FAVORABLE CONDITIONS FOR BACTERIAL GROWTH ^ 



it forms the basis for the Storch test for the efficiency of the pas- 

 teurization of milk or cream for butter-making. This test is 

 described in the chapter on Pasteurization. 



(2) Enzymes developed through the action of germ life 

 bacteria, yeasts and molds. These are many and varied, and 

 cause most of the changes that take place in milk and its products, 

 such, for example, as the ordinary souring of milk or cream, and 

 the development of flavor and aroma in cream ripening. 



(3) Enzymes found in the digestive fluids of animals. All are 

 familiar with the fact that rennet is used in cheese-making. It 

 contains a ferment known as rennin. 



It is the second class of ferments or enzymes, the class due to 

 the action of germ life (principally bacteria) , which is of the great- 

 est importance in connection with dairying, and with the control 

 of which the dairyman concerns himself most. These ferments 

 are capable of working profound changes, some desirable and some 

 very undesirable. 



Size and Shape of Bacteria. In size, bacteria are the smallest 

 organisms that exist, so far as known. The size varies consider- 

 ably. Russell 1 gives the average diameter as 30000 of an inch. 

 They are so inconceivably small and light that nine hundred 

 billions of them would only weigh ^g- of an ounce. 2 



Bacteria also vary considerably in shape. They are as a 

 rule classed into three groups: (i) The bacillus or rod-shaped; 

 (2) The coccus or ball-shaped; (3) The spirillum or spiral-shaped 

 (like a corkscrew) . Some types of bacteria are classified accord- 

 ing to the way in which they adhere to each other. For instance, 

 when two cocci occur together and form a pair, they are called 

 diplococci; when cocci occur in chains, they are called strepto- 

 cocci; when cocci appear in bunches, they are called staphylo- 

 cocci, etc. 



FAVORABLE CONDITIONS FOR BACTERIAL GROWTH 



Food. Bacteria, like other plants, need food for their exist- 

 ence. The food passes into the bacterial cell in solution, but 

 many organisms use materials not in solution by producing 



1 Dairy Bacteriology. 2 Milk, Its Nature and Composition, by Aikman. 



