Curious Questions: Do tear-free onions actually stop you crying when you chop them?

Our intrepid correspondent Martin Fone has put Sunions (and his eyes) to the test.

Curiously, onion, a vegetable known for its strong, pungent aroma and the delicious flavour it brings to cooked dishes, has no smell when it is taken directly out of the ground. It is only when its cells are damaged, either by cutting, slicing, or biting, that a chemical reaction is triggered to produce its characteristic odour and taste.

An enzyme, alliinase, is released, which converts the isoaliin, an odourless compound in the bulb, into flavour compounds, a process that takes less than thirty seconds at room temperature. The intensity of its flavour is determined by the variety of onion and the sulphur content of the soil in which it is grown.

There are hundreds of varieties of onion, or Allium cepa, to give it its botanical name, but in essence they can be grouped into two categories: spring onions which are planted in the autumn and harvested in early spring and tend to be sweeter and less pungent and storage onions, which are sown in the spring and are gathered in the late summer or early autumn.

Onions are a staple feature of most of our diets, ranking just behind potatoes and carrots in a recent YouGov survey of Britain’s most popular vegetables, with men showing a slightly greater preference for them than women.

The onion is also one of the oldest cultivated crops, originating in the wild in Central Asia and first grown by man in Iran and West Pakistan. A Sumerian text from around 2500 BC tells of someone ploughing over the onion patch of a city governor while in Egypt the cultivation of onions was documented a millennium earlier.

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The Egyptians were fascinated by the concentric structure of the onion which they believed was symbolic of eternal life. King Ramses IV was buried in 1160 BC with onions carefully placed in his eye sockets.

Onion Slice

In ancient Greece, athletes competing in the Olympic Games would eat pounds of them, drink copious amounts of onion juice and rub them into their skins in the belief that their performance would be enhanced, although it may just have meant that very few would want to get near them.

Pliny the Elder wrote about the cultivation of onions at Pompeii in his Naturalis Historia. Archaeologists excavating the site have found evidence of onion production on almost an industrial scale, just as Pliny had reported. The Romans also believed that the vegetable had medicinal properties, ideal, Pliny noted, for improving vision, inducing sleep, and healing a variety of complaints, including mouth sores, dog bites, toothache, dysentery, and lumbago.

What we eat is the plant’s bulb, or, more accurately, its underground root system, which has shortened, and compressed to form scale-like leaves. These leaves emerge very fleshy and juicy, but as the vegetable matures in readiness for harvest, the outer foliage dies, becoming dry and brittle. This outer skin preserves the onion, giving it a lengthy storage life, a factor, coupled with the ease with which it can be grown in most types of soil and climatic conditions, that made it an invaluable vegetable, especially when most items in the diet were seasonal and variable in quality and quantity.

They were also ideal vegetables to include in provisions for soldiers embarking upon military campaigns and for sailors voyaging across the seas, providing them with not only something with which to flavour food and supplement diets but also to trade with and plant. It is claimed that the onion was the first vegetable planted by the colonists who settled on the Atlantic coast of North America.

But why do onions make us cry? And how can we protect our eyes?

The production of smell and taste is not the only chemical reaction triggered by damaging an onion’s cells. The release of alliinase allows water and S-1-propenyl-L-cysteine sulphoxide to react together to produce several sulphurous compounds, one of which, 1-propenyl-sulphenic acid, is pounced on by another enzyme, lachrymatory-factor synthase, to produce propanethial-S-oxide. This complex, two-stage process, only recently fully understood and documented in Nature (2002), produces the stinging irritant familiar to most cooks.

As you cut into a regular onion, head bent over in concentration, the irritant rises towards your cornea, causing the brain to send a signal to the lachrymal gland, located between the outer edge of the eyelid and eyebrow. This triggers a reflex action, the production of tears, designed to wash the irritating gases from your eyes. The intensity of the reaction will be determined by the pungency of the onion, with spring onions unlikely to cause the tears to flow, and the sensitivity of the cornea. The production of propanethial-S-oxide is part of the onion bulb’s defence mechanism to deter predators from taking a nibble.

There is no shortage of advice on how to guard against this involuntary reaction. One option is to wear a pair of onion goggles, which not only have a foam seal to shield the eyes from the sting but also come with anti-fog lenses.

Close-up of a mid adult woman cutting an onion

Alternatively, stand away from the onion as you cut, switch on a fan to circulate the air, or use a very sharp knife to minimise the damage done to the cells. Other suggestions include reducing the water content in the onion by putting it in the refrigerator for thirty minutes, or keeping a piece of white bread in your mouth while cutting. Supposedly, the bread will absorb the vapours, although there is no evidence that it works. You could buy pre-sliced onions, an egregious form of cheating, some may feel, or just accept it as a fact of life.

Tears, though, may be a thing of the past if the Sunion takes root in the kitchen. The result of thirty-five years of meticulous research and development, involving the cross-pollination of varieties of onions to reduce their pungency and to enhance their sweetness, the Sunion’s revolutionary quality is that even when its cells are damaged, no irritants are generated to initiate the flow of reflex tears.

Its producers stress that no genetic modification was involved in the Sunion’s development, and the Sensory Lab of BASF’s vegetable seeds business and Ohio State University have certified that, after extensive testing, it is the world’s first and only tearless onion.

Sunions also differ from common or garden onions by changing flavour during storage, becoming milder and sweeter by the day. Unlike other varieties, whose volatile compounds remain stable or increase in intensity during storage, the genetics of the Sunions act in reverse, decreasing their pungency even further over time.

Available to consumers in the United States for over four years, the Sunion has finally reached our shores, with Waitrose announcing that they would be on the shelves of selected stores and available on-line from January 18th.
Intrigued by the sound of Sunions, I unearthed some at my local Waitrose store.

sunions, tear-free onions

Sold in bags of three, they are around ten centimetres in length and 20 in circumference and have a lighter, more golden colouring than the store’s essential onions.

I chopped and diced two, leaning directly over them. By the time I had finished, I was aware of a mild stinging sensation in my eyes, although one not strong enough to incite the flow of reflex tears. For me, the Sunion had met its tearless claim. The only thing guaranteed to bring tears to your eyes is their price – they cost over three times more than Waitrose’s essential onions.

But what about their taste? Eaten raw, Sunions were sweet, quite refreshing, almost like an apple in texture, and not as pungent or as crisp as the store’s common or garden onions. There was barely any aftertaste, and they were strangely moreish.

When cooked, they were mushier and lacking the deep aroma and intense flavour that I associate with onions.

My impression is that they work best uncooked, either in a sandwich or a salad or as an outré snack for the lunchbox, an onion designed for those who do not like onions.

Will the Sunion be the onion to supplant all the others, or will it become just a footnote in our culinary history? Only time will tell. For me, a flood of reflex tears is a part of the joy of cooking and long may it continue.