Office for Science & Society

The most fascinating thing about the email that circulates about the curative properties and dangers of onions is that some people give it credence. Why would anyone believe some unsubstantiated anecdote about onions curing the flu especially when it is clear that the messenger doesn’t know the difference between bacteria and viruses. And are hairdressers really reliable sources of health information. Is it plausible that an onion can draw “germs” out of the body? Of course not.

What about the allegation that onions that are out to get us!  That warning has nothing to do with fouling our breath and destroying our social life.  According to this email sliced raw onions are a “magnet for bacteria” and should not be stored in the fridge even for brief periods.  And watch out for those onions on your hot dog, the message warns.  Better to stay away from such raw onions unless you are keen to explore the protective properties of your immune system.  Alright then, let’s take a look at the science here.  The fact is that onions are not especially prone to bacterial contamination.  In fact, quite the opposite.  Onions feature a variety of sulphur compounds that have antibacterial activity.  (of course that doesn’t mean they can in any way protect against the flu which is a viral disease)  Furthermore, cutting an onion triggers the release of enzymes that initiate a chemical reaction producing propenesulfenic acid, which in turn deconmposes to yield sulphuric acid.  It is the sulphuric acid that makes you cry by irritating the eyes!  But sulphuric acid also inhibits the growth of bacteria.  Also, a cut onion’s surface dries out quickly, reducing the moisture that is needed for bacteria to multiply.  And of course, to have bacteria multiply, you need some source of bacteria in the first place. Where would these come from?  Bacteria are not spontaneously generated.  They have to be somehow present to start with.  Cutting boards and dirty hands are a possible source, but food spoilage bacteria do not become airborne, you need contact.

So unless you have sliced your onions on a contaminated cutting board, or handled them with dirty hands, you can safely put them in a plastic bag and store them and there will not be any bacterial contamination.  Contamination by some sort of mould is possible, because mould spores can travel through the air.  Another point is that bacteria are most likely to multiply on high protein foods and onions are devoid of protein.  Lastly the terminology that onions are “bacterial magnets” makes no sense.  No food attracts bacteria, although of course some are more likely to support bacterial multiplication or viral contamination once infected.  Like any food, poor handling can cause problems.  A couple of years ago raw green onions were fingered as the food that caused a hepatitis A outbreak in a U.S. fast food restaurant.  The virus responsible was traced to onions sourced from Mexico where it may have been present in unclean irrigation water or perhaps on the hands of a worker who did not wash properly after using the facilities.

My conclusion then is that there is no reason to suggest that onions are in any way more risky than other foods and avoiding onions is not only unnecessary but unhealthy.  That’s because onions contain a variety of compounds that have health benefits.  Fructo-oligosaccharides, for example, stimulate the growth of bifidobacteria which suppress the growth of potentially harmful bacteria in the colon.  Eating onions has also been linked with a reduced risk of stomach cancer and flavonoids in onions can lower blood pressure and reduce the risk of blood clots forming.  Some studies have even shown improved lung function in asthmatics who consume lots of onions.  So there is certainly no reason to fear onions, just make sure you exercise the usual safe food handling techniques.