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Bacteria and Microwave Ovens

  1. May 29, 2005 #1
    Hello everyone.

    I thought I might be better off starting in this forum, but I wouldn't be surprised if this thread will be moved to one of the physics boards.

    1. Reheating meat, activates bacteria, which is not healthy for us humans, correct?

    2. While operational, everything inside the microwave chamber is exposed to radiation levels which are hazardous to living beings (except for radiation resistant bacteria, and something else that I probably don’t know about), correct?

    If both are true, than if we were to plot a graph of 'number of bacteria' versus 'cooking time in a microwave oven', wouldn't it actually be a bell curve, rather than a straight line with a positive slope or exponential, or whatever it would be in a regular oven, due to prolonged exposure to radiation killing off the bacteria?
     
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  3. May 29, 2005 #2

    iansmith

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    Reheating will increase the temperature and it might reactivate the bacteria but the rise is to rapid for the bacteria to adpat from relatively cold to warm. the bacterias will be killed if the meat is heated for a proper duration of time.

    The microwave is only harmfull because it boils water. No organism is resitant to water that is being boil inside the cytoplasm. Some bacteria may survive the heating process if the bacteria in spore state.

    the curve could start with a short plateau phase (very few bacteria are killed untill optimal temperature is reached) and follow a drop in the bacteria number. The plateau phase would be longer for longer for bacteria in a spore or for bacteria that resist heat.
     
  4. May 29, 2005 #3
    generally..but not always. The boiling point of cytosol will be higher than that of water due to it's being packed with solutes. So if the water is boiling, and the bacteria shut out the water from entering by closing transmembrane protein channels, the cytosol may not boil.

    some bacteria can live quite well at >100 deg celsuis.

    i would consider 300 deg C (the average energy needed to break a peptide bond) to be the limit of all non-spore bacteria.
     
  5. May 29, 2005 #4

    iansmith

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    The temperature of liquids inside the bacteria at around 100C would be sufficient to denature most protein and render any mechanism of resistance to be ineficient. You do not need to break the peptide bound to kill most if not all the bacteria present in food.
     
  6. May 29, 2005 #5

    DaveC426913

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    The kinds of bacteria that inhabit meat are not the kind that can resist high temps. That's WHY they're living in the meat - it's their optimal temp. 140F is sufficient to kill them. That's the recommended temp to cook meat to, to kill them.
     
  7. May 29, 2005 #6
    i guarantee you that you are not killing every bacteria in that piece of meat, its just that those bacteria don't make you sick.

    after cobalt 60 irradiation, being soaked in antibacterials, and cooking at >100 deg C, bacteria in meat still survive. give some credit to our prokaryotic ancestors.
     
  8. May 30, 2005 #7

    Ouabache

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    I believe quetz was responding to iansmith's statement "No organism is resitant to water that is being boil(ed) inside the cytoplasm."
    Quetz is quite right, there are bacteria that grow in temperatures exceeding 100 deg C. Archaea bacteria are good examples. Some even grow in temperatures up to 115 deg C.
     
  9. May 30, 2005 #8

    DaveC426913

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    Yes, it was not my intention to suggest they were all being killed. It was my intention to suggest that bringing the temp of meat up to 140F is sufficient to kill enough of the right kind of bacteria so as to greatly minimize the risk of illness. Reheating cooked meat will have the same effect. (And if the meat had been stored in temperatures below ~40F, then those same bacteria will not have been able to multiply in the first place.)
     
  10. May 30, 2005 #9

    DaveC426913

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    Yes, though this is slightly off topic. The OP was asking about bacteria in meat, bacteria that make humans ill.
     
  11. May 30, 2005 #10

    iansmith

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    It is now up to 121C but if I remember corectly, the water is not boilling under the condition these bacteria grow. That is the one of reasons hyperthermophile are capable of growing at extreme temperature. So my point still holds.

    How do you know these bacteria don't make you sick. It might be the number is insuffient to cause problem. Also, if the meat was cooked properly the first time, it should be sterile. It is a question of time and temperature. 5 minutes at in boilling water is sufficient to have a 99.999% kill. You are not likely to dectect any bacteria after 5 minutes if the starting number of bacteria was less than 100 000/ml (or g). This I can garatee I have done experiments.
     
  12. May 31, 2005 #11
    it does not matter whether the water boils or not..the same amount of energy has been pumped into the water. it exists at the same temperature as that which is boiling, but without the bubbles.

    they survive because they have a particularly adept system of producing heat shock proteins. these prevent their own proteins from denaturing under these harsh conditions, and also help support the cell membrane.
     
