Why does Onion Makes You Cry And Garlic doesn’t?

I wake up to the beep of my alarm. I jump out of bed, hit the snooze button, and get dressed for work. The days are usually as bright as the sun or as gloomy as a rainy day. But no matter what happens to me that day, one moment of it always fills me with joy.

The aroma of freshly fried onions and ground garlic from the dingy Biryani shop on the corner always fills me with peace as I leave the bus stop at the end of the day and cross the street to my apartment building. The smell immediately transports me back to my cozy bedroom in our house in the village. I miss my small room; it reminds me of my mother and the delicious biryani she used to make.

To remind myself of that magical moment, I decided to cook. I started my preparation with vegetable cutting, so I got out the knife to chop some onions, but at the initial stage, the tears blurred my vision.

Why did onions make me cry, and how could I stop this from happening? A second thought arose: why did cutting garlic feel comfortable even though it came from the same family as onions? I became curious and decided to look into this phenomenon in more detail and came upon the term Lacrimator.

Table of Content

What is a lacrimator?

Lacrimator is a derivative of the Latin word lacrima (a noun meaning tear). It is also related to the word lachrymose, which means prone to crying easily and indicates abundance. Lacrimators are tear producers. It is an irritant that causes tears by stimulating the nerves of the lacrimal gland. Tear gas and pepper spray are two examples. Lacrimators are produced, by plants in the Allium genus, in reaction to a perceived threat or harm.

Do onions and garlic belong to the same genus?

The vegetable section at any grocery store is never without the few common ingredients that make up the Allium genus; onions, garlic, shallots, leeks, and chives. Other Allium varieties that might not be grown in the kitchen garden also exist. Alliums grow as perennials in areas of the globe with cool summers and mild winters. The edible parts can be the bulbs, the flower stems, or the leaves. Onions are eaten raw or cooked in various recipes such as stews, stir-fries, salads, and many other dishes. Garlic bulbs are used primarily in cooking and have a strong odor when raw, which mellows considerably during cooking.

What causes the odor and tearing of eyes in Allium?

Plants in the Allium genus contain amino acid sulfoxides, such as Alliin, Isoalliin, Methiin, Propiin, and others. The chemical notation of the compound is, R-amino acid-sulfoxide. The amino acid group is cysteine. The R group differs. It can be allyl in the case of Alliin, propenyl in the case of Isoalliin, methyl in the case of Methiin, and propyl in the case of Propiin.

An onion in its entirety will never make you cry. You will never get the aroma of garlic before crushing it. So, what happens when you cut it or cause a bruise?

The intact tissues, when they are cut or crushed, are harmed. As a result, a defense mechanism gets activated. The cell began secreting enzyme-containing juices. The enzymes are part of the vacuoles of the intact cells. The vacuoles rupture and release the enzyme into the cytoplasm. These enzymes act on the amino acid sulfoxide compounds in the cell cytoplasm to produce the volatile chemicals that cause odor or eye tearing.

Alliin interacts with an enzyme known as alliinase. It results in the formation of allyl sulfenic acid via a breakdown or lyase, which combines with another allyl sulfenic acid to make diallyl thiosulfate, also known as Allicin. It is responsible for the garlic odor.

Isoalliin, the Lacrimatory Precursor, in the presence of alliinase, creates 1-propenyl-sulfenic acid. Another enzyme, lacrimatory factor synthase (LFS), acts on this molecule to generate Z-propanethial-sulfoxide, or Lacrimatory Factor (LF), a volatile molecule.

What distinguishes garlic from onions in terms of lacrimatory effects?

There could be two explanations for the deferral of behavior. 

First, the enzyme secreted in both situations is similar, but the precursor differs; Alliin in the case of garlic and Isoalliin, the lacrimatory precursor, in the case of onion. Alliin is being changed to Allicin, the molecule that causes odors, and Isoalliin to LF, the crying factor. 

