Most beekeepers are aware that honey is hygroscopic, which describes a material that absorbs moisture from the surrounding air. More accurately, it exchanges moisture with the surrounding air. Water may be gained or lost, depending primarily on the relative humidity. A hygroscopic material seeks to reach moisture equilibrium with its environment, at which point water content stabilizes.
The honey bee brood nest is a rather dry place, despite the requirement of nearly 100% humidity for eggs to hatch. Royal jelly, brood cocoons and the larva itself combine with the efforts of workers to create a very humid microclimate around each individual cell. The remainder of the brood area experiences between 50 and 60 percent relative humidity, regardless of local climate. Beyond the highly regulated conditions of the brood nest, hive humidity trends toward equilibrium with the outside air.
Salt Lake City consistently ranks as one of the four dryest major cities in America. Average year-round relative humidity is 55%, matching that of the honey bee brood nest. Average afternoon humidity during summer drops to 25%. Unlike most parts of the country, Utah is dryer than a beehive.
There are two parts of the ripening process: the inversion of sucrose into simple sugars and the lowering of water content, the latter of which is associated with spoilage and the focus of this review. Bees begin evaporating water from nectar on the flight back to the hive, where it is further concentrated by house bees. By the time it is deposited in the comb, the moisture content of nectar has been substantially reduced. Further ripening takes place in the storage cell, either actively by worker behavior or passively due to environmental conditions in the hive.
At 17% moisture and below honey will not ferment, because the naturally occurring yeasts cannot grow. At 18% moisture there is a low risk of fermentation. At 19% and above, fermentation of honey can be expected. The rate and degree to which it occurs depends on water content, yeast concentration and temperature. Above 80F and below 50F yeast spores are inactive. The liquid portion of crystallized honey carries the highest risk of fermentation. That honey never spoils is mythological.
Equilibrium Moisture Content
At 55% relative humidity (RH), the average found in the brood nest, honey has a water content of 17% when in equilibrium with the surrounding air. Even when uncapped, it will not absorb (or release) moisture, and it will not ferment. Beyond the boundary of the brood nest, at RH levels above 60%, equilibrium moisture content (EMC) of honey rises above 18% and is at risk of fermenting. As distance from the brood nest increases, RH also increases until it matches that of the outside air. Uncapped honey stored above 60% RH will absorb water from the surrounding air and eventually ferment.
But what if the prevailing RH is lower than 60%, as is the case along the Wasatch Front in the summertime? If the air beyond the brood nest only gets dryer, the amount of water honey can hold only gets lower. The average summertime humidity in Utah is 40%, which translates to an EMC of 15%. In other words, if uncapped honey is removed from the hive its moisture content will decrease until reaching 15%, at which point it will be in equilibrium with the surrounding air. Such honey will probably be extracted and jarred in less time it takes to reach equilibrium, resulting in a moisture content somewhere between 15% and 18%. This is easy to verify. Leave the lid off a jar of honey long enough in summertime, and even ripened honey will slowly thicken as it gets further dehydrated by the surrounding air (assuming the air inside your home is not above 60% RH).
So why do workers bother capping honey at all? First, there already exists a need to dilute honey with water for winter consumption. Further drying requires more work and more water, both of which can be problematic for the winter cluster. More importantly, by late October or early November humidity levels along the Wasatch Front will begin to rise above 60%, putting uncapped honey at risk of spoilage.
In Utah, honey does not absorb moisture during the summertime. The possibility of fermentation is extremely low. However, recently deposited nectar may still contain enough moisture to be shaken free from the comb. In this case, spoilage is unlikely but possible, but only for a short period of time. Whether it remains in the hive or is pulled for harvest, the moisture content of uncured honey will passively decrease to safe levels as it moves toward equilibrium with the surrounding air.
In the environments where honey loses moisture to the surrounding air, it seems the phenomenon would be of great use to the colony among efforts to humidify and cool the brood nest. In my own experience, uncapped honey is most often found on the lower half of the frame closest to the brood area. In this scenario, uncapped honey would fall safely below 18% moisture and still shed water to the surrounding air, depending on the humidity level. Uncapped honey in Utah is not synonymous with unripe honey.
The average moisture content of honey produced in the Intermountain West is 16%, the lowest in the country. This is due to our extra dry climate. It would take some effort to extract a batch of honey around here that has a moisture content much above the 18% threshold. If that does happen, just let the honey sit uncovered in a protected area for several days. It will slowly dehydrate on its own until reaching a safe moisture content. The passive summertime evaporation of water from Utah honey makes life a little easier on bees and results in a more concentrated, thicker product that is unique to the region and among the best honey available anywhere.
A Look into the Cell: Honey Storage in Honey Bees
by M Eyer - 2016
Water homeostasis in bees, with the emphasis on sociality
by SW Nicolson - 2009