552 MICROBIOLOGY OF FOODS 



preservative substance in solution, and the whole mass of food sub- 

 stance is permeated by them. Potassium nitrate (saltpeter) reacts 

 with the flesh, being reduced in part to nitrite. This enters into a 

 combination with the coloring matter of meat, which upon cooking 

 produces the characteristic red color of meat cured with saltpeter. 



The various manipulations during the process of pickling or dry 

 curing serve to introduce numerous microorganisms. Many of these 

 may flourish in the pickling fluids, but in a sufficient concentration of 

 salt and at a sufficiently low temperature, decomposition ordinarily does 

 not progress so as to become objectionable, and proteolytic decom- 

 position (putrefaction) is effectually prevented. This protection of 

 the protein depends to some extent upon the acidity of the medium, 

 which in turn is due largely to the bacterial decomposition of the 

 muscle sugar. The powerful putrefactive bacteria (B. cedematis 

 group) flourish only in an alkaline medium. On the other hand, too 

 high a degree of acidity becomes in itself objectionable on account of 

 the sour or rancid taste, and it is, therefore, important that the acid- 

 producing bacteria should be held in check somewhat. In practice, 

 saltpeter has proved of value for this particular purpose, and its 

 action apparently depends upon the antiseptic effect of minute quan- 

 tities of nitric acid (HNO 3 ) and nitrous acid (HNO2) set free from the 

 salt by the excess of organic acids produced by the bacteria. The 

 curing of meats by pickling solutions is often supplemented by desicca- 

 tion and impregnation with the antiseptic substances of wood smoke. 



The dry-salting of codfish is an example of preservation by increasing the osmotic 

 tension. The fish is cleaned and beheaded, split longitudinally, and the vertebral 

 column removed. It is then carefully washed, and all visible blood is removed. The 

 pieces are next covered with dry salt and packed in open casks. The salt rapidly 

 extracts water from the flesh and a strong brine results. After a few days the casks 

 are emptied out, and the pieces of fish, now smaller because of the loss of water, are 

 again thoroughly washed and again packed in dry salt so that the adjacent pieces of 

 fish are completely separated by an intervening layer of solid salt. The contents 

 of the cask are subjected to high pressure to remove air, and the cask is finally closed. 



The curing of ham is an example of preservation by increased osmotic tension 

 combined with the addition of chemical preservatives. After slaughter and chilling, 

 the hams are injected with a solution containing 25 per cent common salt, 15 percent 

 granulated sugar, and 12 per cent saltpeter, and are then stored at a low tem- 

 perature, preferably between o and +4, in a brine containing about 20 per cent 

 common salt, 5 per cent sugar, and i per cent saltpeter. The brine is renewed once 

 or twice at intervals of a week or ten days. After about a month the hams are 



