356 FOOD SCIENCE APPLICATIONS 



because it is converted into ice ; in the salted product free water is bound 

 by the sodium chloride or removed by osmotic action; and in the dried 

 product free water has been removed more or less completely. Heavily 

 smoked products which keep for a long time owe their stability to the 

 removal of water partly as a result of the preliminary salting and partly 

 during the smoking process itself. Bacteria are unable to metabolize in 

 the absence of free water because their assimilative processes are depend- 

 ent on the presence of dissolved nutrients. 



In the case of salted products, the osmotic effects of high salt concen- 

 trations is also important. However, salted products may be subject to a 

 peculiar type of spoilage due to the activities of halophilic bacteria and 

 molds which produce the conditions known as "Pink" and ''Dun" respec- 

 tively. These organisms are capable of growing in nearly saturated solu- 

 tions of sodium chloride. The red pigmented bacteria responsible for Pink 

 are commonly present in solar salts and are difficult to eliminate once 

 they have established themselves in a salt fish processing plant. Some 

 degree of control of Dun can be obtained by use of the fungistatic chemi- 

 cal, sorbic acid, which may be applied to salt fish in the form of a dip. 



The complete inhibition processes are characterized bacteriologically 

 by their reversibility. This is most clearly seen in the case of frozen sea- 

 foods which when thawed immediately show the usual pattern of spoilage 

 as the inhibited bacteria regain their normal capabilities. 



Fermentation and pickling processes occupy a position intermediate 

 between complete inhibition and sterilization. Fermented and pickled 

 products are usually not sterile but have been subject during processing 

 to more or less irreversible changes which preclude the re-establishment 

 of the normal spoilage flora. In each case, the inhibiting substances are 

 acids produced in fermentation by the action of selected microorganisms 

 and in pickling by direct addition of acid or by partial fermentation. 

 Low pH is, like low temperature, a potent general inhibitor of bacterial 

 growth, but microorganisms (particularly yeasts and fungi) exist which 

 can grow at fairly low pH levels and these can cause deterioration of 

 acid fish products. 



Sterilization. There is actually only one commercial seafood processing 

 method which is designed to achieve the objective of sterility. This is 

 canning. However, the experimental technique of high dose irradiation is 

 also designed to achieve sterility. In each of these cases the bacteria on 

 the seafood are killed and the spoilage process can only be reinitiated 

 by recontamination of the foodstuff from an external source. To pre- 

 vent this, the food is enclosed in a hermetically sealed container prior 

 to sterilization. 



Spoilage of such a food can only result from faulty processing or a flaw 



