HEAT IN THE PRESERVATION OF FOOD PRODUCTS 469 



tion may be effected by the intermittent or fractional method of heating 

 on each of three successive days. The time of heating would depend 

 on the nature of the product, and the size of the container. The extra 

 time, labor, and fuel required for this method makes it impractical 

 where a large amount of canning is to be done. The more common 

 method is to sterilize in one period of heating by immersion of the con- 

 tainers in water which is then brought up to the boiling point; or by the 

 use of steam pressure outfits of which the market affords several 

 types. These operate on the same principle as the laboratory autoclav. 



Spoilation of Canned Foods 



Microbial changes occur when the goods have not been proc- 

 essed at a temperature sufficiently high to destroy all the organisms 

 which may have been present in the uncooked food. In some instances, 

 the organisms decompose the contents of the can with formation of gas, 

 causing bulging of the ends of the cans sometimes to the point of burst- 

 ing at the seams. Such cans are designated at the factory as " swells." 

 In other instances, the bacteria in the imperfectly sterilized cans cause 

 an acid fermentation with consequent souring of the contents. The 

 canner terms such cans "flat sours." 



Detection of Spoiled Goods. — In cases of spoilage accompanied 

 by gas production, detection of the spoiled cans is easy from the 

 bulged appearance of the ends of the cans. On account of the exhaus- 

 tion of air from the cans during processing, the ends of sound cans should 

 be slightly concave. If the ends of the cans are convex, it indicates 

 some abnormal condition of the contents, and such cans should be 

 rejected. In the case of sours, detection is not so easy. The can may 

 appear normal, and there may be no change in the contents apparent 

 to the eye on opening the can. Taste, however, reveals a more or less 

 pronounced disagreeable acid flavor. Canned meats, fish, or crus- 

 taceans are Ukewise liable to spoilage if sterilization has been im- 

 perfectly carried out. In these goods the change is generally but not 

 always accompanied by gas production, hence detection is easy 

 because of the swelled appearance of the cans. 



Disposal of Factory Refuse 



The disposal of factory refuse has at times become a serious problem for the 

 commercial camier. Of late years, however, methods have been devised for utiliz- 



