Ultrasonic Extraction: What Mysterious Force Makes Herbs 'Willingly' Release Their Essence?

Ultrasound refers to mechanical waves with frequencies ranging from approximately 20 kHz to 28 KHz, which require a medium for propagation. During transmission, these waves create alternating cycles of positive and negative pressure. In the positive phase, molecules in the medium are compressed, increasing its density. In the negative phase, the molecules rarefy and move apart, decreasing the density. Crucially, ultrasound does not polarise molecules within the sample. Instead, it creates acoustic cavitation in the solvent, leading to the formation, growth, and implosive collapse of microscopic bubbles. This process breaks down solid samples, increasing their surface area contact with the solvent and enhancing the mass transfer rate of target compounds from the solid to the liquid phase.

 

Principles of Ultrasonic Extraction
The superiority of ultrasonic extraction for herbal medicines lies in the unique physical properties of ultrasound. It primarily uses rapid mechanical vibration waves generated by a piezoelectric transducer to reduce the binding force between the target compounds and the sample matrix, thereby achieving solid-liquid separation.

 

(1) Accelerated Particle Motion: When ultrasound, at frequencies above 20 kHz, travels through a continuous medium like water, it causes the particles of the medium (including particles of the herb's active components) to move, granting them immense acceleration and kinetic energy-calculated to be over two thousand times that of gravitational acceleration. This energy transfer enables the active components to rapidly escape the plant matrix and dissolve into the solvent.

 

(2) Cavitation Effect: The propagation of ultrasound in a liquid medium produces a special "cavitation effect." This effect continuously generates countless microscopic cavities with internal pressures reaching thousands of atmospheres. Their subsequent implosion creates powerful microscopic shockwaves that 'blast' the active components out of the herbal material and erode the plant matrix, thereby separating the desired constituents and significantly speeding up the extraction process.

 

(3) Sonication Homogenisation: The vibrating action of ultrasound ensures that all points within the sample medium are subjected to consistent effects, leading to a more uniform extraction throughout the entire sample.

 

Under the influence of an ultrasonic field, the active substances in herbs not only gain tremendous acceleration and kinetic energy themselves as medium particles but are also subjected to powerful external impacts via the cavitation effect. This allows for their highly efficient and thorough separation.