The temperatures are now perfect for a nice soup onions for dinner: hot, steaming and tasty. It’s a shame that, as many have felt, cut onions can be a traumatic experience: the eyes sting, become red, and fill with tears. The fault lies with “tear gas factor”, a real defensive weapon typical of onions useful for warding off and discouraging predators. It does not exist in the whole onion, but is formed after cutting it thanks to the action of two enzymes. It is a volatile compound that stimulates the production of tears in the eyes: in this way our body tries to remove the irritating factor. Surely you have tried many strategies to avoid crying while cutting onions, but in reality there is no evidence to confirm effectiveness of these techniques. Don’t worry: genetically modified onions that lack the enzymes that synthesize the lachrymogenic factor are being studied.
The tear factor is formed after cutting the onion
Onions, scientific name Allium cepain addition to the defense mechanisms typical of the family of Alliaceaethey have a unique weapon that characterizes them: the tear gas factor (LF), or syn-propantial-S-oxide. As with the compounds that make garlic and onion breath smell, the tear factor does not exist in fresh onion, but it is a defense mechanism which is formed only after cutting it: with cutting, or chewing in the case of animals, we break the cells of the onion and we bring some enzymes into contact, usually stored in a plant structure called vacuolewith free molecules in the cell cytoplasm.
The formation path of the lachrymogenic factor starts from a molecule called 1-propenyl-l-cystein sulfoxide, PRENCSO for our friends. When you cut the onion, it reacts with the enzyme allinase to form propenyl sulfenic acid. Until the early 2000s it was thought that this sulfenic acid degraded spontaneously, giving rise to the tear gas factor.
It was later discovered that there is a second specialized enzyme, la lachrymogenic factor synthase (not a very imaginative name), which transforms propenylsulfenic acid to give rise to the actual tear gas factor.
Tears are a technique to remove the irritant
The tear factor is a very compound volatilethat is, it tends to vaporize quickly and spread through the air. This way it reaches our eyes where stimulates nerve endings present in the cornea.
The interaction of the LF with these nerve endings sends a signal to the brain that is interpreted as a sensation of burning. In return, the brain stimulates the lacrimal glands to produce tears remove from eye and dilute the irritating agent, exactly as happens when a bit of dust or smoke gets into the eye.
The research is focusing on onions “no more tears”
Everyone has their own methods for not crying: there are those who wear a diving mask, those who cut onions under water, those who do it by holding something between their teeth or those who freeze them before cutting them. Surely we have all tried many, but the reality is that There are no studies in the literature that confirm the validity of the various methods used.
Some of these techniques may have a basis: since the tear gas factor is a volatile molecule, we must prevent it from getting into your eyesthe (hence the use of the mask), or reduce its ability to spread through the air. Cooling onions, as well as washing or cutting them under water, could theoretically prevent them from spreading and push him away before it becomes volatile.
Luckily, research comes to our aid! It has been shown that if PRENCSO, allinase and synthase are not present, the tear gas factor is not produced. On the contrary! For its production the presence of all three of these components: if even just one is missing, no tear gas factor. Researchers are therefore developing modified onions that they don’t have one of these enzymatic components, for a soup without tears.
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