Propolis and its physical effect in the hive

Composition and effect of propolis

The antibiotic, antiviral and antifungal effects of propolis are well known and can be attributed to the active ingredients of the individual components. Propolis is a mixture that honey bees compose of many different substances. The main components are resins collected from trees and beeswax. Furthermore, it contains essential oils, pollen and saliva. Propolis and beeswax serve as building materials for the bees, of which propolis has a higher melting point as it contains a smaller proportion of wax. The coating of the hive walls with propolis influences the moisture in the hive.

In the literature the following specifications on the composition of propolis can be found:  The proportion of essential oils is usually estimated between 5 and 10%. The proportion of pollen is also at 5-10%. The proportion of natural resins varies between 40 and 70%, the proportion of wax between 20 and 50%, whereas the bees can apparently stretch the propolis with beeswax almost arbitrarily. Natural beeswax can even serve as a substitute for propolis and contains 5-10% propoplis.

The use and processing of propolis

Bees use propolis for puttying and filling smaller cracks and crevices. Additionally they cover all hive walls with a thin layer of propolis. It also serves to narrow the flight hole and to mummify intruders that were killed in the hive.

Propolis bees collect the propolis and partly also use it for puttying. W. Meyer published an extensive article about “Kittharzbienen und ihre Tätigkeiten” in the Zeitschrift für Bienenforschung (issue July 1954), that I gained access to thanks to Sigrun Mittl. I will summarize some particularly interesting observations on the processing of propolis in the following:

“The propolis bee moves into the proximity of a propolis building site with its commodity, where other workers take small pieces of resin (0,5mm³) from its bottom depending on the demand of propolis. These workers then take the propolis to the respective sites and put it on roughly first, chewing it a bit. This is followed by extensive further processing and distribution. As with the execution of wax construction work, puttying implies a constant removal of unevenness, which again results from the attachment of particles of building material. No further work is done on the smooth material. As with the resin particles, wax crumbs that have been gnawed off from the honeycomb material are also processed into the cementation. The resin loses its stickiness and becomes firmer. The proportion of wax in the propolis depends on the amount of required material. It can be stretched with wax arbitrarily.”

Propolis forager returns to the observation hive
propolis bee – foto by Max Westby

Beeswax and propolis serve the bees as building materials for various purposes. The respective composition varies depending on the intended use and the quantity of building material required. As quoted above, bees stretch propolis with wax arbitrarily. However, I also suspect the exact opposite: Depending on its purpose, the bees probably leave out the beeswax almost entirely: When a thin layer is applied on a large surface or when small gaps are filled, a low viscosity and solidity is required. Hence, a lower wax content is advantageous. Possibly, such low wax contents have not been analyzed so far because beekeepers harvest propolis where it is produced in larger quantities (e.g. on grids and nets they specifically laid out). Scrapping off the thin layers from large surfaces would not be very profitable and would inevitably lead to contamination with wood residues. The investigation of such propolis layers is planned for 2019.

Water vapor diffusion through beeswax

Pure beeswax has a very high resistance to water vapor diffusion. According to a study from 1993 (“Water Vapor and Oxygen Permeability of wax films”, Donhowe & Fennema), beeswax (and other natural waxes) is as impermeable as solid plastics. I myself have examined 5 wax samples and, as expected, could not detect any weight changes not resulting from measurement inaccuracy. Based on my experiments I can say that my beeswax samples certainly have a μ value > 20000.

Water vapor diffusion through propolis

The thin propolis layer on the surfaces of the hive walls (approx. 0.1-0.3 mm) allows water vapor diffusion. The resistance of propolis to water vapor diffusion varies greatly, values I determined from an experimental setup based on DIN EN ISO 12572 (see below) vary between μ=150 and μ=8500. It can be assumed that the proportion of wax significantly influences the resistance. The higher the proportion of beeswax, the higher the resistance of the propolis mixture against water vapor diffusion.

So far I have examined samples from 4 different beekeepers/traders. The lowest resistance to water vapor diffusion was found in 5 samples from the company Naturherz. The average μ value of the samples is 330. These samples are therefore open to diffusion in a way that allows for the interaction of wood moisture and humidity to be slowed down only marginally. A 0.1 mm propolis layer of this sample has the same diffusion resistance as a 0.2-1 mm wood layer (the diffusion resistance of wood is different depending on the direction of the fiber and moisture of the wood). The wax content of these samples is so low that it did not separate from the resins during my melting tests. A more exact analysis will follow, however the wax content surely is the lowest of the examined samples.

The highest values of resistance in the 5 samples resulted from the highest wax content: Average μ-value=4400. Again, a more exact analysis of the wax content will follow. A 0.1mm propolis layer with this μ value could just be considered “open to diffusion” from a building physics point of view. The resistance to water vapor diffusion would correspond to that of a 44cm air layer or that of a 3-15mm wood layer, respectively. A 0.1mm coating of the side walls with this mixture would still allow water vapor diffusion, though in a clearly decelerated way.

Building physical purpose and function of propolis in the beehive

The propolis-coated surfaces prevent the transport of liquid water. The beehive is protected from entering water. However, the thin coating still allows water vapor diffusion. And this usually propagates from the warmer to the colder side. I.e. in the months, in which the bees do not ventilate (dehumidify) actively, water vapor diffuses from the inside out. In tree hives and wooden hives this means that the wood absorbes moisture from the hive.

In addition, another physical effect reduces the amount of condensation in the hive:

Propolis prevents capillary condensation on the side of the bees

Through a simple physical effect, the application of propolis shifts the formation threshold for free water in the space of the bees from 60-70% RH to almost 100% relative humidity. Free water is essential for the development and spread of mold and pathogenic germs. In hygroscopic building materials (such as wood), capillary condensation (part of sorption) produces small amounts of free water as early as in the hygroscopic range. Capillary condensation begins at a relative humidity of about 70-80%. That is also the reason why all known building molds grow and thrive on wood above a relative humidity of 80%.

With non-hygroscopic building material surfaces, condensation only occurs when a relative humidity of 100% is reached near the surface. The application of the propolis layer changes the surface properties of the tree hive wall. The surface is no longer hygroscopic, the walls are smoothed. The threshold for the relative humidity at which mold begins to grow is therefore shifted upwards by 20-30 percentage points.

Due to the openness to diffusion (see below), capillary condensation can continue to occur under the propolis layer; it is part of the sorption processes leading to fibre saturation along with chemisorption and adsorption. The low quantities of free water developing on this occasion at the pore-walls of the wood are downright mummified by the propolis layer, a growth of mold should therefore be impossible here, below the propolis layer.

Experimental setup for determining the water vapor diffusion resistance coefficient

In accordance to DIN EN ISO 12572 I carry out measurements on various propolis samples to determine the water vapor diffusion resistance coefficient μ. In an earlier experiment I already discovered the permeability of wood coated with propolis. In the current experiments I consider the propolis separately. The propolis is shaped into a seamless thin specimen. On different sides different relative humidities are produced and measured. Vapor diffusion occurs due to the different vapor pressure. The coefficients of the material are calculated from the change in weight of the specimens.

Experimental setup for determining the wax content according to Hogendoorn

It is possible to determine the wax content of the propolis samples by simple means. I will also conduct a more exact analysis of my samples using Hogendoorn’s method. Until now I have simply melted the samples in the oven at 100°C and weighed the separating layers.