Figure 5. — Soluble tryptophan recovery from raw and 

 cooked hake as a function of enzyme concentration. 

 Alcalase, 5 hr at 55°C and pH 9.5. 



100 



° 80 



60 



<d 40 



J_ 



1 



_L 



2 



J_ 



± 



± 



± 



7 



J I 



3 4 5 6 

 pH OF AUTOLYSIS 



Figure 6. — Percent solubilization versus pH for autolysis 

 of red hake, 24 hr at 50°C. 



raw hake with 0.5% Alcalase, however, would re- 

 quire a 7-fold increase in enzyme concentration. 

 Roughly similar results were obtained from plots 

 of crude protein recoveries versus concentra- 

 tions of added enzymes. 



Autolysis of raw hake. — The optimum tem- 

 perature for autolysis of red hake is in the range 

 of 50° to 55°C. This holds even for a 24-hr au- 

 tolysis although a lower optimum temperature 

 would be expected on the basis of theory. The 

 optimum pH is about 7 as shown in Figure 6 

 which is based on the 24-hr digestion at 50°C. 

 Approximately optimum conditions are obtained 

 without any pH adjustment in which case the pH 

 drops about one-half unit from an initial value 

 a little above pH 7. Rate of hydrolysis increases 

 with lower substrate concentrations. In one 

 test, a 4-hr autolysis at 55°C, reducing the con- 

 centration of fish from 50% of the slurry to 20% , 

 increased degree of solubilization by about 12%. 

 At a concentration of 5% fish solubilization was 

 increased by over 20% . A 50% slurry has been 

 the standard used in our preparation of hydrol- 

 ysate products, but the optimum concentration 

 must be determined on the basis of several as- 

 pects of production economics for a specific pro- 

 cess. 



Different catches of hake differ in autolytic 

 activity, as would be expected. The addition of 

 commercial enzyme preparations has a greater 

 effect on fish with lower native activity but the 

 total degree of solubilization remains lower if 

 both catches are processed similarly. 



Microbiological aspects. — Although red hake 

 are not sterilized prior to hydrolysis, it has been 

 definitely established that solubilization is due 

 to native enzymatic activity and not to the action 

 of bacteria. In one experiment hake were evis- 

 cerated and the flesh and viscera were ground- 

 ed separately. Flesh-water, viscera-water, and 

 flesh-viscera-water slurries were agitated at 

 55°C without pH control. Samples were with- 

 drawn at intervals and the amount of solubilized 

 protein was measured. Soluble protein was al- 

 ways least in the flesh-water slurries and great- 

 est in the slurries containing at least 2% viscera. 

 Total bacterial plate counts were less than 300 

 per gram of raw fish and were essentially zero 

 for all slurries after 24 hr. 



A second autolysis experiment involved agi- 

 tation for 24 hr at 55 °C of slurries containing 

 (a) raw hake, (b) cooked hake, (c) raw viscera 

 plus cooked flesh, and (d) cooked viscera plus 



15 



