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📘 Why do onions sting but shallots less so?

A whole onion can sit on the counter without making you cry. Cut it, and cell compartments rupture — that physical injury switches on defensive sulfur chemistry.

4
lessons
~20 min
to learn
Adults
level
Start the course →

What you’ll learn

  1. Cutting Turns Cells Into A Reaction ChamberExplain how tissue damage activates onion sulfur chemistry and forms the lachrymatory factor.Cutting breaks compartments, letting alliinase and sulfur precursors react while onion LFS directs one branch toward a volatile tear-inducing compound.
  2. The Eye Detects Irritation, Not SadnessDistinguish onion odor from trigeminal irritation and reflex tearing.The volatile factor reaches the tear film, sensory nerves report irritation, and lacrimal and blink reflexes help protect the eye.
  3. Shallots Run A Different Chemical MixExplain why shallots often sting less without treating the comparison as universal.Shallots share Allium defense chemistry but differ in precursor, enzyme, and volatile profiles, so their tear-inducing dose often differs.
  4. The Experience Depends On Dose And ContextConnect cutting technique and airflow to exposure while giving the mechanism-based short answer.The final experience depends on chemical production, aerosol transport, eye sensitivity, and the particular onion or shallot.

Questions this course answers

Put the main onion sting sequence in order.

Tissue damage brings substrates and enzymes together before the volatile product can travel to the eye.

What does onion lachrymatory factor synthase do?

LFS directs an onion sulfur intermediate toward syn-propanethial-S-oxide.

Match each part of the response to its role.

The molecule, sensory pathway, and protective gland are different parts of one chain.

What is the most defensible explanation for shallots often stinging less?

Related Allium plants share chemistry but can differ in compounds, enzymes, and amounts.

Why can ventilation reduce onion tears?

Airflow changes transport and therefore exposure at the ocular surface.

Grounded in trusted sources

  • Imai et al., A lachrymatory factor synthase in onion, Nature (2002) - https://www.nature.com/articles/35079154
  • Shigemi et al., Silencing Onion Lachrymatory Factor Synthase Causes a Significant Change in the Sulfur Secondary Metabolite Profile, Plant Physiology (2008) - https://pmc.ncbi.nlm.nih.gov/articles/PMC2492635/
  • Silvaroli et al., Enzyme That Makes You Cry - Crystal Structure of Lachrymatory Factor Synthase from Allium cepa, ACS Chemical Biology (2017) - https://pmc.ncbi.nlm.nih.gov/articles/PMC5693239/
  • Tomlinson et al., Investigation of Volatiles Emitted from Freshly Cut Onions by Real Time Proton-Transfer Reaction-Mass Spectrometry, PLOS ONE (2013) - https://pmc.ncbi.nlm.nih.gov/articles/PMC3571771/
  • Tsubota et al., Using synthesized onion lachrymatory factor to measure age-related decreases in reflex-tear secretion and ocular-surface sensation, Investigative Ophthalmology & Visual Science (2010) - https://pubmed.ncbi.nlm.nih.gov/20577855/
  • Begum et al., Therapeutic Uses and Pharmacological Properties of Shallot (Allium ascalonicum): A Systematic Review, Frontiers in Nutrition (2022) - https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2022.903686/full

Every Wunder lesson is built from real, reputable sources — never invented.

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