Monday, April 6, 2009

Onions as bacteria magnets.

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Pauline asked: there is an email circulating claiming that cut onions are "magnets for bacteria" and should never be stored for later use (even in the fridge for a few days) as they will cause food poisoning. True?

Snopes (here) rates it as "undetermined".

I am willing to go out on a limb and say "false".

Reason 1: Nothing is a bacteria magnet. Firstly, bacteria have minimal mobility. They usually travel in water droplets, if at all. Sneezes, for example. Moulds can release spores which get blown around but bacteria usually grows in moist environments and are slimy, making getting airborne difficult. Secondly, if there was such a thing as a 'bacteria magnet' it would be enormously useful in the medical field for drawing bacteria away from the ill and infirmed. Not such use has been made of onions.

Contact with unclean hands can introduce bacteria to new surfaces but they need a surface that will support growth, otherwise they will just stay there without multiplying or die.

Reason 2: Bacteria like moist, neutral environments. Not many acidic things grow bacteria. That's why vinegar is used for preserving. The surface of a cut onion is acidic due to the production of sulphuric acid (this is what makes your eyes water). There are some moulds that will tolerate acidic conditions and grow on onions but they are not high risk, they are visible, and any normal person would cut them off or ditch the onion.

Reason 3: High risk foods are usually high in protein and available moisture. Onions are low protein, verging on nil, and what moisture they have is contained in their cellular structure. The surface, as well as being acidic, dries soon after cutting and will not support bacterial growth.

Reason 4: If onions are attracting bacteria, where are they coming from? Somewhere else in your fridge? Perhaps it is time to clean the fridge.

Reason 5: In the 20 odd years I worked in a food laboratory, onions were never mentioned as even a suspect in a food poisoning case.

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Footnote: we did have some onions brought to the laboratory as a food poisoning complaint once. A guy had eaten them and ended up in hospital. Only problem was, they weren't onions.

They were daffodil bulbs.
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89 comments:

  1. Love your informed posts. Love the story about the daffodil bulbs (reminds me of this story :
    http://www.badpuns.com/jokes.php?section=shaggy&name=daffodiljam)

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  2. Thank you for this, Lee. I didn't think it could be true (I've been storing cur onions for years and I've not been poisoned yet). I read the Snopes "undetermined" too, and thought I'd ask One Who Knows.

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  3. I didn't believe the email about the onions either for most of the reasons you stated, but I did start wondering where the bacteria could come from. The outside of the onion is really dirty, most people cut onions without washing them, using the same knife to cut off the outer skins as they do to cut the onion (on the same cutting board)therefore possibly contaminating the onion with the dirt from the outside...this is all I could come up with. However, I too have been storing and using leftover onions for years without illness in my family.

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  4. I've been eating fermented onions and Chinese cabbages for a week now, without landing up in hospital - and I swear I'm only human. :P

    http://chanweiyee.blogspot.com/2009/08/kimchi-proudly-made-in-china.html


    Wei-Yee Chan

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    Replies
    1. Swearing you are "only human" is just what an incognito alien would say. Very suspect.

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  5. I just found this article after reading the afformentioned email sent by a friend. I also work in a lab (ecotoxicology though) and found that email to be completely false. First off, when the source of information is the Mayonnaise "chemist" chances are, he is not going to admit that mayo will spoil. Having both eggs and a specific moisture content. The deception in the email was that he was a "chemist" and surely knew more than us 'common folk'. I find that very laughable, as I know many chemists who do not know a thing about microbiology.

    Great article.

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  6. What kind of onions would you use to prove this myth?

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  7. I don't know of such an onion. I am not in the business of proving myths. Disproving - yes, proving - no.

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  8. Thanks this helps a lot

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  9. Actually onions do work as a "bacteria magnet". I encourage everybody who has posted a comment just to try it, I guarantee it will work! I've been using them for years and nobody in my family has had the flu or any flu symptoms for 8 years. The onions will show bacteria after being left out for just a few days.

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    1. This is pure ignorance. Flu is caused by a virus, not a bacteria. Start over and do some science.
      Anonymous

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    2. I assume you, Anonymous, are a different Anonymous to the one that posted the comment.

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    3. I`ve not had the flu in decades . no onoins involved

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    4. For the last 10 years I have always put my socks on before my pants, I've never had the flu.

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    5. Wow Zook, I'm impressed. Anti-viral socks.

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    6. I haven't had the flu in about 20 years, but I put my pants on before my socks most of the time. Theories?

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    7. what do u do with the onions?

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    8. Put them in your socks, apparently.

