Cold is the short answer...
When the Freezer is left open warm air carrying moisture can get in and ice may form on the outside of the packaging from the moisture in the air - just like ice on your car windows on a frosty morning.
Clarence Birdseye the original frozen food inventor liked round numers and decided that 0 ºF was his ideal temperature - equivalent to -18 ºC
Peccavi, January 2010
