Rice Preparation
Rice Washing (洗米 sen-mai)
The purpose of rice washing is to remove the rice brain (nuka) that accumulates during the polishing process. Nuka contains minerals, proteins, and lipids which will produce undesirable flavors and aromas in the fermentation process. The presence of nuka would also make the rice stickier and more difficult to work with.
Small scale tests are done to determine how long each rice crop and polishing amount takes to get the bran off the rice. I’ll explain this later. Once the timing is determined, the dry rice is split up, by weighing it on a scale, typically into 10-20 kg porous bags, wicker, or wire-mesh containers for more delicate, higher-end sake (tokutei-meishushu) that requires more attention to detail, but there are many automated procedures or large scale operations that are done with bulk-rice washing machines as well.
For high-end sake, the bags may be placed in tubs of water where kura-bito will gently move the rice around to dislodge the particles clinging to the rice. Some breweries will use small, automated machines to carry out this process using a whirlpool-like action inside a tank and in very large facilities, the rice can be washed by
When the washing is considered thorough, the water will not be totally clear, but in an inch or two of water, you should be able to make out the grains quite clearly.
In a brewery, the Toji will instruct everyone when to pull the rice out of the water, and at that point a hose is sprayed over the baskets to free any remaining powder.
Here is a video of kura-bito washing rice together.
When washing rice, there are some important properties that will make an immense difference in how successful you are in washing rice.
Rice:
Polishing Amount (seimai buai)
Moisture Content (before and after)
Storage Temperature
Water:
temperature
mineral content
Rice: Polishing Amount
The polishing amount (% of rice remaining), is an important factor because the more outer layers that are removed, the faster the rice will absorb water. At home, this process isn't very risky due to the low amount of polishing that table/sushi rice has. 90% seimai buai usually will take quite a while to absorb water, so you might wash for 10 minutes and that would not really affect the outcome. However, with rice that has been polished much more (only 35-70% remaining), the process becomes a time-critical experience that brewers will use stop watches and count the seconds till its ready. This is because of how quickly the soft, starchy center of sake rice absorbs water.
Rice: Moisture Content
With the steps that follow, the importance of water absorption rate will become increasingly clear, especially as you try to make your own koji. Brewers attempt to hit a total water content of 28-34% in the rice. With highly polished rice, the absorption rate could be so fast that after just 1-3 minutes it goes from 15% to 25%. If you took 10 minutes to wash, this would actually cause the rice to absorb all the necessary water, but it could actually absorb MORE than is desired, leading to soggy grains, which is absolutely the worst state a grain can be in heading into the steamer. This will produce rice that is very sticky and in the case of koji, could lead to contamination.
Rice: Storage Temperature
It is also worth noting that water temperature will play a huge role here as well. You want to try to reduce the amount of grains that split open (crack) during washing and steeping because it will cause the grains to have inconsistent water absorption. One huge component of that is ensuring that the temperature of the water matches the temperature of the rice (should be within 8-12°C) . The homogenous environment greatly reduces the likelihood of the sudden exposure to cold temps cracking the rice. Check the surface of the grains and look for a white line running down the grain. This indicates the grain has cracked and you should double check all temps to ensure this effect is reduced if possible.
Another key factor is actually how the rice is washed. While it might feel natural to move your hands rigorously, it is far more of a gentle motion, simply to free the stuck powder, and not to induce any intense rubbing together of the grains, which could lead to cracking as well.
Water: Temperature
In addition to the difference between the storage temperature described above, another major concern regarding the temperature of water for washing rice is how warm it is. The warmer the water, the faster the rice absorbs it. Imagine if you are using Yamada Nishiki 40% seimaibuai (with only 40% remaining). This might only need 4 minutes to obtain 30% moisture content. If you wash the rice for 4 minutes, you won’t even need to soak it at all. However, if the water is colder, it could take 5 or 8 minutes and if it’s warmer, say 30°C (86°F), it could only take 1-2 minutes. You could actually go over your intended moisture content before you even finish washing rice. Do tests as described in the next section to determine the best temperature and timing.
Water: Mineral Content
Depending on your water source, city water, well water, reverse osmosis, or others, the content of your water will have drastic effects on the qualities of your brew. This isn’t to say that there is a “good” or “bad” style for that matter. There are however some specific minerals to be aware of that you really don’t want.
