Pre-cut onions are not dangerous

A post claiming that cut onions become dangerous to eat when left overnight is being widely shared on Facebook. This is an old myth, which has persisted in various forms for many years.

According to the post, “it is dangerous to cut an onion and try to use it to cook the next day” because “it becomes highly poisonous for even a single night and creates toxic bacteria which may cause adverse stomach infections because of excess bile secretions and even food poisoning”.

When the fact checking website Snopes looked at this claim in 2009, they found a possible origin in a blog written by Sarah McCann under the pen name Zola Gorgon. The blog describes a visit to a mayonnaise factory, where McCann was told that pre-cut onion was allegedly a common source of food poisoning.

There appears to be no evidence for this, however. Snopes were able to find only one case of food-poisoning linked to bulb onions.

A review of the available scientific research in January 2018 found evidence that “onion volatile compounds” released when cut may actually play a role “as antimicrobials that reduce the rate of microbial growth.”

Overall, the review says that “a shelf‐life of 10 to 14 d[ays] is normally feasible for fresh‐cut onion stored in a refrigerator”. The National Onion Association in the US says onions can be stored safely in a sealed container in a refrigerator for up to seven days after they are cut.