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  10. I haven't had the flu since 1983 and don't use onion bacteria magnets. What does that mean? Nothing really. Sorry Anonymous. It makes no sense logically. It makes no sense scientifically.

    The logic that "I do A and B doesn't happen" does not mean A stopped B happening.

    Scientifically, bacteria are not mobile under their own power (that's why they can grow them on petri dishes and they form colonies) so they cannot travel to you onion.

    What are the signs of 'bacteria' that you see after just a few days? There is a cut red onion on the bench in my kitchen that shows signs of drying out but nothing more. Not even mould is interested in it.

    I'm not convinced but, hey, if it works for you, keep doing it.
    ...

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    1. why is there a cut red onion on the bench in your kitchen? hmmmm, could it be?!
      now i am really confused!

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    2. Dear Confused, The cut red onion is there because a whole red onion would have been too much onion for what I was making and half was left over. :-)

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    3. Do you think garlic would work, my nan swore by it. And yes she swore alot bless her.

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    4. 'Work' as what? A bacteria magnet? No.

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  11. Thanks for the informative common-sense article. I love being able to reply to my mom somewhat better informed when she sends me silly emails that sound suspect.

    I'm more paranoid about hand washing practices when people are preparing my food!

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  12. see this link to good science
    http://www.vegetarian-nutrition.info/updates/onions.php

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  13. How is it then that an onion can be left out to draw away bad odors, just like a bowl of water and vanilla??

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    1. How is it when u cut an onion, u tear? probably some chemistry going on about evaporation and stuff, or it just mask the smell

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    2. Simply put, odours are particles in the air. It's the sulfur content and enzymes in an onion that make you cry when you cut it, especially older onions.

      Although who says that onions can draw away bad odours? It's more often the case that you're meant to keep onions away from other foods which are prone to absorbing the fumes of strong smelling foods

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  14. It was in the Readers Digest years ago that onions were very helpful in a diet to the point they were severed, in one way or another, at a hospital in England at almost every meal.

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    1. You mean 'served'. Try proofreading.

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    2. Not much I can do about the educational standard of Anonymous posters.

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    3. Heh. Maybe they were being severed - it was a hospital after all.

      Onions are highly nutritious and very good for you - how is it confusing that they were served to hospital patients??

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    4. I'd probably dispute 'highly nutritious' but not the 'good for you'. Go figure.

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  15. Thank you for dispelling that useless rumor, I got this email from my elderly grandfather and decided to check it out.
    I have used left over onions most of my time cooking for my family and they've never gotten sick over them. In fact, they are supposed to be good repellents for bugs and many other things,so I will use them, no one in my entire family history has ever ended up in the hospital or dead from onion use.
    I am descendant of the Cherokee Nation, no one from that history has fallen ill due to wild onions either.

    In any case, here is a good link on how to use them to benefit your health as well. Good Luck and happy researching.

    http://www.ehow.com/how_2146427_use-onions-healing.html

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  16. Ingested onions and garlic can be very helpful in preventing illnesses, but sliced and left to be a bacteria magnet? Not plausable....

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  17. Received this email tonight, thought the wording very suspect, as I do with a lot of these so called health scares and such like. Not that I eat raw onions, but that's because they don't like me, shame really as I love those wee silverskin onions.

    I haven't had flu since 1989, not because I have/haven't eaten something or other I'm sure. More like I'm lucky, or work on the I haven't got time to have flu theory.

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  18. Love this... my family tends to send alot of emails such as the one we all have come accross and it starts these long drawn out conversations and it is always great to put an end to them.

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  19. Hmm... thanks for the info. I was wondering about the claim on onions being bacteria magnet...

    I had my doubts about the claim.

    The onions, to me, are very affordable ingredient in many widely used traditional remedies. So it being a bacteria magnet does not make sense to me...

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  20. Do you think it could be true that the gasses emitted by a cut onion, when left out in a room, will mix with the flowing air and hence use their acidic or other powers to kill any germs that might come into contact with the mix?

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  21. jayharle: The gases from a cut onion are sulphur compound that, if they come in contact with water, form sulphuric acid. That is why your eyes sting when you cut onions. Would this upset bacteria? Some, maybe, but not reliably. And this upsetting may just put them in a state of stasis, not killing them as such. It is possible (remotely) that the concentrated fumes may (may) kill bacteria if the bacteria are in a wet environment but it would be unreliable, driven by the strength of the breeze, the amount of onion and, at best, just a surface phenomena.

    Tell me, if you were having an operation, would you be happy for the surgeons to rely on a bowl of chopped onions to keep you free of infection?

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  22. He would if it meant that he could eat that bowl of onions in place of pain killers. That guys loves onions!

