FOOD PRESEEVATION 177 



theory is that the first heating will destroy all except the 

 spores of the bacteria, these will germinate during the next 

 twenty-four hours and be destroyed as vegetative cells upon 

 the next heating or quite certainly upon the third. In the 

 cold pack process the food material is heated in boiling 

 water or flowing steam at approximately 100° c. for such 

 time as will make certain the destruction of all microorgan- 

 isms, or at least those which might bring about deteriora- 

 tion. 



Not all canned foods, even though they may keep per- 

 fectly well, are completely sterilized. It has been deter- 

 mined as the result of many studies within the last few 

 years that the microorganisms which may bring about 

 deterioration in canned foods are : first, those which may 

 enter the food as the result of defective sealing; second, 

 those aerobic spore-producing bacteria which normally are 

 unable to grow in canned foods because of the complete 

 exclusion of air and which may find conditions suitable for 

 growth if air is admitted due to defective sealing; third, 

 anaerobic spore-producing bacteria capable of growing at 

 normal temperature occasionally escape sterilization and 

 produce deleterious changes, frequently accompanied by 

 evolution of gas and the development of malodorous and 

 bad tasting compounds, and sometimes poisons or toxins 

 (such as Clostridium hotulinum) ; and lastly, certain of the 

 most resistant of the spore-forming bacteria belonging to 

 the group of thermophiles. These spores of thermophilic 

 bacteria will not germinate unless the canned food is held 

 at a high temperature for some time. When canned foods 

 are not adequately cooled in a commercial cannery and are 

 stacked kway in a warehouse, they may retain their heat 

 for days or even weeks and conditions be particularly 

 good for the development of the thermophiles. The same 

 may happen when canned goods are stored in hot climates. 



