If I cook up some food and decide to keep it to reheat later or tomorrow, why is it important where I store it? Eg it needs to be refrigerated. I get that bacteria will grow in the danger zones – so room temperature means bacteria will grow. But if I’m heating the food up doesn’t that kill the bacteria that would grow anyway? So why does it matter?
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Not all bacteria are destroyed by cooking things to temperature. Some bacteria can go through a process called sporulation, where they become inactive in harsh conditions by forming temperature resistant spores, which stick around in food waiting for the correct conditions to start growing again. By cooking the food, you’ve destroyed any competition from other microbes so these spores get to grow unchecked if the conditions are correct. Two examples are *Clostridium botulinum* and *Bacillus cereus*, both of which can be potentially lethal. The “correct conditions” for the spores to start growing again differ, but neither will grow at refrigerator temperatures.
Even for bacteria that can’t form spores, cooking doesn’t kill *all* of them, it just kills enough that your body can fight off the rest without getting sick. If you kill say 99% of them, there’s 1% left to grow, and they’re growing exponentially. Next time you kill 99%, the 1% left over is going to be a much bigger number. Let that go on long enough and it’ll take a very long time at high temps for that food to be safe to eat.
Also, as mentioned elsewhere, some microbes produce toxins that are not always destroyed by heat.
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