PLANTS WITHOUT CHLOROPHYLL 147 



by delivery wagons to the consumers. When we remember that 

 much of the milk used in New York City is forty-eight hours 

 old and when we realize that bacteria grow very rapidly in milk, 

 we see the need of finding some way to protect the supply so as 

 to make it safe, particularly for babies and young children. 



This is done by pasteurization, a method named after the 

 French bacteriologist Louis Pasteur. To pasteurize milk we 

 heat it to a temperature of not over 170° Fahrenheit for from 

 ten minutes to half an hour. By such a process all harmful germs 

 will be killed and the keeping qualities of the milk greatly length- 

 ened. Most large milk companies pasteurize their city supply by 

 a rapid pasteurization at a much higher temperature, but this 

 method slightly changes the flavor of the milk. 



Cold Storage. — Man has also come to use cold to keep bacteria 

 from growing in foods. The ice box at home and cold storage on a 

 larger scale enables one to keep foods for a more or less lengthy 

 period. If food is frozen, as in cold storage, it might keep without 

 growth of bacteria for years. But fruits and vegetables cannot 

 be frozen without spoiling their flavor. And all foods after freez- 

 ing seem particularly susceptible to the bacteria of decay. For 

 that reason products taken from cold storage must be used at once. 



Ptomaines. — Many foods get their flavor from the growth of 

 molds or bacteria in them. Cheese, butter, the gamey taste of 

 certain meats, the flavor of sauerkraut, are all due to the work of 

 bacteria. But if bacteria are allowed to grow so as to become 

 very numerous, the ptomaines which result from their growth in 

 foods may poison the person eating such foods. Frequently 

 ptomaine poisoning occurs in the summer time because of the rapid 

 growth of bacteria. Much of the indigestion and diarrhoea which 

 attack people during the summer is doubtless due to this kind of 

 poisoning. 



Preservatives.^ — This leads us to ask if we may not preserve 

 food in ways other than those mentioned so as to protect our- 

 selves from danger of ptomaine poisoning. Many substances 

 check the development of bacteria and in this way they preserve 



1 Perform experiment here to determine the value of different preservatives. 

 Use sugar, salt, vinegar, boracic acid, benzoic acid, formaldehyde, and alcohol. 



