| MadSci Network: Microbiology |
Hello Sally,
It is difficult to know what exactly is going on without more details about the experiment, but I have a hunch about what happened.
Although yeast can make alcohol from sugar, and are exceptionally tolerant of alcohol, it can still be toxic to them. Beer and wine makers discovered a long time ago that fermentation by yeast could produce a maximum alcohol concentration of about 15% (it is likely less than this for yeasts which have not been selected for brewing purposes). It can’t go any higher than this, because that concentration of alcohol stops yeast growth.
It is possible that the amount of ethanol you added was enough to slow the growth of the yeast, even if it was not enough to stop their growth altogether. This would explain why their rate of respiration decreased.
If you grow yeast in apple juice, they will convert some of the sugar to carbon dioxide and some into ethanol. Apple juice is around 10-12% carbohydrate, so even if they converted all of the sugar to alcohol, it still would not reach 15%. Therefore, they can metabolize all the sugar in apple juice without ethanol concentration becoming a problem.
Let’s say you added ethanol to a concentration of 10%. This will probably slow their growth, but they will continue to produce some ethanol on their own (in addition to carbon dioxide). In this case, it is possible that they could produce enough ethanol to bring the concentration to 15% (which will stop their growth) before they have used all of the sugar. If that happens, then the total amount of carbon dioxide produced will be less than if no ethanol had been added, consistent with your observations.
Alex Brands
Lehigh University
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