There is in fact air in cans when you open them.
The reason the food does not spoil is because it is sterile inside the can; that is, there are no living bacteria, fungi, or other organisms inside the sealed can. This is why even a pinprick hole will cause the can to spoil. It is not because air got inside, but because living organisms got inside.
Disturbingly, I have met professors teaching biology at universities who do not know this. This is explained in every introductory biology textbook.
(Economists: "What bubble? Markets are perfectly rational." People really are this incompetent and do not know basic things in their own fields.)
This is also the subject of a truly elegant experiment by Pasteur using swan-necked flasks with broth inside. The necks are very long, and the end points down and is open, but there is no way for bacteria or fungus spores to move up the neck against gravity. Although the soup inside has been exposed to the open air, since no living organisms could get to the broth, the broth was still sterile after years. If the neck were broken or microorganisms got in by other means such as being carried in by an insect, the broth would immediately start to spoil.
Air: Yes. Living organisms: No; food does not spoil.
Air: Yes. Living organisms: Yes; food spoils.
He exposed boiled broths to air in vessels that contained a filter to prevent all particles from passing through to the growth medium, and even in vessels with no filter at all, with air being admitted via a long tortuous tube that would not allow dust particles to pass. Nothing grew in the broths unless the flasks were broken open; therefore, the living organisms that grew in such broths came from outside, as spores on dust, rather than spontaneously generated within the broth. This was one of the last and most important experiments disproving the theory of spontaneous generation. The experiment also supported germ theory.[8]
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