## 7 Cellular Disruption  
The successful extraction of multiple products in a biorefinery will usually rely upon cellular disruption to begin the isolation/fractionation of components of interest. To this end, cell disruption is often a vital step and can be broadly classified in four different approaches: mechanical, physical, biochemical, and biological. It should be stressed that these four approaches are not mutually exclusive, with some downstream processes combining different disruption methods to achieve the best result.  
Mechanical approaches include bead milling and high-pressure homogenization. These are low temperature and neutral pH methods with mild conditions tending to preserve protein bioactivities [81]. The processes, however, tend to be energy expensive and create a dispersion of cell debris that can complicate the purification of the different fractions [82].  
Physical cell disruption approaches include pulsed electric field (PEF) or ultrasound treatment (UT) as well as microwave treatment. PEF is a low shear and low temperature method; it generally has low efficiency on microalgae because of the common presence of a complex, often rigid or even elastic cell wall that protects them from lysis [83]. UT confers high shear stress and can be performed at low operating temperatures, but the cost to control the temperature of the biomass during processing is significant. This disruption method can prove problematic for applications within a biorefinery approach due to degradation of the water soluble components [84].  
Enzyme digestion and alkali/acid treatments represent alternate biochemical approaches to break microalgal cells. Alkali/acid treatments are disruptive methods that, unfortunately, can degrade valuable components. Enzyme digestion is helpful for cell wall weakening and/or degradation, but is generally an expensive approach especially when applied at industrial scale. For a full digestion, proteases can be included to improve efficacy but, if used, protein components can also become degraded in this process. A mix of different degradation enzymes (sulfatase, chitinase, and lysozyme) can increase the permeability of some microalgal cell walls [85] and if coupled with PEF, the approach can be effective [83].  
Biological disruption can be achieved for certain microalgal strains for which a virus has been described. The _Chlorella variabilis_ strain NC64A that has been evaluated for production of high value molecules and biofuels is infected naturally by members of the _Paramecium bursaria Chlorella Virus I_ (PBCV-1) family. Post-lysis, the lipid composition does not seem to be significantly different and the extraction yield is similar to the one achieved with ultrasonic disruption [86]; more studies are needed in terms of the protein composition of _Chlorella variabilis_ biomass before and after infection and lysis. Furthermore, after infection with some virus strains, the cells start producing hyaluron (HA), that could potentially increase the economics of the overall process if the yield of HA was optimised. An interesting aspect of utilising viruses to trigger cellular lysis is that the lytic process itself creates more viruses, which can be utilised for the lysis of the next batch, and so on, aiding process economics by self-generating the lytic agent, albeit potentially at the cost of reducing yields of the current harvest and adding a biological control challenge with regard to managing the production site operationally.