I'm studying for my intro to microbiology test and I have a question asking why a culture of yeast responsible for producing alcohol would be growing, but not producing alcohol?

The multiple choice answers are: a) the maltose is toxic b) O2 is in the medium c) Not enough protein is available in the agar d) The temperature is too low e) The temperature is too high

EDIT: There is not much more context that what I gave here. The question simply asks me to assume the position of a scientist at a wine production company, and then asks the question I posed above - nothing too specific.

I think it is (b) because the presence of O2 would cause the yeast to respire aerobically, but I can't eliminate (d) and (e)

I figure it probably isn't (a) because a toxic chemical would inhibit the growth of the yeast. I don't believe protein is relevant so I don't think it is (c).

  • 2
    $\begingroup$ I think we generally ask people with homework questions to explain their work and reasoning as much as they can first. Look up some stuff about yeast fermentation and you might answer this yourself. And please clarify how the yeast is making bacteria? $\endgroup$
    – user137
    Sep 28 '14 at 0:17
  • $\begingroup$ Changing the temperature would tend to cause the rate of metabolism to increase/decrease, but it would not tend to cause the actual metabolic products to change significantly. $\endgroup$ Sep 29 '14 at 17:48

I think only b.) is true.

  • a. I cannot find evidence that maltose is toxic to yeasts (and I would hardly believe it, because it is a glucose dimer), however I found evidence, that yeasts might need the presence of oxygen to process maltose.

  • b. Alcoholic fermentation is an anaerob process, so it won't work in the presence of oxygen.

  • c. Alcoholic fermentation is for creating energy. Proteins are normally not involved as substrate (maybe by gluconeogenesis, but that's another story). If the yeast grows, then there is enough protein available to build fermentation enzymes as well.
  • d. I found some articles about the temperature dependency of alcoholic fermentation:

    Juices fermented at 10°C exhibited ethanol concentrations between 7.4 and 13.4% and populations of K. apiculata, C. stellata and C. krusei in the range 106-108 cells/ml.

    At low temperatures, alcohol yield was higher. Secondary metabolites to alcoholic fermentation increased as the temperature increased. Glycerol levels were directly affected by temperature.

    The immobilized yeast showed an important operational stability without any decrease of its activity even at low temperatures (1−12 °C). Specifically, at 6 °C the biocatalyst favored wine production within 8 days, which is less time than is required for the natural fermentation of grape must.

    It seems like low temperature does not necessary have an adverse effect, but this could depend on strain.

  • e. Just the same as the previous answer. I think the main fermentation products will be always ethanol in the yeasts chosen for alcoholic fermentation (if the other conditions are right), so it is not temperature dependent in this case. A growing yeast will produce alcohol on both low and high temperatures. The ratio of ethanol and the byproducts can depend on the temperature, but not the presence of the ethanol itself. By too low or too high temperature the yeast will die or will stop to grow. (My experience that CO2 production is faster by high temperature, so it is because everything is faster or because the ethanol/CO2 ratio is lower.) An example of temperature dependence by wine making:

    Temperature control is vital in the production of fine table wines as:

    • High temperatures encourage the loss of alcohol and aroma and flavour compounds due to volatilisation. If the temperature goes above 30 -35ºC, the yeast becomes sluggish and fermentation may stop.
    • Low temperatures will lead to a poor extraction of colour and tannins in red wines and can also cause sluggish fermentations and the production of high levels of ethyl acetate.

    I think "sluggish" means stop growing in this case, and ofc. stop fermenting due to the death of the yeast cells.

  • $\begingroup$ I think you put in way more effort than this question required. Not that that's a bad thing. $\endgroup$
    – canadianer
    Oct 28 '14 at 15:47
  • $\begingroup$ @canadianer Maybe. Btw. your answer is perfectly valid too, and you solved it with pure logic, +1 :-) Ohh I have not known that this is an old question. $\endgroup$
    – inf3rno
    Oct 28 '14 at 17:39
  • 1
    $\begingroup$ Depends on the yeast but some (most notably S. Cerevisiae) readily ferment in the presence of oxygen. So this question is a little ambigious. Although not surprising for biology, Im surprised the microbio prof did not clarify the question. $\endgroup$
    – Cell
    Dec 24 '18 at 22:39

You're right, the answer is b since the presence of oxygen would lead to aerobic respiration and not fermentation.

If maltose was toxic, the yeast wouldn't grow.

Proteins are required for growth, but the yeast is growing.

Any temperature that is extreme enough to prevent fermentation would also prevent other cellular processes and the yeast wouldn't grow.


IMO, the answer to this one is (b). The reason is the (often overlooked) Pasteur effect: oxygen inhibits fermentation.

The Pasteur effect occurs in many cell types, including skeletal muscle, brain, heart, liver, bacteria and yeast (Tejwani,1978) .

The mechanism of the Pasteur Effect has been much studied and debated and it it probably true to say that it has still not been fully explained. However, in abstract terms, it be explained to a good approximation in terms of the allosteric regulation of the the enzyme phosphofructokinase, a key regulatory enzyme of glycolysis, where ATP, inorganic phosphate, and citrate (among other allosteric effectors) play critical roles. (Krebs, 1972, Tejwani, 1978).


Krebs, Hans (1972). The Pasteur effect and the relations between respiration and fermentation. Essays in Biochemistry 8, 1–34.

Tejwani, G. A (1978) The role of phosphofructokinase in the Pasteur effect. Trends in Biochemical Sciences, Volume 3, pages 30-33


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