Onions
are a kitchen staple and
form the basis of diverse cuisines. In addition
to enhancing tastes, they are rich sources of
vitamin C, B6, potassium and dietar y fibre
[1]
.
They are frequently used in a variety of Chinese
dishes, functioning as grease soakers and subtle
sweeteners. However, there is a notorious side-
effect while cooking with this healthy and versatile
vegetable – the juice contained in onions released
during preparation, somehow triggers tears.
Basa l tea r s a re cont i nuous l y p roduced
throughout the day to protect our eyes and
eyelids, keeping them moist and lubricated. Finally,
reflex tears, regulated by the lachrymal glands
located above the eyelids, help to wash out
irritants, such as dust or smoke
[2]
.
On ion j uice contai ns chemical s that are
irritating to the eyes and generate reflex tears in
order to flush away the unpleasantness.
But how do onions synthesize
these chemicals
that no
one cares for? To answer this question, we need to
trace back to the soil in which onions are grown in.
The growth process of onions involves absorbing
sulphur from the soil and converting it into organic
compounds named amino acid sulphoxides. It
is these sulphur-containing organic compounds
which lend onions their uniquely pungent odour
and flavour.
The gaseous chemical originating from oils of
onions is responsible for the tears and is called
propanethiol S-oxide. This chemical is a type of
lachrymator (tear gas), and triggers the lachrymal
glands (tear glands) when it comes in contact with
the cornea. Other common lachrymatory agents
include pepper spray and phenacyl chloride,
used in riot control. Tears are produced in an
effort to dilute the irritant. Propanethial S-oxide is
also very reactive, and even a small amount will
readily react with tears to form sulphuric acid,
which produces fur ther burning and stinging
sensations
[3]
.
For a long time, it was presumed that alliinase,
a crucial enzyme rendered during the process of
synthesising onion flavour, was the only enzyme
required to produce
a group of Japanese scientists discovered the
actual enzyme accountable for the production of
propanethiol S-oxide and subsequently named it
lachrymatory-factor synthase. This opens up the
possibility for onions to be genetically modified
without lachrymatory-factor synthase. In fact, the
inhibition of lachrymatory-factor synthase not only
stops the production of the unwanted irritant, but
also increases the production of thiosulphinates,
which retain the flavour and odour of onions
[4]
.
By Sunjung Lim
林宣廷
N
a
t
u
r
e
’
s
N
a
t
u
r
a
l
T
e
a
r
G
a
洋
蔥
–
自
然
催
淚
劑
Tears are evoked by
three main causes. The most
familiar type, emotional tears,
flow when we are sad, physically
in pain, distressed or moved from
being overly joyful.
