I am looking for any kind of fluid which has a freezing point below the temperature of a regular freezer (about -20 C). My plan is to keep this liquid in my freezer for fast cooling of beverages, so the liquid needs to be harmless to drink in case some is left on the can. Suggestions? Or maybe the liquid will cool down the beverage too fast making it impossible to get a proper even drinking temperature without freezing the beverage?
If brine is either unpalatable or has too high a freezing point for you, other options include a solution of glycerol in water. Glycerol is perfectly safe to ingest in modest quantities -- it's used as a food additive and a medicine -- although it tastes a little weird (a bit like extra-sour sweet and sour sauce).
Although it's mostly been replaced by ethylene glycol now, glycerol used to see a lot of use as an antifreeze, and is still used in domestic applications like ice cream makers. Apparently it's best used in a 60-70% solution, when it can get a freezing point below -40°C.
The paper linked below puts the optimum point at -46.5°C with two-thirds glycerol, one-third water by weight.
- "Freezing Points of Glycerol and Its Aqueous Solutions", Leonard B. Lane, Ind. Eng. Chem., 1925, 17 (9), pp 924–924, DOI: 10.1021/ie50189a017
Aim for a brine solution: mixture of water and NaCl. The lowest freezing point for a brine solution is about -20$^\circ$C, obtained with approximately 23 mass % NaCl, as is seen in the phase diagram below (you want to stay within the "brine" part of the diagram). This is not much below the freezing point of a regular freezer, but you may get away with some brine + ice. Or set your freezer to -18$^\circ$C.
Concentrated salt solutions is toxic, so make sure to rinse and wipe off any residues. In comparison, sea water is around 3,5 % salt, and we all known we should not drink that!