METHODS OF PRESERVING FOODS 407 



spoiled. Inconsistent results were obtained with blackberries, 

 raspberries, and tomatoes, thus indicating that more work is neces- 

 sary before it can be used on a commercial scale. Larson, Hartzell 

 and Diehl found that a direct pressure of 6000 atmospheres kills 

 non-spore-forming bacteria in fourteen hours. A pressure of 

 12,000 atmospheres for the same length of time is required to kill 

 spores. They think sterilization by means of pressure may prove 

 valuable from a medical viewpoint as cultures so killed were found 

 very effective in immunization. They are disposed to attribute the 

 sterilization to the sudden change in the osmotic tension of the fluid 

 in which the bacteria were suspended. However, Bridgman's 

 results indicate that it may be due to the coagulation of the bacterial 

 protoplasm. 



Canning. This process in most cases leaves the food sterile. 

 It is, therefore, a sanitary safeguard, and can be used with most 

 meats, fruits, and vegetables, and if properly conducted yields very 

 satisfactory products. 



The method used and the success met with varies with the 

 different products and their condition at the time of canning. Acid 

 foods or those containing large quantities of soluble constituents 

 are canned with considerable ease as compared with the neutral 

 substances (corn, peas, and beans). 



The various methods used may be arranged under three groups: 



1. The heating of the products under pressure for a sufficient 

 time to sterilize. This method although it requires the use of an 

 autoclave is more efficient and requires less time than the other 

 methods. It is used very extensively in large canneries, whereas 

 the intermittent and continuous methods are used to a greater extent 

 in the home. 



2. The intermittent method consists of heating the products 

 on three successive days, maintaining the food at a temperature 

 between heating such that spores will vegetate. The objection to 

 this method is the time necessary in the preparation of the finished 

 product, and anaerobic organisms may not germinate in the intervals 

 between heating but may later with the production of toxins. 



3. The continuous or cold-pack method is being extensively 

 used of late, but Dickson and later Thorn and coworkers have shown 

 that the temperature is not always sufficient to insure the death of 

 all injurious organisms. 



Sugar and Salt. Sugar and salt preserve by increasing osmotic 

 pressure and are very extensively used as they are without injury 

 upon the health of the consumer. 



Sugar is largely used in the manufacture of jellies and preserves. 

 These substances are cooked in the preparation, and this together 

 with the high osmotic pressure of the solution renders them free from 

 pathogens. 