Side note: What’s all this about MiyaMizu and should I try to match that water profile? Check out the SakeDeepDive MiyaMizu episode
Note: Water Absorption Rate
Usually the head-brewer, or whoever is skilled in this particular task of judging the water absorption, will take a small sample of each batch of rice that came from the polisher and do an absorption test with a stop-watch. Theoretically all the rice of a particular varietal was being polished at the same time, was also from the same field, and also had the same polishing rate, so it should react the same to water. Any variation in these should have separate tests to determine how each batch will absorb water differently.
To conduct this test, the brewer will place rice grains on a small black plate or cup ( dark color is for contrast against the white rice ) and start a stop-watch as soon as it is immersed in water. The experience of the brewer is such that they can almost tell the exact moment the rice has absorbed enough, just by the sight of it,
There are formulas being developed by researchers that can help predict this timing, but it is very complicated because of the variations in grains, polishing amount, etc. For most, if not all, brewers timing must be determined through trial and error by using small samples in the example above to determine the desired absorption amount.
Rice Soaking / Steeping / Immersion
Important Details:
Rice: polishing amount, moisture content, storage temperature
Water: temperature, mineral content
The point of steeping the rice in water is to homogenize the “cooking” which is performed by steaming. If steam were simply applied to dry rice it would cook unevenly and it would not properly gelatinize all the starch.
Gelatinization improves the availability of starch for amylase hydrolysis (source). As will be mentioned many times on this site, amylase or glucoamylase are enzymes produced by Koji mold, which break down this starch into simpler sugars, which will be consumed by Yeast and metabolized into alcohol.
Other factors are at play during the washing and steeping phase as well. “20-40% of the potassium and 20% of the phosphate, which activate fermentation, are washed out of the kernel, along with small amounts of sodium, magnesium, sugars, proteins, amino acids, and lipids. However, ferruginous ions and calcium are absorbed by the rice if they are present in the water.”, Brewing Society Calcium accelerates the modification of rice in a mash and ferruginous ions have a negative effect on the quality of sake, usually in the form of discoloration (iron)
For Koji-mai, or rice that will be used for koji growing, soaking is a critical stage because if the rice absorbs too much moisture, it can cause the rice to become sticky and provide the wrong type of environment for koji mold to grow. If the rice doesn't absorb enough, it can also be problematic, because the koji mold requires a source of water to flourish. The target is usually 30% total moisture content (+/-30% Gautier, +/-33% Brewing Society), and it is extremely important to note that each rice batch (from the polishing mill, and each years harvest) will have a specific timing to reach this percentage.
For Kake-mai, or rice that will make up the majority of the mash, the target water absorption is often lower because while we want the rice to gelatinize homogeneously during steaming, at a larger scale, the additional water in the rice can throw off calculations. As an example, kake-mai typically aims to absorb ( 30-35% Brewing Society, n/a Gautier)
There are many commercial processes for steeping rice including large vats, 10-30kg bags, and many home-made solutions that fit the needs of a brewery. In the home-brewing space, usually people just use mesh brew bags for beer brewing. Coarse mesh is the easiest to clean and allows water and bran to drain out more easily. Ideally most of the bran has been removed during the washing process, but there are always some residuals that cling to the rice.
In a commercial setting, if the batch is not for high-end ginjo or daiginjo sake, then timing is slightly less important. Large quantities of rice (100-300kg) might be placed into a large tank pre-filled with temperature controlled water, they wait for the predetermined time to elapse for it to reach the expected absorption rate, and then the water is drained and the rice pumped via vacuum onto a conveyor belt, which brings the rice to the steamer.
When rice is used for high-end gingo and daiginjo sake, you will often find 10-30 kg bags are used to steep rice in tubs of water. This allows for control down to the second because of how quickly the water drains out of the porous bags. Toji (head brewer) will often stand over the kurabito (factory workers) with a stop watch and call out commands as each bag is being placed into and pulled out of the tub.
The homebrew method is very similar to this bag method. As mentioned above, using a simple mesh brew bag, you can fill a bucket with water and drop the bag in, wait for the predetermined time, lift the whole bag out, and then moving on to letting it drain.
Process
In general, the process is the same regardless of scale.
Using the soak time determined above in the rice washing section, subtract the time you washed the rice, and assume that the remainder will be for soaking (steeping).
Ensure the water you are going to put the rice into is the same temperature to prevent cracking,
Place the bag(s) into the water and start your stop watch.
When the time elapses, take the mesh bag out.
Allow all the excess water to drain off (see the next step for more details)
Rice Draining
The purpose of draining is the fast(er) removal of surface water that hangs on by capillary action, and further resting (covered in the next section) is what allows the water that's left on the surface and in the rice to diffuse into the grain and equilibrate.
