Above is a simple animation showing the ambient air displacing the ice in the storage bin.  What this animation does not show, is that there are many more areas where ambient air can infiltrate the food zone.  Ice machines are not well sealed from the outside air, and to further compound the problem, the condenser fans in ice machines serve to pull air into the machine from the ambient environment, thus circulating contaminated air through the machine as the ice is being made and stored.