kettles has declined significantly, and with them, the number of breweries still using this traditional method has also dwindled.

When using a traditional steamer, achieving even steaming is paramount. Uneven heat distribution can lead to inconsistencies in the steamed rice, which in turn affects the quality of the final sake.

To ensure even steam dispersion, brewers begin by placing koma—wooden pieces with irregular surfaces—over the steamer’s perforated base. A slatted wooden board known as sana is then placed on top, followed by a coarse-weave cloth made of hemp or heat-resistant synthetic fiber to line the base. This setup allows for effective steam circulation while supporting the rice above.

Condensation inside the steamer can lead to a phenomenon known as koshiki-hada or hada-meshi, in which water droplets soften the rice in localized areas, creating undesirable texture. To prevent this, brewers may moisten the inner walls of the steamer, use simulated rice or cloth, and take other steps to avoid direct steam contact with the soaked rice.

When loading the steamer, it's essential to ensure that steam can escape evenly. Two main approaches are used. One involves pre-loading the entire quantity of soaked rice before steaming begins. The other, known as the nukegake method, involves gradually adding rice in stages while allowing steam to rise. This latter method often yields rice with superior texture, but it comes with drawbacks: a higher risk of burns, and it is labor-intensive—especially challenging for small breweries with limited staff.

As a result, many breweries prefer to pre-load rice the day before steaming, though this practice carries risks during warmer seasons when bacterial growth can occur if conditions are not carefully managed.

Once the rice is in place, the steamer is covered with a heavy cloth—typically canvas or a similar material—and the rice is steamed. The process takes approximately 40 to 60 minutes. Toward the end, high-temperature dry steam (102–103°C / 215.6–217.4°F) is applied to ensure a firm, even texture ideal for sake brewing.

While traditional steamers are well-suited to small batches and involve relatively low initial investment, they require significant time and labor, and carry the risk of uneven steaming. However, this variability can also be turned to advantage—particularly in ginjo-style production—by allowing different types of rice, such as koji rice and kake rice, to be steamed simultaneously under slightly different conditions. In this sense, the traditional steamer remains a flexible and invaluable tool in the hands of skilled brewers.

Continuous rice steamer

A continuous rice steamer is an automated machine designed to steam rice in large quantities by continuously moving it through a steam-filled chamber. There are two main types: the belt conveyor type and the vertical type.

In the belt conveyor type, soaked rice is evenly spread across a conveyor belt that moves horizontally through the steamer while steam is applied from below. This allows for high-volume, continuous production with consistent results. The vertical type, by contrast, features a tall cylindrical chamber into which soaked rice is continuously fed from the top. Steam is introduced from the bottom, and the rice moves downward through the chamber, enabling either fully continuous production or a steady, rhythmic output.

While continuous rice steamers offer the advantage of uniform steaming at scale, they require careful operation—especially when producing small batches—to avoid uneven steaming or overcooling during the post-steaming cooling phase. High installation costs and the need for considerable floor space are also cited as disadvantages, making them more suitable for larger-scale breweries.

Cooling steamed rice

After steaming, the rice is cooled to a temperature appropriate for its intended use. Typically, the target temperature is highest for rice used in koji production, followed by rice for the yeast starter (shubo), then the first addition (hatsu-zoe) and final addition (tome-zoe) of the main mash (moromi).

Steamed rice is cooled either by forced cooling using a rice cooling machine or by natural air cooling. Forced cooling is most commonly performed using a continuous steamed rice cooler, in which the steamed rice is placed on a wire mesh conveyor and moved while air is blown over it. This allows for rapid cooling within a short period of time. As a result, the system can accommodate a variety of conditions, including warm climates, seasons, and times of day.

In contrast, natural cooling involves spreading steamed rice over cloth laid atop traditional materials such as mushiro (woven straw mats) or sunoko (bamboo slats), allowing it to cool gradually using ambient air. This method is commonly used in the production of koji rice or in colder regions. One of its advantages is that the rice cools slowly and evenly, allowing the temperature to become more uniform from the surface to the core.

However, in the case of ginjo-style brewing, extended cooling times—often taking several hours—can cause starch degradation. This, in turn, may reduce the effectiveness of the enzymes produced by koji mold, making enzymatic conversion less efficient.

Once the rice has cooled, it is transported to the koji room or fermentation tanks using conveyors, air shooters, or by hand.

History of steaming

As we have seen, steamed rice plays a vital role in sake brewing. While modern production often relies on efficient machinery, the process of rice steaming (jokyo) has deep historical roots and has undergone significant evolution over time.