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  23. Check your source for 'sulphuric acid' in onions. Snopes, the source you paraphrase(but don't cite)in your reason 2 names it as 'sulphenic acids'. Careless citation is where myths spring from. Not a bad review otherwise.

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  24. Onions contain essential oil (named phytoncide). The antiseptic properties lie in the smell. Phytoncides evaporate with odor when we cut onion. They can kill number of bacteria close by.
    I always put a small bowl with cut garlic and onion next to the bed of family member who has some kind of flue. It helps.
    Try: take two glass jars (around 12 oz), boil two eggs and pill them. Use string and pencil to hang eggs in jars. In one jar put chopped onion. You will be able to see that egg in a jar without onion spoiled much faster.

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  25. Vetka: Thanks for your comments. As mentioned above, onions do produce an acid. It is what makes your eyes sting. If you hang a moist egg in a jar with chopped onion, the surface of the egg will become acidic and this will inhibit bacterial growth. You would get the same effect by dipping one egg in weak vinegar.

    But...

    1. This does not make the onion a 'bacteria magnet'.

    2. It only applies to bacteria that is already on the egg.

    3. It does not mean that the onion has any antiseptic properties in a broader sense. If it did, it would be used in formal medicine.

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  26. Lee: I was not saying that onion is a 'bacteria magnet', i was saying that onion vapors kill bacteria.
    If something is not used in formal medicine it does not mean that this is not working. Folk medicine has hundreds of years of experience, formal medicine much less.
    Vapor of onion was actually used to treat bad wounds.
    http://www.sciencemag.org/content/106/2765/625.extract
    try to google "phytoncides".

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  27. Vetka: Certainly, there is something in what you (and the article) say. But remember that the original question to this post related to whether onions were 'bacteria magnets' and therefore unsafe to eat as they will cause food poisoning.

    That the onions produce an acidic vapour and few, if any, bacteria survive below a pH of 4.3 meaning that they may have a potential antiseptic property.

    This property would affect the bacteria where they are and not, magnet-like, draw them to the onion; the original premise.

    I don't dismiss folk medicine out of hand. But I don't assume effectiveness is determined by length of use either.

    There is an old joke: What do you call folk medicine that works?

    A: Medicine.

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  28. Well, i don't want to discuss this issue any more.
    Just couple words: we are not talking about acidic vapors. Terpenes (compounds found in essential oils) are not acidic.
    BTW: if you hang egg above vineger (don't dipping it in vineger) it will spoil though vinegar vapors are acidic.
    I wish formal medicine use more things from folk medicine, but ... things like onion will not get profit pharma companies.

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  29. Well said Vetka. I was hoping someone would come along with a bit of insight into this question to give a ballanced answer instead of everything been scientifically proved to be right or wrong. There is a big thing happening in our world now where they are trying to ban and regulate herbal medicines... I mean come on, they are made from natural ingredients but yet need to be FDA approved before they can be sold? Two words, drug companies.

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    1. I carry no flag for drug companies; in fact, quite the opposite. But I must have logic, it's a personality flaw I have. The onion theory lacks logic.

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  30. when you leave sandwiches that have mayonaise and onions out of the fridge ..they get bad , not because of the mayonaise but onions that pick up bad bacteria in the air ....

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    1. Dear Anonymous, can you perhaps tell me how and where these air-born bacteria originate?

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    2. Oh yes, can you tell me (1) why you would put a mayo & onion sandwich in the fridge, and (2) for how long you left it? Just curious, you understand.

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  31. Onions and garlic definitely keep vampires away! Haven't had one yet!

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    1. Partially true. Only work for some vampires. Foe example: they don't work with Jehovah's Witnesses or people trying to get us to change power company.

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  32. Nothing is a bacteria magnet. Firstly, bacteria have minimal mobility. They usually travel in water droplets, if at all. Sneezes, for example. Moulds can release spores which get blown around but bacteria usually grows in moist environments and are slimy, making getting airborne difficult. Secondly, if there was such a thing as a 'bacteria magnet' it would be enormously useful in the medical field for drawing bacteria away from the ill and infirmed. Not such use has been made of onions. China Sourcing

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    1. Bacteria magnet: Moringa seeds

      Pumice from Moringa tree seeds are a bacteria magnet used in 3rd world countries to purify water. Cool stuff do a search on it and see.
      annie

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  33. asa: Couldn't have said it better myself. You're not me, are you?

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  34. They usually travel in water droplets, if at all. Sneezes, for example. Moulds can release spores which get blown around but bacteria usually grows in moist environments and are slimy, making getting airborne difficult. China Sourcing

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  35. Peeled cloves of garlic between the toes. White cotton socks. No flu. Ten years running. Poof.

