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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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.
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