16 THE CANNING OF FOODS. 



The varying temperatures and methods used in canning produce 

 a certain amount of strain on the cans. These strains also vary with 

 the temperature at which the tipping is done. The contents of 

 a can expand with heat and contract upon cooling. If a can of corn 

 is tipped at 190 and placed in the retort to process, it is at once 

 subjected to an external pressure of from 10 to 13 pounds per square 

 inch, and the expansion of the contents will increase until the internal 

 pressure equals or slightly exceeds the external, or it rises to from 

 12 to 14 pounds per square inch. During a part of the cooking 

 process the strains become equalized, or nearly so, but when the 

 cooking is stopped and the retort is thrown open there will be an 

 outward strain equal to the internal expansion, and this will gradu- 

 ally decrease until the temperature falls to that at which the tipping 

 is done. When the temperature drops to ordinary room temperature, 

 instead of there being an internal pressure it is from without and 

 at 70 will amount to 7 pounds. This accounts for the ends of the 

 cans being drawn in. The can has therefore undergone a strain, first 

 of pressure from without on being placed in the retort, then from 

 expansion from within when the retort is opened, making a total 

 strain of 25 pounds per square inch. The smaller cans, such as No. 

 2 or below, are not much affected by these strains, but in the larger 

 sizes leaks are likely to occur. The more suddenly these strains are 

 applied, as by the wide opening and closing of the steam valve, the 

 greater the danger, and the turning on of a large volume of cold water 

 results in buckling. This principle holds good for all products, the 

 variation from these figures depending upon the temperature used. 



MODERN FACTORY EQUIPMENT AND METHODS. 

 SANITATION. 



A modern cannery is no longer the rough, crude shed that once 

 was thought to be sufficient for this purpose. First of all the loca- 

 tion must be sanitary, away from manufacturing processes which of 

 themselves are objectionable, such as soap making, tanning, render- 

 ing fats, etc., or any other processes which may give rise to noxious 

 odors or be productive of organisms of decomposition. The yards 

 and drives about the factory should be cleaned daily, and in summer 

 dust should be prevented by frequent sprinkling or by the application 

 of crude or specially prepared oil to the drives. The application of 

 oil is especially to be recommended where there is much hauling 

 and there is no pavement, or the factory is to be run for a short season 

 only, as in the case of tomatoes. A single application made a couple 

 of weeks before the season opens will suffice for several weeks; by 

 putting the oil on early it will become incorporated in the earth and 

 not be tracked into the factory to any great extent. The drainage 



