DETERIORATIVE CHANGES 359 



hold the amino acids together may be hydrolyzed by acids or alkalies or 

 by enzymes. Mild conditions of hydrolysis break only a few bonds with 

 the formation of heterogeneous mixtures of large polypeptides. Heat dis- 

 rupts secondary bonds which hold proteins in their stable configurations. 

 With soluble proteins the change in properties called ''denaturation" is 

 observable as subsequent coagulation. Denaturation also occurs in the 

 presence of certain chemical reagents, such as detergents or urea. The 

 changes in texture which occur in frozen fish during storage or in freeze- 

 dehydration are ascribed to denaturation of the labile fish protein. 



The various side chains of proteins may also enter into other types of 

 reactions. Thus, the disulfide bond ( — S — S — ) of cystine may be con- 

 verted to the sulfhydryl ( — SH) of cysteine by reducing agents. The re- 

 verse reaction can also occur if oxidizing conditions are present. 



The free amino groups of proteins are available for reactions with 

 reducing sugars or with some of the products of oxidation of lipid ma- 

 terial. The end result of a complicated series of reactions is the formation 

 of brown colors and changes in flavors. This reaction is called ' 'non- 

 enzymatic'' browning or the Maillard reaction or the carbonyl-amine 

 reaction. The browning which occurs during cooking or during storage of 

 dried fish products under adverse conditions or the ''rusting" of fatty fish 

 during long-term cold storage are probably all of this nature. The brown 

 products of proteins and carbonyl compound reactions are usually indica- 

 tive of a loss in acceptability. Foreign odors and flavors accompany the 

 color changes. 



In cod, browning or rusting has been carefully studied by Burt and 

 Jones'^ A number of reducing sugars and nitrogenous substances may be 

 involved. Their relative importance depends upon their concentration, 

 and this may be dependent upon the nutritive state of the fish, how long 

 it was held in cold storage, etc. The browning reaction can be inhibited 

 by low temperature, by low moisture content (in dehydrated products), 

 and by the addition of sulfites. The latter has, however, undesirable side 

 effects in meat and fish. 



Changes in Oils. Fish oils hydrolyze to form free fatty acids as occurs 

 in other food lipids. Probably owing to the long chain fatty acids which 

 predominate and which, because of relatively high vapor pressure, are 

 without flavor or odor, hydrolyzed fish oils do not present the problem 

 which occurs with butter, for example, where butyric acid and other short 

 chain fatty acids can occur. Oxidation of the fatty acids giving rise to 

 numerous carbonyl types of products is the chief cause of rancidity in 

 fish oils. 



Although little research involving the mechanism of fish oil oxidation 

 has been carried out, there is little reason to believe that such mecha- 



