Obviously if we're going to be cooling to sub-freezing temperatures regular old water just won't do.  We also want to avoid glycol/etc due to the fact that we'll be putting FOOD in this thing!  The solution (its a joke):  pickling salt!  Pickling salt's fine granules dissolve easily and can lower water's freezing point to below -20C, which is more than sufficient for our purposes.  You will of course want to give whatever comes out a quick rinse, but you'd do that anyway!