Dr Michelle Dickinson: nanotechnologist reveals why cutting onions makes you cry

Anyone who has ever sliced an onion, knows the eye stinging feeling and watering that comes with it. 

A new study from researchers at Cornell University, published in the Proceedings of the National Academy of Sciences, has uncovered exactly how those eye-stinging onion chemicals blast into the air and revealed a surprisingly simple way to stop them. 

Onions have long been known to release an irritant called propanethial S-oxide, a sulphur-based compound that triggers your eyes’ tear reflex. But what no one really understood, until now, was how that chemical escapes from the onion and launches itself straight toward your face. 

Using high-speed cameras and computer modelling, the Cornell team watched in extreme slow motion what actually happens when a knife slices through an onion. 

They found a two stage tear cascade 

As your knife presses down, it creates pressure within the tightly packed onion cells. Each onion layer is trapped between two skins, a top and a bottom forming little pressurised chambers. 

When the knife finally breaks through the top layer, all that pressure releases in an instant shooting a fine mist of droplets into the air. 

Then comes stage two - slower streams of fluid form into droplets that continue to spray outward. 

The researchers dubbed this phenomenon the two-stage tear cascade a microscopic explosion that propels irritant droplets at astonishing speeds of 18 to 143 km per hour. 

The scientists discovered that your choice of knife, and how you use it makes a big difference. 

A sharp knife creates a cleaner, slower cut, releasing far fewer droplets. But a blunt blade, especially when used quickly, causes chaos smashing through onion layers, increasing pressure, and launching more mist into the air. 

So, the simple takeaway? Keep your knives sharp and slice slowly. 

Alternatively, coating the onion in oil before cutting can also help trap the mist, though that might be less convenient for everyday cooking. 

Beyond just helping us cook without crying, this research offers fascinating insights into fluid dynamics, food safety, and the everyday physics hiding in your kitchen. 

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Episode Transcript

Speaker 1 (00:06):
You're listening to the Sunday Session podcast with Francesca Rudgin
from News Talks EDB.

Speaker 2 (00:12):
Joining me now with her science study of the week,
Doctor Micheldikinson, Good.

Speaker 3 (00:16):
Morning, Good morning.

Speaker 4 (00:18):
Okay, we're going to clear up the myths around We're
going to get to the bottom of why onions make
us cry today because it's something that we all know
that happens, and we have little techniques for preventing it,
whether it's putting water on the risk and some people
wear those funny glasses. But science has just discovered all
about why cutting onions makes you cry.

Speaker 5 (00:40):
It has and more beautiful than that, there's a bunch
of cool slow mode videos that are totally ridiculous.

Speaker 3 (00:45):
To go with them.

Speaker 5 (00:46):
And then my favorite type of science videos when I'm
looking for topics for the week, what is what is fun?
And I'm like, oh yeah, people chopping onions and somebody
like spending hours taking a video of it.

Speaker 3 (00:56):
That's what this is.

Speaker 5 (00:57):
So this is research from Cornell University and it's just
been published in the Proceedings of the National Cadinity of Sciences,
and it's called drop Outbursts from Onion cutting. And what
they've done is they've gone, Well, we know that there's
a chemical that comes out of onions when you cut them,
and that's been well known. It's called propa an fels oxide,

(01:19):
and we know that it's selfur based and it triggers
your eyes to your reflex. Okay, so we all know
that when you cut inions, you start to cry and
it's pretty horrible, and people try and cut them at water,
people try and cut them with goggles on. But what
we didn't know is how that chemical escapes from the
onion and how does it get to your face so quickly,
Like within one slice you are crying.

Speaker 3 (01:41):
So how does it happen?

Speaker 5 (01:43):
So what they did is they use some high speed
cameras and some computer modeling and they just watched an
extreme slow motion what happens when their knife slices to
the onions. So they've got a person to cut from onions,
different knives, different speeds, different everything, and they found something
that is really useful. I think if you are one
of the people who cuts onions at them because I
love onions, Do you love onions ran chests?

Speaker 2 (02:03):
I do, yes?

