Retrospectacle: A Neuroscience Blog

The Magic of Sake Fermentation

In keeping with my discussion of Japanese foodstuffs (see fugu) this week, I thought I’d post a bit about the alcoholic drink sake. Sake, often called rice wine, has polished rice kernels as its raw material and is produced from the combined effects of a mold and a yeast. However the term “rice wine” is a misnomer since wine is fermented once while sake undergoes multiple fermentations.

Under normal conditions, the starch in rice is completely resistant to degradation by yeast enzymes—therefore it must be converted to sugar first. This feat is undertaken by the common mold Aspergillus oryzae, which easily breaks down the starch which remains after the hard exterior is polished off. During the process of making sake, this mold is cultivated beforehand in a very humid room on Koji rice (or malt rice). The moldy Koji rice, normal rice, and sake yeast mash (shubo) are then mixed together in a large vat. Here’s where it gets interesting.

Within the vat, a parallel process is occurring: as the mold breaks the rice starch down into simple sugars, these sugars are in turn reduced by the yeast to ethyl alcohol and carbon dioxide (to see the synthesis reaction, click here). This mixture is allowed to ferment for about a month, during which time more Koji rice, rice, or water may be added to achieve the desired flavor. At the end of this month, the rice mixture is placed in bags and then pressed to separate the alcoholic liquid from the solids. This liquid is left alone for another 10 or so days, when it is then filtered and pasteurized. The end product is about 18-25% alcohol. There are MANY different types of sake, with nearly infinite variations in the minutia of the brewing process, so the fermentation process I described is just the general picture.

Check out this flowchart on how sake is made:

i-ee533e1291684f0740eeaa31bf90eb07-sake process.gif

Here’s another great source of information about the types and how-to’s of sake.


  1. #1 matt
    September 26, 2007

    You left out the coolest part! The yeast that is used in Sake fermentation isnt the same yeast that is used for making beer. Most mass produced beer uses yeast that sinks to the bottom of the tank for easier filtering of the beer. Sake yeast floats! Although the proteins on the surface of the yeast are similar between the two, the Sake proteins enable the yeast to float on top of the brew and essentially away from the alcohol which is toxic to the yeast! Science is cool.

  2. #2 Left_Wing_Fox
    September 26, 2007

    Most mass produced beer uses yeast that sinks to the bottom of the tank for easier filtering of the beer. Sake yeast floats!

    Actually, whether the yeast floats or sinks is the defining difference between Ales and Lagers. Folks in Canada and Britain would probably be just as likely to get a mass produced ale (I.e. Alexander Keith’s India Pale Ale) as they would a lager. Most homebrew beer kits also use top-brewing yeast (and are thus ales), since they ferment quicker and don’t require the cold-storage of lagers.

  3. #3 brokenseeker
    September 23, 2009

    Wine yeast is also top fermenting. Top fermenting yeast is actualy the original domesticated strain. Lager yeast having actualy resulted as a mutation somewhere in germany.

    Also it is interesting to note that in beer making there is a ‘saccharification’ step, occuring prior to brewing. The dried grain is soaked in water, initiating germination and the release of related hormones, including amylase (Which breaks down starch into maltose) This is malting, the grain is then dried. The mailt is then left to sit in hot water at aproxomately 67 degrees celcius, an the optimal temperature for the amylase to work.

    There is a lot of variation that can be done in this, and grain preperation and usage of roasted grains is one of bigest influences on the flavor of a beer.

  4. #4 ty
    July 10, 2010

    So far this is one of the best explanation on the web for my study.