Onions (genus Allium) have a biochemical pathway that produces volatile compounds associated with tearing of the eyes and a pungent taste.

Tearing is associated with 1-propenyl-L-cysteine-sulphoxide. This is formed from alk(en)yl-L-cysteine-sulphoxide compounds when cellular damage occurs to the onion (cutting, peeling, etc).


1-propenyl-L-cysteine-sulphoxide is hydrolyzed to:

(1) propanethiol-S-oxide (lachrymatory factor) and

(2) thiosulphinate (flavor factor)


Enzymes involved in release of lachrymatory factor:

(1) alliinase

(2) lachrymatory factor synthase


Ways to reduce tearing:

(1) freeze or chill the onion (slows the release of enzymes)

(2) ventilate the cutting area with air moving away from the cook

(3) genetically modify the onion to inactivate the enzymes

(4) wear goggles


Ways to increase tearing:

(1) a warm environment

(2) a large amount of cut onion

(3) poor ventilation

(4) face in close proximity to the onions

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