Propolis is a glue-like, resinous material created by bees and used to block holes and seal cracks in hives. Moreover, propolis contains phenolic acids and their esters, flavonoids (flavones, flavanones, flavonols, flavanonols, and chalcones), terpenes, triterpenes, fatty and amino acids, and other beneficial compounds, which makes it a desirable object of extraction.
As a result, the optimal extraction conditions for propolis are of great interest.
Ultrasound extraction is often preferred for its eco-friendliness, high selectivity, energy- and time-conservation, and efficiency for antioxidants.
On the other hand, ethanol is also a preferred solvent for the extraction of bioactive compounds from propolis and especially for polyphenols, because it has shown to be non-toxic and highly efficient.
So, the study in question examined how temperature, time, solvent concentration, and ultrasonic frequency affect the extraction of several target molecules including balsam content, total phenolic content (TPC), total flavone content (TFC) and individual phenolics including gallic acid, vanillic acid, chlorogenic acid, caffeic acid, p-coumaric acid, rosmarinic acid, myricetin, quercetin, luteolin, and kaempherol.
In terms of ultrasound amplitude, the higher, the better.
“As the amplitude increased from 20% to 100%, the balsam content increased with 23.3%, the TPC increased with 17.5% and TFC increased with 29.1%, respectively.”
Ethanol concentration had a palpable effect, and increasing it from 40% to 80% resulted in increased efficiency across the board – 150% for balsam content, 175% in TFC and 43% for TPC, with the greatest difference being observed “between the concentration of 40% and 60%, while from 60% to 80% the increase was less pronounced.”
Extraction temperature also played a role for TPC and balsam content, but was fairly irrelevant to TFC. The benchmarks were set at 35, 50 and 65°C. The increases in the first two, however, were also comparatively small, which the scientists believe might be “due to the high solubility of polyphenols from propolis into ethanol.”
Time, which was gradually increased, with benchmarks set at 15, 30 and 45 minutes, didn’t have a significant effect on the contents of the three main molecular groups of interest.
“The highest extraction efficiency was found at 100% amplitude of ultrasonic treatment, 70% solvent concentration, 58°C and 30 min.”
 Oroian M, Ursachi F, Dranca F. Influence of ultrasonic amplitude, temperature, time and solvent concentration on bioactive compounds extraction from propolis. Ultrason Sonochem. 2020;64:105021. [journal impact factor = 7.491; times Cited = 20]
Image Credits: Wikimedia Commons