Sake Production

The Brewing Process

Sake is made by a unique process called multiple parallel fermentation, where saccharification and fermentation happen at the same time in the same mash. The secret is something called koji – rice which has been treated with the mould aspergillus oryzae. After milling, soaking and steaming the cooked rice is divided up: and about a fifth is used to make koji, a process which takes place over a period of two days. Aspergillus oryzae is sprinkled over the cooked rice which is then left to grow, in a warm cedar-clad room (rather like a sauna) which is constantly monitored to maintain a temperature of around 30℃.

The koji rice itself will reach a temperature of around 43℃ to 45℃ and, to stop it overheating, and to keep the rice from clumping together, the brewery workers, stripped to their waists and working in teams in the warm koji rooms, turn the rice by hand – sometimes working during the night. On the third day the koji rice is now hard, not sticky, and covered in a fluffy white coat.

Some of the cooked rice which was put aside is mixed with koji, water, yeast and lactic acid to make the starter yeast called shubo. The enzymes in the koji convert the starch into sugar and the result is a sweet sake, called amasake, the sugars of which help the Saccharomyces cerevisae yeast multiply. The type of yeast used, and there are several, is one of the features that gives sake its character. The process of creating the shubo takes around two weeks, until all the sugar has been consumed by the yeast. Then it’s time for it to get to work on the main mash, and let fermentation begin.

In three stages over four days the shubo starter mash, full of sugar-hungry yeast, is combined with the koji and the rest of the steamed rice to make the main mash – the moromi. This is where the delicate balance of multiple parallel fermentation is crucial – in the words of British toji Philip Harper, “sake brewing is walking a tightrope – the two elements must be perfectly balanced”.

Fermentation lasts between two to four weeks – when the toji (head Brewer) decides it is ready, and alcohol content is around 18 to 20 percent. The finished brew is kept in tanks for a few days to settle until the next stage where brewer’s alcohol is added (except for Junmai),which traps volatile fragrances, stabilises the sake and lightens the flavour, after which it is ready for pressing.

Traditionally pressing was done by filling cotton bags and stacking them in a hand-turned press- an extremely labour intensive process which is now done in only a handful of breweries . More usually the sake is pressed by machine, to separate liquid and solids. The left over sake lees, called kasu, is used in food preparation, particularly to pickle fish and vegetables, and imparts a fermented yeasty taste. Filtration, usually through carbon, is the final stage of creating the clear liquid which is sake.

As temperature control is so crucial to sake making, it’s always been a winter activity, as this allows for slow fermentation. So in spring, when fermentation, pressing and filtration are completed, the sake is pasteurized then kept in tanks before it is diluted and bottled ready for shipping in the autumn. Most sake is designed to be drunk within a year, and this historically has been one of the obstacles to its transportation overseas.

Click here to find out more details on how sake it made.

written by Oliver Hilton-Johnson, Tengu Sake