Mechanism of microalgae disintegration by spark discharge treatment for compound extraction

Abstract

Microalgae possess a cell wall with remarkable physical and chemical strength, which has proved to be a bottleneck for extraction techniques. With the application of spark discharges instigated by 100-ns high voltage pulses, a novel method was established that was found to be effective and yet gentle for the extraction of sensitive, especially heat-sensitive, compounds. The spark, which was ignited directly in the submerged algae suspension, is characterised by several physical and chemical processes that might promote a successful disintegration of the cells. Among other characteristics, the conceivably most destructive mechanism, i.e. strong shockwaves, was examined with schlieren diagnostics to evaluate associated pressures. The results were compared with the tensile strength, determined by atomic force microscopy, of Chlorella vulgaris, which was chosen as model organism. A shockwave pressure of 500 MPa in the close vicinity of the discharge by far exceeded the elasticity modulus of the algae with 13.95 MPa, confirming the potential of mechanically breaking the cell wall. At the same time, bulk temperatures could be maintained close to room temperature by adjusting the operating parameters for the spark-application, hence, especially encouraging the extraction of thermally unstable intracellular substances.

Assessment of Phycocyanin Extraction from Cyanidium caldarium by Spark Discharges, Compared to Freeze-Thaw Cycles, Sonication, and Pulsed Electric Fields

“…Spark discharges have been found to be effective against stiff cell walls due to the development of strong shockwaves [ 32 ]. Accordingly, it could be demonstrated that proteins and pigments, e.g., chlorophylls and carotenoids from Chlorella vulgaris are well extractable with this technique.…”

“…Spark discharge treatment of algae suspensions has been shown to be an effective, but gentle tool for the disintegration of microalgae and with regard to pigment and protein extraction [ 30 , 31 , 32 ]. In general, spark discharges generated in aqueous solution provide ultraviolet radiation and shockwaves besides a variety of chemical reactive species, e.g., hydrogen peroxide, hydrogen, oxygen, superoxide or hydroxyl radicals [ 33 ].…”

“…Importantly, the effect of successful cell wall disintegration is mainly based on the generation of shockwaves with a pressure of approx. 500 MPa, which usually exceeds the tensile strength of most if not all microalgae [ 32 ]. With the applied parameters, the bulk temperature did not exceed 27 °C, which are optimal extraction conditions for heat sensitive pigments.…”