Onions and crying

Why do onions make you cry?

It's an interesting question. I've always been wondering about it myself for years. It's comforting to know that there is a simple biochemical reason for this strange phenomena. :)

What others might tell you...

People normally say that there is a chemical that is released from an onion that irritates the eyes. This is kinda right... but not quite! There are a few chemical processes that occur naturally first. The chemical released from the onion is not the irritant: it is not the chemical that irratates your eyes.

Let's get to the biochemistry!

Imagine an onion cell with two compartments. One compartment holds the enzymes which are known as alliinases (yeah, it has two ''i''s in it). Imagine that the second compartment, adjacent to the first, holding the sulfur-containing amino acids (AKA amino acid sulfoxides). The name sulfoxide implies that there exists an S-O bond. The difference in electronegativity causes an overall dipole. The oxygen is more electronegative than sulfur.

This is the structure of sulfoxide which is found in a compartment of onion cells. There are two resonance forms pictured above. Resonance is a type of electron delocalisation. This occurs because electrons repel. In a double bond, they move away from each other to make the molecule more stable. In this picture, we can see the two different arrangements of the electrons.

When you cut an onion, the chemicals in the separate compartments are able to mix and form a type of sulfenic acid. This happens because you break the cells open when chopping an onion. The specific name for the acid produced is 1-propenesulfenic acid. Here's the formula:

CH3CH=CHSOH

Right, now something a little weird happens next. Another enzyme called lachrymatory factor synthase (or LFS for short) assures the rearranging of the acid molecule to form propanethial S-oxide. The rearranging, I believe, is what probably causes a decrease in boiling point in the molecule. It probably makes the structure less regular and thus decreases the number and strength of dipole-dipole interactions between the molecules. Temperature can more easily overcome the now weaker intermolecular forces and the boiling point is consequently lowered (the substance becomes more volatile).




Propanethial S-oxide before exposure to LFS enzyme

Right, so what exactly makes us ''cry''?

The gas diffuses through the air and initiates contact with the eye. Three chemicals are formed from the interaction: hydrogen sulfide, propanol and sulfuric acid. The sulfuric acid is dilute but it is in a high enough concentration to cause irritation to the eye. This triggers a sensory neural response which creates a stinging sensation and tear-production by tear glands. Tears are released as a method of flushing out and diluting the sulfuric acid from the eye.

We need sulfuric acid in onions. It sounds quite deadly, but it's true. Without it, onions wouldn't have the nice scent they they have. They would taste nothing like they do now. The same applies to garlic too.

Old wives' tales

Alright, I did promise to do this last week! And I keep my promises. Here are some ways to cut down on crying whilst chopping onions. Decide for yourself which ones really work:

1. Cover your eyes with safety goggles/specs. This provides a barrier against the gas released from
    the onions.
2. Use a fan to blow away the gas.
3. Put a teaspoon in your mouth.
4. Put a piece of bread in your mouth and let it hang there whilst chopping.
5. Wear contact lenses
6. Use a sharp knife (careful when handling sharp objects!)
7. Chill the onions
8. Chop the onion whilst holding it in water. The water becomes acidic due to the reaction between
    water and propanethial S-oxide. However, your hands could slip whilst slicing - so be extra careful.
9. Breathe with your mouth when chopping onions. You will breathe in the gas released by the
    onions.
10. Close your eyes when chopping onions. I'm kidding! That would be suicide.

Well, I hope I have shed some light on an every-day mystery. Now go and impress your mates! I'll be back next week with some more biochemistry as always! Don't forget to send me feedback too! If you are or know anyone doing or is interested in biochemistry, don't hesitate to contact me. I'm nearly always available for any help and discussion on top of other niceties! :)



Exocytosis