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    I've just done my biology coursework on the decomposition of milk, comparing UHT and ordinary pasturised milk as they go off.
    I measured the Ph of the two types of milk over two days at about 35oc.
    When pasturised milk goes off, the bacterial enzymes in it convert lactose into lactic acid, which reduces the Ph of the milk. So the Ph of the pasturised milk should have dropped over the two days, which it did.

    UHT milk shouldn't have any (or very few) bacteria in it so no or very little lactic acid should be produced and so the ph shouldnt really drop much over the two days.

    However....a couple of us (doing seperate experiments) found the drop in Ph of the two types of milk to be the same (everything was sterialised and the procedure was fine). We used Ph probes and a data logger so it was definatly accurate.

    Does anyone have any explanations for why the ph of the two types of milk dropped by the same amount, or why the UHT milk went off at all (which is shouldn't have really done as its not meant to contain any/many bacteria). It definatly shouldnt have gone off as much as the pastursied milk.??

    Any help would be gratefull recieved......

    Hmm, this is just an initial guess but could the UHT have been contaminated after you opened the milk? I don't know owt about foods really but as far as I'm aware, doesn't UHT stand for ultra-heat treatment? Which would mean that it had been sterilised to remove all the bacteria present at that point, but presumably as soon as you open it you could get bacteria living in it again, which might give you the results you mentioned.

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