192 BACTERIA, YEASTS, AND MOLDS 



this complete destruction. If a housewife finds that a portion 

 of her canned preserves is spoiled, she may infer that the 

 •original heating was insufficient. 



2. It must be remembered that the amount of heat 

 required to destroy different species of bacteria is not 

 always the same. While most actively growing bacteria 

 are destroyed by a moderate heat, and quickly killed by 

 boiling, certain bacteria spores, as already noticed, are 

 capable of standing much greater heat. Some kinds of 

 bacteria produce spores that may be boiled for a few 

 moments, or, indeed, for an hour, without being wholly 

 destroyed. From this it follows that a short boiling 

 is not always sufficient to destroy bacterial life. If the 

 food material chances to contain some of these resisting 

 spores, the brief boiling commonly adopted in the pro- 

 cess of canning will not kill them, and it will inevitably 

 happen that the food, if canned, will undergo putrefac- 

 tion because of the growth of the spores that were left 

 uninjured. If on the other hand the food in question 

 does not chance to contain such spores, a few moments' 

 boiling is quite sufficient to protect the material perfectly 

 from later decay. 



It is a well-known fact that the process of canning is 

 not equally successful with all kinds of foods. Some sub- 

 stances {rhubarb) contain acids that act as partial anti- 

 septics, and can be preserved very easily. Others, like 

 most fruits, require a little more care, but are easily 

 preserved ; while others, in spite of the ordinary precau- 

 tions, will frequently show subsequent signs of decay. 

 The canning of tomatoes has always given trouble to the 

 housewife. In former years it was thought to be an 



