358 FOOD SCIENCE APPLICATIONS 



Chemical Changes 



Early Post Mortem Changes. Ordinarily the most important post 

 mortem change in fish is the changing of the muscle metabolism reactions 

 largely to irreversible ones with the resulting accumulation of lactic acid 

 in the tissue and a decline in its pH. The pH of the living fish muscle is 

 not far from 7.0; as a result of post mortem accumulation of lactic acid, 

 pH values in the range of 5.8 to 6.2 are reached at peak rigor develop- 

 ment. The minimum pH attained varies somewhat with the species of 

 fish, being a function of the initial glycogen content, the buffer capacity 

 of components of the fish tissue, and the rate of the various post mortem 

 reactions which result not only in accumulation of lactic acid but also in 

 its subsequent oxidation and disappearance. Shellfish, such as oysters 

 and clams, which contain relatively large quantities of glycogen attain a 

 much lower pH as a result of post mortem changes with values of pH 5.0 

 or even lower being not uncommonly reached. 



The lowering of the pH is important as a means of decreasing the rate 

 of bacterial multiplication, thus in effect prolonging the keeping quality 

 of the fish to a considerable extent. The pH of the fish at the time of 

 freezing affects the texture of the frozen product because of its relation- 

 ship to ice crystal formation and cell damage which causes variation in 

 expressible fluids 



A second immediate post mortem change important only in certain cases 

 occurs where the fish have been feeding upon certain types of food (e.g., 

 "red feed,'' usually a microscopic shellfish such as Callanus finmarchicus) 

 which have activated the digestive enzymes to a high degree. When this 

 occurs, an extensive proteolysis of the fish tissue adjacent to the digestive 

 tract occurs such that in extreme cases all of the flesh adjacent to the 

 backbone, and sometimes extending for a considerable distance into the 

 muscle, is completely liquefied. This form of autolysis, resulting from 

 action of digestive enzymes, is the only one having serious consequences 

 in the practical handling of fish. 



There are other early post mortem changes which occur in fish tissue 

 as a result of enzymatic changes in the protein and other components. 

 Thus it has been shown that there is a small but definite increase in 

 free amino acids which occurs early in the storage of freshly caught 

 fish^. Other early changes result in some as yet unresolved alteration in 

 flavor components which cause a decrease in the normal species flavor 

 characteristics. 



Changes in Proteins. The nature of proteins has been discussed briefly 

 in the previous chapter. These high polymers are affected in a number of 

 ways by environmental conditions. The primary peptide bonds which 



