One of the earliest memories that stick in my mind is a practical lesson I had in biology. I was in Africa at the time and science resources were pretty minimal, so it was always hugely exciting when we actually had a practical lesson to tie in with the theory lesson. My teacher had a row of materials hidden behind a screen. He asked us to shut our eyes and pinch our noses so that we could not see or smell anything. He came round to each of us and placed a little piece of the test material in our mouth and asked us if we could guess what it was! It was a bit like a feely box game one has at birthday parties to guess what’s in the box.
We must have been very trusting to allow him to put an unknown object in our mouth. Today, it totally wouldn’t be allowed with all these risk assessments, health and safety rules. When it came to my turn, I felt a cool, crunchy and slimy object in my mouth. I had, and still have, a huge phobia about snakes and was convinced that he had placed a piece of chopped up snake in my mouth. I screamed and quickly spat it out, making me let go of my nose that was pinched by my fingers, with eyes wide open in terror. As soon as I did that, I felt such a fool when I realized that it turned out to be nothing but a piece of white onion. It was an experiment on senses of smell and vision. Because I could not smell the onion, I could not taste it. To add insult to injurie, when my smell sense came into action, the fumes from the onion made tears stream down my face uncontrollably! This brings me to the question – ‘why do onions make you cry?’
If you cook, you most likely have had to chop up an onion. Or may be you have been in the kitchen when someone else is chopping or slicing an onion. Either way, if you are in the vicinity of the said culprit, you will probably have experienced burning of your eyes and tears streaming down your face. A broken heart is not the only thing that makes you cry! However, if you want to hide your broken heart and want to have a good old cry, start chopping onions to preserve your dignity. But remember that the tears you shed when feeling emotional are not the same as the ones from cutting up onions. These tears are due to reflexes. Then there are other tears that are there just to lubricate your eyes to keep them comfortable.
Well, what is it about onions that cause this weepy reaction? Onions contain some volatile sulphur containing organic molecules known as amino acid sulphoxides, which are the real tear-jerkers. In an uncut onion, these molecules are contained within the cells and kept separate from other biological molecules like enzymes. When an onion is cut, the cells get broken and the contents are released which are then free to mix. The enzymes released from the cells convert sulphoxides to sulphur containing volatile compounds (Propanethiol S-oxide, a big name!) that waft up to your nose and eyes. One of these substances is called lachrymator (the tear gas that police use to disperse crowds), which is released when the onion is peeled or cut. The vapour combines with the moisture in the eyes to form sulphuric acid.
You must remember from your school days that sulphuric acid is highly corrosive at high concentrations and an irritant at low concentrations. It burns your eyes, which in turn stimulates more tears to wash this irritant away. There are special glands known as lachrymal glands above the eyelids that regulate the release of these tears. If you are not crying due to emotional reasons, you produce reflex tears by external irritants like the vapours from the onions, or dust or smoke. These trigger the nerve endings in the cornea of your eyes to talk to your brain stem. The brain will then register the irritation in the eye and tells the lachrymal gland to produce tears to flush away the offensive invader! The immediate reflex reaction is to try to wipe your tears. Since your hands are covered in the organic sulphoxides compounds with a characteristic smell from the onion, you stimulate even more tears to pour out when you do this.
So, is there any way that you can prevent producing these onion tears? There are many old wives tales as to how to stop these onion-induced tears. There are some scientific explanations as well on how to prevent them. By using a very sharp knife and cutting the onion as quickly as possible, you can minimize this phenomenon. Sharp knife creates less damage to the cells of the onion and so fewer molecules will be released to mix together. Another method is to put the onion in the fridge to cool down before chopping it. This is because the enzymes work best at body temperature and cooling them down will significantly reduce their activity and hence fewer sulphoxide molecules are produced. That is also the reason you don’t cry when the onion is cooked because at high temperature the enzymes are denatured or changed to render them completely inactive or useless! You could also try peeling or cutting the onion under cold running water or in a bowl of water to wash away the volatile compounds before they waft up to your nose and eyes. However, this process can prove to be a bit tricky since it makes the onion even more slippery than it normally is. There are claims that placing a metal spoon or a piece of bread in your mouth can stop you crying! Maybe they absorb these vapours. Of course, you can always wear gloves, goggles or a full-face mask to protect your eyes from meeting these cruel vapours. A bit extreme measures though!!