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Q & A: Yeast Gases

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Most recent answer: 10/22/2007
Q:
i’M DOING A SCIENCE FAIR PROJECT AND I WAS WONDERING IF A CERTIAN AMOUNT OF WATER AND A CERTIAN AMOUNT OF YEAST WAS PLACED INTO A SODA BOTTLE THEN A CORK PLACED IN THE TOP AND THE BOTTLE SHOKEN UP AND THE CORK POPS OUT INTO THE AIR. WHAT KIND OF GAS IS BEING CREATED PLEASE ANSWEAR!!
- jESSIE (age 10)
PITTSBURGH, PA,
A:
Jessie -

When active (live) yeast has both sugar and oxygen available to it, it 'breathes' by a process called aerobic respiration. In this reaction, yeast cells use glucose (sugar) and oxygen (from the air) to produce energy. They also produce water and carbon dioxide (a gas). This is the same chemical process used by humans.

If no oxygen is available, yeast will switch over to a process called anaerobic respiration - in this process, glucose (sugar) is fermented to produce energy, carbon dioxide, and ethanol. Since ethanol is a type of alcohol, which is toxic for yeast cells, anaerobic respiration is a poor second choice to aerobic respiration. (This is, however, the process used to make wine, so sometimes it's not an all bad thing.)

There is a very nice site all about these processes at

As for your experiment, I'm not sure what to say. The thing about respiration is that the yeast cells only do it for the sake of producing energy. And they can only produce energy when the have glucose (sugar). If you are using only water and yeast without adding any sugar, I don't see what could be happening. My best guess is that there is a small amount of sugar naturally dissolved in the water or that the yeast have a small amount of sugar stored in their cells. If this is the case then the gas you are noticing is carbon dioxide, although I can't imagine that there is very much of it.

My suggestion is that you try your experiment again using sugar water in place of regular tap water. I expect you will see much more dramatic results.

-Tamara

(published on 10/22/2007)

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