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So, when I fermented a cider it ended up very dry. I used champagne yeast and only added pure glucose. I then added lactic sugar since I knew that wasn't fermentable by the yeast I used.

I also have understood (though I might be mistaken) that we can produce a higher final gravity by using malt with more complex sugars. Again, this is common sense.

I am confused however, by measurements of a yeasts "attentuation". It seems illogical to me that I would end up with FG 8 if I have a solution of water, nutrients, correct pH and 40 SG of pure glucose and pitch a yeast that has 80% attentuation since glucose is completely fermentable, right?

What I am asking is this: Does attentuation depend as much on the type of sugars as the type of yeast, or maybe actually more on the sugars used?

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I think you mean "lactose" sugar, not "lactic". "Lactic" refers to an acid. – Graham Aug 29 '11 at 17:26
Lactose has a definite taste - I would have used maltodextrin, which also is unfermentable but pretty much tasteless. – mdma Aug 29 '11 at 19:16
up vote 3 down vote accepted

The attenuation rating of yeast is simply a way of comparing one yeast to another given a standard wort or must. It does not necessarily reflect the attenuation you can expect from the yeast. That is far more dependent on the sugar composition of whatever the yeast is fermenting.

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So it is a rough estimate of how many of the different sugars that can be fermented by a specific yeast as well as the make-up in a standard wort between those sugars? Am I right in thinking that if a yeast can ferment a specific sugar, it will ferment it completely? – Max Aug 29 '11 at 17:19
Only if that sugar is completely fermentable. – Denny Conn Aug 29 '11 at 20:28
@Max, but its also a measure of each strains unique tolerance to ethanol and the other by products of fermentation, including CO2 pressures an fermentation environments. Stick with it being a comparison between yeasts. Not a function of sugar types. – brewchez Aug 30 '11 at 12:42
@brewchez Agreed. I'm mainly just trying to understand the chemistry behind the fermentation process, not particularly what the label says - question should probably been stated as Does different yeast strains ferment different types of sugar? – Max Sep 7 '11 at 11:31
Max, as long as you;re talking about about the yeast we use to make beer, the answer is no. But if you include ALL yeast, the answer changes to yes. – Denny Conn Sep 7 '11 at 15:21

As you've seen, even though the wort comprises 100% fermentable sugars, ale yeast does not in fact ferment all of it. As the yeast ferment the sugar, the environment becomes more and more harsh, preventing the yeast from consuming the remaining fermentables. Also, ale yeast can only ferment 1/3 of the maltriose.

If you perform a forced ferment - lots of yeast, kept warm and regularly roused - attenuation is higher, and for lager yeasts, can be complete, if the wort is fully fermentable. There's a great article on homebrewtalk wiki about this - Understanding Attenuation

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This would be correct if you take into account the starting gravity. Most any ale yeast will ferment a 1.030 or less solution of 100% fermentable down to 1.000 right? – brewchez Aug 30 '11 at 13:50
I've not fermented anything less than 1.040, so I can't say for sure. The lowest I've got for an ale yeast was 1.004, but 1.010 is far more typical, but then that's with a mixed composition wort. If the yeast did ferment out all sugars, the FG would be below 1.000 since ethanol has a SG of 0.800. – mdma Aug 30 '11 at 15:35

The ability of a yeast to process sugars is dependent on the strain. Each strain has a slightly different distribution of the enzymes needed to break down the sugars into alcohol and carbon dioxide. Enzymes are very structure specific and there are different enzymes that break down specific sugars in differing amounts in each strain of yeast. Attenuation is different among different strains and is specific to the type of sugar. Another factor is the tolerance of the yeast to the alcohol. Some are more tolerant than others. That is why a strain like cote des blanc or champagne yeast are used to produce high alcohol brews and other strains produce lower percent alcohol. The way to control the ending percent alcohol is by either controlling the OG or choosing a strain of yeast that "shuts down" at a specific alcohol percentage range. Cider yeasts tend to stop processing sugar faster and yield lower ending alcohol percentages. This leaves sugar in the cider what cuts the "dryness" on the tongue. So, to answer your question, yes ... it depends on the sugar and the strain of yeast.

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