Cover Story
Honey’s Magical Power: Four Ways to Slay Microbes
You’ve heard the rumors, right? Honey never goes bad, it’s been found in Egyptian tombs ready to eat, and it’s the only cure for some antibiotic-resistant bacteria. You may wonder if any of this is true, and if so, why. Let’s look at some of the details.
Since the beginning of recorded history, honey has been known to have antiseptic properties. Frequent mention of honey in medicine was made by the Egyptians, Assyrians, Greeks, and Romans. But then, as now, users were well aware that some honeys were better healers than others,1 so a method of grading honey was developed. Beginning in 1937 the underlying cause of honey’s healing power was given the name “inhibine” and a number was assigned to different honey types to indicate how strong the inhibine was by measuring how well it killed specific bacteria like staph.
Related to honey’s antiseptic properties is its extraordinary shelf life. Depending on its source and how it was handled, honey may remain edible for many years. According the National Honey Board, “Honey stored in sealed containers can remain stable for decades and even centuries.”2 Nevertheless, the why of it can be difficult to understand.
A review of the literature reveals four distinct reasons for the medicinal action and stability of honey. Three of them are directly related to things honey bees do to the nectar they collect. The fourth comes from the plants themselves.
The four factors that affect the antiseptic strength of honey are the osmotic concentration, acidity, amount of hydrogen peroxide, and the presence of specialized plant compounds. The healing power of any one sample of honey is simply the sum of all the factors,3 so we will look at each one separately.
Osmotic Concentration
The osmotic concentration of a solution refers to the number of particles dissolved in a unit of liquid. If you’ve ever made sugar syrup, you know that one part of sugar dissolves easily in one part of water. But two parts of sugar in one part of water begins to get tricky. After stirring forever, you may give up and use heat to force the sugar into solution.
But honey is approximately four parts of sugar dissolved in one part of water. We call this a supersaturated solution because the liquid is holding more particles than it could under normal circumstances. A supersaturated sugar solution is unstable; it may suddenly crystallize or it may absorb water from the surrounding environment.
When a substance absorbs water from its surroundings, we say it is hygroscopic. For example, if you leave a jar of honey uncovered on the counter, it absorbs moisture from the atmosphere. Likewise, if you put honey on a bacterium, it will suck the water right out of the cell, killing it by dehydration. This hygroscopic action is one key to honey’s long shelf life and its ability to heal wounds—it simply dehydrates any microbe it touches.
But the osmotic concentration of honey changes as water is absorbed. Once the honey absorbs enough water to reach equilibrium, it no longer absorbs more. That jar of honey that you left uncovered will eventually absorb so much water from the air that it is no longer supersaturated. At that point, a microbe such as a yeast spore can land on it and germinate, causing the honey to ferment.
You get a similar result when you extract honey frames that contain many uncapped cells. Because uncapped cells contain excess water, they can lower the osmotic concentration of the entire batch, leading to fermentation. The inverse relationship between osmotic concentration and the amount of water in the honey means this mode of microbial suppression is temporary.4
The next two modes of microbial suppression, acidity and the presence of hydrogen peroxide, are both due to the action of one enzyme, glucose oxidase.
Acidity
The hydronium ion concentration, or pH, of honey varies from about 3.2 to 4.5. This high acidity is partially due to acids found in the nectar, including acetic, butyric, formic, lactic, and malic. But the major source of acidity in honey is …
