Why do ginkgo seeds smell so bad?
― The smell is on the outside. What you should worry about is the inside.
Walking under a row of ginkgo trees in autumn, a powerful smell rises up from the ground. The culprit is acids — mainly butyric acid — that build up in the outer coating of the ginkgo seed. It takes only a trace to reach your nose, so stepping on just one fallen seed can leave the whole area smelling. And yet the part that really needs caution is the odourless kernel inside.
It's an October morning and you're walking along a ginkgo-lined pavement. The leaves are still mostly green, but the ground is scattered with round yellow seeds.
You step on one by accident, and a heavy smell rises up — something between old cheese and sweaty socks, hard to put into words.
Yet from that same seed, once cracked open and roasted, comes a nutty, fragrant nibble. How does that stinking fruit on the pavement connect to the soft green kernel in a bowl of chawanmushi?
Only two reasons it smells so bad
The soft outer layer of the yellow seed is rich in acids such as butyric acid. Butyric acid belongs to the same family of compounds as rancid butter and foot odour.
Butyric acid is a substance the human nose is especially sensitive to. Even a tiny trace in the air is enough to smell, which is why stepping on just one seed can make the whole area reek.
In other words, the smell isn't just about how much is released. It's also about how little it takes for us to notice it. Let's first look at what's actually inside the seed.
The ginkgo "seed" is made of four layers
What we call the "ginkgo seed" is, in fact, the seed of the ginkgo tree itself. The soft outermost layer is where the smell comes from. Inside that is a hard white shell, and inside that again a thin brown skin wrapping the edible part.
Look at Figure 1. In the cross-section on the left, only the outer yellow layer is responsible for both the smell and the skin irritation. The kernel that comes out once you crack the shell has almost no smell at all — instead, it contains a different kind of compound.
Why does butyric acid stick to your nose so much?
Butyric acid is a small acid molecule. Because the molecule is small, it drifts out into the air little by little even at ordinary temperatures. Crush the seed, and the part that was sealed in is suddenly exposed to air all at once, so the smell surges.
On top of that, the human nose is extremely sensitive to butyric acid. It's said we can detect it when there's only about 0.0002 molecules of butyric acid for every million molecules of air — in other words, roughly one molecule in five billion.
That's why even a tiny scrap of the fleshy coating is enough to bring the surrounding air up to a "smellable" concentration. If a bit sticks to the sole of your shoe and comes home with you, that's why your entryway ends up smelling too.
Interestingly, if you combine butyric acid with an alcohol, it turns into a compound with a sweet, pineapple-like scent. A tiny difference in molecular shape flips the smell's impression completely.
The edible kernel needs a different kind of caution
Inside the shell, the ginkgo kernel contains a compound shaped very much like vitamin B6. This compound "impersonates" vitamin B6 inside the body and gets in the way of its job.
Vitamin B6 is essential for making a substance that calms down excitement in the brain. When that job is interfered with, the brain can become overexcited, leading to nausea and convulsions. This is what causes ginkgo food poisoning.
The tricky part is that this compound is heat-stable. Roasting, boiling, or cooking it into chawanmushi barely reduces it. Cooking it does not make it safe.
Children need particular caution. The smaller the body, the larger the per-kilogram dose even from the same single seed. Try changing the number of seeds eaten with the slider in Figure 2.
Most reported cases of adult poisoning involve eating 40 or more seeds. In children, however, cases have been reported after as few as about 7 seeds — a figure that broadly matches the calculation in Figure 2. It's safest to assume that children should eat no more than a few seeds.
Ginkgo trees are either male or female, and only female trees produce ginkgo seeds. To avoid the smell, some cities deliberately plant only male trees along their streets. Incidentally, Sakugoro Hirase, who studied how ginkgo trees are pollinated, discovered in 1896 that ginkgo produces swimming sperm — the first such discovery in any seed-bearing plant, anywhere in the world.
So what should you actually do?
- If children eat them at all, keep it to a few seedsAdults, too, shouldn't eat a large amount at once.
- Wear gloves when collecting themThe outer flesh can cause skin irritation. Avoid touching it bare-handed.
- If vomiting, trembling, or convulsions occur after eating, get medical help right awayIf there are convulsions or altered consciousness, call emergency services (119) without hesitation.
