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Name: Richard Y.
Status: Student
Age: 14
Location: N/A
Country: N/A
Date: N/A 

What is the most efficient way of testing yeast fermentation? eg collecting co2 etc.

Yeast cells will use oxygen if it is present, and break down sugars all the way to CO2 and H2O. In the absence of oxygen, yeast will switch to an alternative pathway that does not require oxygen. The end products of this pathway are CO2 and ethanol. The first pathway yields a lot more energy per sugar molecule consumed, and so it is the "preferred" pathway if oxygen is present. So, the first thing you need to do is set up a yeast culture that can grow anaerobically, i.e., in the absence of oxygen.

A low-tech way of doing this is to use a plastic soft drink bottle. Fill it with warm, NOT HOT (which will kill the yeast) water and dissolve a few tablespoons of sucrose (table sugar) in it. Dump in a packet of baker's yeast and mix. Fill the bottle right up to the top of the neck with water, and fit a deflated balloon over the neck.

Incubate the culture in a warm place; even room temperature will work. (If you have ever made bread, you will know that it's possible to overheat the yeast and kill the culture.) Within a day you should see the balloon start to puff up with the CO2 that is expelled as a waste product from the yeast. You won't get tons of gas, since the resistance of the balloon prevents it from expanding to a large size, but you will get carbon dioxide.

For a more elegant collection method, replace the balloon with a stopper that has a short piece of tubing inserted through it. Attach a couple of feet of flexible tubing to the tube on the stopper. Now the CO2 will be expelled out the tubing. How to capture it..... Fill a test tube or similar item FULL of water, put your finger on top to seal it, and insert it upside down into a beaker or glass that is half full of water. You now have a setup that is similar to the waterers found on bird cages, for example. Run the flexible tubing so that the end of it is located underneath the end of the test tube, in the water. As gasses escape from the tubing, they will bubble up into the test tube, and displace the water in the test tube. Once sufficient gas has collected, you can remove the test tube and test for CO2 by seeing if the gas in the test tube will support combustion (what happens to a glowing ember slid into the tube?).

The hardest part about the second setup is just getting everything clamped to stay in place. Good luck!

Paul Mahoney, Ph.D.

There is a piece of glassware called a fermentation tube but if you don't have that, you can turn a small test tube upside down inside of another bigger test tube. As the gas is generated it will be caught in the small tube. You can measure the height of the bubbles with a metric ruler.


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