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Why Do Onions Make Us Cry? · So That's Why

• 0:04
  Jenny Oliver: Welcome to the, so that's Why podcast where we unpack
• 0:07
  the science behind everyday questions.
• 0:10
  I'm Jen.
• 0:11
  Chris Smith: I'm Chris.
• 0:12
  Matt Edmundson: And I'm Matt.
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  Jenny Oliver: Okay.
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  So today we are tackling something that pretty much everyone who's
• 0:18
  ever cooked has experienced.
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  Why do onions make us cry?
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  And why does the reaction vary so wildly between people?
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  Matt Edmundson: This is a great question 'cause I'm definitely
• 0:29
  one of the extreme reactors.
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  Let me tell you.
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  You know, I can start chopping onions and after a few minutes I'm
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  genuinely questioning my life choices.
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  Uh, you know, is this pasta dish really worth the emotional trauma?
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  Uh, or should I go and get some goggles?
• 0:44
  Maybe.
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  I don't know.
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  Jenny Oliver: I reckon it is worth it.
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  But what I've always found curious is that cooking them.
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  It completely eliminates the problem.
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  Yeah, it's the same vegetable, same knife, but you put, you apply
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  heat and suddenly there's no tears.
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  So there's clearly some specific chemistry happening at room
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  temperature that heat disrupts.
• 1:06
  Chris Smith: Well, I feel like I should be wearing a lab coat
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  and a chef's hat for this one.
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  Matt Edmundson: Um, I, I, I, for one, wanna see that?
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  Jenny Oliver: Yeah.
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  Yeah.
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  Chris Smith: You really don't.
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  So the chemistry involves multiple enzymatic reactions that only trigger
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  when you damage the onion cells.
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  Jenny Oliver: Okay?
• 1:23
  So by the end of this episode, you'll understand the two enzyme
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  system that creates this volatile irritant, how it travels from the
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  cutting board to your eyeball at speeds of up to 89 miles per hour.
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  And why your genetic makeup determines whether you're a heavy
• 1:40
  crier or you are mysteriously immune.
• 1:43
  Matt Edmundson: Plus, we'll look at whether any of those fog crees
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  actually work, or whether I've just been holding bread in my mouth for
• 1:51
  absolutely no reason whatsoever.
• 1:54
  Chris Smith: The stats are enough to make you cry, ping.
• 1:58
  Um, research shows that about 70% of people experience significant
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  tearing when cutting onions.
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  And of course that means a lot of people affected are likely to avoid
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  like group cooking sessions or classes or to cook when others aren't around.
• 2:12
  Matt Edmundson: Yeah.
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  Which is absolutely me, right?
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  I, I have time.
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  I have to time actually my meal prep to when the family aren't around.
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  Otherwise they're just gonna laugh at me the whole time.
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  Jenny Oliver: Well, apparently the reaction intensity doesn't correlate
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  with cooking experience, so you can't build up tolerance through
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  exposure because the stimulus acts directly on re specialized receptors
• 2:33
  in your cornea every single time.
• 2:37
  Matt Edmundson: Well, thanks Jen.
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  That's all cheery news.
• 2:40
  Jenny Oliver: It's a problem you can't escape from.
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  Matt Edmundson: Yeah.
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  To done, done, done.
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  So what, actually, let's look at it then.
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  What actually happens The instant I make the first cut into that onion.
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  Chris Smith: So onion store, different chemical compounds in
• 2:52
  separate cellular compartments.
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  An enzyme called ese sits inside one part of the onion while reactive
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  substances called amino acid.
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  Sulfoxide sit in another.
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  Jenny Oliver: Yeah.
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  So they're physically separated preventing a premature reaction like storing
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  oxidizer and fuel in separate tanks.
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  Chris Smith: Exactly.
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  And when your knife ruptures the onion cells, those walls
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  break and it all goes off.
• 3:16
  Jenny Oliver: It all goes off.
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  Chris Smith: And the enzyme reacts with slf oxides producing unstable
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  compounds called snic acids.
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  Jenny Oliver: And what happens to those snic acids?
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  Chris Smith: So this is where onions have something special.
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  It's a second enzyme called a atory factor.
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  Synthase, or LFS, let's call it LFS.
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  Matt Edmundson: Thank you.
• 3:35
  Jenny Oliver: Yeah.
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  Chris Smith: And uh, this was discovered in 2002 by Japanese researchers
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  and finally explained the mechanism behind white onions make us cry.
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  Matt Edmundson: So what then does this LFS, 'cause I'm, I
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  am never gonna try and produce
• 3:52
  Jenny Oliver: Oh, on go, go on try.
