Learn about Rice Washing and Soaking in Sake Brewing: Summary of Methods, Purposes, and Precautions

In the world of sake brewing, it’s now widely recognized that rice plays a central role. From media coverage to fan discussions, topics like polishing ratios frequently come up, and the image of freshly steamed rice has become an iconic symbol of the sake-making process.

Yet nestled between polishing and steaming are two lesser-known but critically important steps: senmai (washing) and shinseki (soaking). Washing is essential for removing the nuka or rice powder left behind after polishing, while soaking controls the rice’s water absorption—a key factor in determining the quality of the steamed rice and ultimately the koji. Though they may seem like straightforward tasks—washing rice and soaking it in water—these steps have a profound impact on the final character of the sake. In this article, we take a closer look at the purpose and techniques behind senmai and shinseki, two subtle but vital processes in sake brewing.

Purpose of Washing

The purpose of rice washing is to remove nuka—the fine powder left on the surface of the rice after polishing. Nuka consists of the outer layers and bran that were intended to be milled away, including fragments of the husk and endosperm.

While nuka is rich in nutrients such as proteins, fats, and vitamins—essentially a concentration of what makes rice nourishing—these same components can lead to unwanted flavors in sake. Left unchecked, they contribute to “zatsumi” (off-flavors or roughness), making the careful removal of nuka through washing a crucial step in achieving clean, well-balanced sake.

How to Wash Rice

Let’s take a closer look at how rice washing is carried out in sake brewing. Since sake rice is typically polished more extensively than table rice, the washing process differs significantly from how one might rinse rice at home before cooking.

Hand Washing: Tradition and Precision

The most traditional method is hand washing—sometimes even using the feet in the past. This labor-intensive process involves washing rice in cold water by hand, and while physically demanding, it offers distinct advantages. It minimizes the risk of grain breakage and allows for precise control over soaking times, which is why some brewers continue to favor this method for its artisanal precision.

Mechanical Washing

Technological advancements have brought various types of rice-washing machines to the brewery floor, enabling greater efficiency and consistency compared to manual methods

Large-Scale Washers

These industrial machines automate the entire process—from washing and soaking to transferring rice to the steaming unit. Often used in conjunction with continuous steamers, they are ideal for high-volume production. However, they can pose challenges: the mechanical agitation used to clean the rice can lead to broken grains, uneven nuka removal, and less control over water absorption.

Some large-scale models employ a no-rinse system using brush-like components to reduce wastewater.

Water Pressure Washers

Smaller breweries often use water-pressure washers, which offer a balance between efficiency and care. While not as fast as large machines, they enable gentler handling of the rice, better control over water absorption, and reduced breakage. For years, many brewers still preferred hand washing, arguing that it removed nuka more thoroughly. These machines now serve as a middle ground between manual and fully automated methods.

MJP (Mixed Air Jet Pump)

One of the most significant innovations in the past decade is the MJP system, developed by Woodson Co., Ltd. This machine uses fine air bubbles in water—jet microbubbles—to gently remove nuka. MJP machines can often outperform even hand washing in cleanliness, with minimal grain damage and excellent absorption control in a short amount of time. Its introduction has contributed notably to the overall rise in sake quality in recent years.

The Role of Kakenagashi

Kake-nagashi (running-water rinsing) is a supplementary step that comes between washing and soaking. It involves a controlled flow of clean water over the rice for a specific duration to rinse off remaining nuka. Sometimes this method is used as part of the soaking phase itself, where the rice is left in gently running water to further reduce the presence of soluble bran components—helping ensure a clean fermentation later on.

Purpose of Soaking

The Purpose and Role of Soaking in Sake Brewing

Following the rice-washing step is soaking—a process in which polished rice is immersed in water to allow it to absorb moisture. The primary goal of soaking is to prepare the rice for effective breakdown by koji mold during fermentation.

When rice is steamed, the starch needs to undergo gelatinization (or alpha conversion) to form a soft, sticky texture that enzymes from the koji can easily break down. This transformation requires sufficient internal moisture, which must be absorbed during the soaking stage—making this process essential for successful saccharification.

That said, too much water can be just as problematic. Over-soaked rice can become overly soft when steamed, making it difficult to produce high-quality koji. Achieving the right balance—optimal water absorption—is the key.

