Science Questions

Fri, 24th May 2002

Part of the show What is a Virus and How do Viruses Cause Disease - Stacey Efstathiou

Question

Anita, by email asked:

Why do onions make us cry when we cut them and is there any way to prevent this ?

Answer

Onions are a member of the Allium family of vegetables that also includes leeks, shallots, chives, garlic and sharp scallion. The tears come from volatile oils containing substances called amino acid sulphoxides, which give the members of this family of vegetables their characteristic smells. So why do we cry ? Well, when you cut the onion you also release substances called allinases which break down the amino acid sulphoxides to something called sulphenic acid which then turns into something called syn-propanethial-S-oxide which irritates nerve fibres on the surface of the eye, triggering tear production. Because it takes time for this substance to form, this explains why we don't start to cry until at least 30 seconds into cutting up an onion. So how do you solve the problem ? There are 3 ways. 1) Heat the onions before you cut them. This destroys the allinase which causes the reaction to happen in the first place, so the onion cannot make you cry. 2) cut the onions outside in a well-ventilated area 3) cut them under water.

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