SAPROPHYTES 419 



they begin action on the food in a can, they soon liberate suffi- 

 cient gas to cause the ends of the can to bulge out and become 

 convex. Cans which thus show this swelling are discarded 

 before they leave the factory. Some kinds of bacteria, how- 

 ever, do not produce gases by their action and so the failure 

 of a can to swell is not an absolute guarantee that the food 

 is perfectly preserved. 



In ordinary practise in canning factories, the food is heated 

 to a temperature of from 110C. to 12oC. for from 20 

 minutes to a half hour. The operator determines the tempera- 

 ture by noting the reading of the steam gauge. What pres- 

 sure is required forllOC.? for 125C? (Table VI, page 135.) 

 The time and temperature vary with the kind of food that is 

 being preserved and with the size of the cans. Equipped with 

 this power to destroy by heat the most resistant forms of 

 microorganisms, the canning factories are able to preserve 

 any kind of food that is not seriously injured by the high tem- 

 peratures. Fruits and vegetables of every kind, milk, meats, 

 soups, and many other forms of food prepared ready for use are 

 now preserved by canning. 



These foods are preserved when they are in season and 

 generally in the part of the country where they are produced. 

 They then become available throughout the year and in every 

 part of the country. Before the introduction of canning, the 

 food of the people naturally varied considerably at different 

 times of the year. Fruits and vegetables were abundant in 

 the summer time and scarce in the winter, but now we may 

 have practically the same variety of foods throughout the year 

 at very little added cost. 



Canning factories, like most other modern factories, have 

 many forms of labor-saving machinery which help to cheapen 

 the cost of their products. A discussion of these, however, 

 would take us outside our present topic, and besides this, you 

 could learn more in an hour's visit to a neighboring factory than 

 we could tell you in several pages. 



483. Sterilization and Pasteurization. It is obvious from 



