PRODUCTS OF MILK. 



149 



removed by the separator, the skim-milk is still quite sweet, 

 but when we allow the milk to rise in shallow pans, or even in 

 deep cans, the milk may become a little soured, unless we 

 keep ice or cool water around it. When liquids become sour 

 it is because of the forming of what we call an acid, like the 

 acid of vinegar. The acid that is formed in milk when it first 

 sours is called lactic acid (from the latin word lac, meaning 

 " milk "). This lactic acid is formed from the sugar of the 

 milk or the lactose. Since the sugar is very soluble, any water 

 that goes off in the cream will contain lactose, so that lactic 

 acid will form also in cream. This acid at once acts upon the 

 casein, changing it from a soluble substance to an insoluble 

 substance ; therefore, as soon as acid begins to form, the milk 

 will begin to curdle. This lactic acid is not nearly so valuable 

 for food as the sugar from which it is formed ; therefore, sweet 

 skim-milk and sweet whey are always more valuable for feeding 



than sour milk or sour 

 whey, and care should 

 be taken to keep them 

 as sweet as possible. 

 But why does milk sour? 

 What causes the sweet 

 sugar to change to the 



Fig. 79.-Yeast plant, magnified. sour ac j<J p Y OU knOW 



that yeast causes bread to "work" or to ferment. The yeast is 

 a mass of little plants, each plant very simple and very small. 

 These plants feed upon the substances in the dough, changing 

 them and making new compounds, some of which are gases. 

 These gases push out in all directions, and make little air-holes 

 all through the bread, causing it to be light, as we say. Now, 

 yeast is only one kind of these minute little plants. They are 

 found everywhere floating about in the air by millions, too 

 small to be seen. Some of them, we have already stated, 

 grow in little knots or balls on the roots of clover and peas. 