• 3:53
  Matt Edmundson: Yeah.
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  No, it's not gonna happen.
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  Uh, what does that do differently?
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  Chris Smith: So this LFS intercepts, those snic acids produce when
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  you cut the onion and converts them into a volatile compound.
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  Matt Edmundson: A volatile compound sounds.
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  Jenny Oliver: It does not sound, sounds dangerous, doesn't it?
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  Matt Edmundson: Uh, and I guess I'm guessing then that this
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  has also got a name that I'm not gonna be able to pronounce,
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  Chris Smith: which is why I called it a volatile compound.
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  Um, yes, it's a simPRO Panol, S oxide.
• 4:23
  Matt Edmundson: Very good.
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  Chris Smith: Excellent scrub.
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  Matt Edmundson: Very good.
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  Jenny Oliver: Well done.
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  He's been practicing.
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  Matt Edmundson: Yeah, you've been practicing.
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  I'm not gonna try and pronounce that either.
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  Chris Smith: So anyway, that's the responsible volatile compound for
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  the onion tears, and this entire reaction completes within seconds
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  of your knife hitting that onion.
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  Matt Edmundson: So what you're saying then is I'm basically creating this
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  sort of tiny chemical reaction or chemi chemical factory that reacts
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  faster than I can dice my onion.
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  Jenny Oliver: So once this writes my turn, now send Propan the S oxide forms.
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  Well done.
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  How does it travel to my eyes?
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  I mean, you said it's volatile, but it's gotta be, it's like it,
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  it feels like you've barely cut your onion and it's taking up.
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  Chris Smith: Yeah, absolutely.
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  So studies have shown like using cameras that cutting the onion, ejects
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  these tiny droplets and they move at speed up to 40 meters per second.
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  Um, like knife sharpness and cutting speed dramatically affects the number
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  and velocity of those, um, particles.
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  But it's fair to say they're rapid.
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  Jenny Oliver: I mean, I guess that explains why they say sharper knives
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  help, because cleaner cuts through the onion mean it's less disruption maybe.
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  Matt Edmundson: So I'm, I'm, I'm not actually missing the point then
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  when I'm so OCD about sharpening my knives, uh, they're, they're,
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  you know, they're gonna help me cry less is what we're basically saying.
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  Jenny Oliver: Yeah, essentially, yeah.
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  So you, you need to get those knife sharpeners out.
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  Um, not just to look like an expert, but actually,
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  Matt Edmundson: which I clearly do, obviously,
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  Jenny Oliver: you know, made me to cry a bit less.
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  So when the droplets reach the eye, what's the chemical reaction
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  to cause us to actually then cry?
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  Chris Smith: So that volatile compound that I'm definitely not gonna try and
• 6:02
  pronounce again, but Matt, before the end, you've got to give it one go.
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  Um, it dissolves in your, the water, in your eye and reacts with
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  it to form dilute sulfuric acid.
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  Matt Edmundson: SFU sulfuric acid, I can't even pronounce that.
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  So I've got no chance for the other one, but that on my eyeball,
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  sulfuric acid on my eyeball, that sounds genuinely dangerous.
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  Where is the government's, you know, like safety warning?
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  Chris Smith: Yeah, well it is.
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  I mean, sulfuric acid, even in tiny amounts, it's super irritating.
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  But one of the reasons for that is the cornea in your
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  eye has tons of nerve endings.
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  Um, that are really good at spotting harmful chemicals and tears are
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  your natural safety response.
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  Hm.
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  Jenny Oliver: And is this linked to the so-called wasabi receptor that we have?
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  Matt Edmundson: The wasabi receptor?
• 6:49
  Jenny Oliver: Oh yeah.
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  We have wasabi receptors.
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  Matt Edmundson: That actually genuinely sounds a lot cooler
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  than what it probably is.
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  Right?
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  Chris Smith: Of course.
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  It's cool.
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  It's called the wasabi receptor, TRPA one to give it his other boring name.
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  Jenny Oliver: Yeah, that doesn't sound as cool.
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  Chris Smith: Um, it gets called the wasabi receptor because it reacts to
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  pungent chemicals, so it's things like mustard, horseradish, garlic, onions.
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  And of course, wasabi.
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  Matt Edmundson: So this wasabi receptor gets triggered by this
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  sulfuric acid on my eyeball and reacts.
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  That's, that sounds a little bit painful, right?
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  Chris Smith: Yeah.
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  So, and then what happens next is your, your eyes reacted.
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  The signal response travels along the nerves to the brainstem.
