Freezing Fish With Salt

Hopefully someone may be able to explain this for me. I have heard many stories from fishermen who claim that adding salt to a sealed, insulated but unrefrigerated container of ice and freshly caught fish (melting I presume as not refrigerated) will result in the fish freezing. I understand that the super salty water has a lower freezing temperature than the salty fish but as it is an unrefrigerated container and the salt was added after the ice was frozen this could not possibly lower the temperature of the fish to freezing? The salty ice slurry couldn’t become colder than the original pure iced water by the addition of salt!? Even if the fish is less salty than the salted ice/brine and therefore has a higher freezing temperature how could they freeze? Even if there was pure (unsalted) water in the fish tissues surely the temperature could only increase with the addition of salt and the fish not the opposite?
- Chris (age 45)
Darwin NT Australia

This is one fish story that's probably true. Dissolving ordinary NaCL salt in water causes only a slight decrease in temperature. It takes energy for the salt ions to break away from their crystal and dissolve in the water. Then as as the salt causes the ice to melt, that soaks up a lot of heat. It takes energy for water molecules to shake loose from the ice crystal and join the liquid. (That's called the latent heat of the melting/freezing process.) So yes, the brine and the ice really do get colder than they were initially, dropping down to the new, lower, melting point, at least if there was enough ice to start with. That could freeze the fish.

You could try the experiment. Make a slurry of water and ice and see how its temperature changes when you stir in some salt. You may have to be careful and do a control experiment stirring in sand or something to a similar slurry, just to allow for any gradual changes not connected with the salt.

Mike W.

(published on 10/31/2012)