The application of acoustic energy and ultrasound to a liquid medium results in the formation of cavities if the power and acoustic pressure wave is sufficient. At the expansion phase of the sound wave the liquid can be torn apart at structural discontinuities (particles, gas bubbles, foreign bodies) to create these cavities. Upon compression the cavitational bubbles collapse violently to produce regions of transient high pressure and temperature. At lower acoustic pressure (lower power) cavitation can be avoided and instead streaming effects can be seen. The cavity can collapse whereupon the energy is dissipated as transient heat (5000K) and pressure (2000 ats). These extreme shear forces can facilitate biomass treatment and biorefining.
The application of acoustic energy and power ultrasound of 10 - 100 kHz in biocatalysis is in its infancy. However, when applied under low-power conditions, ultrasound (20 - 100 kHz) can substantially benefit reaction kinetics. Normally ultrasound is applied as a destructive force to break open cells and release required proteins for recovery. Used under milder conditions, faster cell growth, faster reaction rates and higher products yields can be achieved in fermentations.
In biotransformations and biocatalysis that use isolated enzymes faster reactions can be readily achieved. The physical effects derived from ultrasound phenomena such as cavitation and acoustic streaming are clearly implicated. These effects in turn can have profound effects on:
mixing and mass transfer
facilitating the entry into, and exit out of dissolved molecules (substrates, products and enzymes) from cells
kinetics of gas transfer
removal of inhibitory fermentation product gases
The challenge and skill lies in identifying which processes could benefit from this technology and being able to design and apply the appropriate ultrasonic treatment regime with respect to power input and frequency, together with assembling the correct equipment available at scale.
Previous attempts to develop scaleable ultrasound systems for broad application within the fine chemicals and pharmaceutical industry have met with limited success as a result of significant reliability, scalability and contamination issues. Through its preferred ultrasonic engineering partners Celbius will develop manufacturing scale Sonobioprocessing equipment suitable for use with sensitive enzymic and cellular biocatalysts.