Why doesn't honey go off?

April 27, 2022

Why doesn't honey go off?

Why Doesn’t Honey Spoil? – The Chemistry of Honey

Honey's low moisture content keeps bacteria from surviving.

And without bacteria at work, honey just doesn't spoil. Plus, honey is acidic enough to ward off most of the bacteria and organisms that spoil other food. What's more, the bees add their own enzymes to honey, and these enzymes produce hydrogen peroxide. The answer is as complex as honey’s flavour–you don’t get a food source with no expiration date without a whole slew of factors working in perfect harmony.

The first comes from the chemical make-up of honey itself. Honey is, first and foremost, a sugar. 

Sugars are hygroscopic, a term that means they contain very little water in their natural state but can readily suck in moisture if left unsealed. As Amina Harris, executive director of the Honey and Pollination Center at the Robert Mondavi Institute at Univeristy of California, Davis explains, “Honey in its natural form is very low moisture. Very few bacteria or microorganisms can survive in an environment like that, they just die. They’re smothered by it, essentially.” What Harris points out represents an important feature of honey’s longevity: for honey to spoil, there needs to be something inside of it that can spoil. With such an inhospitable environment, organisms can’t survive long enough within the jar of honey to have the chance to spoil.

The chemistry of honey

 

Honey is something of an oddity, in that, unlike most foods, it doesn’t spoil over time.

In fact, the oldest known sample of honey, found in an Ancient Egyptian tomb and dated to approximately 3000 years ago, was still perfectly edible (supposedly*). Honey can last for thousands of years in sealed pots. What is it, then, that gives honey this unusual property?

To answer this question, we need to understand how bees make honey in the first place. Honey derives from plant nectar, which is a mix of various different sugars, proteins, and other compounds, in a water solution.

There are a few other examples of foods that keep - indefinitely - in their raw state: salt, sugar, dried rice are a few. But there’s something about honey; it can remain preserved in a completely edible form, and while you wouldn’t want to chow down on raw rice or straight salt, one could ostensibly dip into a thousand year old jar of honey and enjoy it, without preparation, as if it were a day old. Moreover, honey’s longevity lends it other properties - mainly medicinal - that other resilient foods don’t have. Which raises the question - what exactly makes honey such a special food?

The secret to its long life lies in the bees’ honey-making process.

Forager bees collect sugary nectar from flowers and transport it back to the hive. Here, the bees transfer the nectar to other worker bees, which repeatedly drink and regurgitate the liquid, reducing its water content. During this process, an enzyme in the bees’ stomachs breaks down the nectar’s glucose into gluconic acid – which helps to make honey acidic (pH of around 4) – and hydrogen peroxide.

Whilst nectar composition varies from plant to plant, and a range of chemical compounds are commonly present, often the dominant sugar is sucrose. This is actually exactly the same sugar as is found in your kitchen, as table sugar. The varying concentrations of the different components of nectar in different nectars is the reason that a range of types of honey are available, depending on which type of nectar the bees have predominantly been collecting.

Bees are the key intermediate step between nectar and honey.

Worker bees will collect the nectar from flowers, and store it in their honey stomach – distinct from their normal stomach. Enzymes secreted from glands are then mixed with the nectar; these enzymes begin the breakdown of the sucrose in the nectar to simpler sugars. Sucrose is what we refer to as a disaccharide; it actually consists of two different simpler sugars, glucose and fructose, joined together. In the bee’s honey stomach, the sucrose molecules are gradually split by the enzymes into glucose and fructose.

Glucose and fructose can also be referred to as dextrose and levulose respectively. These two sugars are actually structural isomers, as they have the same chemical formula. The names ‘dextrose’ and ‘levulose’ refer to their effect on plane-polarised light. Due to the difference in the arrangement of the atoms, dextrose rotates plane-polarised light to the right, whilst levulose rotates it to the left. The prefixes ‘dextro-‘ and ‘levulo-‘ come from the latin for right and left respectively.

Once the worker bee returns to the hive, it will regurgitate the nectar solution and pass it on to one of the house bees, who remain in the hive.

Once the nectar is deposited in the honeycomb, the bees fan it furiously with their wings to speed up the water’s evaporation. The honey’s low water content and high acidity are the two main reasons it doesn’t spoil – the bacteria that cause food to go off can’t thrive in these conditions. The hydrogen peroxide also has antibacterial properties. So the honey stays fresh for the bees during the cold winter months – and for much longer inside our jars.

The house bee will continue the process the worker bee started – for up to 20 minutes, it will regurgitate and re-drink the nectar, continuing to mix it with enzymes and breaking it down further. Whilst some sucrose will remain, the majority is broken down into glucose and fructose.

Once suitable breakdown has been achieved, the house bee deposits the nectar into the honeycomb in the hive. Then another important step in the process begins. Nectar can be up to 70% water, and this water must be evaporated in order to produce the consistency of honey that we’re all familiar with.

The bees achieve this by fanning the honeycomb with their wings in order to encourage rapid evaporation of the water from the nectar mixture.

Eventually, the water content of the solution will drop to around 17%, vastly reduced from the content of the original nectar. The conversion of the watery nectar to syrupy honey takes between 1-3 days.