Target Absorption Rate and How It's Measured

But how does a brewer determine what qualifies as "optimal"? The answer lies in measuring the water absorption rate—a numerical indicator that reflects how much moisture the rice has taken in. It is calculated as follows:

Water absorption rate = (Weight of soaked rice after draining − Original dry rice weight) ÷ Original dry rice weight

The target absorption rate varies depending on how the rice will be used—for example, kakemai (rice added to the mash) might require 25%, while kojimai (rice used for koji-making) may aim for 35%. These rates also differ depending on the desired flavor profile and the yeast strain being used.

Soaking time can range from just a few minutes to several hours, depending on the polishing ratio and absorption goals. Highly polished rice, such as that used for ginjo or daiginjo, absorbs water much more quickly, meaning even a few seconds’ difference can significantly impact the final product. As a result, precision timing is critical during this stage of the brewing process.

How to Soak Rice

Standard Soaking Practices

The most common soaking method involves submerging the rice in large vessels—either tubs or soaking tanks—filled with water. This bulk soaking process can take several hours or even overnight, depending on the desired water absorption.

Gentei Kyusui (Precision Soaking)

For highly polished rice, water absorption begins almost immediately upon contact with water. This makes precise timing critical; a delay of even a few seconds can result in over-absorption, affecting both texture and fermentation outcomes. To manage this, some brewers employ a technique known as gentei kyusui, or "limited/precision soaking."

In this method, rice is divided into small 10 kg batches and placed in mesh bags or colanders. Each batch is soaked individually in a water-filled tub, with the timing tracked down to the second using stopwatches or timers. By handling the rice in small, controlled units, brewers can precisely calibrate the absorption rate to achieve the exact moisture content required for the target sake profile.

Draining After Soaking

In gentei kyūsui, draining the rice quickly and uniformly is essential to avoid uneven moisture distribution. Methods may include spinning the colander, using a commercial-grade vacuum, or employing a dedicated dehydration machine. Unlike natural air-draining, these tools help minimize moisture differences between the top and bottom of the batch.

For standard soaking methods, draining is typically done overnight. This allows the moisture to stabilize throughout the rice and ensures optimal steaming quality. However, careful temperature control is essential—on cold days to prevent the rice from freezing, and on warm days to avoid bacterial growth that can cause discoloration known as akameshi ("red rice"). Proper sanitation and time management during draining are critical to maintaining rice quality.

What Brewers Pay Attention to During Washing and Soaking

When it comes to rice washing and soaking, what exactly are sake brewers looking out for?

One of the most important steps begins before the process even starts: testing how quickly and how much water the rice absorbs. Even if washing and soaking times are kept constant, small differences in the rice’s initial moisture content can lead to significant variations in the final absorption rate.

Water absorption is affected by a range of factors—including the time elapsed since polishing, storage conditions, the rice’s temperature, ambient temperature, and humidity. Brewers must take all of these into account and adjust soaking times accordingly.

Another critical factor is water temperature. Ideally, the difference between the water temperature and the rice temperature should be less than 5°C (9°F). If the temperature gap is too large, absorption slows down. Conversely, if the water is too warm, the rice can absorb water too quickly—resulting in uneven moisture distribution, with the surface absorbing more than the core.

Finally, brewers pay close visual attention to the rice during the soaking process, especially in gentei kyūsui (precision soaking). One key indicator is the medama, or “eye,” a translucent spot at the center of the grain that hasn’t yet absorbed water. When a small “eye” remains, the rice tends to steam to the ideal firmness. Even with the rise of automation and data-driven soaking protocols, sensory judgment—especially by eye—remains indispensable in the hands of skilled brewers.

Secondary Effects of Washing and Soaking

While washing and soaking serve clear technical purposes in sake brewing, they also bring about additional, lesser-known effects.

During washing, the rice’s surface is slightly abraded, effectively mimicking the process of polishing. For this reason, senmai is sometimes referred to as the “second polishing.”

Moreover, both washing and soaking cause some components—particularly minerals—to leach out from the rice. In this way, these early-stage processes subtly influence the flavor profile of the finished sake by altering the rice’s composition before fermentation even begins.

Summary

Washing and soaking may not be the most talked-about stages in sake brewing, but they are essential processes that have a profound impact on the final product. Even with the rise of advanced machinery and automation, these steps still rely heavily on human judgment and sensory skill—making them a true showcase of craftsmanship.

For anyone seeking to understand the nuances of sake production, washing and soaking are key points worth remembering.