Lactobacilli (bacteria present in milk) perform lactic fermentation, where the end product is lactic acid.
For this they use as their starting point the lactose, milk sugar, which is broken down by enzymatic action that occurs outside the bacterial cells, into glucose and galactose. Monosaccharides then enter cells where fermentation occurs.
Each molecule of pyruvic acid is converted to lactic acid, which also contains three carbon atoms.
The sour taste of fermented milk is due to the lactic acid formed and eliminated by lactobacilli. O pH lowering caused by lactic acid It causes the coagulation of milk proteins and the formation of rennet, used in the manufacture of yogurt and cheese.
Lactic Fermentation in Man
You may have heard that lactic acid is commonly produced in a person's muscles on occasions when there is exaggerated muscle strain. The amount of oxygen that muscle cells receive for aerobic respiration is insufficient to release the energy needed for intense muscle activity.
Under these conditions, while muscle cells keep breathing, they begin to ferment part of the glucose in an attempt to release extra energy.
Lactic acid builds up inside the muscle fiber producing pain, tiredness and cramps.
Then a portion of this acid is carried through the bloodstream to the liver where it is converted to pyruvic acid.
Acetobacteria make acetic fermentation, where the end product is acetic acid. They cause the souring of wine and fruit juices, being responsible for the production of vinegars.