## 4 Preparation of Single-Cell Protein (SCP) for Use in Food
### Cell Disruption  
For the extraction of biomass, various mechanical and non-mechanical methods can be used [82]. Each method has its unique advantages and limitations; therefore, the choice of method significantly affects the extraction efficiency, the quality of the final product, and both fixed and variable process costs [83]. Mechanical methods generally are characterized by the ease of scaling up and low operational cost. However, due to a non-selective disruption, they allow the simultaneous release of compounds from the cell wall, which may cause an increase in operation steps. One of the most applied mechanical methods is bead mill (also known as bead beating), in which cells are disrupted by shear forces during the rotatory movement of the cells and the glass, ceramic, or steel beads [82]. During high-pressure homogenization (HPH), a cell suspension is forced to pass through a narrow gap, also known as a homogenizing nozzle or a high-pressure valve, at high pressure. Cell disruption is mainly based on the fluid impingement through the high-pressure valve and the shear forces, and its effectiveness depends on the pressure, temperature, and the number of passes through the valve [82]. In the case of ultrasonication (US), bubble cavitation, which is assumed to be the main mechanism of cell disruption, is generated through the conversion of sonic into mechanical energy, in the form of intense elastic shockwaves [83]. Due to the operational and economical limitations of ultrasonication methods, such as amplitude and energy consumption, bead milling and HPH are widely favored at the industrial scale [83]. Another mechanical method that has been considered for industrial-scale applications is microwave extraction. It is perceived as a green method characterized by a short processing time and a low energy requirement. It involves the heating of polar solvents in contact with solid samples. The temperature and pressure increase caused by the heating contributes to the release of compounds from the cells. This method has an ability to effectively disrupt the cell wall with a short processing time and low-energy requirements [84]. Furthermore, [PERSON] et al. [84] noted that microwave treatment combined with three-phase partitioning (TPP) can enhance the protein recovery yield from microalgae by more than 2.5 times in comparison to the TPP process alone. Figure 3: The process of obtaining single-cell protein. The non-mechanical methods can be divided into four distinct groups: physical, electrical, chemical, and enzymatic methods. They are usually more selective, less severe towards the cells, and less energy-intensive in relation to mechanical strategies [82]. Freeze-thawing involves subjecting the microbial cells to slow freezing, during which large ice crystals inside the cell are formed, and this leads to its disruption, while in osmotic shock, the cell disruption occurs by submitting the cells at high osmotic pressure. Both methods are applicable only at the laboratory scale due to the high operating costs caused by considerable time and high energy consumption (freeze-thawing), or large volumes of water and a cooling system (osmotic shock). In turn, thermolysis, which consists of thermal decomposition, in which the reagent decays into at least two new substances, is easily scalable but the thermal degradation of selected bioactive compounds should be considered [85]. An interesting research area is the use of electrical methods that are based on the breakdown or disruption of the cell by applying electric fields. Both pulsed electric field (PEF) and high-voltage electrical discharge (HVED) techniques are considered low-temperature and energy-efficient, and thus environmentally friendly. PEF uses high-voltage pulses through an aqueous environment between two electrodes.