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  Okay?
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  Jenny Oliver: Okay, so let's recap.
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  The chemical from the onion reaction gets into the eye forms sulfuric acid.
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  Which triggers receptors in the eye and sends signals up to our brains
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  telling it to start the tear response.
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  Matt Edmundson: I love that you two are just basically speaking another language
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  at whilst D tear thinking, ah yes, the crying button just gets pressed.
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  Jenny Oliver: It's actually a perfect summary.
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  So we've got the complete pathway now, but what determines why some people react
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  to onions more strongly than others?
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  Chris Smith: Yeah, so first there's the onion itself.
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  So different varieties produce vastly different amounts of these
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  enzymes that start the whole process.
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  So there could be a threefold variation just in one type of onion.
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  Matt Edmundson: Well, so you can't, you, you, you can't even predict the
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  credibility then, can you really, uh, just by selecting the onion type
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  Jenny Oliver: and any other factors why some people might react more.
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  Chris Smith: Uh, so other factors are mainly about your individual biology.
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  So there's individual variation in the sensitivity of those
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  receptors in the eye and different baselines in actual tear production.
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  But plus there's, uh, various between people in how quickly your
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  body can flush out those irritants.
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  Jenny Oliver: So the discovery of LFS in 2002 opened up possibilities for
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  breeding tailless onions, didn't it?
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  It's quite interesting.
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  Chris Smith: It did.
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  And so researchers have been looking at ways of using RNA interference
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  to silence the LFS gene and ways of cross-breeding, to obtain onion
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  varieties with lower enzyme activity.
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  Matt Edmundson: Cross-breeding onions, man, these scientists have really pushed
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  the boundaries to solve what is in effect a first world problem, right?
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  Yeah.
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  I'm, I'm loving it.
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  But did, did it work well?
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  Chris Smith: So both work scientifically, but I think it's fair to say that
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  genetically modified onions aren't really gonna catch on in the today's marketplace.
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  But the cross-breeding approach is promising.
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  And, um, there's a plant breeder called Rick Watson who developed onions.
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  These are so-called tearless onions, and he did this having
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  spent over 30 years selecting for lower pungency, uh, types of onion,
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  Matt Edmundson: 30 years.
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  Of selective onion breeding.
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  That's a life cause right there, isn't it?
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  Yeah.
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  Just to prevent kitchen tears.
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  I am loving the commitment of these guys to solving the inconveniences that
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  I face so I totally can respect that.
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  Chris Smith: Yep.
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  And and you can still get them.
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  So they've been around since, uh, in the UK it's about 2020 and the US
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  a little longer, although it's fair to say they are significantly more
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  expensive than the standard onions.
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  Matt Edmundson: Oh, okay.
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  So for those then that maybe wanna stick with regular onions,
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  um, 'cause they're a bit cheaper.
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  What actually does work to reduce our tears?
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  Chris Smith: Safety goggles are a full on hazmat suit.
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  That's all you can do.
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  Jenny Oliver: And when that's not an option, um, we, we
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  know that sharp knives help.
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  Um, I guess making, chopping them a bit more slowly, slow controlled
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  cuts, produce fewer droplets, and that's something that's been
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  quantified with high speed cameras.
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  Secondly, I was reading that refrigerating or freezing the onions for a short
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  time before cutting them helps 'cause it slows the enzymatic reactions
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  and reduces the compound volatility.
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  Matt Edmundson: Well, that makes sense.
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  So I, I guess lower temperatures must reduce or calm down any reactions
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  that are taking place, right?
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  Chris Smith: Yep, certainly do.
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  And the other thing that's worth trying is cutting the onion under
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  running water because that volatile compound dissolves in the water that
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  you're running rather than evaporating and traveling up to your eyes.
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  And of course ventilation or, you know, cutting under a, near
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  a window or an extraction hood is definitely gonna help, but the
• 11:09
  hazmat suit is still the best option.
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  Matt Edmundson: You know what, the other day, um, we were with my son
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  and his fiance going through the photos of when he was younger, right?
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  Pulling out all the embarrassing photos, and there was a photo of him when he
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  was about 10 or 11 wearing goggles.
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  Uh, which I thought was quite cute.
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  Um, but one of the things that I do, and again, is this, an old wive's tale.
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  I leave the root intact as I'm cutting the onion.
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  Yeah.
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  It's sort of like the last thing that I do.
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  Does that work?
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  Chris Smith: So, to be honest, um, all parts of the onion
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  contain the necessary enzymes and precursors to cause that reaction.
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  So I wouldn't say the difference is significant.
