KILLING BACTERIA BY CHEMICALS 115 



food, but it does not protect the food against bacteria which 

 get in it after the heating has ceased. Food that is to be pre- 

 served should be put into sterilized jars (i.e. jars washed in scald- 

 ing water) and the jar should be sealed while hot. If the jars 

 of hot preserves are not promptly sealed, bacteria from the air 

 drift into the rich food substance and develop there and undo 

 the work of the canner. Successful canning consists in cooking 

 fruits and vegetables until all bacteria and spores are killed, 

 and then sealing the cans so that bacteria and spores from out- 

 side cannot find entrance. 



Killing bacteria by chemicals. All sorts and kinds of bac- 

 teria make their way into foods. Some of them produce 

 decomposition products of disagreeable odors and tastes and 

 cause rapid change in the appearance of the food. Others 

 produce decomposition products without odors and tastes 

 and cause no visible change in the food for a long time. Both 

 types of bacteria may be injurious to health. Those which 

 give warning of their presence by disagreeable odor and taste 

 and by altered appearance in the food are often less dangerous 

 than those which give no such warning. 



Manufacturers and dealers have learned that certain chemi- 

 cals check the growth of the bacteria which cause disagreeable 

 odors and taste. They therefore add these chemicals to foods 

 and keep them in an apparently fresh condition. But the food 

 is not wholesome; it contains dangerous bacteria! For ex- 

 ample, milk doctored with formaldehyde remains sweet, because 

 formaldehyde destroys the lactic acid bacteria which cause 

 souring ; but milk doctored with formaldehyde, although sweet, 

 is not healthful because it contains the poisonous formaldehyde 

 and a multitude of colon bacteria which are dangerous to health. 

 Milk which does not sour within a few days is probably chemi- 

 cally preserved and is dangerous because of the colon bacteria 

 which it contains. 



CLARK INTRO. TO SO.-" 8 



