326] PRESERVATION METHODS 



operations by controlled dehydration in dryers or smoke houses) or by 

 the osmotic action occurring during salting whereby moisture is removed 

 at the same time that salt enters the fish. To a very great extent, the 

 preservative effect resulting in both salted and smoked fish, and to a 

 lesser extent with pickled fish, is due to the reduction in moisture content. 



The action of specific chemicals in retarding bacterial spoilage is the 

 second most important factor. Except in the case of pickled fish, this 

 effect is often of minor importance. With pickled fish the presence of 

 considerable acetic acid (vinegar) lowers the pH of the flesh to a point 

 where bacterial action is largely arrested. Salt possesses some bacterio- 

 static action, although a few organisms grow readily in its presence. Other 

 components of wood smoke or spices added in pickling have a mild re- 

 tarding effect on bacterial spoilage but are usually not the major factor 

 in extending the storage life of the product. 



Drying. Drying is carried out not only to prepare dried fish but even 

 more important in this country as a preliminary step in producing smoked 

 and salted fish. In drying fish, important variables include humidity of 

 the air, air velocity past the fish, and temperature during drying. Natural 

 sun drying requires a climate with reasonably low humidity and also 

 either a fairly high temperature or a windy condition. Uniformity in 

 drying requires use of artificial dryers which give more or less complete 

 control of these variables. The humidity of air is frequently reduced by 

 first chilling it, which reduces the moisture content (cold air dissolves at 

 saturation much less moisture than warm air), and then reheating. Heat- 

 ing during the drying of fish is commonly carried out by a heat exchanger 

 in the air supply, by fires under the fish (in connection with smoking), or 

 sometimes by infrared heating. In some cases moisture removal is also 

 achieved by pressing the fish, especially after moisture has been partially 

 released as during salting. 



Salting. In the salting of fish the objective is to rapidly remove moisture 

 and, at the same time, to cause salt to penetrate uniformly into the flesh. 

 The principal preservation action is achieved by the lowering of the mois- 

 ture content. This is enhanced by the fact that at high salt concentrations 

 most bacterial action is halted or at least greatly retarded, although the 

 halophilic bacteria are not so affected. 



In salting fish it is generally highly important to select methods which 

 facilitate rapid removal of moisture and penetration of salt; this is espe- 

 cially important in hot climates where onset of spoilage is rapid unless 

 the preservative effects of the salting process are expedited. 



Penetration of salt into the fish is more rapid when relatively pure 

 sodium chloride is used than when impurities are present. Excessive 

 quantities of magnesium salts, such as the sulfate, which are often pres- 



