Why does honey crystallise

Some time after you open a jar of honey, often a few weeks and occasionally just a few days, you notice it isn't pouring the way it used to. It turns cloudy, then thick, then solid enough to stand a spoon in.

This is the question we're asked about more than any other, so here's the complete answer.

Crystallised honey hasn't gone off

Start with the reassuring part. Solid honey is not spoiled honey. It's the same honey, with the same flavour, arranged differently. You can eat it as it is, spread it like butter, or turn it back into liquid in a few minutes.

It's so normal, in fact, that some honey is sold crystallised on purpose. Set honey and creamed honey are simply honey whose crystallisation has been controlled to produce a fine, smooth texture.

What's happening inside the jar

Honey is mostly sugar with very little water, usually less than 20%. That's far more sugar than water can normally keep dissolved at room temperature. Chemists call this a supersaturated solution, and supersaturated solutions don't stay settled for long. Given time, some of the sugar comes out of solution and forms crystals.

The sugar responsible is glucose. Honey's two main sugars are fructose and glucose, and glucose is considerably less soluble in water than fructose is. So glucose is the one that drops out first. Each new crystal gives the next one something to grow on, and gradually the process spreads through the whole jar.

Fructose stays dissolved. That's why a crystallised jar sometimes has a thin runny layer on top or around the edges. It's the part of the honey that hasn't set.

Why some honeys set in weeks and others stay runny for months

Every honey crystallises in the end. How quickly depends mainly on three things.

The balance of sugars. A honey with plenty of fructose relative to glucose has less glucose available to crystallise, so it stays liquid for longer. Acacia honey is the classic example: it's naturally high in fructose and can remain pourable for many months. At the opposite end, honey from oilseed rape is high in glucose and can set within weeks, sometimes days.

Tiny particles. Pollen grains, specks of wax and even small air bubbles all give glucose crystals a place to start forming. Honey that has been heavily filtered has fewer of these starting points. Honey that hasn't tends to crystallise sooner.

Temperature. This is the one you can control, so it gets a section of its own.

The temperature that matters most

Honey crystallises fastest between about 11°C and 16°C. That's a very common temperature for a cool larder, a cupboard on an outside wall, or a kitchen in winter, and it's why so many people notice their honey setting once the weather turns.

To slow crystallisation down, keep your honey somewhere steady and reasonably warm. We recommend 20 to 22°C, which in most homes simply means an ordinary kitchen cupboard, away from the back door and out of direct sunlight.

There are two places worth avoiding:

• The fridge. There's no food safety reason to keep honey cold. Chilled honey is stiff and difficult to spoon, and keeping it there gains you nothing.
• Beside the oven or on a sunny windowsill. Warmth won't make honey unsafe, but sustained heat slowly darkens it, dulls its flavour and raises its level of HMF, a compound laboratories measure to tell how much a honey has been heated or aged.

How to make crystallised honey runny again

All you need is a bowl of warm water and a little patience.

1. Loosen the lid of the jar.
2. Stand the jar in a bowl or pan of warm water, at around 40°C. That's comfortably warm to the touch, like a bath, rather than hot.
3. Leave it for 10 to 20 minutes, stirring now and then once the honey begins to soften. Top up the water if it cools.
4. Dry the jar, tighten the lid and return it to the cupboard.

And a few things not to do:

• Don't use the microwave. Microwaves heat unevenly, so some of the honey ends up far hotter than you intended.
• Don't use boiling water or put the jar on the hob. Honey's natural enzymes, including diastase and invertase, break down with heat. So do its more delicate aromas, such as the herbal notes in linden honey, which are usually the first thing to go.

Warmed honey will crystallise again in time. You can repeat the process whenever you need to. Just keep it gentle each time.

The one jar you shouldn't warm

If you have a jar of our Organic Blossom Honey with Honeycomb, leave it out of the warm water. Beeswax softens at fairly low temperatures, and a warm bath will slump the comb. If the liquid honey around the comb sets, cut through it with a knife instead. Set honey alongside a piece of comb is very good on hot toast. There's more on serving it in our honeycomb guide.

White streaks and frosting

Now and again a jar develops white streaks, patches or a lacy pattern on the inside of the glass, usually near the top. This is known as frosting. It happens when tiny pockets of air become trapped as the honey crystallises, or when the honey shrinks very slightly and pulls away from the glass.

Frosting is purely cosmetic and makes no difference to the honey at all.

Does crystallisation prove honey is real?

You'll see this claim a lot online. It isn't quite right.

Crystallisation is normal for genuine honey and nothing to worry about. But it isn't a purity test. Some real honeys, acacia in particular, stay liquid for a very long time. And honey that has been mixed with syrup can still crystallise, depending on what's gone into it.

Whether a honey is genuine can only be settled in a laboratory, which is why every batch of our honey is tested four different ways. We explain those tests in our article on honey labels and honey fraud.

Can honey go off?

Sealed honey is remarkably stable. Its low water content and natural acidity make it a very poor home for the microbes that spoil most foods. When honey is diluted, an enzyme it contains called glucose oxidase also produces small amounts of hydrogen peroxide.

The thing that can go wrong is moisture. Honey draws water in from the air around it. Leave a jar open in a steamy kitchen, or keep dipping a wet spoon into it, and the top layer can take on enough water to begin fermenting. You'd notice a sour or slightly boozy smell, and possibly some foam on the surface. Keep the lid on and use a clean, dry spoon, and it won't happen.

Storing honey: the short version

• Keep it at 20 to 22°C if you can.
• Avoid the 11 to 16°C range, which sets honey fastest.
• Lid on, a dry spoon, and out of direct sunlight.
• If it sets, use warm water at around 40°C. Never boiling water, never the microwave.
• Leave the honeycomb jar out of the water altogether.

If you'd like a honey that stays runny for as long as possible, choose Organic Acacia Honey. For how it compares with the rest of the range, see our guide to choosing a honey.