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The article making_ginger_beer mentions using bread yeast so you get more carbonation and less alcohol. Isn't carbonation a by-product of the process that makes alcohol? Wouldn't that mean more carbonation means more alcohol? What is the relation between type of yeast, carbonation, and alcohol content?

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4 Answers 4

Carbonation is indeed a by-product of fermentation, the yeast will consume sugar and produce alcohol and co2.

Some yeast strains will consume more sugar before the alcohol concentration gets too high and they go into 'hibernation' (floculate and settle to the botton).

Lower temperatures will also cause some strains of yeast to floculate and stop fermenting, which is the key to your recipe.

According to it, you let the yeast do its magic for a few hours, then put it on the fridge to stop fermentation. All you want is enough co2 to produce bubbles so if you follow the recipe and put the bottles in the fridge once they feel 'hard' you shouldn't get too much alcohol.

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It says it in the article

Wine yeasts are engineered to generate less CO2,

So, according to it, for the same amount of alcohol produced (and fermented sugar), bread yest will generate more CO2 than wine yeast.

I am not an expert, so I do not know if the statement is correct or not. Fermentation chemical reaction is

C6H12O6 → 2 C2H5OH + 2 CO2

In order for the statement to be true, alongside "main" fermentation reaction there should be some other reaction with effect on alchool or CO2, for example generating different ratio of CO2 and alcohol.

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Indeed, I can't think of any reason that bread yeast or wine yeast would produce different amounts of CO2 for a given amount of fermentation. It would have to be a separate metabolic pathway. Bread yeast has been selected over the years to start producing gas quickly, though it stalls out early. Wine yeast, on the other hand, has been selected to ferment more completely, though it may have a longer lag phase. In either case, only a very trivial portion of the sugars will actually ferment in something like ginger beer. –  MalFet Feb 13 '13 at 15:54
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That sounds suspect to me. The article suggests that wine yeasts are selected for low CO2 output sound right. The primary selection criteria for yeasts in brewing is taste and alcohol tolerance while the primary selection criteria for baking yeast is that it starts off faster.

Secondly if you think about chemistry, you'd have to do something with the carbon other than producing CO2. If you could reduce the CO2 from the yeast the very serious question would be "what else is it producing instead?" and that would lead in my mind to health concerns.

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Beer yeasts use the Crabtree effect to undergo fermentation in the presence of (some) oxygen. On the other hand, bread yeasts prefer to undergo respiration rather than fermentation in the presence of oxygen. A byproduct of the TCA cycle is CO2-but not alcohol. If you limit your oxygen levels then you will get alcohol produced as the yeast will switch from a respiration to a fermentation pathway to produce energy.

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This is interesting. Do you have any references to the Crabtree effect, especially regarding difference between beer yeast and bread yeasts? –  Tobias Patton Apr 10 at 5:24
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