The Unique Process of Sake Brewing
The first stage of sake brewing is polishing, or milling, the rice. The grains are fed into a very large machine called a seimai-ki (精米機) which carefully removes the outer surface of the rice kernels. Depending on the degree of polishing, this process can be very time consuming. For example, a rice that has been polished down so that 70% of the grain remains will take roughly 7-10 hours. However, if polished to around 50% it can take up to 2 days. Furthermore, the higher the degree of polishing the easier it is to crack the rice kernels. Therefore, for sake from the premium categories, like ginjo-shu, very often special cultivars are used that are tougher and allow for higher degrees of polishing. For more on this please see Raw Ingredients.
The reason for removing the surface of the kernel is to reduce the fats, vitamins and proteins that are said to produce less desirable flavours in the sake. On the other hand, the core of the kernel, or shinpaku, contains the starch that will later be converted into sugar to produce alcohol. The final degree of polishing will greatly influence the style of the sake, and will in most cases determine where it sits amongst sake's various legal definitions (see Sake Classifications)
Washing and Soaking Rice
Polished rice is then washed and left to soak for a pre-determined period of time. This is done in order to allow the rice to absorb the desired amount of water in preparation for the next stage. However, far from being a simple step in the process, establishing the ideal amount of time for soaking is subject to numerous different factors. These include the degree of polishing, the type of rice being used, and the temperature of the water on that particular day. Finding the perfect time requires both experience and some trial and error. Timings can range from being left to soak overnight for rice with very low degrees of polishing, to just minutes for highly polished rice for use in premium sake. For more on this fascinating process see the full blog post The Devil is in The Detail.
Steaming Rice 蒸米
With the polished rice washed and soaked, it is finally ready for steaming. How close it is to the desired condition after these first stages will greatly affect how the rice will steam during this stage. In order to steam the volumes required for sake production, a large steal vat known as a koshiki (甑) is used. Unlike the preparation for table rice, no water is used at this stage. Instead, steam is fed into the bottom of the vat which works its way up through the rice. For this reason, attention must also be paid to how evenly the rice is laid out in the steamer, and to making sure it has had the proper time to dry off. Getting either of these things wrong can result in inconsistencies in the rice which can go on to have negative effects on the final brew. Once steaming is complete, the rice will be divided up for various different uses in the later stages of brewing.
The next stage is one of the most crucial in the process, and also the one that requires the most care and attention. Between 15-25% of the steamed rice is cooled before being taken to a special temperature controlled room used for the production of koji (麹). As rice doesn't contain the necessary sugars required for fermentation into alcohol, like for example grapes do, rice is sprinkled with a type of mould (Aspergillus oryzae) that provides crucial enzymes that convert the starch into sugar. Over the following 24-36 hours, the koji will constantly be monitored and adjusted to keep it at an ideal temperature that suits the rice variety being used, or the toji's preference for that particular sake. The whole process is extremely complicated and requires all of the toji's experience to perfect. Once completed it will be used straight away in various different stages of the brewing process.
Making the Yeast Starter
With rice and koji prepared, the task of brewing tangible sake begins when a small amount of these ingredients are mixed together in a starter tank along with water and yeast. This stage is known as either preparing the moto or shubo (酛,酒母) in Japanese. Although there are various ways of doing this, nowadays the vast majority will be by the quickest and least labour intensive method known as sokujo (速醸). This takes around two weeks to complete and again requires constant monitoring of the tank's temperature. More labour intensive methods do exist, with the two most popular being kimoto and yamahai (生酛,山廃) both of which take an additional two weeks to complete.
Making the Main Mash (moromi)
When the moto is ready, it will be moved to a much larger tank in preparation for what is known as sandan shikomi (三段仕込), or three stage mashing. Over the next four days, three increasingly larger volumes of water, koji and steamed rice will be added to the mixture. Roughly speaking, the first addition will double the volume of the moto, and the second will double it again, and so on. All four days of this process have special names which are: Day 1 Soe(添), Day 2 Odori (踊り) (Dance in Japanese), Day 3 Naka (仲) and Day 4 Tome (留).
