No. 6. DEPARTMENT OF AGRICULTURE. 6C5 



fciderably above 212 degrees F., and eveu then require one 

 to three hours. Dry heat is less effective than moist heat. 

 Live steam, therefore, affords a most efficient means of de- 

 stroying bacteria. All bacteria are rendered inactive by low 

 temperatures and some may be killed by intense cold. Many 

 bacteria may retain life even on being dried and become active 

 again when placed under favorable conditions of moisture and 

 temperature. Su«light kills many bacteria when they are exposed 

 directly to the sun's rays for a few hours. Bacteria are either 

 checked in growth or killed by many chemical compounds. Those 

 compounds that simply retard the rapidity of growth of bacteria 

 are called antiseptics; those that destroy bacterial life are called 

 disinfectants. Bacterial activity is stopped by an accumulation of 

 the products formed by it and, in some cases, by the products of 

 the activity of other bacteria. In the course of their growth, bac- 

 teria produce great changes in the materials in which they grow, 

 and the processes by which these changes are brought* about are 

 known, as previously stated, under the geoeral name of fermenta- 

 tions. Bacteria are found distributed nearly everywhere in the soil, 

 in the air and in water. They are always present in large numbers 

 wherever vegetable or animal matter is undergoing decay. They 

 are, therefore, always closely associated with dirt and filth. While 

 some are the cause of dreaded diseases, most of them are either 

 harmless or actively helpful in many ways. 



(2.) Unorganized FGrme4its or Enzymes are chemical substances 

 or ferments, vvithout life, capable of causing marked changes in 

 many complex organic compounds, the enzymes themselves under- 

 going little or no change. Many enzymes are produced directly by 

 bacteria, while many are formed in higher plants and in animals. 

 The pepsin found in the. human stomach is an enzyme; its special 

 activity enables it to change proteid compounds from insoluble to 

 soluble forms. The ptyalin contained in saliva is another enzyme, 

 and is capable of changing starch into sugar. Babcock and Ku^sell 

 discovered in milk, a few years ago, an enzyme called galactasc 

 which has the power of coagulating and then dissolving milk-caseittv 

 Another enzyme of interest in dairying is rennet, a small amount o^ 

 which can coagulate many thousand times its own weight of milk 

 Enzymes are destroyed by high temperatures a«d by many disinfect- 

 ants. Some substances, like ether and chloroform, do not seriously 

 interfere with the activity of enzymes, while tliey do destroy bac- 

 terial activity. 



10. Sources of Bacteria in Milk. 



Milk, when drawn with careful precautions from the udder of a 

 cow. contains comparatively few bacteria; but milk obtained and 



