Name: Richard Y.
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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Update: June 2012