Surely the bacteria that grew after the expiration date will have been killed in the cooking process?
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Often times the bacteria doesn’t have to be alive to cause problems.
Some bacteria produce toxins precisely when they die. Others, it’s their poop that’s dangerous, and heat doesn’t fix that.
Sealed heat treated cans/jars of products are a different story. Ill focus more on refrigerated products and sort of excluding raw meats.
Pre-cooked and cured meats (uncured meats with celery powder is still cured technically), will still end up with damaged or extremely small amounts of bacteria still living or repairing itself. Refrigeration helps slow the repair and multiplication of spoiling and harmful bacteria. Given enough time (expiration being an educated guess when a product may be harmful and/or just not meet their quality standards) the bacteria will grow large enough to start to spoil a product.
Main point: The smaller amount of bacteria, the larger amount is killed in a smaller amount of time.
When a product ‘expires’ the bacteria may have had enough time to multiply so much, that a normal cooking time wont kill enough. You’d have to essentially make it unpalatable/undesirable for consumption before it is actually safe.
The higher the temp in cooking the faster bacteria die, but given a large amount of bacteria you would have to keep it at such a high temp for so long, now the product is burnt/dry etc.
Of course the larger amount of bacteria the more waste products they produce. Both bacteria that survive and waste products can both make you sick given the amount of either.
When you cooked your food, you killed all the bacteria on it. You didn’t kill all the bacteria on your plate or your utensils. You didn’t kill all the bacteria on the container you stored your leftovers in. Bacteria, and fungal spores, are everywhere and on almost everything.
Because some strains of bacteria produce toxins, and heat doesn’t always destroy those toxic compounds.
If the food has already spoiled, it means the bacteria already ate it and their waste products typically aren’t good for us.
Quick distinction, btw: “expired” isn’t the same as “spoiled.”
At lot of “expiration/expiry” dates on food are actually just “best-by” dates after which the manufacturer no longer guarantees quality, and suggests it’ll be past its peak – it doesn’t mean it automatically goes bad on or after that date (although some stuff, like fresh milk seems to pretty reliably go off soon after that).
ETA: Normal cooking heat doesn’t kill all kinds of bacteria, but in the vast majority of cases, it kills the kinds that make us sick.
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