One key reason steamed rice is used in sake making is that steaming was the traditional method of rice preparation in Japan. Until around the Heian period (794–1185), rice for everyday consumption was not boiled as it is today, but rather steamed in large vessels called koshiki, producing a firmer style of rice known as kowa-meshi. Archaeological findings confirm that koshiki were already in use by the Nara period (710–794), indicating that steamed rice was a well-established part of daily life.

Further evidence comes from the Harima no Kuni Fudoki (The Topography of Harima Province), compiled in 716 during the early Nara period. The text contains one of the earliest references to sake brewing using rice koji, which is considered the prototype of modern sake. It describes how steamed kowa-meshi offered to a deity dried out and became moldy, leading to the accidental creation of alcohol: the people then used it to brew sake and held a celebratory feast. This account provides one of the earliest documented connections between steamed rice and sake production in Japan.

In the Edo period, illustrations in Settsu Meisho Zue (Famous Places of Settsu, 1796–1798) depict the sake brewing process, including scenes of rice being steamed in a traditional koshiki.

Significant changes in rice steaming methods began to appear in the Meiji period. In 1892 (Meiji 25), Den Shichi Itō, a sake brewer from Mie Prefecture, introduced a boiler as a heat source for rice steaming. By 1902 (Meiji 35), the Seishu Jozoho (The Method of Sake Brewing), compiled by the sake brewing research institute in Oyama Town, Yamagata Prefecture, documented the use of the nukegake method—an early step toward more controlled and efficient steaming.

In the Taisho era, further records of this method appear in a 1917 brewing manual from the Marugame Tax Bureau and in Nihon Jozoho (The Brewing Methods of Japan, 1919) published by the Osaka Finance Research Association, indicating that nukegake had become a common practice by that time.

Following World War II, during Japan’s period of rapid economic growth, the mechanization of rice steaming advanced significantly. In 1957 (Showa 32), a steamed rice cooling machine was introduced and exhibited at the All-Japan Brewing Equipment Exhibition, quickly gaining nationwide adoption. In 1962 (Showa 37), the Okura Sake Research Laboratory (now Gekkeikan Co., Ltd.) developed a conveyor-type continuous rice steamer, which entered full-scale industrial use. Later, in 1971 (Showa 46), Hideo Aya developed a vertical continuous rice steamer.

However, as brewing volumes expanded and mechanization progressed in the 1950s and beyond, traditional Japanese kettles (wagama) were rapidly replaced by these modern machines. By the early 1970s, the production of large wagama had ceased, marking the end of an era and the near-total disappearance of traditional steaming methods.

Summary

For over a thousand years—from ancient times through the Edo period—rice in Japan was steamed using koshiki and traditional wagama, with firewood as the heat source. This long-standing method endured until the waves of modernization and postwar economic growth swept the nation. Even a single step like rice steaming reflects the remarkable pace of mechanization and efficiency-driven innovation in sake production.

Today, most breweries rely on modern equipment such as continuous rice steamers and automated cooling systems. Yet, some continue to brew using wooden koshiki and natural cooling methods. The choice of tools and techniques—each with its own strengths and limitations—often reflects a brewery’s particular philosophy and commitment to tradition or innovation.

References

• Brewing Society of Japan (Ed.). Revised and Expanded New Edition of Sake Brewing Techniques (Brewing Society of Japan, 2009)
• Brewing Society of Japan (Ed.). Latest Sake Brewing Manual (Brewing Society of Japan, 2007)
• Brewing Society of Japan (Ed.). Sake Brewing Textbook (Brewing Society of Japan, 2009)
• Central Union of Sake Brewers of Japan (Ed.). Introductory Course Textbook on Product Manufacturing Knowledge (Central Union of Sake Brewers of Japan, 2020)
• Keiichiro Katsuki, Katsuki Column, Episode 8: Steamed Rice (Accessed: September 26, 2024)
• Yuichi Akiyama, “IV. Steaming,” Journal of the Brewing Society of Japan, Vol. 70, No. 4, 1975
• National Tax Agency, Survey Report on Traditional Japanese Sake Brewing Using Koji Mold (2021)
• Norihiro Okuda, “On the Brewing Machinery Industry,” Journal of the Brewing Society of Japan, Vol. 70, No. 12, 1975
• Editorial Department of the Journal of the Brewing Society of Japan, “Research Achievements in Alcoholic Beverages in Fiscal Year 1971,” Journal of the Brewing Society of Japan, Vol. 67, No. 4, 1972
• Masaru Yamashita, “Traditional Sake Steaming Kettles: Large wagama and Sanshūkama,” Journal of the Brewing Society of Japan, Vol. 98, No. 3, 2003