For centuries, beekeepers assumed that bees sealed the beehive with propolis to protect the colony from the elements, such as rain and cold winter drafts. However, 20th century research has revealed that bees not only survive, but also thrive, with increased ventilation during the winter months throughout most temperate regions of the world.
Propolis is now believed to:
The composition of propolis will vary from hive to hive, district to district, and from season to season. Normally it is dark brown in color, but it can be found in green, red, black and white hues, depending on the sources of resin found in the particular hive area. Honey bees are opportunists, and will gather what they need from available sources, and detailed analyses show that propolis chemical composition varies considerably from region to region, along with the vegetation. In northern temperate climates, for example, bees collect resins from trees, such as poplars and conifers (the biological role of resin in trees is to seal wounds and defend against bacteria, fungi and insects). Poplar resin is rich in flavanoids. "Typical" northern temperate propolis has approximately 50 constituents, primarily resins and vegetable balsams (50%), waxes (30%), essential oils (10%), and pollen (5%). In neotropical regions, in additional to a large variety of trees, bees may also gather resin from flowers in the genera Clusia and Dalechampia, which are the only known plant genera that produce floral resins to attract pollinators. Clusia resin contains polyprenylated benzophenones. In some areas of Chile, propolis contains viscidone, a terpene from Baccharis shrubs, and in Brazil, naphthoquinone epoxide has recently isolated from red propolis, and prenylated acids such as 4-hydroxy-3,5-diprenyl cinnamic acid have been documented. An analysis of propolis from Henan, China found sinapic acid, isoferulic acid, caffeic acid and chrysin, with the first three compounds demonstrating anti-bacterial properties. Occasionally worker bees will even gather various caulking compounds of human manufacture, when the usual sources are more difficult to obtain. The properties of the propolis depend on the exact sources used by each individual hive, therefore any potential medicinal properties that may be present in one hive's propolis may be absent from another's, and the distributors of propolis products cannot control such factors. This may account for the many and varied claims regarding medicinal properties, and the difficulty in replicating previous scientific studies investigating these claims. Even propolis samples taken from within a single colony can vary, making controlled clinical tests difficult, and the results of any given study cannot be reliably extrapolated to propolis samples from other areas.
Some of these claims are being clinically investigated and several studies are published in the biomedical literature. Since the chemical composition of propolis varies depending on season, bee species and geographic location, caution must be applied in extrapolating results (above). Depending upon its composition, propolis may show powerful local antibiotic and antifungal properties, and studies indicate that it may be effective in treating skin burns. Propolis also exhibits immunomodulatory effects and is a subject of recent dentistry research, since there is some evidence that propolis may actively protect against caries and other forms of oral disease, due to its antimicrobial properties. Propolis can also be used to treat canker sores, and its use in canal debridement for endodontic procedures has been explored in Brazil.