Why Honey Keeps Almost Forever

There is a famous story about honey. Archaeologists, it is said, opened ancient Egyptian tombs and found jars of honey thousands of years old that were still perfectly edible. It is a wonderful tale, and it gets repeated everywhere. But the chemistry site Compound Interest adds an important note: although it is an oft-cited story, the thing it often lacks is an actual citation. The real science of why honey lasts so long, however, is well documented, and it is every bit as impressive as the legend.

The first reason is how little water honey contains. According to Compound Interest, honey’s water content is about 17 percent, much lower than that of bacteria or fungi. Honey also has a low water activity, which is a measure of how much of the water in a food is actually available to support microbial growth. Because honey is so dry in this sense, it draws water out of any bacteria that land in it. Compound Interest explains that this dehydration of bacteria is a major reason honey resists spoiling.

Sugar plays a part in that too. A review in the Asian Pacific Journal of Tropical Biomedicine explains that honey’s antibacterial property comes partly from the osmotic effect of its high sugar content and low moisture content. In simple terms, a microbe sitting in something this sugary and this dry loses water to its surroundings and struggles to grow. For most of the tiny organisms that make other foods go off, a jar of honey is a very hostile place to be.

Honey is also acidic. The same review describes it as characteristically acidic, with a pH between 3.2 and 4.5, which it says is low enough to inhibit several kinds of disease-causing bacteria. Compound Interest reports an average pH of around 4, and explains that the main acid is gluconic acid, produced when bee enzymes act on some of the glucose in the honey. So part of what protects honey is added by the bees themselves while they are turning nectar into honey.

There is one more defence. According to the review, most types of honey generate hydrogen peroxide when they are diluted, because an enzyme called glucose oxidase converts glucose into gluconic acid and hydrogen peroxide, a substance that can stop bacteria from growing. The authors say these factors can work alone or together. So what keeps your jar of honey safe is not one secret ingredient, but a whole team of dryness, sugar, acidity and chemistry working at once.

What about the honey that has turned solid and grainy? Compound Interest explains that over time glucose can come out of solution and form crystals, but that the honey is still perfectly fine to eat, and that this does not count as spoiling. So if you find an old, crystallised jar at the back of the cupboard, there is a very good chance it is still good. Warm it gently if you prefer it runny, and enjoy one of the most durable foods in your kitchen, legend or no legend.