THE TUNA INDUSTRY 103 



some cans made from long-lined fish, especially where the muscle has a 

 pH greater than 7.2. The crystals are fairly soft and dissolve in the 

 digestive juices, but their glass-like appearance is alarming to consumers. 

 Their formation can be reduced or eliminated by the addition of small 

 amounts of harmless additives to the can before closing which appear to 

 sequester the magnesium ion. 



Another undesirable condition in raw tuna is excessive salt content. 

 Bad handling practices, especially the use of strong freezing brine at too 

 high temperatures and slow thawing, may lead to salt levels as high as 

 4 per cent in the outer layers of tuna. This makes accurate control of 

 the salt level in the final product virtually impossible. Low quality salt 

 containing excessive iron or copper may result in a gray ''metal stain" 

 forming on some pieces of fish after canning, as may contact with copper 

 or brass during processing. 



Prolonged storage of the raw fish, especially in the case of skipjack, 

 may lead to dehydration and oxidation of subcutaneous fat, which shows 

 itself as a yellow to orange coating on the surface of the cooked loin. 

 This appears to be an insoluble complex of oxidized oil and protein. It 

 is costly and wasteful to remove entirely, but if particles remain in the 

 fish, they give a poor appearance and strong, bitter taste to the canned 

 tuna. 



Chemical methods for the measurement of spoilage, in particular, the 

 volatile reducible solids (V.R.S.)^" and volatile acids^^, have been pro- 

 posed for tuna. They have never gained a wide acceptance as processing 

 controls, mainly owing to their relatively lengthy nature, but might be 

 developed as referee methods. 



In practice, most spoilage and quality assessments, especially on the 

 production line, are organoleptic, relying on the skilled judgments of 

 trained personnel. ij 



Quality in Tuna 



The quality of the raw tuna as received is probably by far the biggest 

 single factor influencing the final quality in the can. Efforts to predict 

 the quality of the canned fish from its raw appearance have met with 

 little success so that it is not feasible at present for a processor to select 

 and buy only premium fish. In any case, canneries have to buy the 

 entire load of a boat, good or bad, except for spoiled fish, and so must 

 often accept very variable material. 



The result is an end product which, in spite of all leveling processes, 

 varies widely in color and texture and, to a somewhat lesser extent, in 

 taste and odor. It is, therefore, necessary to separate canned tunajnto 

 grades of quality corresponding to different market outlets, each demand- 



