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Usually when left in hot temperature, the milk curdles (which I know when I boil it and it starts separating).

If the re boiling does not curdle the milk, can that be taken as an indicator that the milk hasn't developed the bacteria (and their toxins)?

This is the milk that I use.
http://www.amul.com/products/amul-taaza-info.php

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Just to clarify: when you say reboiling, you mean boiling pasteurized milk right? –  Jay Dec 14 '12 at 10:59
    
@Jay Yes, I get milk in packets which IS pasteurized. Thanks. –  TheIndependentAquarius Dec 14 '12 at 11:09
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Interesting question. I know that heated milk will separate when it is acidified. Another take on this question would be "Do all harmful bacteria acidify milk?" –  Sobachatina Dec 14 '12 at 14:55
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@Sobachatina: Even if all bacteria did acidify (which, to a layman in microbiology, sounds like a difficult claim to prove), wouldn't absence of curdling potentially just mean that whatever bacteria are there have not acidified it enough to curdle? So in order to answer "yes" to this question we'd need to not only prove that (a) all harmful bacteria acidify milk, but also (b) that acidification to the point of curdling will take place before growth to a harmful degree, and not some time after. Without a mountain of evidence I think it's easier to just say "no". –  Aaronut Dec 16 '12 at 18:30
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Doesn't separation refer to the fat separating from the water component? When protein is removed, I've heard it referred to as denatured or "burnt" or curdled. I would think you can re-pasturize the milk to sterilize it, but if it was previously contaminated, you would not be able to remove bacterial toxins. One toxin that may remain would be endotoxin which causes illness in humans and is only denatured at temperatures in excess of 300ºC. –  leonardo Dec 25 '12 at 23:01
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1 Answer

Fresh milk does not separate when boiled. I use it in sauces and pan gravies and it always boils and never separates.

My suspicion is that you are separating it on purpose, for example to make cheese. In that case you have to add acid or rennet to separate it. If you then re-boil it to separate it again, you get very little solids. I have made cheese at home then tried to 'ricotta' the left over whey and all I get is a tiny bit of solids.

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"My suspicion is that you are separating it on purpose," No need to suspect anything. Please read the question again. –  TheIndependentAquarius Dec 16 '12 at 15:54
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