572 U. S. BUREAU OF FISHERIES 



in at about the same time, while in brine the iish pass through the 

 process in small lots; but, other things being equal, as much time 

 would be required to freeze a day's capacity by one process as by 

 another. 



The means of freezing fish may be classified for the purposes of 

 this discussion as (a) those methods in which fish are immersed 

 directly in or exposed directly to brine, and (b) those methods in 

 which fish are inclosed in molds, cells, or containers, the walls of 

 which separate the fish from the brine but make good contact with 

 both. These two classes of freezing methods will now be discussed 

 at some length. 



FREEZING IN CONTACT WITH BRINE 



If the liquid to be used for freezing is to come in direct contact 

 with the fish, we are limited virtually to sodium chloride or common 

 salt brine. It is the only chemical substance that is tolerated by the 

 human palate and stomach, while being cheap, harmless, and afford- 

 ing a solution of sufficiently low freezing point. Calcium-chloride 

 brine has an acrid, disagreeable taste, and any trace left on the fish 

 would be objectionable. The same may be said of magnesium 

 chloride. Glycerin and alcohol are excessively expensive and are 

 otherwise objectionable. A mixture of salt brine and glycerin has 

 been proposed and is being used to some extent. 



STERILIZING EFFECT OF BRINE. 



Strong brine has a distinctively destructive effect on bacteria, as 

 is well known from its use as a preservative for meat, fish, etc. 

 Coming in direct contact with the slime on the skin and gills of fish, 

 where decomposition starts very easily, the strong brine used in 

 freezing plasmolyzes and kills many putrefactive bacteria. Green 

 has investigated, bacteriologically, the keeping quality of brine-frozen 

 fish in comparison with those frozen in air. Her report on this work 

 is as follows: 



Samples of brine and dry frozen fish were brought out of store and placed on 

 trays with an equal number of perfectly fresh herrings straight from the 

 drifter. These were left away from the sun and wind at a temperature of 

 about 60° F. for two or three days, and then bacteriological samples were 

 taken from them and inoculated into fish broth. 



Observations. — (a) Brine-frozen herrings produced the least amount of 

 bacterial growth, {b) Dry-frozen herrings produced by far the greatest amount 

 of growth. This experiment, like the first, shows that bacteria do continue 

 to multiply at a temperature of 18 to 20° F., whereas the brine-freezing method 

 not only inhibits growth but kills many of the forms of bacteria present before 

 they have time to spore. 



RAPIDITY OF FREEZING IN BRINE 



Figures that show the speed of freezing in brine as compared with 

 that in air already have been given (p. 571). The larger the fish to be 

 frozen, the smaller the difference, as shown in Table 17. It is easily 

 understood why the larger objects freeze more slowly than the 

 smaller ones, when we remember that the volume of a solid object 

 increases as the cube of its diameter, while its surface increases onlv 



