Fermentation

In 1856 Monsieur Bigo, a distillator from the North of France presented Pasteur with a problem: the wine in some of his vats had turned sour.  Pasteur noticed that the wine that was not sour consisted of circular yeast cells that produced ethanol (alcohol) but the sour vats also contained long, black bacteria that produced lactic acid.

After much experimentation, Pasteur resolved Monsieur Bigo's Problems, and had managed to perfect the method of fermentation.  He found that by heating a broth of yeast and a little sugar he was able to kill the bacteria and produce wine or beer that was not acidic.  During this process carbon dioxide, produced by the yeast must be allowed to leave the system, but at the same time oxygen must be prevented from entering.  During fermentation the yeast transforms the glucose found in grape juice to pyruvic acid, this acid is then transformed to ethanol by a method called glycolysis, shown below.

From his findings Pasteur was able to classify microorganisms into two main categories.  Aerobic - those that cannot survive without oxygen such as the bacteria that produced lactic acid and anaerobic, that can only live without oxygen like the yeast cells.  It is for this reason that oxygen must not enter the system during fermentation. 

As a result of his research on fermentation Pasteur was able to prove that  microorganisms enter the broth from particles in the atmosphere.  He proved his hypothesis with the help of a swan-necked flask.  After sterillisation the broth was placed in the flask but the particles were prevented form entering the solution by the moist walls of the flask.  On tipping the flask, however, the sterile contents then became contaminated as they touched the sides.  In 1862 Pasteur was thus able to confirm that germs do not appear spontaneously but that they appear in the atmosphere and begin to multiply only when the conditions are favourable.

Following this research, Pasteur discovered that the method sterillisation, by the addition of heat lead to the pasturisation of milk - hence its name.  He found that by heating milk at a moderate temperature (about 60 degrees) for about 15 minutes it was possible to kill the bacteria responsible for turning milk sour.  Several years later it was found that this temperature also destroyed the bacteria responsible for Scarlet fever, tuberculosis, typhoid and diphtheria that are found in numerous samples of milk.