ANALYTICAL METHODS 371 



Results are usually somewhat different than when the entire flesh is used. 

 This procedure is useful particularly when relative rather than absolute 

 values are involved (for example, it is useful for preparing samples for 

 comparison of the freshness of two batches of fish but where no legal or 

 other standard is involved). 



Proximate Composition 



Proximate composition of fish is generally determined by standard 

 procedures such as those of the A.O.A.C^. The only component under 

 proximate composition which may present special problems with fish is 

 that of determination of the oil content. Fish oils are highly susceptible 

 to oxidation. Oxidized oils may be only slowly extracted by conventional 

 fat solvents. In some cases, also, the fish oils in fish flesh or other fishery 

 products may be combined with protein or other components and are 

 then only partially extracted by ethyl or petroleum ether. This is the 

 case, for example, with fish meals which have been stored for a long time. 

 Use of acetone^ or acetone containing acid (which partially hydrolyzes 

 the complex lipids) ^^ is helpful in such cases. Or a mixed solvent such as 

 chloroform and methanol^ can be used. Certain portions of fish such as 

 roe and milt have most of their oil in phosopholipid or similar complex 

 form. Such products must be analyzed using one of these techniques if 

 the total oil content is to be determined. 



Freshness Tests 



Under this section will be considered tests for deteriorative changes in 

 the nitrogenous portion of fish (mostly as a result of bacterial decompo- 

 sition). Tests for rancidity are discussed in the section on fish oils. 



While many chemical and bacterial tests have been proposed as giving 

 a measure of the freshness of fish, none of these are universally applicable. 

 Actually such tests usually are expected to correlate with various organo- 

 leptic conditions such as flavor and odor. Flavor and odor are subjective 

 tests and their organoleptic measurement suffers all the limitations such 

 as lack of reproducibility that are common to such tests. Nevertheless, 

 these organoleptic evaluations are the ultimate standard against which 

 any scientific tests for freshness must be compared. In particular, flavor 

 and odor, which are the most important criteria for fish freshness, are 

 dependent upon the presence or absence of not one but many chemicals. 

 It is not surprising, therefore, that no single freshness test has been found 

 which is both an accurate measure of freshness of fish and fairly uni- 

 versally applicable. Chemical freshness tests for fish can be classified^- as 

 specific tests (tests for a single chemical component) and group tests 

 (tests for a group of related substances). From what has just been said, 