Second, the activity of LFS, or lacrimatory factor synthase, is essential. The enzyme is substrate-specific, meaning it only acts when Isoalliin and Allinase enzymes are present. The absence of this enzyme and the substrate combination will never result in the formation of an LF.

How are tears produced?

One avoided the word “tear” because it evoked associations with pain. It could be caused by any foreign particle or induced by emotional jargon. Tears are something uncomfortable to experience, but they have other purposes too.

One interesting fact about tears is a secretion of a gland called the lacrimal gland. The principal function is to provide the cornea with a protective layer to keep it moist and nourished.

Tear formation is a continuous process. It is associated with the blinking of the eyes. We generate tears while awake or asleep.

How is blinking related to tear production?

Blinking is due to a set of reflexes known as the Corneal Reflex. Irritation to the cornea triggers this reflex. It leads to two responses happening at the same time.

• The Blink Reflex causes both eyes to squint shut, protecting the eye from further irritation. It facilitates the spread of the tear film throughout the eyeball and channels the spent tears by squeezing action through the lacrimal apparatus into the nasal opening.

• Tears are produced, preventing the irritation from causing an infection. Both responses occur in a similar pattern.

What is a Lacrimal Apparatus?

The lacrimal apparatus carries the expended tears to the nasal cavity. It prevents the blurring of the vision due to the accumulation of tears. Maintain the continuity of the process of tear production.

• The lacrimal gland is a secretory organ located on the upper eyelids. This organ secretes tears in response to the stimulation of nerves.
• The lacrimal gland receives parasympathetic nerve VII.
• The lacrimal ducts are the channels via which tears from the lacrimal glands flow. They are six to ten in number. With each blink, the tear spreads across the surface of the eye.
• Tears accumulate in a triangular area from the eye’s surface called a lacrimal lake.
• From the lacrimal lake, tears get channelized through tiny openings in the eyelids. These apertures are known as lacrimal punctae. There are two punctae present on the upper and lower eyelids.
• The two channels connecting the punctum to the nasal opening are the lacrimal canaliculi.

How to prevent the discomfort of the onion lacrimators

The previous studies suggest that suppressing the lacrimatory component can resolve the issues of tears. There are a few techniques that can prove to be remedial in preventing the LF

• Chopping onions can be easier if they are chilled beforehand. The lower the temperature, the less volatile the LF.
• LF is water soluble. So, peeling or cutting the onion under water is an effective method.
• Cutting the roots last might also be beneficial since it contains a high and wide range of allinase enzyme activity.

Well, to continue with my nostalgia. I chose the two preventive methods, chilling and chopping the onion with the roots. I found it to be reliable and convenient.

 Citation:

• Rabi A. Musah, Quan He, Roman Kubec, Discovery and Characterization of a Novel Lachrymatory Factor Synthase in Petiveria alliacea and Its Influence on Alliinase-Mediated Formation of Biologically Active Organosulfur Compounds, Plant Physiology, Volume 151, Issue 3, November 2009, Pages 1294–1303, https://doi.org/10.1104/pp.109.142539
• Jane E. Lancaster, Martin L. Shaw, Meghan D. Pither Joyce, John A. McCallum, and Michael T. McManus, A Novel Alliinase from Onion Roots. Biochemical, Characterization and cDNA Cloning, Plant Physiology, April 2000, Vol. 122, pp. 1269–1279, www.plantphysiol.org
• Eric Block, The Chemistry of Garlic and Onions, Scientific American, Volume 252, No. 3, March 1985, Pages 114-119, https://www.researchgate.net/publication
• Josie A. Silvaroli, Matthew J. Pleshinger, Surajit Banerjee, Philip D. Kiser, and Marcin Golczak, Enzyme That Makes You Cry–Crystal Structure of Lachrymatory Factor Synthase from Allium cepa, ACS Chemical Biology, 2017 12 (9), 2296-2304
• Diana C. Peterson; Renee N. Hamel, Corneal Reflex, National Library of Medicine, StatPearls, July 25, 2022