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  No.
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  Jenny Oliver: But if it works for you.
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  Matt Edmundson: You do you?
• 11:50
  Jenny Oliver: You do, right?
• 11:50
  Yeah.
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  Chris Smith: I'm basically trying to get to wear a hazmat suit.
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  I'm trying my best to,
• 11:55
  Jenny Oliver: we know what we're getting for Chris.
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  Yeah.
• 11:57
  Matt Edmundson: From Chris hazmat suits.
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  Yeah.
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  I'd rather wear the hazmat suit than try and pronounce that.
• 12:01
  Thing that you were pronouncing earlier.
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  Jenny Oliver: So what about garlic?
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  So that's from the same family of plants as onions, but you
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  don't get that same response.
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  What's the difference?
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  Chris Smith: Yeah, so garlic lacks that LFS enzyme and without LFS, the
• 12:15
  snic acids just condense rather than react to produce the volatile compound.
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  And this is what causes garlic's, uh, characteristic odor.
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  Matt Edmundson: Yeah, so onions and are basically the overachievers of the allium
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  family when it comes to making us cry.
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  Right?
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  Jenny Oliver: Yeah.
• 12:31
  So let's go back to that exciting wasabi receptor.
• 12:33
  Um, there's a broader connection to how it mediates pain and inflammation
• 12:38
  responses generally, isn't there?
• 12:40
  Chris Smith: Absolutely.
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  So this receptor responds to loads of irritants, so including compounds
• 12:45
  in mustard and cinnamon and wasabi, as we mentioned, but also through to
• 12:49
  environmentally irritants such as smoke.
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  Um, but because it mediates pain and inflammation, it's become an
• 12:55
  important drug target for chronic pain and asthma treatments.
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  Matt Edmundson: So I'm gonna tell myself then that my, my sort of kitchen crying
• 13:04
  is actually advancing medical research to make me feel a little bit better.
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  Right.
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  Makes it feel a bit more noble, I guess.
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  Jenny Oliver: Yeah, it does.
• 13:12
  So to recap, when it comes to onions from knife contact to tears,
• 13:18
  how fast is this entire process?
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  Chris Smith: So the enzymatic reaction takes a few seconds, but once the compound
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  reaches your eye, the response and signal to produce tears happens in milliseconds.
• 13:29
  Matt Edmundson: And didn't you say earlier is 89 miles an hour?
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  Mm-hmm.
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  So I'm thinking back to the future.
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  88 miles an hour takes me back in time.
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  So this is, it can't be a coincidence, surely, maybe.
• 13:40
  We could use onions as the fuel for the flux capacitor.
• 13:47
  Chris Smith: There'd be younger people in this audience who've got
• 13:48
  no idea what they're talking about.
• 13:52
  Matt Edmundson: That's probably true.
• 13:53
  I don't, I don't even know what I'm talking about right now.
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  So let's move on.
• 13:56
  Chris Smith: But I'm with you all the way.
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  Jenny Oliver: Yeah.
• 13:59
  Matt Edmundson: So then why do onions make me cry?
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  Chris Smith: So onions have a two enzyme system where the onion being cut causes a
• 14:06
  reaction to produce a volatile compound.
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  This gets into the eye reaction with the water to form sulfuric acid and activates
• 14:14
  receptors to trigger tear production
• 14:17
  Jenny Oliver: and variation between people.
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  Comes down to onion variety, individual eye receptor sensitivity, baseline
• 14:25
  tear production, and how efficiently your eyes can flush out the.
• 14:30
  Matt Edmundson: And so those droplets flying at 88 miles an hour from my
• 14:34
  enthusiastic chopping are creating sulfuric acid on my eyeballs and my wasabi
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  receptor, which is definitely a cool name.
• 14:43
  It's, it's a really good name for a band.
• 14:44
  Chris.
• 14:45
  Uh, my wasabi receptor is hitting the emergency button,
• 14:48
  Jenny Oliver: so that's why.
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  We cry when we cut onions.
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  It's, I think it's well worth it though.
• 14:56
  'cause they are tasty.
• 14:57
  I think it's well worth the, the crying they are.
• 15:00
  So that's it for this week.
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  Make sure you hit that subscribe button.
• 15:03
  We're unpacking the science behind everyday questions each week.
• 15:07
  Tell us in the comments what you'd like us to explore next.
• 15:11
  Thanks for geeking out with us, but for now, that's it from me.
• 15:14
  Matt Edmundson: That's it from me.
• 15:15
  Chris Smith: And that's it from me.
• 15:16
  Jenny Oliver: Bye for now.