A balance is established between the water content of the honey and the surrounding air humidity. The balancing moisture content of honey at a relative humidity of 55% is about 17.5%. At a relative humidity of 80%, honey has a water content of over 30% (source: The hygroscopic properties of different dilutions of Honey, Doull & Mew 1977).
The wax capping of the honey cells slows down the speed of these water transports, but does not prevent diffusion. The wax layer is so thin that the resistance of this cap to water vapour diffusion is just as high as that of a 3.3 cm thick layer of air. The experimental setup for determining the diffusion density is explained below.
Thus, an increase in the humidity around the honeycombs also leads to an increase in the water content of the honey. Honey with a water content above 18% is extremely at risk of fermentation (“Wassergehalt im Honig”, LAVES – Institut für Bienenkunde, 2014). Such an increase in relative humidity is, for example, the result of poor thermal insulation. The question is whether such possible fermentation processes in autumn or spring do not also pose a health risk for the bees.
The water vapour diffusion resistances of the wax capping were determined using the following experimental set-up:
The water-soaked clay jug acts as a climatic chamber. With the lid closed, a relative humidity of almost 100% is achieved. A relative humidity of 33% is produced in the glasses by means of a saturated salt solution. This difference in relative humidity causes water vapour diffusion through the specimens. The diffusion current density is measured by the change in weight of the glasses. The test setup corresponds in principle to the design according to DIN EN ISO 12572, however, the diffusion area is about 10x smaller and the accuracy is therefore lower. This smaller diffusion area had to be chosen because it is almost impossible to remove a closed wax cover with a circular area of more than 9cm diameter.
Assuming that the cap is about 0.1 mm thick, this results in a water vapour diffusion resistance factor µ of 330 for the cap wax.
In addition to the 5 samples of the wax capping, samples with cast middle walls, samples with bee-built middle walls and samples with propolis were also examined.