There’s nothing like cracking a cold beer on a hot summer day, but what if your six-pack isn’t chilled? You’ve got to find a way to get those brews frosty, lest you face the horror of a lukewarm libation. That’s why we’re trying a couple methods to cool your beer quickly.
Part of what is needed for the towel (though it may need more time too) is air circulation.
If the air cannot move around the towel, it won’t be able to chill the towel and contents of the can. The can keeps the towel warm enough to stay floppy. It is similar to the non-electric freezers that can get cold enough to freeze water in the middle of summer heat. Short summary, you take a sealed vessel to hold whatever needs to be chilled/frozen, surround it with a porous material that can hold moisture (soil works well), then place all that in a container that can allow the air move freely over most of the surface. Provided the moisture remains, all that heat will get sucked out by the water.
Why would adding salt to the ice bath accelerate cooling of the beer?
Freezing water is around 0° C, but freezing salt-water can go as low as -21° C. In early experiments, Gabriel Fahrenheit was able to get freezing salt water down to about -18 C, or what he called 0° F.
Side note: Fahrenheit soon realized that freezing salt water and blood made lousy reference points (cough Celsius cough) and ended up using freezing and boiling water.