# Microalgal Proteins and Bioactives for Food, Feed, and Other Applications
## 2 Isolation of Proteins and Functional Peptides from Microalgae
### Protein Extraction  
In general, microalgae cell disruption techniques can be divided into mechanical and non-mechanical techniques. The main advantages and disadvantages of selected techniques suitable for microalgae protein extraction are listed in Table 9. Mechanical methods are suitable for cell disruption; where the bioactive in question is not heat sensitive, a fast process method is required (Table 9). For heat sensitive actives such as proteins and peptides, enzymatic methods are preferred. In addition, enzymatic methods are considered to be more environmentally friendly. Enzymatic methods and the physical methods are both scalable. The yields of proteins obtained following the use of different disruption methods are shown in Table 10. Ultrasound and high-pressure homogenization combined with enzymatic treatment have resulted in protein yields of between 74-90% when applied to microalgae previously.  
Figure 1: Production of bioactive peptides.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{3}{c}{**Mechanical Techniques of Cell Disruption**} \\ \hline
**Technique** & **Advantages** & **Disadvantages** \\ \hline \multirow{4}{*}{Bead mills} & Low dependence on cell wall & \\  & composition [20] & High energy consumption [118] \\  & High efficiency [117] & Difficult/energy consuming \\  & High biomass loading [118] & control of temperature [118, 119] \\  & Easy scale-up [118] & Low selectivity [20] \\  & Short processing time [119] & \\ \hline \multirow{4}{*}{High-pressure homogenization} & Low dependence on cell wall & \\  & composition [20] & \\  & High efficiency [119] & Difficult/energy consuming \\  & Easy scale-up [117] & control of temperature [120] \\  & Simple [117] & Low selectivity [120] \\  & Applicable to highly concentrated & High energy consumption [20] \\  & microalgae pastes [119] & \\ \hline \multirow{4}{*}{Microwave} & High efficiency [118, 119] & Intensive heat production [118] \\  & Short processing time [119] & Formation of free radicals [118] \\  & Easy scale-up [118] & \\ \hline \multirow{4}{*}{Osmotic shock} & Simple [119] & Low efficiency [117] \\  & Low energy consumption [20] & High cost of salt [117, 119] \\  & Easy scale-up [119] & \\ \hline \multirow{4}{*}{Pulsed electric field} & Easy scale-up [118] & \\  & Mild conditions [118] & Medium has to be \\  & Selective extraction of water-soluble & non-conductive [117, 118] \\  & compounds [119] & \\ \hline \multirow{4}{*}{Subcritical water hydrolysis} & \multirow{2}{*}{Possible to scale-up [121]} & \multirow{2}{*}{High capital cost [121]} \\  & & Intensive heat production [118] \\  & & Low efficiency [118] \\  & & Low selectivity \\  & Simple [20] & Formation of free radicals [118] \\  & & High energy consumption \\  & & [117, 119] \\  & & Difficult scale-up [117] \\ \hline \multicolumn{3}{c}{**Non-Mechanical Techniques of Cell Disruption**} \\ \hline
**Technique** & **Advantages** & **Disadvantages** \\ \hline \multirow{4}{*}{Chemical disruption} & \multirow{4}{*}{Low energy consumption [117]} & High dependence on cell wall \\  & & composition [118] \\  & Low energy consumption [117] & Risk of protein \\  & & degradation [119] \\  & & Contamination by solvents [118] \\ \hline \multirow{4}{*}{Enzymatic disruption} & Low energy consumption, & \\  & biological specificity, mild & High cost of enzymes [117, 118] \\  & operational conditions, & Long processing time [118] \\ \cline{1-1}  & low capital investments [118] & Low production capacity [118] \\ \cline{1-1}  & Suitable for thermo-sensitive & Product inhibition [118] \\ \cline{1-1}  & & High efficiency [119] \\ \cline{1-1}  & Environmentally friendly [119] & \\ \hline \hline \end{tabular}
\end{table}
Table 9: Mechanical and non-mechanical techniques of microalgae cell disruption.  
Extraction techniques used for protein extractions of selected microalgae species together with gained protein recovery/yield are listed in Table 10.