How sake is made
Sake is brewed from rice, water and yeast, with the aid of a mould known as koji. Production was once restricted by law to the colder months (October onwards), and this continues to be the practice for most breweries. However, refrigeration has made possible the practice of year-round brewing.
A special kind of large-grained white rice called Sakamai is used for making sake - there are about a hundred different types of Sakamai. Costing two to five times as much as table rice, it has a higher starch content with little protein and fat. Only 5% of the rice grown in Japan is for sake.
Most of the all-important starch lies in the centre of the rice grain with fats and proteins around the outside. It is this starchy 'white heart' of the grain that sake brewers use.
The outer surface is milled off, to leave the starch-rich middle exposed in a process known as polishing. The degree the grain is polished depends on the category of sake being made, typically 30% to 50% of the outside is removed but for higher quality, the rice is reduced to less than half its original size sometimes to even less than 10%.
The mills used to polish the rice seimaiki are programmed to remove the desired percentage and this is monitored by constantly comparing the weight of the remaining rice against the original weight of that batch of rice. As the rice is milled the powder muka is constantly vacuumed away and stored for sale as a food ingredient.
After polishing the rice is thoroughly washed to remove any powder left by milling. The washed rice is then steeped in water for a length of time dependant on the quality of the sake being made and how much the rice has been milled. The more the rice is milled, the faster it will absorb water and the more important it is for the brewer to steep for precisely the right length of time. Rice may steep for as little as a minute, or as long as a whole night.
After steeping the rice is steamed. Traditionally this is done in a koshiki in small batches of about one tonne, but larger breweries can use large, continuous steamers. The bulk of the steamed rice is cooled to 5°C and used in the fermentation process, while the smaller portion left is used for making mouldy rice or koji rice.
Koji rice (komekoji) is made with yellow koji, a mould (Aspergillus Oryzae) which is crucial to sake production as it is this mould that converts the starch in the rice to fermentable sugar.
Yellow koji mould spores (koji-kin) are sprinkled over the steamed rice in the kojimuro, a wood-lined room where the humidity and temperature are kept high. This process is closely monitored and takes about 36-45 hours: the koji rice looks slightly frosted and smells sweet, and vaguely chestnutty.
Next, a 'moto', or seed mash, is made. Yeast is added to the koji and mixed with water in a tank, then steamed rice is added. Most brewers also adjust the acidity with lactic acid, to help avoid microbial spoilage during this stage, but some sakes achieve this protection using older techniques that rely on bacteria to create lactic acid. These older methods are called yamahai and kimoto and result in sakes with higher acidity and more depth of flavour. This highly yeasty mash will form the basis for the rest of the batch. The koji and yeast simultaneously work together to convert the rice starch into glucose and the glucose into alcohol. This 'parallel combined fermentation' is unique to sake production.
The 'moto' is termed 'matured moto' after the fermentation process has begun. It is transferred to a larger fermentation tank, where batches of water, rice and koji are then added to the seed mash, each time approximately doubling its volume. This main mash (moromi) then ferments for 18 to 32 days. The elegant, fragrant styles are fermented most slowly, at the lowest temperatures.
Once the fermentation reaches the desired level of alcohol, neutral alcohol at around 30% alc./vol. is commonly added to the moromi. Cheap sakes can more than double their volume this way. When used in very small quantities for premium sake, neutral alcohol can help enhance the fragrance and flavour while allowing the brewer more control. (Although 94% of all sake brewed in Japan is made with the addition of alcohol, the US federal government does not allow the brewing of this type of sake, and imports are taxed at the same rate as spirits, making it disproportionately expensive.)
Once fermentation is complete and any neutral alcohol has been added, the moromi is filtered to separate the liquid from the lees (kasu), which comprise dead yeast and large amounts of solid rice particles. This filtration process is a legal requirement, but the law allows the use of filters with holes large enough to allow the solid particles to pass through. (The result of this is cloudy nigori sake.) The liquid is then either allowed to settle or filtered, resulting in 'fresh sake'. The lees can be used for all manner of Japanese classic dishes.
Fresh sake is usually then pasteurised at approximately 60°C. to denature enzymes and kill any remaining yeast or bacteria that might later alter the flavour profile. For most sakes, this is done by passing through the pipes of a heat exchanger. Some premium sakes are pasteurised by immersing the bottles into hot water. This is a gentler process. After pasteurisation, sake is matured (in tank or bottle) to allow it to mellow for several months.
Unpasteurised sake is referred to as 'Namazake' and tends to be fresher and more fragrant than pasteurised sake. Because it is delicate and unstable, namazake has a short life and must be kept continually refrigerated from production until it is drunk. It is usually released as a seasonal product.
At this stage the sake is over 20% alc./vol. and is usually watered down to 15% to 16% alc./vol. prior to bottling to make finer flavours more discernible. Sake is not generally aged for more than the standard few months maturation and unlike wine, vintages are not traditionally featured on packaging, though some brewers are beginning to state a vintage on their labels and a 'produced on' date is a legal requirement for sake made and sold in Japan. This generally appears on the back label.