Probably preservatives.
I still have an unopened tin of Spam that was given to me as a
souvenir when I toured the Hormel factory down in Austin, MN, back
in the early 90s. I wonder if the "meat" inside is still
edible. 
If the tin is not dented/rusted/otherwise damaged, the meat
should be edible.
I remember reading about some scientists who opened a 120 year
old can of beef and discovered that it was still in good
condition.
