Fermentation literally means a chemical change accompanied by effervescence. The anaerobic breakdown of glucose to carbondioxide and ethanol is a form of respiration referred to fermentation. It is normally carried by yeast cells and accounts for the production of alcohol in alcoholic beverages.
In fermentation process, if glucose is converted into ethanol then it is called ethanolic fermentation.
When glucose is converted into organic acids such as lactic acid, then this type of fermentation is known as lactic acid fermentation. It is carried out by the bacterium Bacillus acidilacti.
Kuhne's fermentation tube consists of an upright glass tube and a side tube with a bulb. 10 per cent glucose solution mixed with baker's yeast is taken in the Kuhne's tube and the tube is completely filled. After some time, the glucose solution is fermented and gives out an alcoholic smell. The level in the upright tube will fall due to the accumulation of CO2 gas. It is because yeast contains the enzyme zymase which converts glucose solution into alcohol and CO2. When a crystal of KOH is introduced into the tube, the KOH will absorb CO2 and the level of the solution will rise in the upright tube.