How a Vegetable Brings Us to Tears
Start chopping up an onion and soon enough your eyes will be brimming with tears, but how does this bulbous vegetable have the power to cause such a response?
Firstly, lets discuss what happens when you cry.
Tears are made of a salty fluid which contains protein, water, mucus and oil. When we cry, tears are released from the lacrimal gland in the upper, outer region of your eye.
You may be surprised to learn that there are in fact three types of tears, all with different purposes:
1) Basal tears are normally present in our eye, and the human body produces 5-10 ounces per day. These tears drain through the nasal cavity which is the reason many of us develop runny noses after crying. The purpose of basal tears is to prevent our eyes from drying out.
2) Reflex tears protect the human eye from irritants such as smoke, dusty wind, and onions which we will go on to discuss. When an irritant is detected, the sensory nerves in your cornea communicate this to your brain stem, which in turn sends hormones to the glands in the eyelids. These hormones cause the eyes to produce tears, and rid the eye of the irritant.
3) Emotional tears are triggered when sadness is registered in the cerebrum (a principle part of the brain). The cerebrum then triggers the endocrine system which releases hormones and causes tears to form.
How an onion induces reflex tears.
The moment you chop into an onion, you change its chemistry by rupturing its cell walls. Cell walls are important structures that prevent the contents of a plant cell from escaping, and mixing with other chemicals outside the cell walls. The moment the cell walls are ruptured, molecules and enzymes escape. Some of those escaped enzymes break down a sulfur compound which is present in the onion - this generates sulfenic acid. Further chemical reactions involving several molecules (ammonia, pyruvic acid and lachrymatory factor synthase) then convert sulfenic acid into a volatile and irritating gas called onion lachrymatory factor. When this gas wafts into the eyes, certain neurons instruct your tear ducts to flush out this potentially harmful substance, and hence induce reflex tears!
Methods to avoid tears.
Store the onion in the fridge or freezer for 30 minutes before chopping. This slows down its enzymes, and as such should slow down the chain of events that lead to the irritant gas.
Briefly boiling an onion should have the same effect, as high heat can deactivate enzymes, thus preventing the gas being formed at all.
Another strategy is to cut the onion under running water to prevent the gas from reaching your eyes.
The easiest solution is a pair of sunglasses, though this may then impair your vision which could be dangerous with sharp knives!
Scientists are trying to tackle the problem too - researchers in Japan and New Zealand have collaborated to create a genetically modified (GM) tear free onion. They have silenced one of the genes that makes onion lachrymatory factor (the gas that irritates your eyes). However, as GM crops aren't currently accepted as a viable solution, this onion is not yet available on the market. Other scientists have used selective plant breeding to create the 'ever mild' - a yellow onion with low levels of the volatile gas. These are however not widely available yet either.