# Extraction of Pigments from Microalgae and Cyanobacteria--A Review on Current Methodologies
## 4 Pressurized Systems  
The use of pressure as a cell disruption technique is one of the most promising methods for the extraction of pigments from microalgae and cyanobacteria. High-pressure homogenization (HPH) and pressurized liquid extraction (PLE) have high extraction capacity and are both scalable and able to be applied in large-scale processes. The effects of pressurized extraction processes on the cell are represented in Figure 3.  
Figure 2: Schematic representation of an enzymatic extraction followed by a solvent extraction. HPH forces the suspension (biomass + solvent) through a small orifice, increasing the pressure drastically; the consequent mechanical effects of turbulence, shear stress, and cavitation induce cell disruption [30]. The system must take into account the loading pressure, solvent viscosity, biomass concentration and cell wall composition. However, the increase in the pressure and the shear in the orifice induce a significant increase of the temperature in the system, requiring cooling to prevent pigments degradation [31]. The system is also limited to the high-energy requirement and high cost of implementation and operation [30]. Thus, the pressure required depends on the cell wall composition and varies according to the species. [PERSON] et al. [32] showed that in the microalgae _Nannochloropsis_ sp., _Chlorella_ sp., and _Tetraselmis suecia_, the pressure needed to disrupt 50% of the cells per cycle is ca. 2000, 1000, and 170 bar, respectively. A similar result was found by [PERSON] et al. [33], where the optimal condition for lipids extraction from _Chlorella_ saccharophilia_ was 800 bar with 10 cycles through the system. Moreover, [PERSON] et al. [34] showed that HPH using 1000 bar for 4 cycles in _Nannochloropsis_ sp. biomass has enhanced the bioaccessibility of carotenoids for food products, while decreasing the pigment content due to degradation, in particular violaxanthin. On the other hand, PLE is an alternative to classical thermal solvent extraction. In this method, the suspension (biomass + solvent) is placed in an oven at high temperatures (50 to 200 \({}^{\circ}\)C) and increased pressure (100 to 140 bar). The high pressure prevents the solvent from boiling, as the boiling temperature increases; in the liquid state and with high temperature, the solvent has a high diffusion coefficient, low viscosity, and higher solubility strength. At the same time, the high temperature reduces cell wall stability and increases the solvent flux into the cell. After the extraction time, the liquid is pumped out of the extraction chamber by the addition of compressed nitrogen, and the biomass is retained by a filter, reducing the processing steps. The main limitations of this system are, on the one hand, the use of high temperatures, and on the other, the high costs of the facilities [35]. [PERSON] et al. [36] used PLE for the extraction of carotenoids and chlorophylls from _Chlorella vulgaris_. The optimization was performed through a response surface methodology using temperature and time as factors, and the optimal extraction conditions varied  
Figure 3: Schematic representation of high\(-\)pressure homogenization (HPH) and pressurized liquid extraction (PLE) effects in microalgae and cyanobacteria cells. between the target products: 148 \({}^{\circ}\)C, 35 min for lutein; 117 \({}^{\circ}\)C, 25 min for \(\beta\)-carotene; 173 \({}^{\circ}\)C, 15 min for chlorophyll \(a\); and 170 \({}^{\circ}\)C, 3 min for chlorophyll \(b\). The pressure in this study was kept constant at 100 bar. Although high temperatures were used, the extraction was more effective when compared to maceration, Soxhlet, and ultrasound extraction. Similar findings were reported to \(D\). _salina_, [PERSON] et al. [37] optimized PLE also in terms of temperature and extraction time for the recovery of antioxidant compounds, and in special \(\beta\)-carotene, where the increase in temperature also increased the extraction rate, reaching the optimal extraction at the conditions 160 \({}^{\circ}\)C, 30 min, and 100 bar [37].