Q: can leftover cut onions prevent illnesses?
The straight answer: no
The facts: it has long been thought that leftover onions are a magnet for bacteria and viruses. As far back as the 1500s, raw onions were placed in and around homes all over Europe, in the belief that they prevented diseases - including bubonic plague - by absorbing the "elements of infection". The Chinese, Ancient Greeks, and Native Americans, used onions to ward off infections and relieve respiratory congestion.
Today, because raw onions are thought to attract bacteria, it is assumed that one should never keep half an onion, not even in the fridge, because it can spread illnesses like colds and flu. Based on this belief, many people avoid eating leftover onions.
This ancient folklore might stem from the fact that raw onions contain compounds that kill or inhibit bacteria when ingested, says associate professor Lee Yuan-kun from the Department of Microbiology at the National University of Singapore. One such compound, allicin is thought to have anti-fungal properties, while quercetin, a bioflavonoid, has been found to be anti-fungal as well as anti-bacterial.
However, there is no scientific evidence that cut onions, when placed around the house, are a flu remedy. In the first place, bacteria and viruses do not "fly around" from surface to surface. They are transported by contact - that is, if you cut contaminated meat and then use the same, unwashed knife to chop vegetables, bacteria will be transferred from the meat to the vegetables.
In the case of airborne viruses and bacteria, it is not true that these agents of infection can direct themselves to land on an onion, so the notion that half an onion can draw dangerous, illness-causing germs from the air like a sponge is far-fetched.
Bottom line: Ingesting raw onions may boost your resistance to infection, but placing a cut onion in a room in which everyone is coughing and sneezing will not.
When left for a prolonged period, whether at room temperature or in the fridge, onions, like any other produce, decay. During this process, which speeds up when the environment is warm and moist, bacteria that are already present on the onion multiply naturally. Certainly, consuming the contaminated onion - especially when it is in an advanced stage of decay - can cause infections, as well as diarrhoea, vomiting and other symptoms associated with food poisoning, says Lee.
Handling onions with utensils that have had contact with contaminated food can also cause food poisoning. This is why food hygiene is so important.
Avoid cross-contamination by using separate cutting boards for raw and cooked foods, and always wash your hands well before and after preparing ingredients for a meal.
It also helps to store both raw and cooked foods the right way. Raw onions can be stored at room temperature, but once they have been cut, it is best to keep them in a sealed container or in a plastic bag in the fridge.