Saturday, June 3, 2023

Why do onions make you cry?

It is always sad to see someone cry. When someone cries it almost always means someone has done them wrong.
And, of course, like getting home at 2 a.m (it’s so late it’s early), there are also tears of joy when delight comes full circle and tickles those tear glands. We have seen it before, especially at weddings.

And I have seen it before, too, at kitchen sinks when someone just starts ‘crying’ while they are cutting onions.
What is an onion and why does it make you cry?

The Latin word unio, from which the word ‘onion’ is derived, means “single” or “one” because the onion plant produces a single bulb, unlike its cousin, the garlic, that produces many small bulbs. The name also describes the union (also from unio) of the many separate, concentrically arranged layers of the onion.

“When you cut into an onion, you slice through cells which contain
sulfur compounds. Sulfur is quite a noxious (bad smelling)
element,” says the US Department of Energy. “The compounds released are highly volatile, meaning they quickly enter the air in their vicinity. Your eyes produce tears because your body wants to wash or dilute the chemicals before they affect your eyes. The tears are a natural way the body can wash the offensive chemicals away.”

They go on to say that since there are a variety of onions, we would expect there to be a variety of amounts and types of sulfur compounds produced by them. Some are really bad smelling, some only mildly bad smelling.

The onion on the supermarket shelf or sitting in the little basket in your kitchen is actually quite harmless. It gives off no smell and causes no tears. But when you cut into it, you release a gaseous sulfur-containing chemical called the lachrymatory factor (propanthial S-oxide) that spreads in the air and comes into contact with your eyes.

“The lachrymatory factor activates the nerve endings of pain fibres in the top layer of the cornea,” explains Professor Jonathan Dostrovsky, director of the University of Toronto’s collaborative program in neuroscience and a professor in the Department of Physiology. “When these pain fibres become active, they send signals to the brain and give rise to the sensation of pain in the eye,” he says. “At the same time, these signals also activate the part of the autonomic system (the system responsible for the optimal functioning of the body) that controls the lacrimal gland (the gland that produces tears) leading to increased production and release of tears.”

The same effect is produced, he notes, when other types of toxic or irritating chemicals ÔÇö such as a squirt of lemon juice or drop of shampoo ÔÇö get in the eye.

“The pain leads us to avoid or minimize further contact with these chemicals; the increasing tearing helps to wash away the irritating chemicals.”

Onions are good for our health. They are said to ‘cut heart disease risk.’ The BBC’s Health and Science says research shows that eating a meal rich in compounds called flavonoids reduces some early signs of heart disease. Onions are rich in quercetin.

“An Institute of Food Research team focused on one of the compounds, quercetin, which is found in tea, onions, apples and red wine,” said the BBC report. “The Atherosclerosis study examined the effect of the compounds produced after quercetin is broken down by the body.”

It goes on to say that they were shown to help prevent the chronic inflammation which can lead to thickening of the arteries. Previous research had shown quercetin is metabolised very quickly by the intestine and liver and is not actually found in human blood.

World’s Healthiest Foods says evidence suggests that onions may be effective against the common cold, heart disease, diabetes, osteoporosis, and other diseases because “they contain anti-inflammatory, anticholesterol, anticancer, and antioxidant components such as quercetin.”

“In many parts of the world,” says Science Direct, “onions are used to heal blisters and boils. A traditional Maltese remedy for sea urchin wounds is to tie half a baked onion to the afflicted area overnight. In the morning, the spikes will be in the onion. In the United States, products that contain onion extract (such as Mederma) are used in the treatment of topical scars, though studies have found no evidence that they are effective.”

Wikipedia says that onions, like garlic, are members of the Allium family, and both are rich in powerful sulfur-containing compounds that are responsible for their pungent odors and for many of their health-promoting effects. “Onions contain allyl propyl disulphide, while garlic is rich in allicin, diallyl disulphide, diallyl trisulfide and others. In addition, onions are very rich in chromium, a trace mineral that helps cells respond to insulin, plus vitamin C, and numerous flavonoids, most notably, quercetin.”

The (British) National Onion Association says ancient Egyptians worshipped the onion believing that its spherical shape and concentric rings symbolized eternal life… they believed that if buried with the dead, the strong scent of onions would bring breath back to the dead.

So, what can be done to reduce the tearing when you cut into an onion? There are several tried rememdies while the Internet has some of the most ouragious remedies. A look at a few:
* “If you cut onions under water, the lachrymatory factor dissolves in the water and is greatly diluted,” Dostrovsky says. “Tearing, therefore, will not be as pronounced.”

You can also try moving your head as far away as possible from the onion while slicing and dicing so there is a smaller amount of chemical reaching your eye. Or, you could always just wear goggles!
* Ian Beyer, writing on Lifehacker, Tech Tricks and Tips, says using a knife, cut a cone out of the bottom of the onion (where the roots come out).

“The diameter of this cone should be about a third of the diameter of the onion, and about 1/3 deep. This piece contains the part/gland that makes you cry when you’re chopping it up. Once you’ve gotten that piece out, chop off the top, peel, and slice the onion. Been doing it for years, and it works like a charm. You know your cone is too small if it doesn’t work, because you’ve cut into that teargas grenade.”

* Yet another contributor on Lifehacker says: “Refrigerate the onion for 30 minutes or chill it in ice water first. The chill slows down the action of the onion’s trigger enzyme and saps some energy from the vegetable’s volatile molecules…. It reduces the tendency for the sulfur compound to volatize.

One further bit of information. The chemicals which are described above
are both volatile and water soluble. This means if you cut the onion
while holding it under a spray of water, the chemicals will wash down
the drain and will not enter the air in as great an amount to cause
your eyes irritation.

However, help is on the way.
Scientists in New Zealand and Japan have created a “tear-free” onion using biotechnology to switch off the gene behind the enzyme that makes us cry.
Margot Staunton, of Agence France-Presse, said the discovery could signal an end to one of cooking’s eternal puzzles: why does cutting up a simple onion sting the eyes and trigger teardrops?

AFP said in January, the research institute in New Zealand, Crop and Food, used gene-silencing technology to make the breakthrough which it hopes could lead to a prototype onion hitting the market in a decade’s time.

Colin Eady, the institute’s senior scientist, said the project started in 2002 after Japanese scientists located the gene responsible for producing the agent behind the tears.

“We previously thought the tearing agent was produced spontaneously by cutting onions, but they proved it was controlled by an enzyme,” he told AFP from his home outside Christchurch.
“Here in New Zealand we had the ability to insert DNA into onions, using gene-silencing technology developed by Australian scientists.

“The technology creates a sequence that switches off the tear-inducing gene in the onion so it doesn’t produce the enzyme. So when you slice the vegetable, it doesn’t produce tears.”

Eady said that by stopping sulphur compounds from being converted to the tearing agent and redirecting them into compounds responsible for flavour and health, the process could even improve the taste of the onion.

“We anticipate that the health and flavour profiles will actually be enhanced by what we’ve done,” he said.
Sources: Ask A Scientist; BBC, AFP, Internet.


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