Science Questions

Why should we not re-freeze defrosted meat?

Sun, 1st Nov 2009

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Question

Ian Mellor asked:

I've always been taught, an early age, that if meat has been frozen and thawed, you shouldn’t re-freeze it because of food poisoning. But why is this?

Answer

Well the answer is that some bacteria don't actually get to be bad for you because they infect you.  They actually put things into the food that are toxins and the toxins are not broken down by heat.  So, the bacteria multiplying in the food leads to the accumulation of toxins in the food which will then make you ill even though the bacteria may be long gone by reheating the food.  So if you keep cooling and warming the food, the food might spend enough time at a certain temperature which encourages the bacteria to grow and put toxins into the food whilst not themselves actually really posing much of a threat.  That’s one way.  Another way is that if you keep on warming up and cooling down food, some bacteria will just end up flourishing and they’ll go from being at very low level in the food, where they're not growing very fast because the temperature is low, to getting to a very high population in the food where that might be an infectious dose.  So to catch Salmonella for example, you actually need to eat about a million organisms, 106 particles of Salmonella.  That’s an infectious dose.  Other bacteria infect you at much lower doses.  So it really depends on what the pathogen is and what the way in which it makes you sick is – but the bottom line is, if food spends time at higher temperatures, there’s a higher chance that bacteria will grow and therefore, make you sick.  So, the best advice is to either cook it and eat it, cool it and eat it, but don't keep reheating it because that could be bad.

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Ian Mellor asked the Naked Scientists: A question for you:   We are always taught from an early age that if meat has been frozen and then thawed, we should not re-freeze, because of the danger of food poisoning. But what is the scientific reason for this?   My understanding is that freezing slows down the replication of bacteria, so what goes wrong the 2nd time the meat is frozen?   Regards   Ian What do you think? Ian Mellor , Thu, 29th Oct 2009

I do not offer this as a scientific reason but while the meat is unfrozen, it picks up additional bacteria and they multiply to the point that, the second time it is thawed, it would be possibly very harmful.  Joe L. Ogan Joe L. Ogan, Sun, 22nd Nov 2009

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