Propolis is a sticky, resinous substance collected from different kinds of trees and plants. The bees mix in substances derived from pollen and active enzymes to form a potent, naturally therapeutic substance that beekeepers call 'bee glue'.
The bees collect propolis by biting off scraps of the plant resin and packing them into the pollen baskets on their hind legs. Because it is so sticky propolis gathering is a slow business. It is only collected when the temperature is above 18°C and each bee can only carry about 20mg in one journey. Sometimes bees collect man-made materials of similar consistency such as road tar or varnish.
Propolis is a very stable substance, variable in colour and composed of resins, waxes, volatile oils, pollen, vitamins, minerals and plant chemicals especially concentrated flavonoids, which are active plant-derived essential oils that are thought to be mainly responsible for the therapeutic properties of propolis. Over 180 distinct compounds have been identified in propolis with researchers expecting to find more.
Honeybees use propolis to keep their homes dry, cosy and hygienic. The propolis coating makes the walls of their nesting place waterproof and draught-proof. Propolis is used to seal up any cracks or gaps where micro-organisms could flourish and to decrease the size of nest entrances, which makes them smooth for passing bee traffic and easier to defend from intruders. A thin layer is used to varnish inside brood cells before the queen lays eggs into them. This provides a strong, hygienic unit for developing larvae while the volatile oils in propolis serve as a kind of antiseptic air-freshener.
Different races of Apis mellifera use propolis to different extents, with some races known for their high collection of propolis. Latterly beekeepers have been inclined to breed queens that show a reduced propensity for collecting propolis in order to make beekeeping easier. However, not all honey bee species collect propolis. The Asian species Apis cerana does not use propolis at all. By contrast Apis florea, another Asian honeybee species, uses rings of propolis (like grease bands) to coat the branch from which its single-comb nest is suspended to deter predators.
One method of harvesting propolis is to place a perforated plastic grid, similar to a queen excluder, in the hive. The holes in the plastic are small - not more than 6 mm - consequently the bees will seal them up with propolis. The grid is put into a freezer so the cold propolis pieces drop out when the sheet is flexed. It might be possible to harvest 50 g per hive per season this way. Normally beekeepers just scrape off propolis while going about their beekeeping inspection activity. Tempting though it is, harvested propolis should never be rolled into a ball. This gives propolis that may have debris, dust and wax in it. Debris can be removed by heating the collected propolis in a pot over a heat source (fire etc). When the water is boiling remove the pot and allow it to cool. The debris floats on the top while the propolis sinks to the bottom. Pour off the surface layers carefully leaving the propolis behind. Strain the propolis through a clean cloth, dry it and store in a cool place.
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