commercial load. (A rate of approximately 

 0.02 inches per hour.) 



For comparison, the Codex Alimentarius pro- 

 posed draft provisional standard for frozen 

 gutted Pacific salmon specifies a minimum 

 freezing rate of 0.25 inches per hour. Although 

 there are some differences in the operating cir- 

 cumstances, a higher rate for freezing, or hold- 

 ing, at lower temperatures would improve 

 either the yield (the number of cans of tuna per 

 ton of raw tuna) , the quality, or both. 



Freshly caught yellowfin and skipjack were 

 still-air frozen at sea, stored at temperatures 

 of 14°, 8°, and —5° F for the time periods 

 of approximately 30, 60, and 90 days, and then 

 canned. These were compared with tuna of 

 the same size and species caught in the same 

 sets, which had been stored for approximately 

 30 days in a commercial brine well before can- 

 ning. The quality of the quicker air frozen 

 product showed some improvement over the 

 brine frozen controls, although there was no 

 remarkable diflference in the quality between 

 fish frozen and stored at different times and 

 temperatures (Finch and Crawford, unpub- 

 lished). These samples had been still-air froz- 

 en and the rate of freezing was fairly slow. 

 Therefore another experiment was carried out 

 in which yellowfin, skipjack, and bluefin were 

 frozen very rapidly by immersion directly in 

 liquid dichlorodifluoromethane (Freezant 12), 

 (Crawford et al., 1969). The time taken to 

 reach 0° F was measured and found to run 

 from 3 hr for 61/2-lb. skipjack to 7 hr for 52-lb. 

 bluefin. The freezing rate conformed to the 

 equation, Z = 67.5 W where Z is the time in 

 minutes to freeze to 0° F and W is the weight 

 in pounds. Some of the tuna were frozen im- 

 mediately when taken aboard the vessel and 

 some held in brine wells for 11 and 25 hr before 

 freezing. After freezing, some of the fish were 

 stored at 0° F and some at —50° F. On re- 

 turning to the cannery, the fish were thawed 

 in running seawater and canned at the same 

 time as samples of fish from the same sets which 

 had been frozen commercially in the vessel's 

 brine wells. The fish which were frozen rap- 

 idly in Freezant 12 showed quality improve- 

 ments over those frozen in the ship's well, espe- 

 cially in the case of the 16-lb. yellowfin, /md in 

 the case of skipjack which had been stored at 

 — 50° F after freezing. It was also noted that 



the blood spots and flecks were quite prominent 

 in the rapidly frozen yellowfin and bluefin tuna 

 as compared with the brine well controls, which 

 showed little or none. This was the same effect 

 we had found with the very fresh albacore and 

 bluefin before brine or ice storage. It would 

 be premature to come to a final conclusion on 

 the basis of one experiment, but a first impres- 

 sion is that the improvement by rapid freezing 

 produced a definite but fairly small quality in- 

 crease over tuna which is properly brine frozen. 

 It must be emphasized in relation to these re- 

 sults that so little is known about consumer 

 reactions to higher quality that it is not pos- 

 sible at present to tell what level of improve- 

 ments are needed to be significant to marketing. 



Relation of Refrigeration Experiments 

 to Present Practices 



In looking at the present quality of the com- 

 mercial pack in relation to the improvements 

 obtained by better freezing methods, it is clear 

 that there are two areas of potential upgrading. 

 One is by improvement of the present refriger- 

 ation procedures to improve the poorer commer- 

 cially landed tuna until it is equal in quality 

 to the best that is brine frozen by present meth- 

 ods. The second is by improvement of the 

 overall level of quality beyond this present 

 point by the application of new methods. The 

 first of these is likely to be more feasible for 

 immediate exploration. While the important 

 longer term prospects cannot be neglected, we 

 should concentrate our activity upon learning 

 more about the exact points of weakness and 

 strength of brine freezing techniques so that 

 we have a sound knowledge upon which to base 

 improvements. 



During a review period we made a start with 

 both these activities. We worked out a speci- 

 fication for a small brine-freezing unit capable 

 of being carried on the deck of a tunaboat. This 

 is a simplified compact unit, designed for pro- 

 grammed chilling and freezing, so that a series 

 of precise refrigeration circumstances can be 

 studied in a manner similar to shipboard con- 

 ditions. Once its operation has been worked 

 out, more of these units could be put aboard 

 commercial tuna vessels so that, with the co- 

 operation of the engineers, there can be col- 



