520 U. S. BUREAU OF FISHERIES 



ally lowered. When the body of the fish reaches the freezing point 

 of body juice, it begins to freeze. In general, the outer parts of the 

 fish are colder than the inner parts, though there is no sharp line 

 of demarcation. The fish gradually becomes firm, and finally hard 

 throughout. In this case the factor controlling the speed of freezing 

 is the removal of heat from the surface, for the heat flows from the 

 inside of the fish to the surface as fast as it can be carried away by the 

 air, which is a very poor conductor of heat. When heat is removed 

 very rapidly, as by immersing it in very cold, rapidly moving brine, 

 the factor that limits the rate of freezing is the conductivity of the fish 

 itself. The surrounding brine, being a good conductor and in rapid 

 motion, removes the heat from the surface as fast as it can be 

 conducted from the inner parts. The outer parts of a fish may be 

 frozen hard, while the innermost parts are still quite unfrozen. 

 The outer frozen shell becomes thicker and thicker as freezing pro- 

 ceeds inwardly, always sharply demarked from the inner unfrozen 

 core until freezing is complete. The slowly frozen fish is highly 

 solidified, while rapidly frozen fish are of a more waxy consistency. 

 Between the two there are other profound differences in microscopic 

 structure that will be referred to later. 



CHANGE IN VOLUME 



Water in freezing expands by about 8.8 per cent of its volume. 

 Fish expand accordingly, in proportion to the amount of water pres- 

 ent and the amount of that present which is frozen. It is not all 

 frozen in ordinary practice. It has been shown by measurements of 

 expansion that in gels not all the water is frozen until extremely low 

 temperatures are reached (about 103° F. below zero). It is sup- 

 posed that some of the water remains diffused in the " capillary " con- 

 dition. As fish contain from 65 to 80 per cent water, the expansion 

 may be estimated at from 5.7 to 7.1 per cent of volume. Whether 

 this expansion is responsible for any of the structural changes in fish, 

 such as rupture of gall bladder or destruction of cell membranes, is 

 as yet uncertain. It is not now considered to be so important as it 

 was some years ago. Expansion is of importance where fish are 

 frozen confined in molds or cans, where allowance must be made for 

 expansion. It can not be prevented or resisted. 



COAGULATION OF PROTEIN GELS 



As stated previously, the cell contents of fish are a semiliquid gel 

 of protein in water, with small amounts of numerous substances in 

 solution. It has been reported by various investigators that such 

 gels, on being frozen and under certain conditions, become coagu- 

 lated. In the report of the Food Investigation Board of Great 

 Britain for 1923 it is shown that egg albumen, if frozen at a tem- 

 perature not colder than 21° F., will defrost as a liquid similar to 

 what it was in its unfrozen state, but that if frozen at a moderately 

 lower temperature it can not be returned to its original condition by 

 defrosting but will be coagulated. However, if frozen with extreme 

 rapidity in liquid air and defrosted with great rapidity in warm 

 mercury it is not coagulated. These experiments indicate that there 