Ginkgo food poisoning is often reported to appear as nausea and convulsions within a few hours of eating. Roasting or boiling barely reduces the causative compound. If a child has eaten a large number of seeds, don't just wait and see even without symptoms — consult a medical professional. If vomiting continues, watch for dehydration too. If convulsions occur, call emergency services (119) immediately.
Summary
Ginkgo seeds smell bad because acids such as butyric acid build up in the soft outer layer, and because the human nose can detect even a minuscule trace of it. The smell comes only from the outside — the edible kernel inside the shell has no smell. But that kernel contains a compound that interferes with vitamin B6, and cooking doesn't reduce it. The smaller a child's body, the fewer seeds it takes to cause an effect.
The smell is on the outside; the caution is about the inside.
10 seeds for a child equal 40 for an adult.
For more on autumn scents, see "Why do you suddenly notice the scent of osmanthus "one morning"?"; for how ginkgo leaves turn yellow, see "Why do autumn leaves turn red and yellow?"
- Along a row of ginkgo trees, look for trees with fallen seeds underneath and trees without. The ones with fallen seeds are female. See whether male and female trees are mixed together, and try counting them.
- Standing upwind and then downwind of a spot with fallen seeds, compare the smell. How many metres away does the smell stop being noticeable?
- Wearing gloves, rinse the outer flesh off a collected seed under water. Once only the white shell is left, check that the smell has almost disappeared.
※Don't touch the fleshy coating bare-handed, and discard or wash any gloves you use right away. If you eat seeds you've collected, keep it to a small amount, whether you're an adult or a child.
Want to know more? ― Terms, formulas, and textbook connectionsWe label each part by level, from junior-high science to university-level specialist courses
- Junior highCovered in junior-high school science
- High schoolCovered in high-school "Chemistry" or "Biology"
- High school+Advanced high-school content, or textbook sidebar material
- UniversityNot covered in high school — university-level specialist subjects (biochemistry, pharmacology)
- ResearchNot yet settled "textbook fact" even at university — an active area of research
Junior highTerminology: this phenomenon has names
- Outer seed coat (sarcotesta): The outermost layer of the seed. In ginkgo, it's soft and yellow, and looks like fruit flesh.
- Butyric acid: A small acid made of four carbon atoms. Also the compound behind rancid butter and foot odour.
- Odour threshold: The lowest concentration at which a person begins to detect a smell.
- Dioecious: Describes plants where male and female individuals are separate trees. Ginkgo is one of them.
Junior highHigh schoolWorking it out: how many grams of butyric acid does it take to make a room smell?
Let's calculate how much butyric acid is needed to bring the air in an 8-tatami-mat room up to "just detectable" concentration. Gas amounts are easier to handle in moles (a unit counting numbers of molecules) rather than in grams.
| As symbols | m = V × c × M ÷ Vm |
| In words | Mass of butyric acid needed = volume of room air × detectable concentration × mass of 1 mole ÷ volume of 1 mole of gas |
| Where it comes from | This combines Avogadro's law — "at the same temperature and pressure, one mole of any gas occupies the same volume" — with how concentration (a volume ratio) is defined. |
| Symbol | Meaning and unit |
| m | Mass of butyric acid needed to reach the odour threshold (grams) |
| V | Volume of air in the room (litres) |
| c | Odour threshold (ratio per million parts of air) |
| M | Mass of 1 mole of butyric acid (grams) |
| Vm | Volume of 1 mole of gas (litres) |
| Air in an 8-tatami-mat room (V) | about 30,000 litres (30 cubic metres) |
| Odour threshold of butyric acid (c) | said to be about 0.00019 per million parts of air |
| Mass of 1 mole of butyric acid (M) | about 88 grams |
| Volume of 1 mole of gas (Vm, at 25°C) | about 24.5 litres |
| Volume of butyric acid gas (in millionths of a litre) | 30000 × 0.00019 = 5.7 |
| How many moles that is (in millionths of a mole) | 5.7 ÷ 24.5 ≒ 0.23 |
| Converting to mass (in millionths of a gram) | 0.23 × 88 ≒ 20 |
| What fraction of a gram this is (1 gram = 1,000,000 micrograms) | 1000000 ÷ 20 = 50000 |
To make an entire room smell, you need only about 20 micrograms of butyric acid — one fifty-thousandth of a single 1-yen coin (1 gram). It's no wonder a tiny scrap of the fleshy coating is enough to make your entryway smell.
| Formula (symbols) | q = N ÷ W (per-kg amount = number of seeds ÷ body weight) |
| A 15kg child eating 10 seeds | 10 ÷ 15 ≒ 0.67 |
| A 60kg adult eating 40 seeds | 40 ÷ 60 ≒ 0.67 |
| How many times heavier is the adult | 60 ÷ 15 = 4 |
If body weight is a quarter as much, the number of seeds needed to produce the same effect should also be roughly a quarter. That said, actual risk also depends on things like the body's existing vitamin B6 levels, so this calculation is only a rough guide.
