Ultrasound-Assisted Extracellular Polymeric Substance Removal from the Diatom Navicula sp.: A Route to Functional Polysaccharides and More Efficient Algal Biorefineries

Extracellular polymeric substances (EPS) from microalgae have potential as functional polysaccharides for various food and industrial applications. This study investigates the use of ultrasonication to remove EPS from the diatom Navicula, first as a potentially useful byproduct of lipid and protein production and second in terms of the effect of EPS removal on downstream processing, including slurry rheology, dewatering, and lipid extraction. Ultrasound (20 kHz) was effective at removing EPS; however, cell rupture also resulted, particularly at high yields involving the removal of bound EPS. Increasing the power intensity/density did not affect the cell rupture at a given EPS yield but halved the energy requirement. Importantly, EPS removal improved the rheology and dewaterability of the cell suspensions, by interrupting intercellular connections and reducing the overall hydrodynamic volume of the cells. For a 5 wt. % cell slurry, EPS removal by sonication for 30 s reduced the shear-dependent viscosity by about 10-fold, and increased the solid concentrations following centrifugation by >sevenfold. Subsequent biphasic lipid extraction was also accelerated by the removal of EPS as a barrier between the cells and the solvent. The decreased viscosity, increased concentration, and improved extraction kinetics should enable greatly intensified downstream processing.