Propolis and its physical effect in the hive

Composition and properties of propolis

The antibiotic, antiviral and antifungal effects of propolis are well known and can be attributed to the ingredients of the individual components. Propolis is a mixture of honey bees made of many different substances. The main components are collected resins and beeswax. Furthermore, essential oils, pollen and saliva secretion are contained. Propolis and beeswax are used by bees as building materials, whereby propolis has a higher melting point due to the lower wax content. The coating of the hive walls with propolis influences the moisture in the hive.

In the literature you can find information about the composition of propolis as follows: The proportion of essential oils is usually between 5 and 10%. The proportion of pollen is also 5-10%. The proportion of natural resins varies in the literature between 40 and 70%, the wax proportion between 20 and 50%, whereby the propolis can apparently be stretched by the bees almost at will with beeswax. Natural beeswax even can be used as a substitute for propolis and has a propoplis content of 5-10%.

Intended use and handling of bee glue

Propolis is used by bees for filling smaller cracks and crevices. In addition, all hive walls are covered with a thin layer of it. It also serves to narrow the flight hole and to mummify killed intruders in the beehive.
The bee glue is collected by the propolis bees. There is an extensive article about “Kittharzbienen und ihre Tätigkeiten” by W. Meyer in the “Zeitschrift für Bienenforschung” (issue July 1954), which I could see thanks to Sigrun Mittl. I find the observations on the processing of bee glue, which I summarize here, particularly exciting:

“The propolis bee goes with its goods to the near of a propolis construction site, where other workers remove small pieces of the resin (0.5mm³) from the propolis bee depending on the propolis requirement. These then run to the interfaces and first tack it on roughly, chewing it through a bit. This is followed by extensive further processing and distribution. As with the application of wax, putty means a constant removal of unevenness, which is created again and again by attaching particles of building material. No further work is done on the smooth material. In the same way as resin particles, wax crumbs that have been gnawed off from the honeycomb material are also processed into the putty. The resin loses its stickiness and becomes firmer. The proportion of wax in the putty resin depends on the amount of putty required. It can be stretched with wax as desired.”

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

Beeswax and propolis are used by bees as building materials for various purposes. The respective composition varies depending on the intended use and the quantity of building material required. As already quoted above, propolis is stretched with wax by the bees at will. However, I also suspect the exact opposite: Depending on the application, the bees probably leave out the beeswax almost completely: thin applications on large surfaces and for filling small gaps require a low viscosity and strength. A lower wax content would be advantageous here. It is easy to imagine that such low wax contents have not yet been analysed, because propolis is picked by the beekeeper where it is applied in larger quantities (for example on specially designed grids and nets). Scrapping off the thin layers on the large surfaces would not be so productive and would inevitably lead to contamination by wood residues. The investigation of such propolis layers is planned for 2019.

Water vapor diffusion through beeswax

Pure beeswax has a very high water vapor diffusion resistance. 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 outside the 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 strongly, values determined by me after a test setup based on DIN EN ISO 12572 (see below) vary between μ=150 and μ=8500. It can be assumed that the resistance is significantly influenced by the proportion of wax. 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 had 5 samples of the company Naturherz. The average μ-value of the samples is 330. These samples are so permeable that the interaction of wood and air humidity is only slightly slowed down. A 0,1mm thick propolis layer has here about the same diffusion resistance as a 0,2-1mm thick wood layer (the diffusion resistance of wood is different depending on grain direction and humidity 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 still follows, the wax-content is the lowest of the examined samples, however, surely.

The highest resistance values were found in the 5 samples with the highest wax content: Average μ value=4400. A more precise analysis of the wax content will also be carried out here. A 0.1mm thick propolis layer with this μ-value would be evaluated as “permeable” from a building physics point of view. The resistance against water vapor diffusion would correspond to that of a 44cm thick air layer, and/or that of a 3-15mm thick wood layer. A 0.1mm coating of the side walls with this mixture would still allow a water vapor diffusion, even if clearly slowed down.

Building physics function of propolis in beehives

The propolis-coated surfaces prevent the transport of liquid water. The beehive is protected against water penetration. However, the thin coating still allows water vapor diffusion. And this usually takes place from the warmer to the colder side. I.e. in the months, in which the bees do not ventilate (dehumidify) actively, a water vapor diffusion from inside to outside takes place. In tree hollows and wooden hives this means that moisture from the hive is absorbed by the wood.

In addition, there is a further physical effect of less 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 limit for free water in the room of bees from 60-70% RH to almost 100% relative humidity. Free water is absolutely necessary for the development and spread of mould and pathogenic germs. In the case of hygroscopic building materials (such as wood), capillary condensation (part of sorption) results in small amounts of free water already in the hygroscopic area. Capillary condensation begins at a relative humidity of about 70-80%. This is also the reason why all known moulds in buildings grow on wood from a relative humidity of 80%.

In the case of non-hygroscopic material surfaces, condensation only occurs when a relative humidity of 100% is reached near the surface. The application of the propolis coating changes the surface condition of the tree hollow or hive wall. The surface is no longer hygroscopic, the walls are smoothed. The limit value for the relative air humidity, from which a mold danger exists, is shifted therefore by 20-30 percentage-points upward.

Due to the diffusion permeability (see below), capillary condensation can continue to occur under the propolis coating, which is part of the sorption processes up to fibre saturation with chemisorption and adsorption. The low quantities of free water at the pore walls of the wood that arise on this occasion are literally mummified through the propolis layer, a growth of mold should be excluded thus here, under the propolis layer.

Holes in propolis

I consider the theory about holes in propolis absurd. Apart from the missing evidence and the assumption of wrong physical correlations, the theory also makes hardly sense: The thin propolis coating is open to diffusion, even without holes. The shown partially capillary breaking holes would make a penetration of liquid water possible, which would not be in the sense of the bees certainly.

Experimental set-up for the determination of the water vapor diffusion resistance factor

In the spirit of DIN EN ISO 12572 I carry out measurements on various propolis samples to determine the water vapour diffusion resistance value μ . Already in an earlier experiment I determined the permeability of wood coated with propolis. In the current experiments the propolis is considered separately. The Propolis is formed to a gapless thin test piece. On the different sides different relative air humidities are produced and measured. Due to the different vapor pressure, vapor diffusion occurs. The material properties are calculated by changing the weight of the specimens.

Experimental setup for the determination of the wax content according to Hogendoorn

It is possible to determine the wax content of the propolis samples by simple means. A more exact analysis of my samples will be done in this way according to Hogendoorn. So far I have simply melted the samples at 100°C in the oven and weighed the separating layers.