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I'm a little confused about this and can't find an answer that satisfies my curiosity... I've found a helpful thread (What does high attentuation actually mean in terms of types of sugar fermented?) but it doesn't really sort me out. Perhaps someone can help break it down for a noob... :)

I understand that your wort will contain a certain amount of fermentable sugars, and some unfermentable. I believe this to be around 60%-70% (depending on your grain choice, mash temperature, etc.).

Then we have yeast attenuation rates, which appear to be anything around 60%-85% depending on the strain, but this appears to be a mechanism for comparing different yeast types in a 100% fermentable sugar solution.

Does this mean that any particular yeast can only ferment, say, 75% of the fermentable sugars? So somewhere around 50% of the actual sugars in the wort will be converted to alcohol?

I'm sure this can't be the case and I must be missing something important here... what is the importance of understanding yeast attenuation?

I intend to buy BeerSmith at some point but I'm just at the beginning of the road at the moment so I'd like to get my head around the entire process as much as I can.

Thanks!

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Related - homebrew.stackexchange.com/questions/11013/… –  mdma Nov 28 '13 at 11:57

3 Answers 3

Yeast attenuation cannot be used on its own to determine how much residual sugar, maltose is left in beer. You must take into consideration other factors, specifically the following three: 1. unfermentable sugars such as carapils, and lactose 2. Mash temperature. 3. pitch rate of yeast.

Keep in mind if you use wild yeast or other bacteria all of what I am about to say is meaningless. Also simple sugar, such as table sugar and corn sugar is completely fermentable,and will muck up an attenuation calculation.

Unfermentable sugars has the greatest impact on your attenuation than any other factors. You can turn a dry stout into a sweet stout by adding lactose only. The rule of thumb i use for lactose is that whatever points of gravity lactose adds, just add it to your FG.

Secoondly, and the most interesting in your case I beleive, is the mash tempature. Take a look at wlp002 http://www.whitelabs.com/yeast/wlp002-english-ale-yeast, great yeast. Its attenuation is 65-70%. If you mash your beer at 145 F, you will achive attenuation of 70%. If you mash at 156, your attenuation will be arround 65% or so.

Finally, pitching rate. If you pitch more yeast, not only will your beer ferment more quickly, you will also archive a higher attenuation, by a point or two. Conversely, if you under pitch your beer, you will get a lower attenuation

Unfortunately, pitch rate takes practice, and its difficult to predict what its effect is unless you brew the same beer frequently.

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"what is the importance of understanding yeast attenuation?"....Yeast attenuation is determined by the yeast lab under laboratory conditions, and it is only a way of comparing one yeast relative to another using the same wort. You may or may not achieve the attenuation that is listed for the yeast. It may be less, it may be greater than listed. That depends on the fermentability of your wort.

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Here's an article from Danstar on yeast attenuation and how it's measured. It's not using 100% fermentable wort, although according to the article there is no industry standard.

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