fected by scorch and caramelization. Scorch 

 is the tan to brown color seen on that surface 

 of the canned tuna which is not covered with 

 liquid (headspace) during the retort process. 

 In most cases the commercial pack is randomly 

 dropped into retort baskets and so the head- 

 space scorch can occur in any position in the 

 can. It is usually easy to identify. The second 

 type of browning reaction, caramelization, is a 

 general browning occurring throughout the 

 whole of the tuna in the can and occurs usually 

 only under the more severe conditions of re- 

 torting: at a low level it ijiay only have the 

 effect of generally dulling the appearance of the 

 canned product. The former type of browning 

 appears to be due to Maillard reactions (non- 

 enzymic browning) , which occur widely in food 

 processing. This reaction is due to the inter- 

 action of a carbonyl compound, usually a re- 

 ducing sugar such as glucose, fructose, or ri- 

 bose, with amino compounds, usually proteins 

 or amino acids. Such reactions have been de- 

 scribed as occurring during the prolonged heat- 

 ing or during the drying of fish (Jones, 1957). 

 Sometimes carbonyl compounds from fish oils 

 and other sources may be involved. 



We found that by adding increased amounts 

 of glucose and ribose and some of their nat- 

 urally occurring phosphorylated derivatives 

 to tuna before canning, progressive scorching 

 and caramelization could be developed. Small 

 amounts of added sugars first produce a head- 

 space scorch. As the amount of added sugar 

 is increased, it becomes more intense and then 

 an overall browning appears in the tissue. At 

 higher concentrations, a bitter flavor is also 

 noted. About three times as much glucose as 

 ribose is required for the same scorch intensity, 

 and the scorch increases with prolonged re- 

 torting, corresponding to the increased process 

 time required for institutional packs. Replace- 

 ment of the air in the headspace with nitrogen 

 gives partial protection during long periods of 

 retorting. 



We have not made extensive measurements 

 on tuna, but, in general, glucose is normally 

 present in freshly caught fish and has been 

 found at quite high levels in tuna (Crawford 

 et al., 1970; Tarr, 1954; and Jones, 1957). 

 The amounts of glucose present are likely to 

 depend on various factors relating to the cir- 

 cumstances of catching. On storage the glucose 



decreases, and ribose, liberated during the last 

 stage of the breakdown of the purine com- 

 pounds, progressively increases. The ribose 

 in tuna may decrease depending on the activity 

 of the spoilage bacteria or other factors. In 

 the case of tuna, there is likely to be some loss 

 of sugars and soluble nitrogen compounds in 

 the stickwater during the precook. 



These general observations tie in with results 

 on chill storage of albacore (Crawford and 

 Finch, 1968). Fresh caught samples showed 

 considerable scorch after canning, presumably 

 due to high glucose content, although the sugar 

 contents were not measured. In later samples 

 the scorch was considerably less, but with the 

 tuna canned during the final week of storage 

 (between day 27 and day 35), scorch became 

 more severe, presumably due to the increase 

 in ribose. It is not clear at present how these 

 observations can be turned to advantage except 

 perhaps that sugar measurements on the raw 

 fish may serve as another index of the quality 

 to be expected after canning. It is quite pos- 

 sible that a more complete understanding of 

 the way in which factors such as catch method, 

 exhaustion of the fish, chilling, freezing, stor- 

 age, and thawing conditions, aff'ect the devel- 

 opment of sugars may enable suggestions of 

 control measures to reduce scorch and discolor- 

 ation due to this cause. This would be espe- 

 cially valuable in the institutional pack which 

 is increasing in importance to the industry, 

 since the prolonged retort process makes it 

 especially vulnerable to this kind of quality loss. 

 The sugar content of skipjack tuna, and their 

 relation to scorch, was therefore chosen as one 

 of the studies to be included in this investiga- 

 tion. 



Purine compounds. — In the living fish, en- 

 ergy for muscular action is provided by a series 

 of high energy phosphate compounds. The 

 most important is adenosine triphosphate 

 (ATP) which is converted to adenosine mono- 

 phosphate (AMP) as it gives up its energy 

 during muscular contraction and then is rebuilt 

 to ATP by creatine phosphate. When the fish 

 dies these compounds go through a series of 

 degradations to successively simpler compounds 

 at rates which depend on the storage tem- 

 peratures. The sequence of degradation is 



11 



