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I've been reading about Sake, and from what I gather, Sake brewers use a particular method to coax the yeast into producing 15-20% ABV. The method goes something like this:

  1. Combine a mold called Koji with a small quantity of rice to break the starches into sugar, and add yeast.
  2. After some time, add more koji rice to the fermenter, and then add double the rice originally used.
  3. After some time, add more koji and double the amount of rice used in step 2.

Apparently, if a sake brewer were to add all of the koji, yeast, and rice into one step, the ABV would not exceed 5-10%. However, by doubling the amount of rice after the former step has been fully fermented, the yeast can somehow survive the highly alcoholic environment and keep producing more alcohol, up to 15-20% ABV.

Could this same method be used in the production of beer to create a 15-20% ABV homebrew?

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While it wouldn't be literally the same process (in saké production starch degradation and alcohol production take place simultaneously, in the presence of two different types of organisms which have different and not necessarily compatible optimum conditions), something similar is definitely practiced in brewing, in which yeast are encouraged to make more alcohol by a sort of delayed feeding schedule. I've seen this practiced before, though I've never really experimented with it myself. My recommendations therefore would be limited to 'try it a few times and see what happens'.

A bit of theory as to why this method is sound, however: in higher gravity (meaning higher sugar) worts, yeast are increasingly subject to osmotic pressure, caused by the high difference in concentration of dissolved materials on either side of the yeast's outer membranes. It's been suggested * that at wort gravties above 18°P the osmotic pressure is enough to suppress yeast reproduction.

Imagine you plan a beer to be 15% ABV. You'd need a starting wort gravity of ~35-40°P (assuming 75% attenuation, which may be generous) to hit 15% ABV. This would put enormous stress on your yeast from the start. This stressed yeast then has the task of fermenting into a high alcohol range, as well. Evey the most alcohol-tolerant yeast might not do so well.

Imagine instead you start with a weaker wort (say 20°P) and once the yeast gets going, you start feeding it either very concentrated wort, or, probably more practically, a sugar or extract solution, every 12-36 hours. This way you're catching the yeast at its most vigorous (in the exponential growth phase) and, if dosed properly, are never putting the yeast in an environment dense enough to really affect its physiology. Minus the osmotic stress, the yeast will most likely always be able to ferment to a higher degree of attenuation than if you started off in a super-dense medium.

As long as you take account of all your additions (gravity and volume) you should have no trouble figuring out the total ABV% once it has end-fermented.

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  • FWIW, this kind of step feeding is used pretty frequently when making sack meads. Those typically use wine yeasts, which have much higher a alcohol tolerance than beer yeasts. Some Belgian strains, like Wyeast 1388, can hit ABV's that high, given the right conditions and care. – valverij Apr 11 '16 at 15:05
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Yes

Its all about gradually allowing the yeast to strengthen its cell walls for the target ABV. Hitting the yeast all at once with all the fermentables is like having a body builder eat 1/2 a cow of meat and expect him to lift twice as much the next day... Eating the meat alone will kill him on the first day.

For most beers above 10% they benifiet from feeding sugars and nutrients and oxygen during fermentation.

White labs has Super High Gravity yeast than can do 15% without too much help, but can hit 27% if nurtured.

If you ask any homebrewer, going above 20% is kinda like breaking the sound barrier, it can be done but it's hard.

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