## 4 Extraction of _Citrus_ Seed's Phytochemicals  
### Traditional Extraction Methods  
#### 4.1.1 Solvent Extraction  
The solvent extraction technique is an extraction technique that uses solvents, high temperatures, and long extraction times to obtain compounds. Thus, solvents are chosen considering the compounds of interest to be extracted, evaluating chemical properties such as polarity, affinity, and selectivity. For the _Citrus_ seed compound extraction, the incubation of dried ground _Citrus_ seeds in a solvent at a warm temperature is needed [1]. Considering the oil nature of _Citrus_ seeds, non-polar solvents are commonly used, such as n-hexane, petroleum ether, and ethyl ether. However, methanol is also applied to obtain free phenolics such as flavones and glycosylated flavones. Moreover, alkaline hydrolysis is suitable for the bound phenolic recovery of phenolic acids and flavonoids. For the total polyphenols and flavonoids extraction, ethanol was applied in grapefruit seeds and trifoliate orange (_C. trifoliate_), showing suitability for the limonoids recovery (limonin and nomilin). Furthermore, t-butyl methyl ether was also applied for the limonoid extraction from bitter orange [1]. By applying this methodology, a high oil yield and fatty acid extraction is achieved. Nevertheless, the phenolic extraction is limited [1]. Disadvantages of this technique include loss of volatile compounds, residues of toxic substances, and oxidation of fatty acids induced by high temperatures [23].  
A study performed by [PERSON] et al. run a trial comparing the % yield of fatty acids, ocopherols, and sterols obtained with both solvent extraction and cold-press extraction applied in lemon seeds [63]. Results showed no significant differences between solvent and cold-press extraction techniques on fatty acids. Moreover, slightly differences were achieved regarding sterol content. Nevertheless, the alpha-tocopherol content was significantly higher when cold-press extraction was applied. Both characteristics and % of yield of two oil _Citrus_ species seeds were compared applying conventional solvent extraction and supercritical fluid extraction using carbon dioxide (SFE-CO\({}_{2}\)) [63]. Results in Table 2 showed significant differences between both extraction methodologies, being solvent extraction with hexane 95% the one with higher yield obtained. Nevertheless, the antioxidant activity of the oils extracted (measured by 2,2\({}^{\prime}\)-azino-bis(3-ethylbenzothazoline-6-sulfonic acid) (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) assays) was slightly higher when SFE-CO\({}_{2}\) was applied in both operational conditions (200 bars, 45 \({}^{\circ}\)C = 36.82%; and 250 bars 60 \({}^{\circ}\)C = 36.25%) than when hexane was used as extraction solvent (32.82%).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Matrix** & **Operational** & **Compounds** & **\%Yield** & **Activity** & **Ref.** \\ \hline \hline \multicolumn{6}{c}{_Solvent extraction_} \\ \hline _Citrus limon_ & n-hexane, 3 h, 45 \({}^{\circ}\)C, 3 times & Fatty acids, sterols, & 71.3 & - & [63] \\ \hline _Citrus limon_ & n-hexane, 8 h, 40 \({}^{\circ}\)C & Fatty acids & 66.46 & - & [64] \\ \cline{3-6} \cline{5