The common name of Allium Cepa is known as onion, a normal type of vegetable which can be easily found in our daily dishes. The question is, why cutting an onion will make us tear?
Onion contains sulphur compounds for their chemical defence mechanism in order to prevent the onion form attacking by pests and insects. The pungent smell released from onion is mainly due to these compounds. The cellular structures of onion are destroyed once we cut it apart, which then resulted the enzyme Alliinase and a class of molecules called amino acid sulfoxides to be released from the broken cells. The amino acid sulfoxides are then broken down by the enzyme Alliinase to to produce sulfenic acids. 1-propenesulfenic acid is a one type of sulfenic acid formed when the plant cells of onions are damaged. This sulfenic acid is rearranged to form syn-Propanethial-S-oxide by another type of enzyme named lachrymatory factor synthase(LFS). Syn-Propanethial-S-oxide is in gas from and also a type of lachrymatory agent. Lachrymatory agent is a type of chemical which can stimulates the corneal nerves in the eyes cause tearing effect. When the gas diffuses into the air and gets contact with the eyes, the corneal nerves are stimulated and taking actions by carrying out lachrymation. The purpose of this reaction is to clean away the irritants which entered the eyes by tears flowing out from the eyes.
STOP CRYING for ONION!
Lachrymation can be restrained by cooling the onion before preparing it for dishes and dipping it into water for a while after peeling off the first layer of it. Why? Cooling the onion will consequently changes the optimum temperature for the enzyme to be active, thus the speed of chemical reactions will slow down. Secondly, dipping the onion into the water after peeling can allow the sulphur compounds to be reacted with the water and thus there is no irritating gases are formed.
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