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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Bacteria etc. are not only harmful by themselves.
Some also excrete stuff that is toxic as well and doesn’t get killed or rendered harnless by being cooked.
Basically poison that can resist heat.
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Bacteria themselves are killed by heat, you’re right. But **bacteria poop can be toxic too, and it’s NOT removed by heat**. The toxic chemicals are much less sensitive to temp than living bacteria are.
So if you leave food out a room temp for days then reheat it, sure that kills all the bacteria on it, but *all the toxic poop they have pooped over those few days remains in the food* and gets eaten! That’s the bad part. For food to be safe, there can’t *have been* bacteria either. Killing them and then eating their dead bodies and all their poop isn’t good enough.
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In order to inactivate bacteria in your food you need to bring it up to a boil (or close to it 160F 70C) for 1 min. When you microwave your burrito back up to eating temp you usually aren’t getting the whole thing that hot as it would badly burn you mouth and you aren’t holding it at that temperature for long enough. Because of this a large percentage of germs remain, if you refrigerate that percentage will represent a much smaller number than if you didn’t.
Bacteria can be mostly easily be dealt with… as you said heating.
But when bacteria grows, it eats, when it eats, it poops… literally… and those byproducts are not that easily be cleaned out by just heating.
So your best approach is prevention. Don’t let bacteria poop on your food.
If you eat my shit, you’ll get sick. If you kill me and eat my shit, you’ll probably still get sick. That’s why heating food doesn’t protect you. The bacteria may be dead, but it’s shit is still harmful.
Okay, so there are basically three reasons for this.
# Reason 1 – Spoilage #
The most important mechanism of spoilage in most foods are fermentation by microbes. By not storing food in the fridge spoilage will happen MUCH quicker, thus changing the characteristics of your foods in a short time. When this change is seen as negative for the sensory quality (ex a bad smell) it is called spoilage.
# Reason 2 – Microbes that produce toxins in food #
Some of the pathogenic (causing disease) microbes we see in foods produce toxic compounds while in the food. Some of these toxins are quit heat stable, which makes it hard to inactivate them by heating.
# Reason 3 – Killing microbes by heat is not as effective as you think #
Most of us seem to think that if we heat up our food to a certain temperature, all the microbes will suddenly die – this is however not what happens. Microbes starts to die above a certain temperature, and this temperature depends on the microbe type and the environment that it is in. However, the rate at which they die depends on the number of bacteria that is in the food at the moment. A great way to explain this is by the D-value:
The D-value for Clostridium botulinum (a pathogenic bacteria) has been reported to be as high as 1.38 minutes at a temperature of a 121°C. This means that if I heat up a batch of C. botulinum to a 121°C for 1.38 minutes I will have reduced the number of living cells by 90%. There will however still be living cells in the food. Also, the D-value is very dependent on things like fat content etc.
This should – hopefully – demonstrate that the so called death rate kinetics of microbes is not as simple as one would think. It is hard to guarantee that the number of microbes are below a safe limit in a home kitchen by heat killing alone. Therefore we as a society have devised a set of “rules” that ensures safe food practices in the home kitchen – one of them being the rule about low temp. storage.
I hope this gave some insight without being too confusing or technical. I apologize for any spelling mistakes – english is a second language.
Edit: Spelling and formatting.
Bacteria poop.
Heat kills the bacteria but their poop remains.
The poop can make you sick.
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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