Past its 'Use-By'.

Don asks "How important are the 'sell by' dates on food? Clearly something like milk gets nasty. Bread can easily mold. But what about something like canned beans? Would that be more a textural issue; like they get soft or something? Some people in the U.S. stockpile food thinking something awful might happen. I don't agree but what would you expect of an out-of-date can of beans or even tuna?"

Researching it a bit, it seems there are lots of different date marking legislations.

In Australia, perishable food must have a USE-BY date.  Consumption beyond this date may present a health hazard.  But it presupposes that storage conditions have been complied with.  Leave your milk in the back of your car for a day and all guarantees are off.

Food that has a shelf-life of less than two years must have a BEST BEFORE date.  Consumption beyond this date may mean that the product is no longer of good marketable quality.  It may be stale or aged but does not imply a health hazard.  And it doesn't change from OK to dodgy on the BEST BEFORE date.  It is best before that date but can be passable for quite some time afterwards.  Again it depends on storage conditions.

Food expected to last beyond two years does not require a BEST BEFORE date but must have some identifiable marking to permit a recall (Batch code, manufactured date etc.).

So, where doe that get us with Don's question?

Canned beans will last a long, long time.  Canning produces a bacterially sterile product.  Enzymes are destroyed too.  Generally speaking changes happen at the time of cooking (flavour, texture) and then the product is in a kind of suspended animation.

With canned beans, assuming they were of good quality when canned, there is nothing much that can happen to them.  There is no mechanism for deterioration: free of bacteria and enzymes, protected from physical damage by a thick sauce, they are pretty indestructible.  That goes for most canned product.

The main issue will be the external deterioration of the can over time, resulting in the formation of small holes that permit the entry of bacteria or the possible failure of the lacquer or other coating on the inside of the can.  The only protection you really have from that is to buy good product.  If you are planning to cater for the end of civilization, don't do it with cheap, plain-wrap stuff.

If the people planning for disasters want to be safe they could rotate their stockpile, eating the oldest product and replacing it with fresh product, using it like a well-stocked pantry.
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