## 3 Extraction of Phenolic Compounds
In this method, methanol, ethanol, acetone, water, and ethyl ethanoate in different proportions are commonly utilized for extraction. The selection can be done based on polarity. Due to the hydrophilic nature of these compounds, hydroalcoholic solvent is the most effective for this process. Some previous studies have mentioned the combination of solvents, with acids such as citric acid, tartaric acid, or HCl potentially improving the extraction of phenolic compounds [236; 237]. In traditional procedures, Soxhlet extraction provides better results of extraction in terms of yield, although this technique also presents some demerits such as the degradation of temperature-sensitive compounds as some phenolic acids, tannins, and anthocyanins require a large number of solvents and are time-consuming. Besides, this classical Soxhlet extraction method is a continuous process; the solvent can easily be recycled, and less time and less solvent are used than in macceration and percolation [238]. Moreover, the used extract of the selected algae is constantly being heated at the boiling point of the solvent and it may damage some temperature-sensitive components, which may affect further analysis [239]. [PERSON] et al. [236] reported the use of the Soxhlet method in the extraction of lipophilic compounds. Moreover, heat-assisted extraction can be divided into two steps. The first step is the faster step and the second one is slower. The faster method consists of a compound's transference from the matrix surface to the solvent whereas the slower method consists of diffusion from the matrix to the solvent. Extraction is mainly affected by the type of sample, type of solvent, temperature value, and time. The main disadvantage is that it requires filtration, decantation, or clarification to separate the solid parts when the extraction is done. It also requires a large number of solvents and takes a long time. Hence, these classical methods are not efficient and environmentally friendly due to the high requirements of the organic solvents [240]. With advancements, new techniques have evolved to improve the efficacy and accuracy of extraction. Figure 5: Techniques of extraction and characterization for phenolic compounds. Source: [PERSON] and [PERSON] [232]. A non-conventional technique, Pressurized Liquid Extraction (PLE), also known as extraction with pressurized solvent, includes high pressure (1 to 15 MPa), short processing time, and temperature ranges of about 50 to 200 \({}^{\circ}\)C using a low volume of nontoxic solvent and thus being considered a green technology. [PERSON] et al. [241] observed a highest extraction yield of 37% for 80 \({}^{\circ}\)C and 52% for 160 \({}^{\circ}\)C using diluted ethanol from the brown alga _Laminaria chloroleuca_ (Phaeophyceaeae) at 100 bars. Microwave-assisted extraction is mainly used for the extraction of polyphenols and polysaccharides. This method can be performed in open (at atmospheric pressure) or closed (higher than atmospheric pressure) vessels. In this method, electromagnetic waves cause changes in cell structures. Two mechanisms, ionic conduction, and dipole rotation, transform electromagnetic energy into calorific energy [242; 243]. There are many affecting factors such as the type of extraction, frequency of microwave, solid-to-solvent ratio, temperature, pressure, and time. Besides, the demerit is that high microwave power and elevated temperature may degrade phenolic compounds [244]. [PERSON] et al. [243] showed the merits of ultrasound-assisted extraction (UAE) for the extraction of phenolic compounds from algae including low temperature, short times, and low amounts of solvent. However, the ultrasonication time can increase the temperature, which may affect the stability of the phenolic compounds. Besides, there are some other affecting factors such as frequency, power, time, temperature, and solid: solvent ratio [245]. This method applies ultrasound waves with a frequency between 20 kHz and 100 kHz, which creates bubbles due to pressure differences.