METHODS OF PRESERVING FOODS 405 



1. Physical The more important methods of this class are heat, 

 cold, and drying. The last two are regarded by the sanitarian as 

 antiseptic rather than germicidal, as they usually arrest the growth 

 of the organisms, but do not kill them. 



2. Chemical. Under this class we have two groups. 



(a) Those chemicals which preserve through their influence 

 upon the medium in which they are placed. The majority of these 

 act by increasing the osmotic pressure of the solution, and the 

 important constituents used are salt and sugar. These are without 

 objection from the hygienic standpoint. 



(b) Those substances which bring about a chemical change within 

 the medium in which they are placed or which combine chemically 

 with the living protoplasmic substance, or inhibiting their natural 

 functions. Their function is then to prevent bacterial digestion. 

 They are for the most part injurious, for if bacteria have attempted 

 to digest food and failed, man need not try. 



Cold. Cold has come in recent times to be of inestimable value in 

 food preservation, and such food usually commands a higher market 

 value than food preserved by other means. This is due to the fact 

 that refrigerated foods in most cases very closely resemble in 

 appearance, taste, and nutritive value the fresh article. Food 

 can be kept in a satisfactory condition in cold storage for a very 

 long time. The time varies with the specific article, its condition 

 when placed in cold storage, and the temperature at which it is kept. 

 Moreover, some foods such as meats pass through a stage of ripening 

 while in cold storage so that when removed they have developed a 

 tenderness and even more delicate flavor than that of the fresh 

 product. 



The temperature at which foodstuffs must be kept varies with 

 different articles. Fish are usually frozen, dipped in water, and 

 refrozen. The formation of a coating of ice acts as a. protection 

 against bacteria and also prevents dessication. They are then 

 stored at 16 C. Smith has shown that fish may be so kept for as 

 long as two years without depreciating in nutritive or sanitary value. 



Meats are usually surface-dried before they are placed in cold 

 storage. This prevents the formation of a film of water on the 

 surface which under some conditions may act as a good cultural 

 medium. Fggs and milk are materially injured by freezing; hence, 

 they are ordinarily kept at a temperature just above C. 



Cold is a disinfectant and not a germicide, and although a tempera- 

 ture of C. will prevent the growth of pathogens, some saprophytes 

 may actually multiply at this temperature, but as the temperature 

 is decreased the pathogens slowly succumb. Nevertheless, cold 

 alone should not be relied upon as a protection against pathogenic 

 bacteria. 



Most animal parasites die If kept in cold storage long. Rosenau 



