FERMENTATION Fermentation in the widest sense of the term includes all forms of de composition in both vegetable and animal substances when exposed to air and moisture at temperatures be tween the freezing and boiling point of water. But in common language, the word fermentation is more often confmed to those processes by which vegetable juices are transformed into alcoholic liquors. These processes, however, are entirely similar to pu trefaction, or the decomposition of organic matter which sets free foul smelling gases; and decay, or the change by which without moisture, the trunk of a tree molders into dust. Fermentation does not ordinarily take place much below 32° F. or much above 140° F. It usually causes liquids to rise in temperature and to give off gases with considerable internal mo tion, to become turbid, to form a scum and to deposit a sediment.
Among the useful results of fer mentation are the raising of bread with yeast; the preparation of al coholic beverages and certain food products, as sauerkraut; the curdling of milk by means of rennet to form cheese; the manufacture of vinegar, etc.
Among the injurious results of fer mentation are the souring of milk and vegetables, the putrefaction of meat, the becoming rancid of fats and the decay of articles of wood or textile fabrics.
Fermentation is caused by the vital action of microscopic plants, the germs of which may be present in the fermenting substance, or may be de posited on their surface from the air or from contact with water or other substances containing them. Or they may be introduced intentionally, as when yeast is used for brewing, or for making vinegar or bread. These small plants feed upon fermentable substances and bring about various chemical changes. Thus the subject of fermentation has two phases: i.e., (1) how to induce those forms of fer mentation that are useful, and (2) how to prevent those that are inj urious.
The promotion of fermentation falls under such various subjects as fermented beverages, the making of vinegar, cheese, bread, etc. The pre vention of fermentation falls under such subjects as the preservation of food, the preservation of timber, etc.
As fermentation occurs from the presence and development of germs, it is evident that its prevention de pends upon the destruction of any germs that are present, and keeping away others, or the removal of con ditions favorable to germ life. Hence, in general, fermentation and putre faction may be prevented by drying heat; by cooling below the point at which fermentation takes place; by heating or cooking substances to a point sufficient to kill the germs pres ent, and then hermetically sealing them to exclude others; and by the employment of various antiseptics, as alcohol, common salt, saltpeter, sugar, sirup, smoke, borax, and many other substances.
Putrefaction. — This change is a decomposition of animal or vegetable substances with the liberation of ill smelling gases. It can only take place at a temperature between the freezing point of water (32° F. and 140° F.), in the presence of moisture and after exposure to the air. Gen erally speaking, the more moisture and the greater warmth present, the more rapid is the process. The germs of the bacteria which cause putrefac tion are heavier than the germs of yeast and mold, and hence do not float in equal numbers, as dust in dry air. They are more often communi cated by contact with water or moist surfaces. For this reason in dry cli mates meats and vegetables may be preserved simply by drying or curing them by exposure to sunlight. But it is well known that if these sub stances are left out after the dew falls, and allowed to become mois tened, they may be covered with tt coating of mold. Most of the bac teria that cause putrefaction are killed by exposure to a temperature of 140° F. for a number of hours; to a temperature of 212° F., the boiling point of water, for ten to fifteen min utes; or to a temperature of 215° F. for 4 or 5 minutes. The activity of these bacteria ceases at the freezing point, but they cannot be killed by freezing, and again become active when warmed to a temperature of 40° F. Hence, in general terms, boil ing in water kills bacteria and freez ing suspends their activity.