  13. Jun 1, 2005 #12
    I would have to agree for iansmith for the following reason.

    The microwave directly targets the water molecules inside the cytosol (as well as anywhere else in the microwave). In a sufficient amount of time the water in their cytoplasm will boil. It might be at a higher temperature due to increase solutes, but it will boil if it is continued to be heated via microwaves. If one places a cup of water with dissolved salt inside the microwave, it will boil! It will boil at a higher temperature then pure water due to the solutes but it does boil.
     
  14. Jun 4, 2005 #13
    This is an interesting discussion but doesn't quite answer my question...

    Doesn't the radiation exposure itself kill the bacteria over time? And how long would the bacteria have to be "cooked" for before it is being killed off?
     
  15. Jun 5, 2005 #14

    Monique

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    Only ionizing radiation, such as UV, will kill bacteria itself by producing harmful free radicals that attack proteins and the DNA. Sound is radiation, it does not kill bacteria. Microwave radiation kills bacteria, since the water molecules take up the energy and release it as heat, heat denatures proteins and dna.
     
  16. Jun 5, 2005 #15
    So that means that re-heating MEAT in the microwave is for longer periods of time is NOT bad for health (well... there is of course the other factors associated with it as well). So if I reheat my launch (roast let's say) for 2 minutes or 5 minutes... the 5 minutes is actually better?
     
  17. Jun 5, 2005 #16

    iansmith

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    I will depend on a many bacteria are in your lunch, the location of the infection, and the size and water%/weight ratio of your lunch.

    The larger the amount of bacteria in your lunch, the longer the heating should be to obtain a "safe" number. 2 minutes might be sufficient to cross the safety threshold and 5 minutes is just too much.

    The lunch is not heated uniformely and it depends of the size and the water %. So killing bacteria on top will required a different time than for bacteria inside. Usually, the longest heating time is the one choosen and it will ensure that the safety threshold is passed.
     
  18. Jun 6, 2005 #17
    I am still extremely confused.

    My co-workers keep on telling me (common sense they say!) that heating meat for longer periods of time activates more and more bacteria that is harmful to our bodies. They usually heat the their launches for 2 minutes, where I put my launch on 5 minutes or more.

    What if the meat in my launch was re-heated in a conventional oven? Would the story be different?

    And I still have trouble understanding why is it that the longer period of cooking defeats the general acceptance of the fact that re-heating meat is harmful due to beacteria activation.

    Should re-heating meat be longer? Is this true for both microwave and ovens? Does the readiation exposure have anything to do with it?
     
  19. Jun 6, 2005 #18

    iansmith

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    They are wrong. It is all about temperature most bacteria that are harmfull for your body will not be activated at temperature reached in a microwaves. The temperature are high enough to kill most microorganisms but a certain time must be reached/pass to cross the safety threshold.

    There would be no difference it terms of killing but the time might differt.

    I do not know where that general acceptance your talking about comes from. It is just wrong. Longer cooking time will kill more bacteria. Most bacteria do not get activated by heat that are used for cooking and reheating food. High heat will cause cytoplasm damage and RNA degradation and the longer the heating the more damage the cells are and the higher the number of cells get damage. It does not matter where the heat source comes from, oven or microwave. And again, the radiation of the microwaves do not kill the bacteria, the radiation heats up water and that heat is transmitted to the food and the heat kills the bacteria.

    As far as reheating goes, most bacteria should of been killing during the initial cooking. The time of heating should be sufficient for you to fell it is hot. If the meal is still cold nothing got killed and most bacteria are still alive and might be reactivated.
     
  20. Jun 7, 2005 #19
    So let me get this straight. Re-heating meat (whether it is in a microwave or a conventional oven) is NOT harmful? This idea is all wrong? Even in the media?

    The reason why I mention the media is because there are articles in the newspaper and stories in the news (especially after Christmas), about what kind of Turkey sandwiches you can make, and as a side note the anchors and the authors always mention the fact that you should NOT re-heat any meat leftovers due to bacteria activation.

    What I do know for sure is that meat should NOT be left in room temperature (the fact that bacteria will multiply in such “pleasant” conditions is obvious).

    P.S. Does this idea of bacteria in meat apply to meat ONLY? Or is it the same idea for everything else that may be in your typical launch?
     
  21. Jun 7, 2005 #20

    iansmith

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    As far as I know, the media side note just does not make any sense. If you could point the article it would be useful.

    As far as leaving food at room temperature, it may apply to all "wet" food but meat products are more likely to contained dangerous bacteria. For example, poultry and eggs are virtually always infected with salmonella. Meat also is more of a rich growth medium for bacteria when compare to some other food. Fruit and vegetable could also be contaminated will fecal matter (fertilizer) and it may contain dangerous bacteria or it could be contaminated with soil that has Clostirium. However, cleaning your fruit and vegetable usually reduce significantly the risk.
     
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