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Why Do Onions Make Us Cry?
Why do onions make us cry every time we cut them — and why does cooking them make the problem disappear completely?
About 70% of people experience significant tearing when cutting onions, and no matter how many you've chopped over the years, the reaction doesn't get easier. In this episode, Jen, Chris, and Matt unpack the full chemistry behind one of cooking's most universal frustrations. They explore the two-enzyme system inside every onion, the volatile compound it produces, the sulfuric acid that forms on your cornea, and why some people are hit far harder than others. They also look at what actually reduces the reaction — and what the so-called folk remedies get wrong.
Timestamps00:00 Introduction 01:42 The Two-Enzyme System Inside Every Onion 03:25 The Second Enzyme Discovered in 2002 05:58 Sulfuric Acid and the Wasabi Receptor 07:55 Why Some People React More Than Others 10:09 What Actually Works to Reduce Tears 12:04 Why Garlic Doesn't Make You Cry
The Science of Onion TearsThe Two Chemicals Inside Every Onion That Only React When You Cut ItTimestamp: 01:42
Every onion stores two chemical components in completely separate cellular compartments, physically prevented from interacting. One compartment holds an enzyme called alliinase; the other holds reactive substances called amino acid sulfoxides. As Jen explains, it's like storing oxidiser and fuel in separate tanks — perfectly stable when apart, but the moment a knife ruptures those cell walls, the compartments break open and the reaction begins. Alliinase reacts with the sulfoxides to produce unstable compounds called sulfenic acids, and from there, a second enzyme takes over.
The Enzyme Nobody Knew About Until 2002Timestamp: 03:25
For most of human history, nobody could fully explain why onions make us cry. In 2002, Japanese researchers finally identified the missing piece: a second enzyme called lachrymatory factor synthase (LFS). This enzyme intercepts the sulfenic acids produced in the first reaction and converts them into a volatile compound — syn-propanethial-S-oxide — that immediately becomes airborne. High-speed camera studies have clocked the tiny droplets ejected from a cut onion at speeds of up to 40 metres per second, roughly 89 miles per hour. The entire two-step reaction completes within seconds of your knife making contact. This is also why sharper knives genuinely help: cleaner cuts rupture fewer cell walls, producing fewer droplets at lower velocity.
"The stats are enough to make you cry." — ChrisSulfuric Acid, the Wasabi Receptor, and Why You Can't Build a ToleranceTimestamp: 05:58
When those airborne droplets reach the eye, the volatile compound dissolves into the moisture on the surface of the cornea and reacts to form dilute sulfuric acid. The cornea is densely packed with nerve endings designed to detect harmful chemicals, and tears are the body's immediate safety response. The specific receptor triggered is TRPA1, better known as the wasabi receptor, because it responds to the same class of pungent compounds found in mustard, horseradish, wasabi, and onions. When sulfuric acid activates this receptor, a signal travels along the nerves to the brainstem, which fires the tear response — all within milliseconds of the compound arriving.
Crucially, this receptor doesn't adapt. Reaction intensity has no correlation with cooking experience. Every cut into a raw onion triggers the same chemical sequence, every time.
Why Some People React Far More Strongly Than Others
Timestamp: 07:55
The same onion can leave one person unaffected while reducing another to tears. Two factors drive the variation: the onion itself, and individual biology. Different onion varieties produce vastly different enzyme concentrations — there can be up to a threefold difference even within a single variety. Beyond that, people vary in their corneal receptor sensitivity, their baseline rate of tear production, and how efficiently their eyes flush out the irritant. None of these factors are within our control, and none change with repeated exposure.
"I have to time my meal prep to when the family aren't around. Otherwise they're just gonna laugh at me the whole time." — MattAbout So That's Why
So That's Why is a weekly podcast where Jen, Chris, and Matt unpack the science behind everyday health questions. No jargon, no judgment — just genuine curiosity and proper research.
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By VegetologyWhy do onions make us cry every time we cut them — and why does cooking them make the problem disappear completely?
About 70% of people experience significant tearing when cutting onions, and no matter how many you've chopped over the years, the reaction doesn't get easier. In this episode, Jen, Chris, and Matt unpack the full chemistry behind one of cooking's most universal frustrations. They explore the two-enzyme system inside every onion, the volatile compound it produces, the sulfuric acid that forms on your cornea, and why some people are hit far harder than others. They also look at what actually reduces the reaction — and what the so-called folk remedies get wrong.
Timestamps00:00 Introduction 01:42 The Two-Enzyme System Inside Every Onion 03:25 The Second Enzyme Discovered in 2002 05:58 Sulfuric Acid and the Wasabi Receptor 07:55 Why Some People React More Than Others 10:09 What Actually Works to Reduce Tears 12:04 Why Garlic Doesn't Make You Cry
The Science of Onion TearsThe Two Chemicals Inside Every Onion That Only React When You Cut ItTimestamp: 01:42
Every onion stores two chemical components in completely separate cellular compartments, physically prevented from interacting. One compartment holds an enzyme called alliinase; the other holds reactive substances called amino acid sulfoxides. As Jen explains, it's like storing oxidiser and fuel in separate tanks — perfectly stable when apart, but the moment a knife ruptures those cell walls, the compartments break open and the reaction begins. Alliinase reacts with the sulfoxides to produce unstable compounds called sulfenic acids, and from there, a second enzyme takes over.
The Enzyme Nobody Knew About Until 2002Timestamp: 03:25
For most of human history, nobody could fully explain why onions make us cry. In 2002, Japanese researchers finally identified the missing piece: a second enzyme called lachrymatory factor synthase (LFS). This enzyme intercepts the sulfenic acids produced in the first reaction and converts them into a volatile compound — syn-propanethial-S-oxide — that immediately becomes airborne. High-speed camera studies have clocked the tiny droplets ejected from a cut onion at speeds of up to 40 metres per second, roughly 89 miles per hour. The entire two-step reaction completes within seconds of your knife making contact. This is also why sharper knives genuinely help: cleaner cuts rupture fewer cell walls, producing fewer droplets at lower velocity.
"The stats are enough to make you cry." — ChrisSulfuric Acid, the Wasabi Receptor, and Why You Can't Build a ToleranceTimestamp: 05:58
When those airborne droplets reach the eye, the volatile compound dissolves into the moisture on the surface of the cornea and reacts to form dilute sulfuric acid. The cornea is densely packed with nerve endings designed to detect harmful chemicals, and tears are the body's immediate safety response. The specific receptor triggered is TRPA1, better known as the wasabi receptor, because it responds to the same class of pungent compounds found in mustard, horseradish, wasabi, and onions. When sulfuric acid activates this receptor, a signal travels along the nerves to the brainstem, which fires the tear response — all within milliseconds of the compound arriving.
Crucially, this receptor doesn't adapt. Reaction intensity has no correlation with cooking experience. Every cut into a raw onion triggers the same chemical sequence, every time.
Why Some People React Far More Strongly Than Others
Timestamp: 07:55
The same onion can leave one person unaffected while reducing another to tears. Two factors drive the variation: the onion itself, and individual biology. Different onion varieties produce vastly different enzyme concentrations — there can be up to a threefold difference even within a single variety. Beyond that, people vary in their corneal receptor sensitivity, their baseline rate of tear production, and how efficiently their eyes flush out the irritant. None of these factors are within our control, and none change with repeated exposure.
"I have to time my meal prep to when the family aren't around. Otherwise they're just gonna laugh at me the whole time." — MattAbout So That's Why
So That's Why is a weekly podcast where Jen, Chris, and Matt unpack the science behind everyday health questions. No jargon, no judgment — just genuine curiosity and proper research.