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  36. No garlic between toes. Usually no socks. No flu. Nineteen years. Poof complete.

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  37. O Just one other ingredient needed, faith.

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  38. Eat lots of onions and garlic... no colds and flu and mosquito bites. Girlfriend loves garlic breath.

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  39. Moringa seeds ARE a bacteria magnet

    I must correct you on "Nothing is a bacteria magnet," statement. There is something. It is the pumice made from seeds of the moringa tree. This is used particularly in Africa to purify water. No kidding, no urban legend; do a search on it and see. By the way, you will love this tree, called the miracle tree by many.
    annie

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    1. Thanks aloecalico. Close but no bacteria magnet - they are shown to have flocculant properties - so, instead of adding aluminium salts to the water, if you add Moringa seeds they will cause the colloidal clay to aggregate to where it will settle out and the water is then clear. Not bacteria free, just clear.

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  40. @Lee - Sounds like you got everything figured out. If not, I'm sure you'll let me know.

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    Replies
    1. 'Everything' is a big ask. Little bits of lots of little things.

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  41. It does work. I work in a lab. Wanted to try it when my daughter got sick. Cut an onion placed it next to her bed and the next day wrapped it up and took it with me to work. It was filled with bacteria's. Coincidence??? Yea maybe but she felt better the next day. Try it

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    Replies
    1. Great story. Can you suggest a mechanism for the bacteria to get from your daughter to the onion? No legs, can't fly. Love to know.

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    2. If you work in a lab I doubt you would spell "bacteria's". Really?

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    3. My experience is that working in a lab gives you experience at working in a lab. (Note: I have owned a lab.) It has no causal relationships with a person's spelling. Nor would I expect it to.

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    4. Your dumbass! Bacterium. How did you owned it? theft?

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    5. Goodness; so early in the conversation and you have so run out of ideas that you have to do the 'personal attack' thing. That's sad, really.

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    6. Ignore him. He only likes bacteria because they are his intellectual superiors.

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    7. Are you sure? I have known some really stupid bacteria.

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  42. Maybe its a misconception about it being a magnet. However, dust particles which are made up of human skin cells, dirt, hair, and bacteria viruses and mold spores could possibly land on the onion as the onions own water vapor enters the atmosphere creating the slightest draft drawing dust to itself. If it is a magnet its not a very strong one. I believe that the health effects actually are occurring within the nasal passages and the sinuses by increasing mucosal production thus drawing out and increasing the immunal response by more efficient mucous (more white blood cells in the mucous) and placing the bacteria in stasis or possibly even the stasicizing the virus so that the bodys own immune system has a chance to surge ahead and therefore aiding in the recovery process by decreasing the amount of time you are sick. Something similar to cold-eeze lozenges.

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    1. Btw I'm not the anonymous mom that works in a lab.

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    2. OK. Get a nice polished wooden table, put an onion in the middle of it and see it the dust distribution is different across the table. (Control for drafts).

      I don't dispute that onions (and lots of other things) contain chemicals that can have antibiotic properties. It is the notion that the bacteria can somehow move or be drawn to the onion that I dispute. If it works, why has it not been commercialised?

      If a loved one was really sick, would you rely on onions?

      Placebos work. Double strength placebos work better.

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    3. THIS IS NICE WAY TO KILL TIME AT WORK---WE WERE HAVING THE SAME DISCUSSION HERE.

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  43. I just offered to bring chopped onions to a work frito pie potluck. Someone mentioned the above question.Thanks for your post, I'm going to print this out and tape it next to my big bowl of onion!

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  44. I ate a couple raw onion slices last night and ended up with a stomach ache and cramps from hell.
    Not sure it was the onion for sure but I do know because of the lack of sleep I am cancelling my drive from Dallas to Houston today.
    EEEErrrrrr!

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  45. Because onions are a root vegetable I always wash and peel them with a paring knife before cutting them with a larger knife on my cutting board. I've been doing that since I was a child raised on a farm. And, I've eaten raw onions and leftover sliced onions without a problem for the past 60 years. We kept our leftovers in the spring house or a fridge once we got one. Onions don't kill people, eggs do!

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  46. shut up your opinions if it is not published: http://link.springer.com/article/10.1007%2Fs13337-012-0114-z#page-1

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    Replies
    1. And how, pray tell, does a virus attacking the production of onions mean that the onion in my kitchen is a 'bacteria magnet'? Did you read the article? No, didn't think so. Think about it. How are they related?

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