Why Doesn't Honey Go Bad?
The Reason It Stays Safe by "Stealing" Water
You find a jar of honey at the back of the cupboard, bought years ago. It is long past its best-before date, but when you open the lid, there is no mould and no sign of rot. Most other foods would have spoiled long ago. So why does honey last so well?
Bread or rice left out for a few days grows mould or starts to smell off. Honey, though, is said to almost never spoil, even when kept at room temperature for a long time. Over time it may turn white and hard (it crystallises), but this is not spoilage. It is thought to be an effect of the way sugar behaves.
In fact, the secret of its long life is said to be that honey is a food that pushes one single property to the limit: keeping water to a minimum.
Why does having so little water make food harder to spoil?
Honey is said to be about 20% water or less, and most of the rest is sugar.
When a microbe gets into concentrated honey, osmosis is thought to draw water out of the microbe.
Let's look at this "water-stealing" mechanism step by step.
Why Little Water Means Less Spoilage: How Osmosis Works
Microbes such as moulds and bacteria cannot live or multiply unless they have a certain amount of water inside them. This is where a phenomenon called "osmosis" matters. When two liquids of different concentrations are separated by a thin membrane (such as a cell membrane), water is said to move from the weaker side to the stronger side.
Now put a water-rich microbe cell in a setting like honey, where the sugar around it is extremely concentrated. The water inside the cell is drawn out into the thick honey. A microbe robbed of water loses what it needs to stay active, and is thought to become unable to live and multiply.
It simply cuts water down so far that microbes cannot live in it.
How Bees "Concentrate" Flower Nectar
The nectar that bees collect from flowers is said to be a much thinner, more watery liquid than honey. After carrying the nectar back to the hive, bees fan it with their wings to blow air across it and evaporate the water, concentrating the sugar little by little. Through this work, watery nectar is thought to turn into very thick honey with about 20% water or less.
A commonly told story says that honey found in ancient Egyptian sites was still edible after an extremely long time. It is hard to confirm every detail, but the story is widely known as a symbol of how well honey keeps.
Acidity Also Helps Prevent Spoilage
Honey is said to be weakly acidic. Many microbes are known to grow poorly in strongly acidic conditions, and this acidity is thought to be another factor, along with the low water content, that helps prevent spoilage.
Things You Can Check Yourself
- Prepare two small containers. Put pure honey in one and honey mixed with a little water in the other.
- Leave both uncovered in the same place at room temperature.
- For several days up to a week, watch how each one changes in look and smell. (Do not plan to eat them. This is for observation only.)
- Check whether the one with added water has become less concentrated and more prone to mould.
When water is added and the concentration drops, the "water-stealing power" of osmosis weakens, and microbes can grow more easily. You can see for yourself how much the concentration of pure honey matters.
Summary
Honey is thought to resist spoilage because it has very little water and a very high concentration of sugar. Thanks to this, microbes that touch honey have their water drawn out by osmosis and can no longer live and multiply. The effort bees put into fanning and concentrating nectar is exactly what turns it into a long-lasting food.
Behind the sweetness of honey lies a quiet, clever way of staying safe: taking water away.
For Those Who Want to Know More: Terms, Numbers and Links to TextbooksFrom middle-school science to topics under active research, with the level of each part made clear
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry"
- High school+High-school "Biology," or material treated as advanced or a sidebar in textbooks
- UniversitySpecialist university content (food science) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Middle schoolTerms: Words Around Honey
- Osmotic pressure: the force that makes water try to move between liquids of different concentrations.
- Concentration: making the amount of a substance in a liquid stronger by removing water and the like.
- Water activity: an index showing what share of the water in a food microbes can actually use.
High schoolChecking with a Formula: How Much Is Nectar Concentrated?
Using simplified rough figures, let's calculate how bees concentrate nectar.
Total weight after concentrating = weight of sugar ÷ sugar share after concentrating
| Weight of sugar | Assumed not to change before and after concentrating |
| Sugar share after concentrating | The share that sugar makes up once the water has been removed |
| Total weight after concentrating (g) | 200 ÷ 0.8 = 250 |
| Water evaporated (g) | 1000 − 250 = 750 |
| Share of the original nectar | 750 ÷ 1000 = 0.75 |
| Convert to a percentage (%) | 0.75 × 100 = 75 |
| Result | Removing water equal to 75% of the original nectar leaves 250 g of thick honey |
By evaporating water equal to three quarters of the original nectar, honey this thick is made.
* The figures here are rough values chosen to keep the calculation simple. The actual concentration of nectar and honey is said to vary with the kind of flower and other factors.
High school+"Water Activity": An Index of How Easily Food Spoils
In food science, the share of water in a food that microbes can actually use is expressed by an index called "water activity." Most bacteria are said to need a water activity of about 0.9 or higher to grow, and most moulds need about 0.7 to 0.8. The water activity of honey is said to fall well below these guidelines, and it is thought to be an extremely harsh environment for microbes.
UniversityOther Ingredients Besides Sugar Also Help Prevent Spoilage
In food science, it is known that while bees process nectar, an enzyme called "glucose oxidase" produces a very small amount of hydrogen peroxide. This hydrogen peroxide is also thought to help hold back the growth of microbes.
ResearchWhat Is Still Unclear
- How the kinds and strength of antibacterial substances in honey change with the flowers the nectar comes from is a topic still being studied in food science.
- Research is also under way on using honey from certain flowers, said to have especially strong antibacterial power, for wound care in medical settings.
- Ongoing studies are also looking at how the ingredients and quality of honey change in detail over long-term storage.
Even a single ordinary jar of honey holds a rich subject, where chemistry and food science meet and research still goes on.
Links to Textbooks (by Level)
| Level | Subject / Unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of aqueous solutions | Basic terms: osmotic pressure, concentration, water activity |
| High school | Basic Chemistry: concentration of solutions | Calculating how much nectar is concentrated |
| High school+ | Biology: osmosis (advanced) | The idea of water activity as an index |
| University | Food science | Hydrogen peroxide produced by glucose oxidase |
| Research | Food science and medical use (under research) | Detailed study of antibacterial substances, medical applications |
- Explanations of osmotic pressure and food preservation in food science textbooks and materials.
- Explanations of water activity and conditions for microbial growth in food microbiology materials.
- Explanations of how bees concentrate nectar in beekeeping science materials.
- Research reviews in food chemistry on glucose oxidase and hydrogen peroxide production in honey.
- Reviews of wound-care research in medical fields that make use of honey's antibacterial properties.
* Figures such as water content and water activity are commonly cited rough values. Actual values are said to vary with the type and origin of the honey.
* This article is a general-audience science explainer. Giving honey to infants under one year old is said to carry a risk of infant botulism. Please do not give honey to children under one year old.