I was just watching this program on the food network here and they were talking about the Spanish fermented pork sausages. It made me think of sausages that are hung to dry and even though its raw meat (which would go bad very very quickly if not refrigerated properly), it doesn't go bad. Why is that? Is it because no air touches the meat inside the casings?
It is due to a combination of several factor, depending on how the particular sausage was made:
Meat for dry cures sausage is also often frozen to specific temperatures for prescribed periods of time to kill any trichinosis which may be present.
As the sausage cures, it becomes too dry and concentrated in salt to foster pathogens, as well as too acidic from the action of friendly bacteria, and thus is relatively shelf stable. It may also simply have acid ingredients as part of the initial recipe.
Modern cured sausages may also be subjected to irradiation, cooking, or another "kill step" to satisfy the needs of regulatory agencies, according to the New York Times.