This method is used in order to maintain a healthy yeast population in the tank. For example, if everything was added all at once the yeast would likely thin out, leaving the moromi susceptible to other unwanted micro-organisms. During this stage we can also see a unique aspect of sake brewing: In beer production, the conversion of starch to sugar and then sugar to alcohol is done sequentially. However, in sake production this conversion occurs simultaneously. This is known as Multiple-Parallel Fermentation (平行複発酵). This complex process is what makes brewing sake so fascinating and challenging.
With the moromi finished its long cycle of fermentation, the process of separating the sake from the solid lees begins. There are typically three ways of doing this, however the most common is the use of a large accordion like press called a assaku-ki (圧搾機) as seen in the opposite picture. The moromi is first pumped into the machine using air-pressure before being passed through a series of large panels that separate the liquid from the solids. Once all the moromi has passed through the press, the solids will be collected from the screens and boxed or stored for usage later.
An important point to mention regarding this stage is that it is not filtration. Very often with premium sake, the freshly pressed liquid will be left to settle in a tank for just a couple of days and then bottled. Sake prepared in this way will most often display muroka (無濾過) on the label as an indication to consumers that it has not been filtered. The other two methods of pressing are more laborious and tend to only be used for super premium daiginjo. The details of both of these methods will be discussed in a future blog.
As mentioned above, premium sake will often be bottled soon after pressing. Depending on the brewery, there are various different machines that are used for this, varying from extremely large automated devices that also affix labels and bottle caps, to very small manual devices that require lots of time to complete. Sometimes a brewery will have both, as often the more gentle time consuming machines are favoured over the larger ones when making premium varieties like ginjo-shu . Although bottling is not a particulary difficult stage in the process, knowing by what method it took place can sometimes be a good indicator as to how carefully it was brewed .
Any sake that displays the Japanese character for fresh (生) will have skipped this next stage entirely. However, those without are known as pasteurised or hi-ire (火入れ). Unpasteurised sake still contain active enzymes from the koji which can produce unwanted flavours in the sake. Therefore, in order to stabilise the sake it is heated to around 60-62°C.
As before, there are several possible methods a brewer may chose in order to do this. However, the one considered the most appropriate for premium sake is known as bin-kan (瓶爛 ) or bottle heating. As the name suggests, the sake is bottled, then placed into a large tank filled with water. This water will then have steam passed through it until the required temperature is reached. The bottles will then be removed, sealed, and quickly put back into the tank and left to cool down. This method is extremely laborious and time consuming but is considered the most careful method.
Although there are exceptions, most sake undergoes some degree of ageing. The length of time varies from brewery to brewery, but is usually somewhere between 6-12 months. Once again, there are a few methods of doing this. Variations include, storing before or after pasteurisation, the temperature at which it is stored, and lastly whether it is aged in the bottle or in a larger tank. In the case of the latter, a special closed top storage tank is used, as seen in the opposite image. These tanks are several times the size of the moromi tanks which allows the toji, if he so desires, to blend together several batches of the same type. This is often done in order to maintain a consistency across that particular line of sake.
Although on the decline, a substantial amount of sake undergoes a process of filtration. The desired outcome of this stage depends on a wide array of circumstances. For example, for lower grade types, filtering just before bottling is often done to improve the colour of the sake. Also, less desirable aromas and tastes can be removed by doing this step. However, for higher classifications it is usually done for completely different reasons, like removing sediment.
The most common method uses charcoal and a device called a roka-ki (濾過機). The charcoal is first washed through the machine and collected by paper filters. This liquid is then pumped through the same device, with the undesirable elements collected in the filters.As mentioned previously, there are a wide range of reasons a brewer might decide to apply this step. Luckily, there is also an equally wide variety of different charcoals available for them to use. For example, some will only remove sediment and will leave the aromas and tastes intact, whereas others will significantly affect the final profile of the sake.
Please note that the above is just one possible brewing process. There are countless variations and styles depending on which brewery is applying them.