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What Gives Onions Their Distinctive Smell?

What Gives Onions Their Distinctive Smell?

I’m sure you’ve been there—cutting up onions in the kitchen, only to have visitors show up.

Now what? How do you hide the tell-tale onion whiff that fills your home, leaving it smelling like a demon farted in the kitchen, leaving everyone weeping for no reason?

Just what is it that gives onions their distinctive smell?

After my guests had left, sniffing their noses at me and wiping their eyes, I decided to settle the question once and for all.

I phoned my son-in-law, who’s a chemistry professor, and what he told me about onions made it all clear to me.

What Gives Onions Their Distinctive Smell?

The powerful onion smell is a result of the powerful sulfur-containing acids (propenylsuplhenic acid) being broken into lachrymatory-factor synthase and thiosulfinate due to the enzyme alliinase. The alliinase then creates the foul smelling sulfurous smell of onions. Alliinase, while responsible for the smell, is also responsible for many health benefits.

The Allinase in Allums and Why They Smell

Onions have a distinctive smell that’s somewhere between the smell of a tar pit and a hard-boiled egg.

The smell of onions is so powerful that it affects all the soft membranes of the eyes, nose, and mouth.

This means you don’t just smell onions, you taste them too.

The enzyme alliinase is responsible for breaking down the sulfur-rich propenylsulphenic acid, and this causes the release of the onion’s distinctive smell.

The onion juices are loaded with this scent, which is why when you’re chopping onions or garlic, you will have this smell ingrained in the skin of your hands.

Why Onions Smell More When Cutting

I was somewhat puzzled while talking to my son-in-law as to why only the onions I was cutting had the terrible onion smell while the ones that still had their skins on were quite mild smelling.

After all, the only difference was that I had cut the ones that were now smelling. What made them different enough to stink while the other onions were quietly minding their own smell-business?

So, my amazing science professor son-in-law had answers to this too. It seems that during the conversion of sulfur by the alliinase enzyme, a sulphuric gas is released.

This happens due to the introduction of the atmosphere to the onion’s flesh.

By cutting the onion, I was introducing air to the onion’s cells, which leads to the stinky onion gas (syn-propanethial-s-oxide) forming.

Tips to Make Onions Stink Less

My next question was how to stop this gas being released and keep my kitchen smelling lovely and not like a stinky sulfurous bog. Surprisingly, there are a few ways to limit the onion smell.

Slice Cold Onions

By putting your unsliced onion in the freezer for 10-20 minutes before you slice them, you will greatly limit the amount of sulfur gas that can be formed.

In basic chemistry terms, most chemical reactions need some heat or at least room temperature to happen. The cold retards the chemicals, preventing them from reacting. Thus, cold temperatures stop the alliinase from reacting to make the gas.

Cut off the Onion Roots

Another great way to reduce the onion smell is to cut off the bottom or root end of the onion before you start peeling and slicing. Place this part of the onion in a packet or sealed container to limit the onion smell. The root section of the onion is where most of the sulfur compounds are stored.

Use Water

Another tip that you can use to reduce onion stink is to wash the onions, peel them under water, wash them again, and then slice them. The water will form a barrier between the onion cells and the atmosphere, also reducing the amount of sulfuric gas that can be released.

3 Ways to Remove Onion Smell From Your Hands

The onion smell is also quite prevalent in the onion juices. When you slice onions, you will find your hands take on that particular aroma we all know to be completely onion. Getting rid of it can be challenging.

Method One: Soap and Salt Scrub

One way to get rid of onion smell is to mix equal parts liquid soap and salt and scrub your hands with this for at least a minute. The onion smell should be gone and your hands will feel silky smooth.

Method Two: Acid Wash

Not pool acid, but you can use vinegar or lime juice to neutralize the onion smell on your skin. Simply pour a generous amount of these acidic liquids onto your hands, then scrub vigorously for a minute before rinsing.

Method Three: Stainless Steel Rub

The final method you can use to remove onion smell from your hands is to rub your hands under running water with a stainless steel spoon or stainless steel bar. Stainless steel is known to eliminate the sulfur that causes the onion smell.

Frequently Asked Questions About What Gives Onions Their Distinctive Smell

What gives onions their distinctive smell?

The sulfur-containing chemicals that onions absorb from the soil produce the sulfurous smell of onions.

What causes your mouth to smell like onions?

Some bacteria that live in the mouth produce the same sulfurous compounds that we smell on onions, making the mouth smell like onions. Be sure to brush your mouth thoroughly, and if the smell persists, see your doctor to prescribe antibacterial medications and rinses to use.

Why does my mouth smell for hours after eating onions?

Onions contain natural sulfur compounds, and when you eat these, these compounds interact with the natural bacteria that your mouth has. This causes a chain reaction of sulfur production, which leads to bad breath. To remove the smell, be sure to brush your whole mouth with toothpaste as soon as you’ve eaten onions.


I will always love onions, but now I am careful to first freeze them before peeling and slicing onions.

I also keep a plastic container handy to place the root section in to reduce that tell-tale onion smell in my kitchen.

Since taking these steps, my home no longer smells of onions when

I’m cooking, and I can receive guests in my home while I prepare meals without a worry in the world.

And have I mentioned how impressed I am with my culinary chemistry-wise son-in-law?