The health benefits of bee glue: Extracting phenolic compounds from bee propolis
- Published: Mar 20, 2017
- Author: Jon Evans
- Channels: Sample Preparation
Propolis is a glue made by honey bees from a mixture of saliva, beeswax, and tree resin or plant sap, which the bees use to fix up the hive. Humans have found an alternative use for it: as a treatment for a wide range of medical conditions, including minor infections and dry skin.
Even though humans have used propolis as a medical treatment for thousands of years, scientists are still working to identify the active ingredients responsible for its medicinal properties. What makes this process so difficult is that not only is propolis a highly complex substance, comprising many different compounds, but its composition can differ substantially between different hives and seasons. This is because bees produce it using resin or sap from whatever trees or plants are growing near their hive.
One specific class of organic molecules that scientists are focusing on is phenols, as they are known to have medicinal properties and are abundant in propolis. Because of the complexity of propolis, scientists first need to extract the phenolic compounds from any samples before they can identify them with a technique such as liquid chromatography-mass spectrometry (LC-MS). Existing methods for doing this are slow and not particularly environmentally friendly, because they tend to involve extracting the phenolic compounds with large volumes of organic solvents.
Ionic liquids and cellulose
So Jun Cao and his colleagues at Hangzhou Normal University in China decided to try developing a faster, greener extraction method, by replacing the large volumes of organic solvents with an ionic liquid. These are salts that are liquid at room temperature, consisting entirely of anions and cations, and have proven to be very effective solvents. Cao and his colleagues planned to use an ionic liquid to release the phenolic compounds from the propolis. They would then extract the released compounds by adding an absorbent material to the ionic liquid solution, before removing the material by filtration. Finally, the phenolic compounds would be released from this absorbent material for analysis by LC-MS.
Cao and his colleagues began by testing a variety of different ionic liquids, finding that one consisting of a bromine anion and a methylimidazolium cation comprising a chain of 12 carbon atoms proved most effective at releasing phenolic compounds from propolis. This was probably due to a combination of the bromine anion interacting strongly with the phenolic compounds and the long carbon chains promoting the formation of micelles in the ionic liquid solution. They also used ultrasound to enhance the release of the phenolic compounds.
After deciding on an ionic liquid, Cao and his colleagues then tested various absorbent materials, including silica, mesoporous carbon, carbon nanotubes and microcrystalline cellulose (MCC). They found that MCC was best at absorbing the phenolic compounds, probably due to a combination of it being highly porous, having a large relative surface area and possessing abundant hydroxyl groups for forming hydrogen bonds with the phenolic compounds. Finally, they tested various different organic solvents for releasing the phenolic compounds from the MCC, finding that ethyl acetate was most effective.
Different regions, different times
Using this method, Cao and his colleagues were able to extract 20 phenolic compounds from three different propolis samples; all 20 of these compounds could then be unambiguously identified by LC-MS. Investigating three of these compounds in more detail – pinocembrin, chrysin and galangin – they confirmed that the extraction method could recover 85–98% of the compounds, allowing their concentrations to be determined accurately by LC-MS. Interestingly, this revealed that the concentrations of these three compounds varied quite widely between the three propolis samples, presumably reflecting the fact they were obtained from different regions at different times.
As hoped, this extraction method was not only more effective than existing versions, allowing the phenolic compounds to be detected at lower concentrations, but was also faster – taking just one minute – and also greener. Although it still employed an organic solvent, the amount was greatly reduced, with just 0.1mL of ethyl acetate required to release the phenolic compounds from the MCC.
Analytica Chimica Acta, 2017, 963, 24–32: "Ultrasound-assisted ionic liquid-based micellar extraction combined with microcrystalline cellulose as sorbent in dispersive microextraction for the determination of phenolic compounds in propolis"
Article by Jon Evans
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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