Fill the closed arm of a Kühne fermentation tube with glucose solution mixed with yeast and keep it warm, and after a while gas starts to collect at the top of the closed arm. The yeast is breaking glucose down into ethanol and carbon dioxide, and the carbon dioxide rises as bubbles. This reaction is alcoholic fermentation, and it uses no oxygen.
Add sodium hydroxide solution to the tube and let it touch the gas, and the collected gas shrinks. Sodium hydroxide absorbs carbon dioxide, so this shows that the gas was carbon dioxide.
Switch to lactic acid bacteria and lactate builds up in the liquid, but no gas collects. In lactic acid fermentation none of the carbon in glucose leaves; all of it stays in the lactate.
Both kinds of fermentation start with glycolysis, just like respiration, and part ways after pyruvate. Without oxygen, the hydrogen that NAD+ picked up in glycolysis cannot be passed on to the electron transport chain. Yeast hands it to acetaldehyde, made by removing carbon dioxide from pyruvate, while lactic acid bacteria hand it to pyruvate itself, and in both cases NADH turns back into NAD+. With NAD+ back, glycolysis can take in glucose again.
The ATP gained is only the 2 from glycolysis per glucose. That is about a fifteenth of the roughly 30 from respiration, so far more glucose has to be broken down to get the same amount of ATP.
Raising the temperature speeds up the reaction, but if it gets too hot the enzyme proteins lose their shape and fermentation stops. An enzyme stopped this way does not recover when it is cooled again.
Click a part of the tube or the cell to read about it in the column on the right. Choose the organism, the liquid and the temperature in the panel below the figure.