Speaker 3 (02:05):
But do you cry when you cut them? No?

Speaker 2 (02:07):
Not always? And I cannot work out why sometimes I
do and sometimes I don't. Might you do with the
kind of onion?

Speaker 3 (02:17):
Maybe the kind of onion, but it might be your knife.
And the reason why I say this is because I'm ridiculous.
I have the most blunt nose in my house, and
it is what it is, right.

Speaker 5 (02:25):
But when I read this, I'm like, oh, I have
to buy a knife sharpener.

Speaker 3 (02:29):
So here's what happens.

Speaker 5 (02:30):
As your knife presses down on the onion, it creates pressure,
and so the onion cells are tightly packed inside the onion.
And you know that an onion has layers, and each
layer is trapped between these two skins, So there's a
top skin and a bottom skin, and so you imagine
you've got all this onion juice inside these two skins.
So as you start to push down with your knife,

(02:50):
it creates this massive, pressurized sort of chamber area. So
when you and I finally break through the top layer,
all of that pressure releases. So the onion juice, including
this horrible kepicle that makes you cry, shoots a fine
mist of droplets into the air. Now, the researchers, because
they measured it with this high speed camera, saw that
this can actually.

Speaker 3 (03:11):
Come out of your onion. Its speeds of one hundred
and forty three kilometers per hour. So basically you've cut
it and it's in your face.

Speaker 2 (03:20):
That's so interesting.

Speaker 4 (03:21):
But it is also interesting why it's it's because it
can aggravate your eyes quite for long.

Speaker 2 (03:25):
I mean, you know, it's amazing. It doesn't aggravate our
skin or something else.

Speaker 5 (03:30):
Yeah, No, it's the chemical actually has something to do
with your tar. That's and it's that reaction there. But
what they discovered it not only how fast this is going,
which is why you're on your first cut you've already
start weeping.

Speaker 3 (03:41):
They said that your choice of knife makes a difference.
So when they used a very sharp knife, it created
a cleaner, slower cut and far fewer droplets were released.
And you can imagine if you've got these two layers
and you're creating a pressurized chamber, the more you push
down in your onion because your knife is blunt, the
faster it's going to shoot out when you actually open

(04:02):
that top layer. So trick number one is make sure
your blade are really sharp if.

Speaker 5 (04:07):
You're cutting an onion, because it's less likely that more
droplet's going to get to your face.

Speaker 3 (04:12):
With a sharp life.

Speaker 5 (04:13):
So I've literally gone and sharpened all my knives now
because I realized that I have terribly blood knives. And
then the other one is to slice your onions slowly,
So don't put too much pressure on your onion, which
is going to create this pressure ized pressure coming up.
But slice it slowly with a reallyose, sharp knife, and
then there's not going to be as much mist coming up,
and it's also not going to be as bad for

(04:35):
your eyes. And so those are the two suggestions from
the science. They said, you know, sharp knives, cut slowly,
and they said, oh, just cover your onion in oil.
And cover your onion in oil before you cut it.
It traps the mist. And if you're going to cut it,
if you're going to cook it in oil anyway, then
it doesn't matter. So so that was trick number three
using signs, and yes, it's going to help you stop crying,

(04:59):
but also it's it's been a really cool way that
they modeled it to help people understanding fluid dynamics and
also food safety, because if there was bacteria on that onion,
they were looking at how bacteria might fly to somebody's
face on food products and so it's a cool little
physics story with an onion.

Speaker 3 (05:16):
But actually something's really helpful that I like.

Speaker 2 (05:18):
Well, you've totally answered a question for me. Thank you
so much, Michelle, because yes, I'm too terribly blunt knives
and my brother gave me a beautiful one for my
birthday about a year ago. And actually it's probably.

Speaker 4 (05:27):
The last year that I haven't hit the problem, so
I believe that, or I do like the idea of
sort of mindfully slowly cutting an onion.

Speaker 1 (05:35):
For more from the Sunday session with Francesca Rudkin, listen
live to News Talks it'd be from nine am Sunday,
or follow the podcast on iHeartRadio.