Sour honey is usually fermenting honey. Every jar of raw honey already carries wild yeast, sitting dormant, and it wakes up when there's enough water in the jar to feed on.
That's the answer most people are looking for. There's a second one that catches them out, which is that some honey is supposed to taste sour.
Why honey ferments
Honey doesn't spoil the way most food spoils. It's a supersaturated sugar solution, which means the sugar binds up almost all the available water and leaves nothing for microbes to live on. That's why a sealed jar can sit in a cupboard for years.
The yeast is still in there though. It's a sugar-tolerant type that survives the conditions rather than dying off, and it stays inactive as long as the water content stays low.
Below 18.5% moisture, honey is unlikely to ferment. The National Honey Board puts the lower bound tighter still: under 17.1% water, honey won't ferment within a year no matter how much yeast is present. Above 19% it can ferment even with barely any yeast in the jar at all. The gap between those numbers is small, which is why beekeepers pay so much attention to it.
Nectar arrives at the hive somewhere between 60% and 80% water. Bees spend weeks driving that down, passing it between themselves and fanning it with their wings, and they only cap the cell in wax once it's ready. Honey that gets harvested before the bees have finished that work goes into the jar wetter than it should be.
Humidity does the rest. A jar opened and closed repeatedly in a Singapore kitchen picks up moisture from the air every time. A wet spoon does it faster.
How to tell if your honey is fermenting
Look for foam or bubbles sitting on the surface. The smell changes too, usually toward beer or wine rather than anything rotten. The texture goes thinner and runnier than you remember. Taste comes last, and by then the other signs have usually been there a while.
If the lid pops when you open it, that's carbon dioxide. Fermentation has been underway for some time.
Is fermented honey safe to eat?
Our position is that it depends entirely on whether the fermentation was meant to happen.
Fermented foods are a category humans have eaten for a very long time, and controlled fermentation can be good for you. Honey that has been fermented deliberately, under conditions someone was managing, sits in that category.
Honey that fermented by accident in your cupboard is a different thing. It won't hurt a healthy adult, but nobody controlled what happened in there, and the flavour is usually the first casualty. Most people find it too sharp to want.
Use it in cooking or baking if the taste doesn't bother you. Marinades handle it well, since acidity is doing useful work there anyway.
The one rule that doesn't bend: no honey of any kind for babies under twelve months. That's a separate issue from fermentation and it applies to every honey on the shelf.
Sour, or just crystallised?
These get confused constantly, and they're unrelated.
Crystallised honey has gone grainy or solid. That's glucose coming out of solution and it's completely normal, often a sign the honey hasn't been heavily processed. It doesn't taste sour and it hasn't gone off. We've gone through what crystallisation actually tells you separately.
Crystallisation can eventually lead to fermentation though, and the reason is worth knowing. When glucose crystallises out, it leaves the surrounding liquid with a higher proportion of water than before. That's the wet layer the yeast needs. So a jar left crystallised at room temperature for a long stretch is more exposed than one that was used up.
Some honey is supposed to be sour
Now the part that surprises people.
Not all sourness is a fault. Stingless bee honey, the kind produced by Trigona bees across Southeast Asia, is naturally sour and always has been. In parts of Yunnan it's simply called sour honey.
Three things drive it, and none of them are spoilage.
The sugar content is lower. Ordinary honey runs high in sugar. Trigona honey sits somewhere around 17% to 20%, varying with the season and with where the hive is, and less sugar means less to mask the acidity underneath.
The organic acid content is higher. Gluconic acid is the main one, produced when an enzyme the bees add breaks glucose down. Lactic and acetic acids are in there too. The result is a lower pH than ordinary honey and a tang you can taste immediately.
And the flowers matter. Stingless bees forage across a wide range of tropical plants, some of them high in organic acids themselves, some growing in the acidic soils common to tropical forest. That acidity travels into the honey. It's part of why Trigona honey from one region tastes noticeably different from another.
If you've bought stingless bee honey and it tastes sharp, that's the product working as intended. We've written about the chemistry behind it in more detail.
What to do with the jar in front of you
If it's ordinary honey and it's bubbling, it fermented. Cook with it or replace it.
If it's stingless bee honey and it tastes sour, nothing has gone wrong.
Either way, the storage advice is the same. Keep the lid tight, keep it out of the light, use a dry spoon every time, and don't leave it somewhere the temperature swings. Honey has no fixed shelf life, which makes how you store it the thing that decides how long it lasts.
Common questions
What is sour honey?
Two different things go by that name. One is honey that has fermented, which is a fault. The other is stingless bee honey, which is naturally acidic and sold as sour honey in parts of Southeast Asia. The label doesn't tell you which you have. Bubbling does.
Can real honey be sour?
Yes. Pure, unadulterated honey can taste sour either because it fermented or because it came from bees that produce it that way. Sourness on its own says nothing about whether honey is genuine.
Why does my honey smell sour?
Smell usually arrives before taste. If it's gone yeasty or reminds you of wine, fermentation started a while ago. A sharp smell with no bubbles and no change in texture is more likely the honey's own character.
Is sour honey safe to eat?
Deliberately fermented honey and naturally sour honey are both fine. Honey that fermented by accident in your cupboard won't harm a healthy adult, but the flavour has usually gone. No honey of any kind for babies under twelve months.
Why does my honey taste bitter rather than sour?
Bitter is a different signal. That usually points to the flowers the bees worked rather than anything going wrong. Some nectar sources produce genuinely bitter honey, and buckwheat and chestnut are the ones people notice most.
Does honey go bad?
Not in the way most food does. Honey has no fixed shelf life. What changes is moisture, and moisture is what fermentation needs, so storage decides how long a jar stays good rather than any date on the label.