There is no mystery to this. Onions
contain a water-soluble organic sulphur compound called alliin,
referred to in onions as the lacrimatory precursor - the harbinger of
tears. This is present in varying amounts in different onions, hence
some varieties will reduce you to tears more readily than others.
When the onion bulb is cut the enzyme
alliinase acts on the alliin to produce an organic sulphur compound
which reacts with water (the moisture in your eyes as you are standing
over the onion with your knife) to produce sulphuric acid. It is the
acid which produces tears and makes your eyes sting.
Tips: Slicing an onion right through,
keeping its root intact, helps to prevent tears when chopping or
slicing.
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