The onion family
The fearsome chemical defenses of the members
of the onion family make them treasured kitchen
companions for cooks seeking pungency, flavor, and a
powerful punch of health-boosting phytochemicals.
Garlicky goodness
Like all of the onion family, garlic produces
sulfur compounds designed to irritate and
ward off herbivores, but which can also
boost human health. Garlic’s sulfur
defenses consist of the antioxidant
allicin, among others. As with onions,
the defensive chemicals are produced
by enzymes released when cells are
damaged. So to get the full nutritional
benefits of garlic, it is best to crush
the garlic and leave the enzymes to
work for a while before destroying
them in the cooking pan.
Meet the relatives
Onions and their relatives are edible members
of the Allium genus, which store their energy
in swollen leaf bases or scales. Crucially, their
energy stores are not of starch, but of chains
of fructose sugars, such as inulin, which
break down with long, slow cooking to
produce sweet flavors.
Edible Alliums
Onion family members
are popular worldwide and
range from garlic to leeks.
Combats “bad”
cholesterol
Allicin protects bad
cholesterol from
oxidation (which
increases the risk it
will clog up arteries).
It also helps the body
to expel the bad
cholesterol faster.
Lowers blood pressure
Since garlic relaxes small
blood vessels, it should also
reduce blood pressure, and
indeed there is evidence of
a small but significant effect.
Widens blood vessels
Garlic has been shown to
relax peripheral blood vessels,
producing a “warming” effect
that boosts circulation and
improves nail health.
Fights colds
Traditionally used as a cold
treatment, garlic does have
antiviral properties, but
more study is needed
to confirm that
garlic works.
Reduces blood stickiness
Sulfur compounds in garlic
help to reduce the stickiness of
platelets in the blood, reducing
the risk that they create
unwanted blood clots and
subsequent blockages.
LEEKSCALLIONCHIVEGARLIC SHALLOT ONION
30 SECONDS
THE TIME BETWEEN
CUTTING AN ONION
AND IT MAKING YOU CRY
The bulb of an onion
is not a root, but a
mass of enlarged
leaf bases
Scallions are harvested
before they grow
a large bulb
US_118-119_Onion_Family.indd 118 18/01/2017 09:44
Why do onions make us tearful?
Onions release chemical weapons when they are
damaged. Their chemical cascade begins with
alliin, like garlic, but the important product is not
allicin, but lachrymatory (“tear-jerking”) factor,
intended to sting the eyes of would be onion-
munchers. Chefs wishing to avoid tears can try
cooling the onion before cutting or using an
extremely sharp knife to minimize cell damage.
Undamaged onion
The onion is loaded with odorless precursor
chemicals, such as alliin and propiin. Onion cells
also contain enzymes to change these precursors
into pungent volatiles, but the enzymes are locked
away in chambers called vacuoles.
Damage starts chain reaction
Damage to the cell breaks open the vacuole
and mixes the alliinase enzymes with the alliin, and
the damage response sequence is set in motion.
Enzymes produce
volatile chemicals
A further enzyme makes a
chemical called lachrymatory
factor, meaning simply
a substance that produces tears.
It evaporates, along with some
other volatile chemicals produced.
Flush it out!
The acid triggers
the eye’s defense
mechanisms to swing
into action, producing
tears to wash out
the irritant.
Onion chemical
forms acid in eye
The lachrymatory factor
diffuses quickly through
the air to reach the eyes.
It dissolves in the layer of
fluid that coats the eye and
some of it forms sulfuric
acid, stinging the eye.
1
2
3
5
4
Lachrymatory
(tear-jerking)
factor evaporates,
forming a gas
Pungent chemicals
stimulate the mouth
and nose
Tears used to
flush out acid
Alliinase enzyme
is locked inside a
vacuole in the cell
Precursor molecule
(alliin) waits to be
activated
SULFENIC
ACID
LACHRYMATORY
FACTOR
ENZYME
ALLIIN
UNDAMAGED CELL
DAMAGED CELL
ENZYME
VOLATILE
CHEMICALS
ENZYME
ALLIIN
PAIN SIGNAL
TO BRAIN
“CRY” SIGNAL
FROM BRAIN
BRAIN
More enzymes wait
for their part in the
series of reactions
Alliinase
converts alliin
into sulfenic
acid
The chemical cascade
makes other pungent
defensive chemicals
The next enzyme
in the chain produces
lachrymatory factor
US_118-119_Onion_Family.indd 119 23/02/2017 11:27
Why do onions make us tearful?
Onions release chemical weapons when they are
damaged. Their chemical cascade begins with
alliin, like garlic, but the important product is not
allicin, but lachrymatory (“tear-jerking”) factor,
intended to sting the eyes of would be onion-
munchers. Chefs wishing to avoid tears can try
cooling the onion before cutting or using an
extremely sharp knife to minimize cell damage.
Undamaged onion
The onion is loaded with odorless precursor
chemicals, such as alliin and propiin. Onion cells
also contain enzymes to change these precursors
into pungent volatiles, but the enzymes are locked
away in chambers called vacuoles.
Damage starts chain reaction
Damage to the cell breaks open the vacuole
and mixes the alliinase enzymes with the alliin, and
the damage response sequence is set in motion.
Enzymes produce
volatile chemicals
A further enzyme makes a
chemical called lachrymatory
factor, meaning simply
a substance that produces tears.
It evaporates, along with some
other volatile chemicals produced.
Flush it out!
The acid triggers
the eye’s defense
mechanisms to swing
into action, producing
tears to wash out
the irritant.
Onion chemical
forms acid in eye
The lachrymatory factor
diffuses quickly through
the air to reach the eyes.
It dissolves in the layer of
fluid that coats the eye and
some of it forms sulfuric
acid, stinging the eye.
1
2
3
5
4
Lachrymatory
(tear-jerking)
factor evaporates,
forming a gas
Pungent chemicals
stimulate the mouth
and nose
Tears used to
flush out acid
Alliinase enzyme
is locked inside a
vacuole in the cell
Precursor molecule
(alliin) waits to be
activated
SULFENIC
ACID
LACHRYMATORY
FACTOR
ENZYME
ALLIIN
UNDAMAGED CELL
DAMAGED CELL
ENZYME
VOLATILE
CHEMICALS
ENZYME
ALLIIN
PAIN SIGNAL
TO BRAIN
“CRY” SIGNAL
FROM BRAIN
BRAIN
More enzymes wait
for their part in the
series of reactions
Alliinase
converts alliin
into sulfenic
acid
The chemical cascade
makes other pungent
defensive chemicals
The next enzyme
in the chain produces
lachrymatory factor
US_118-119_Onion_Family.indd 119 23/02/2017 11:27
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