The water content of honey is a key factor in why it doesn’t spoil. At 17%, its water content is much lower than that of bacteria or fungi. Honey also has a low water activity; this is a measure of the amount of water in a substance that is available to support microbial growth. Water activity is on a scale of 0 to 1, with most moulds and bacteria being unable to grow under a water activity of 0.75. Honey has a water activity of 0.6. This, combined with the fact that its low water content dehydrates bacteria, makes it resistant to spoiling.

Another factor that helps honey avoid spoiling is its acidity. Its average pH is around 4 

Honey is also naturally extremely acidic. “It has a pH that falls between 3 and 4.5, approximately, and that acid will kill off almost anything that wants to grow there,” Harris explains. So bacteria and spoil-ready organisms must look elsewhere for a home–the life expectancy inside of honey is just too low. This acidity is contributed to by a number of acids, including formic acid and citric acid, but the dominant acid is gluconic acid, produced by the action of bee enzymes on some of the glucose molecules in the honey.

This further boosts honey’s antibacterial properties, as many bacteria thrive in neutral rather than acidic conditions. Hydrogen peroxide is also produce by the production of gluconic acid – this too can inhibit the growth of bacteria. Honey’s antibacterial properties are actually potent enough for it to be effective as an impromptu wound dressing.

But honey isn’t the only hygroscopic food source out there. Molasses, for example, which comes from the byproduct of cane sugar, is extremely hygroscopic, and is acidic, though less so than honey (molasses has a pH of around 5.5). And yet–although it may take a long time, as the sugar cane product has a longer shelf-life than fresh produce, eventually molasses will spoil.

So why does one sugar solution spoil, while another lasts indefinitely? Enter bees.

“Bees are magical,” Harris jokes. But there is certainly a special alchemy that goes into honey. Nectar, the first material collected by bees to make honey, is naturally very high in water–anywhere from 60-80 percent, by Harris’ estimate. But through the process of making honey, the bees play a large part in removing much of this moisture by flapping their wings to literally dry out the nectar. On top of behavior, the chemical makeup of a bees stomach also plays a large part in honey’s resilience.

Bees have an enzyme in their stomachs called glucose oxidase (PDF). When the bees regurgitate the nectar from their mouths into the combs to make honey, this enzyme mixes with the nectar, breaking it down into two by-products: gluconic acid and hydrogen peroxide. “Then,” Harris explains, “hydrogen peroxide is the next thing that goes into work against all these other bad things that could possibly grow.”

Honey has been used for centuries as a medicinal remedy.

Because it’s so thick, rejects any kind of growth and contains hydrogen peroxide, it creates the perfect barrier against infection for wounds. The earliest recorded use of honey for medicinal purposes comes from Sumerian clay tablets, which state that honey was used in 30 percent of prescriptions. The ancient Egyptians used medicinal honey regularly, making ointments to treat skin and eye diseases. “Honey was used to cover a wound or a burn or a slash, or something like that, because nothing could grow on it – so it was a natural bandage,” Harris explains.

What’s more, when honey isn’t sealed in a jar, it sucks in moisture. “While it’s drawing water out of the wound, which is how it might get infected, it’s letting off this very minute amount of hydrogen peroxide. The amount of hydrogen peroxide comes off of honey is exactly what we need–it’s so small and so minute that it actually promotes healing.” And honey for healing open gashes is no longer just folk medicinein the past decade, Derma Sciences, a medical device company, has been marketing and selling MEDIHONEY, bandages covered in honey used in hospitals around the world.

On a final note, you might also notice that, over time, honey tends to crystallise and solidify.

Because honey’s water content is so low, it can be considered to be a super-saturated solution of the various sugars; a solution is considered saturated when as much solid as is possible has been dissolved in it. Over time, glucose will precipitate out of the solution, forming solid crystals. The honey is still perfectly fine to eat, and this doesn’t constitute spoilage – to revert it back to its liquid form, all that’s required is immersion in warm water for a few minutes.

A slew of factors - its acidity, its lack of water and the presence of hydrogen peroxide - work in perfect harmony, allowing the sticky treat to last forever.

If you buy your honey from the supermarket, that little plastic bottle of golden nectar has been heated, strained and processed so that it contains zero particulates, meaning that there’s nothing in the liquid for molecules to crystallize on, and your supermarket honey will look the same for almost forever. If you buy your honey from a small-scale vendor like Two Busy Bees Honey, however, certain particulates might remain, from pollen to enzymes. With these particulates, the honey might crystallize, but don’t worry - if it’s sealed, it’s not spoiled and won’t be for quite some time.

A jar of honey’s seal, it turns out, is the final factor that’s key to honey’s long shelf life, as exemplified by the storied millennia-old Egyptian specimens. While honey is certainly a super-food, it isn’t supernatural–if you leave it out, unsealed in a humid environment, it will spoil. As Harris explains, ” As long as the lid stays on it and no water is added to it, honey will not go bad. As soon as you add water to it, it may go bad. Or if you open the lid, it may get more water in it and it may go bad.”

So if you’re interested in keeping honey for hundreds of years, do what the bees do and keep it sealed–a hard thing to do with this delicious treat!

 

 





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