Some breweries, and many home-brewers, use a conical funnel and a wet-vacuum to pull any surface water off the grains to prevent further absorption and ensure a specific rate of 30% (or whatever their target is). However, if you have a sieve (commonly used in cooking), put the rice into it and allow water to drain off the rice (tilting avoids any grains soaking in the water that collects at the bottom).
Post-Soak Resting (another Karashi)
The purpose of resting is to allow water absorbed from soaking to fully diffuse into the center of the grain, permeating the semi-crystalline starch structure of
When water is present at sufficient high temperatures (such as during the steaming process), the starch in the endosperm undergoes a gelatinization reaction. If the water does not fully diffuse ( If you don't allow the rice to sit long enough after the soak) it is possible that you will get grains after you steam with hard centers. This is not ideal for many reasons. The biggest is that those hard centers are not gelatinized and the koji spores will not be able to consume the starch. In addition to that, the rice will not liquify as easily, leaving a greater a mount kasu (lees) in the take afterward.
To ensure enough time to rest, once the rice is done soaking, hang the mesh bag (if you are using a sieve as described in the previous section tilt the sieve on it's side) and wait for at least 45-60 minutes. Many books discuss the practice of leaving the rice out overnight. This practice should be tested for your needs and environment to ensure you do not lose too much moisture to evaporation. If you weighed the dry rice, you can now weight it again to get a percentage of how much moisture is present after the soak. If you weigh it after 45 minutes, then after several hours, and then again overnight, you can get a sense of how much moisture is lost per hour and then adjust accordingly
Rice Steaming
The purpose of steaming is to gelatinize the starch in the rice and denature the proteins, which makes them receptive to the koji enzymes
This will also increase the water content of the rice for koji-mai + moto making (41-43% Gautier + Brewing Society), kake-mai (39% Gautier, 39-40% brewing society). It is noted by the Brewing Society that these rates would be much lower for gingo-shu, but no information is provided. (few of us are still investigating this, but if you have information, please comment).
The rice should exit the steamer at 100°C with a firm exterior and a bouncy, almost spring-like interior. It should not be too sticky, so ideally when it is picked up, some of the grains might stick together, but they shouldn't stick to your hand very well. If they do, they should fall off if you rub your hands together or touch them. If it is too early, the grains will be too try and unable to form a mochi-like mass when a few grains are pressed together. This is a test of sorts to determine the done-ness of the rice. If you take about 15-30 gains and smush them together, they should form a ball with a texture similar to gum. You can maybe stretch it to the size of a poker chip and give it a little tug with your fingers to see if it stretches. If you can still see the individual grains very clearly it is not ready. On the flip side, if you can see no gain definition at all, you might have over steamed the rice. It should not feel like putty. Typically this won't mean you make bad koji, but it could cause baka-haze if it was soaked for too long, and worse, it could allow bacterial to contaminate the grain because of the soft exterior. Koji-kin (spores) are able to penetrate the hard surface of well-steamed rice and bacteria and wild yeasts are not, but if you over steam the rice, you are giving them better chances of survival.
When the rice cools again to about 29°C, it will have lost most of the water it absorbed during steaming, dropping it 10% from 40% down to 30% again with the only difference being the effects of gelatinization.
More on Gelatinization
Gelatinization provides an ideal condition for liquefaction and saccharification (depolymerization to a sweet-tasting product, similar to chewing rice and saliva breaks down the starch into simple sugars making it taste sweet ). Liquefaction is required for the particular enzymes to work because they perform hydrolysis (using water to break chemical bonds). Gelatinization transforms the partial crystalline structure of the starch granules. More specifically, “amylose [and Amylopectin] molecules leach out of the starch granule network and diffuse into the surrounding aqueous medium [(water)] outside the granules[6] “. This has been correlated with the observed grain swelling that occurs during the steaming process, significantly increasing the volume of the rice. ”Amylose is the major factor influencing the physicochemical properties of rice starch. Upon heating in aqueous solutions, starch swells irreversibly and its crystalline structure collapses, a phenomenon known as gelatinization. Starch swelling is a property of amylopectin, whereas amylose has been known to restrict it (Tester and Morrison 1990a). Gelatinization converts starch into a physical form that is desirable in many food systems. … Upon cooling, starch molecules re-associate in a complex recrystallization process known as retrogradation, which is often associated with water separation from the gel (syneresis) (Yeh and Yeh 1993; Hoover and Manuel 1995). These changes may result in textural and visual gel deterioration (Thomas and Atwell 1999; Fredriksson and others 2000). The pasting properties are used in assessing the suitability of starch as a functional ingredient in food and other industrial products.", source.