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Can i kill milk bacteria in a microwave

  1. Sep 23, 2010 #1

    I am about to start making bulgarian yoghurt with unpaseurised gersey milk. The food forums say that the bacteria in the milk will compete with the yoghurt bacteria and so should be reduced (not necesserilly killed altogether) without destroying the enzimes and milk protiens.

    In bulgaria the traditional way is to lightly simmer for an hour. The food forums have all sorts of different ideas what to do. I thought a scientist might be able to advise me better than foodies.

    Would putting the milk in a microwave kill the bacteria without destroying the good stuff? Should I use a low setting for a long time? I would be making a litre at a time.

    if i had to use a stove instead, how long should i boil it and what temperature is best to achieve my aim??
  2. jcsd
  3. Sep 23, 2010 #2


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    I don't think there is a difference between using microwave and stove - in both cases what kills the bacteria is the elevated temperature.
  4. Sep 23, 2010 #3
    yeah, both heat, but microwaves tend to heat the top more than the bottom - uneven heating, so I would stir it often (the top usually gets really hot and foams up and spills out while the bottom is still cold) and heat it until its all hot.
  5. Sep 23, 2010 #4


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    On a stove it is much easier to control the heat and monitor the process. As said, it is the heat that kills the bacteria, there is nothing magical that happens in a microwave that would preserve the regular milk proteins.
  6. Sep 23, 2010 #5


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    What do you mean "destroying" the enzymes and proteins?

    In a microwave, or on a stove, they will at most denaturate. They still have the same nutritional content (i.e. amino acids).
  7. Sep 23, 2010 #6

    jim mcnamara

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    I think the OP is referring to vitamin content. Cooking does degrade some vitamins, so does a microwave.

    This article asserts that there is a positive effect of consuming live culture yogurts, the benefit (creation of new micronutrients) varies with the variety of the LAB used to make the yogurt. Since the bacteria in yogurt grow post-heating, getting rid of other bacteria is essential to making quality yogurt. The yogurt bacteria don't need competition.

  8. Sep 23, 2010 #7

    Andy Resnick

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    I think you are getting conflicting answers because heating will both degrade vitamins and proteins as well as kill bacteria.

    Side note- is 100 C really high enough to kill bacteria? AFAIK, food processing occurs at higher temperatures (pressure cookers), in the presence of acid or salt (water bath canning), or some other extra biocide-thingy.

    In principle, there's no substantive difference between bringing milk to a boil in the microwave or on a stove. Processing times may vary, tho.
  9. Feb 9, 2011 #8
    Ok I understand the basis for this comment in light of the fact that the bacteria is immersed in lots of liquid. Would it be true if it were in a sponge/ dish cloth that were dry. Would the microwave energy not focus on the "molecules" in the bacteria?
  10. Feb 10, 2011 #9


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    Cells are full of water, that is they have an aqueous cytoplasm. Regardless of the medium, microwaves "heat" water molecules. The microwave doesn't care if this is water in a cell or water in a drink.

    The heat from that is what kills the bacteria, in the case of the microwave, you're just heating them from the "inside".
  11. Feb 10, 2011 #10
    I think one other element here is that you have all of the "downsides" of a stove top from your perspective, but there's one other issue:

    Uneven heating. You might actually have to cook that sucker more than you think to get what you would with a brief pasteurization. I suggest a pressure cooker for ease and speed, but you can't have it both ways: you get it raw, or not.

    I've never made yogurt... intentionally... but couldn't you just get pasteurized milk and inoculate it with the proper amount of the bacteria you want? When you "skim" or "fatten" milk, first the whole mess is separated, then combined and homogenized for precision. Why start with something you need to reduce, when you can ADD precisely?
  12. Feb 11, 2011 #11
    I think there's something else you might want to consider.

    The reason why you simmer the milk, on the stove, as opposed to letting it boil, is because the milk might curdle if you boil it too long. This is because of the Casein protein in mammalian milk.

    Since the microwave is going to be difficult to control... I just recommend doing it the old fashioned way. The results will be better I think since you have better control.

    Milk pasteurization is usually done below boiling point (or for a very short time) to prevent this curdling.
  13. Feb 11, 2011 #12
    Oh that would be so unpleasant: a mixture of curdled or scorched milk, and cold spots. If it's going to be this milk, your method with a make-shift double boiler sounds like it would be the safest bet.
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