HEAT IN THE PRESERVATION OF FOOD PRODUCTS 525 



In the earliest days of canning, glass jars were used exclusively, but 

 were gradually abandoned as it was found that they could not readily 

 withstand the extremes of temperature, and were expensive, bulky, 

 and costly in transportation. In 1825, Thomas Kensett secured a 

 patent on the use of tin cans in preserving food, and in the same year 

 began using the process in his factory. The early manufacture of tin 

 cans was by hand and crude, the bodies being cut with shears and the 

 side seams made with a plumb joint (that is meeting but not over- 

 lapping), and then soldered together. Heads were made to set into 

 the body and were soldered in place in a very crude manner. The 

 making of 100 cans was considered a good day's work for one man. 

 Improvements were gradually made, however, in their manufacture, 

 until finally can making became a distinct industry and now all the 

 parts are made and put together by machinery. 



In the original Appert process, the goods were cooked in open kettles, 

 the highest temperature obtainable by this method being the boiling 

 point. A little later common salt was used to aid in securing a higher 

 temperature, and this was followed later by the use of calcium chloride 

 which made possible a temperature of 115. In 1874, a closed kettle 

 was invented for superheating water with steam, and this was imme- 

 diately followed by another improved kettle in which dry steam was 

 used, the principle employed being that of the modern autoclav, by 

 which method any desired temperature may be obtained and modified 

 to suit the requirements of different classes of food. 



ECONOMIC IMPORTANCE 



FROM STANDPOINT OF HEALTH AND DIETETICS.- -The value of a 

 variety of foods, especially fruits and vegetables, is recognized by 

 dietitians. Unfortunately, however, the season for fresh fruits and 

 vegetables is comparatively short. Moreover, many foods grown 

 exclusively in one section of country will not withstand shipping in a 

 fresh condition to other sections. In spite of improved methods of 

 refrigeration, it is not practicable to ship fresh sea foods to far inland 

 towns, or to send some perishable products of warm climates to cold 

 countries. The canning and preserving industry overcomes these 

 difficulties by supplying pure, clean, wholesome fruits, vegetables, 

 meats, and fish to any region the year round, and at prices com- 

 paratively low. 



