rates until the tuna can is opened. 



These changes are, of course, much 

 slower after canning. The changes in qual- 

 ity which occur at the cannery prior to 

 actual canning have also been investigated, 

 particularly the changes which occur during 

 butchering, precooking and cooling. These 

 results will be reported elsewhere and will, 

 therefore, not be subject to comment here 

 except to state that the changes at the 

 cannery are easier to recognize and identify 

 and may be followed experimentally without 

 much difficulty. 



The situation is different when it 

 comes to an investigation of the quality 

 changes in tuna while in transit. It is 

 evident that a thorough knowledge of the 

 general method of handling of tuna at sea 

 is indispensable for an inquiry into the 

 nature of quality changes of tuna. Further- 

 more, it is important to use methods of 

 analysis which will permit the measurement 

 of such changes. But until we know what 

 the nature of these changes is, it is dif- 

 ficult to select the proper analytical 

 procedure. In the past and to a certain 

 extent at present, the organoleptic test 

 has been predominantly used. The subjec- 

 tivity of this test, however, always leaves 

 room for difference of interpretation and 

 such a method does not reveal very well the 

 cause of the changes observed. Several 

 chemical tests have been used by various 

 investigators in connection with quality 

 evaluation of tuna, and as used by us, they 

 have thrown light on the changes which oc- 

 cur during one stage or another of the 

 several steps into which the preservation 

 of tuna on board the vessel may be divided. 

 By simulating in our San Pedro experimental 

 freezing plant, the freezing procedure which 

 takes place on board, we have acquired a 

 deeper insight into the relative influence 

 that each step of the refrigeration process 

 has on the overall change in quality. 



Many chemical analyses have been made 

 of samples of tuna taken from returning 

 clippers, and have been compared with ana- 

 lytical results obtained from fresh tuna 

 which has undergone similar treatment in 

 our experimental freezing plant. The ana- 

 lytical work has covered investigations of 

 oxidative changes in the fatty matter con- 

 tained in the tuna muscle, oxidative changes 

 in the protein substance of the tuna^ bac- 

 teriological examinations of skin, muscle 



and viscera of tuna, and the determination 

 of volatile substances released in the tuna 

 muscle tissues as a result of incipient 

 spoilage. Finally, the problem of salt 

 penetration into the tuna muscle has been 

 investigated, and we have established that 

 the thawing of the tuna often causes more 

 salt penetration and spoilage than does 

 the cooling, chilling and freezing. 



These studies, which by no means have 

 been completed, have led us to differentiate 

 between quality changes caused by spoilage 

 factors and quality changes caused by de- 

 naturation. 



By denaturation, we mean the undesir- 

 able quality changes which may occur when 

 tuna is refrigerated in brine. As a result 

 of refrigeration in brine, certain changes 

 in the muscle tissue of the tuna take place 

 which might affect the texture, the taste 

 and the yield of the end product. The 

 process of denaturation is a complicated 

 one, and involves a partial change of state 

 of the muscle protein such as dehydration, 

 reduced solubility in certain electrolytes, 

 and a change in its colloidal properties. 

 Much work has been done by others on the 

 denaturation of protein by heat, but less 

 on denaturation of protein by freezing. 



Little is known about protein dena- 

 turation in fish in general and practically 

 nothing is known about the denaturation 

 which takes place when tuna is frozen in 

 a brine solution under conditions such as 

 prevail in a clipper ship. Some efforts 

 to relate degree of denaturation of fish 

 protein to its solubility in sodium chloride 

 solutions, of given strength, have been made 

 by the British. Their method has seemingly 

 been successful in its application to muscle 

 protein from cod, and we are trying to apply 

 this method to tuna. Some work has recently 

 been done in our laboratory which aims at 

 relating the electrode potential of a tuna- 

 muscle homogenate to degree of denaturation. 

 It is too early to express any opinion about 

 the success of the method. 



While this distinction between dena- 

 turation and spoilage as a cause of quality 

 change, which we have just made, is not a 

 sharp one, it is our belief that denatura- 

 tion of tuna plays a greater role in quality 

 of landed tuna than does spoilage. 



When in the thirties the percentage of 



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