High schoolHigh school+Carboxylic acids and esters: same skeleton, opposite smells
High schoolButyric acid (butanoic acid) is a "carboxylic acid" — a compound with a carboxyl group. Carboxylic acids with few carbons are called "lower fatty acids"; many evaporate easily and have a sharp, unpleasant smell. React butyric acid with ethanol and you get ethyl butyrate, an ester with a fruity scent.
High school+The outer seed coat of ginkgo is also said to contain other fatty acids besides butyric acid, such as heptanoic acid (enanthic acid). The compound known to cause the skin irritation is ginkgolic acid, whose shape resembles urushiol, the compound behind the rash caused by Japanese lacquer tree (urushi).
UniversityGinkgotoxin and vitamin B6: how convulsions happen
The compound responsible for ginkgo food poisoning is 4'-O-methylpyridoxine, also known as ginkgotoxin. Its structure closely resembles pyridoxine, a form of vitamin B6. In the body, vitamin B6 is converted into its active form, pyridoxal 5'-phosphate (PLP), which supports many enzymes. One of these is glutamate decarboxylase, which produces GABA, a neurotransmitter that calms excitement. When this compound interferes with vitamin B6 metabolism, PLP runs short, GABA decreases, and convulsions become more likely. This is why treatment often involves supplementing vitamin B6.
📖 For the derivation of the formula and further reading: Pyridoxal phosphate (Japanese Wikipedia) ／ Avogadro constant (Japanese Wikipedia)
ResearchWhat's still not fully understood
- Why smell so bad in the first place? One idea is that the smell lures animals in to disperse the seeds. Ginkgo's ancestors date back to the age of dinosaurs, and one hypothesis is that large animals once carried the seeds around — but this hasn't been confirmed. In present-day Japan, raccoon dogs (tanuki) have reportedly been observed eating them.
- Why symptoms vary between people. Some people develop symptoms after eating the same number of seeds, and others don't. This is thought to be related to the body's existing vitamin B6 levels, but there's no clear line for exactly how many seeds becomes dangerous.
- Variation in how much toxin is present. The amount of the causative compound is said to vary by tree, by region, and by harvest time. Accurately estimating the amount per seed remains difficult.
In other words, everything in this article reflects "what's currently understood." In particular, treat any number like "how many seeds are safe" as a rough guide only.
Textbook connections, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: "Plant classification (gymnosperms)" | Structure of the ginkgo seed, male and female trees |
| High school | Chemistry: "Amount of substance", "Carboxylic acids and esters" | Calculations using moles, butyric acid and ethyl butyrate |
| High school+ | Chemistry: "Natural organic compounds" | Ginkgolic acid and lacquer-tree skin irritation |
| University | Biochemistry / pharmacology: "Coenzymes", "Neurotransmitters" | Vitamin B6 and GABA, the mechanism of convulsions |
| Research | Plant ecology / food hygiene | Role of the smell, individual variation in symptoms |
| ― | Everyday relevance | Gloves for collecting seeds, safe amounts for children to eat |
- Ministry of Health, Labour and Welfare (厚生労働省), "Natural Toxin Risk Profile: Higher Plants: Ginkgo Seed"
- Nagata Yoshio, Takeuchi Norifumi (永田好男・竹内教文), "Odour threshold measurements of odorous substances by the triangle-odour-bag method," Bulletin of the Japan Environmental Sanitation Center (1990)
- Wada Keiji et al. (和田啓爾ほか), "Isolation and identification of the toxic component 4'-O-methylpyridoxine from Ginkgo seeds," Chemical and Pharmaceutical Bulletin (1985)
- Wikipedia (Japanese): "Butyric acid"
- Wikipedia (Japanese): "Ginkgo"
※This article is a general-audience science explainer. The figures given are approximations meant to illustrate the underlying mechanism. If you feel unwell after eating ginkgo seeds, do not rely on self-judgement — consult a medical professional and follow the guidance of your doctor or local health authority.