HEAT IN THE PRESERVATION OF FOOD PRODUCTS 539 



at eleven revolutions per minute. The cans in the curves marked 

 "C ' and "D" were processed according to the practice of one of the 

 typical commercial cookers. It will be noticed that whereas the center 

 of a #3 can of tomatoes required about twenty-eight minutes to reach 

 the temperature of 180 without rotation, it reached the same tempera- 

 ture in three or four minutes with rotation. In general it may be said 

 that a product consisting of solid particles surrounded by a relatively 

 clear liquor heats to the center of the can almost as quickly as if the 

 particles were not there, that is, almost as quickly as water in a still 

 retort so that agitation is not of particular value in such products. 

 A product of a viscous nature which interferes with convection cur- 

 rents, however, heats to the center of the can much more quickly if 

 there is some form of agitation which supplements the convection 

 currents. By rotation as mentioned above is meant the rotating of a 

 can on its axis as done by some of the commercial cookers now widely 

 used with tomatoes and some other products. 



~t 



HOME CANNING OF FOODS 



The successful canning of foods in the home depends upon the same 

 principles as those employed in commercial canning, namely, cleanli- 

 ness, soundness of raw material, and thorough processing. Aside from 

 the universally used open kettle method of handling the foods, two other 

 methods are now widely used: the cold-pack method, and the vacuum- 

 seal method. In the cold-pack method, the products are cleaned, 

 blanched by dipping in hot water, and immediately packed into the 

 glass jars or other containers. To fruits hot syrup may be added; to 

 vegetables and greens hot water and a little salt is added. Preserva- 

 tion of fruits and some vegetables may be effected by the intermittent 

 or fractional method of heating on each of three successive days, 

 the time of heating depending on the nature of the product and the 

 size of the container as described above. The extra time, labor, and 

 fuel required for this method makes it impractical where a large amount 

 of canning is to be done. Furthermore, as Bigelow and Estey have 

 shown, foods may carry organisms the spores of which are not destroyed 

 by fractional processing, and these may cause spoilage of the canned 

 product. The more common method of home canning is to process the 

 food in one period by immersion of the containers in water which is then 

 brought up to the boiling point; or by the use of steam pressure outfits 



