And how do you tell an expiry date
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Depends on the food.
Some foods have “use by” dates because they’ll eventually spoil from bacteria and fungi, rendering them hazardous to eat and pretty nasty. You’ve surely smelled spoiled meat before, or had a loaf of bred turn blue on you.
More processed/preserved foods often have a “best by” date instead. There’s no imminent danger of actual spoilage as long as they’re still unopened, but chemical breakdown of the complex starch and proteins will still gradually occur until the food is just dust or goo.
Canned goods can be safe to eat for a very long time as long as the can isn’t damaged, but sometimes the contents have been reduced to sludge by a decade of chemical degradation.
These dates are selected based on data from previous spoilage tests.
It depends on the product. Some products expire when the amount of bacteria and fungi in it might reach a significant toxic level. Some products react with oxygen in the air and become rancid. Some products have chemical reactions between various chemicals in it changing their properties. We know which issues is the worst with each product and can measure this in lab experiments. So for example you can measure how fast the fat in the product reacts to the oxygen in the air to become rancid and you can do taste tests for when people notice that the food taste bad. Using this you can calculate when the food goes rancid.
It should be noted that these are predictions. They are not usually right. Manufacturers try to put the expiry date as soon as possible so that if the manufacturing process is a bit off and if the product is stored badly it will still not expire before the date. If there is a risk that the product expire before being sold they would rather put inn stabelizing agents, for example vitamin-c as an antioxidant making the fat take longer to become rancid. So you should not take the expiry date for gospel. Most food is perfectly eatable after the expiry date, especially if it have been stored properly and is prepared properly.
It has simply reached the point where it’s quality is not up to sellable standard. There is no regulation, at least in the USA, on what expiration dates even mean. There’s sell by, use by, best by, etc. It is entirely possible for the product to be perfectly edible for some time post expiration. The key here is that none of these mean the product is magically bad on that date.
You tell by testing it, then adding some margin of error. What happens depends a lot on the product. Milk goes sour, bread goes stale, vegetables wilt or rot, harmful bacteria multiply, or a host of other bad things. Or maybe the ingredients just separate. The white “bloom” on old chocolate is sugar migrating to the surface, but it’s still perfectly edible.
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