FISH MEAL QUALITY 257 



and of individual amino acids^^ as well as of unidentified growth factors^ 

 However, chemical tests are more practical for routine tests, and a num- 

 ber of methods have been developed for in vitro measurements''. 



Several factors affect quality and no one test will give complete infor- 

 mation. Both protein and oil in the meal can be affected by processing 

 and storage and are used for laboratory tests^'^. If the oil has oxidized, the 

 meal will show a low ratio of ether extract to the total oil extracted by 

 acid hydrolysis. This ratio decreases with oxidation or polymerization of 

 the oil in the meal, and it will vary with the length of meal storage and 

 with the species of fish used as raw material. 



Several tests are used to indicate the value of the protein. The pepsin- 

 digestibility method^' ^ is often used, but the range covered by commercial 

 meals is too small to show most differences^^'^^ The method can be used 

 to detect an extremely poor meal, but this rarely occurs, and noting a 

 high amount of charred material by visual observation gives just as 

 reliable results. 



Lysine availability is sometimes used to indicate heat damage in fish 

 meals. Although lysine usually is not the limiting amino acid, if lysine 

 has been damaged, other amino acids will also have been damaged. An 

 in vitro method for ^^available" lysine has been developed^-^*®. 



Digestibility of the meal is affected by physical and chemical factors 

 and is diflScult to measure. Methods for in vitro analysis do not give the 

 same results as those for in vivo analysis. Part of the reason that knowl- 

 edge of chemical composition cannot be used to predict nutritional value 

 is that it does not give a clue to digestibility of the product for the animal 

 being fed. However, digestibility alone cannot be used as a criterion of 

 nutritive value. 



A chick assay ^^ to measure unidentified growth factor (UGF) activity 

 was developed and used to determine correlation between processing 

 variables and UGF activity of fish meals^^ The UGF activity appar- 

 ently was not affected by drying at 390°F for 3 hours or by storage at 

 room temperature for 18 months. Consequently UGF activity should be 

 stable to commercial processing and storage conditions, which are less 

 severe than the above experimental treatments. A wide variety of fish 

 meals and fish solubles were assayed and all had significant amounts of 

 UGF activity. 



Factors Affecting Quality 



The type of raw material used will affect both composition and quality 

 of the meal. The species of fish will help to determine the amount of oil 

 in the meal. The amount of protein will be greater in meals from whole 

 fish than from fillet waste. If the raw material has spoiled, the amount of 



