Friday, 14 October 2011

The Mysteries of Sake Brewing with Aspergillus

What makes the Japanese alcoholic drink different to alcoholic drinks traditionally brewed in the west? What  makes the taste of Sake distinctive? Why do different makes of Sake taste different? The answer to all of these questions is Aspergillus oryzae.

Malted barley
Brewing in the west is usually begun by the breakdown of the complex carbohydrates (starch) in grain using the natural process of germination. Prior to growing a seedling needs to use simple sugars for energy so part of the process of germination involves the release of enzymes within the seed that turn the starch stored in the seed (starch is great for energy storage because it is difficult to use and thus won't get 'accidentally' eaten!) into simple sugars which are easy for the germinating plant to use. This is of course a completely natural process (referred to as 'malting') and has been used for many centuries.

Different drinks tend to use difference sources for their starch - whisky uses barley, wheat or rye and these can impart a characteristic taste to the drink. Other sources of starch can be used e.g. potato, sweet potato and other vegetables though not often commercially - the brewer tends to use starch sources readily available in the part of the world they are working.

Moromi, the main mash
In the East the principle starchy food is rice so that forms to basis for its brewing industries e.g. Sake. The grain that is used for Sake brewing is that which is unsuitable for eating and is often weak. Germination is not carried out as perhaps this weaker grain would not germinate well, but all the parts of the seed are also thought to be contaminants that impair the flavour of the drink. Only the pure, polished starch grains are used. This leaves the brewer with a problem - how to convert the starch into simpler sugars for consumption and conversion into alcohol?

Grain of rice on which
koji is propagating
Aspergillus oryzae is a 'domesticated' species of Aspergillus that has been used by the Sake industry for centuries (named koji). Over this time each company has jealously guarded its own strain of koji, resulting in multiple slightly different strains gradually evolving all over Japan. Koji is seeded onto the rice starch and proceeds to release enzymes to break down the starch into simple sugars. Starting two days later yeast is added which now finds a plentiful supply of sugar to turn to alcohol and proceeds to grow along with the koji over the next few days.

One effect of this 'co-culture' technique is that the yeast continues to work for longer, producing more alcohol and a stronger drunk compared with western brews (beers 5-7%, wines 11-14%, sake 14 - 20% alcohol). Another is that the use of a 'pure' source of starch gives a distinctive flavour, and that flavour is contributed to by the particular strain of koji in use in each factory.

The use of Aspergillus to brew the beverage is more efficient, allows more starch to be turned into alcohol and gives the drink a distinctive taste!

 An introduction to Sake (Esquire)

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