728 Rural School Leaflet. 



o %i OP-^ f^ ^^^y ^ small number of these acid-producing 



^%^ cfb ^ofu^ bacteria, but they multiply very rapidly, and 



■(9qOo '^^o[j -2 when they have produced enough acid, the 



milk begins to taste sour, and as the bacteria 



^c:::) grow and the amount of acid produced in- 



^ ,, ^ creases, the milk becomes more and more 



Fig. ^i.—The form of some sour until it finally curdles. 



of the bacteria common in These organisms which cause the milk to 



milk. . ' , . . ^ 



become sour belong to a group 01 mmute 



plants. They are the smallest plants that we know anything about ; so 



small, in fact, that it takes many thousands of them placed side by side 



to make a row an inch long. They are so small that we cannot see them 



with the unaided eye, and if we want to look at the individual bacteria 



plants, we must look at them through a high power microscope. It is 



because of their extreme smallness that we cannot see them in the milk. 



All we can see are the changes which they produce in the appearance of 



the milk and in its taste. Note Fig. 51. Three forms of bacteria are 



given : i causes milk to sour and is the organism used for ripening cream ; 



2 produces gas and is the cause of gassy cheese ; 3 causes the milk to 



putrify. 



Besides being the smallest in size, bacteria are also the simplest in 

 form of any of the plants. A mature, full-grown bacteria plant consists 

 simply of a little cell or sack filled with protoplasm. Some of these 

 plants are round like a ball, while others are cylindrical in form, as shown 

 in the illustration. But no 'matter which shape the organisms are, the 

 structure is always very simple. 



Not all bacteria are capable of producing acid as a result of their 

 growth, and in a later lesson we will discuss some of the other things 

 which bacteria do. But in this lesson we will consider only the organisms 

 which can produce acid in milk. 



The growth of the acid-forming bacteria in milk can be observed in 

 the following way : 



Secure a cjuantity of milk, mix it thoroughly, and pour equal quanti- 

 ties into each of four pint bottles or glass fruit jars. (These jars should 

 be thoroughly washed and scalded before the milk is poured into them.) 

 Cover the jars or bottles with paper to prevent the entrance of dust. 

 Then place one bottle in a dish of ice-water, one in water at 55° or 60°, 

 one at 70° to 75°, and the other at 90° to 100° Fahrenheit. It will be 

 well to shake the bottles frequently when first put into the water, until 

 the milk becomes the same temperature as the water. 



Now keep the water in the dishes at the above temperatures, and 

 notice when the milk in each jar first tastes sour, and also when it curdles. 



Like the higher plants, bacteria grow best at warm temperatures, and 



