## 3 Extraction Techniques
### Conventional Methods  
Solid-liquid extraction (SLE) and steam distillation have been applied for extracting bioactive compounds from hops. For this purpose, several organic solvents such as ethanol, methanol, methylene chloride, ethyl acetate, acetone, hexane, and their mixtures with water have been used. The extraction efficiency is greatly affected by the type and polarity of the solvent, the experimental conditions of time and temperature, and the extraction number of cycles [41]. Several phytochemicals have been extracted from hops using conventional methods. For instance, hydrodistillation was applied to extract essential oils from the cone powder, obtaining a yield of 6.3 mL/kg of dry cones [10]. Using gas chromatography coupled to mass spectrometry (GC-MS/MS), the authors identified the presence of 16 compounds, of which myrrene, trans-caryophyllene, and \(\alpha\)-humulene were the three main components of hops essential oil [10]. In another study, [PERSON] et al. [42] evaluated the yield and compound profile of essential oil extracted from whole hop cones via steam distillation using a sequential elution. The results indicated that 83.2% of the oil was extracted during the first hour, in comparison with control performed during 4-h noninterrupted distillation [42]. Furthermore, the profile of eluted compounds was also significantly different, observing that monoterpenes were eluted before sesquiterpennes.  
The effect of four organic solvents with different polarities (ethylene chloride, acetone, ethyl acetate, and methanol) on the recovery of flavonoids from spent hops was studied by [PERSON] et al. [43]. According to their results, the increase of polarity of the extractant improved the efficacy of extraction, obtaining the highest yield with methanol (92.95 g/kg). The authors found that the most abundant extract compound was xanthohumol, and they indicated that the highest efficiency and selectivity to recover this compound from spent hops was ethyl acetate (3.51 g/kg) and the solvent, which led to the lowest extraction yield being methylene chloride (1.33 g/kg) [43]. In hydroalcoholic extracts obtained by maceration from hop cones, two main groups of compounds were identified by reverse phase HPLC-UV: prenylated chalcones and acylphloroglucinol derivatives [10].  
[PERSON] et al. [44] indicated that the dynamic maceration of hops with MeOH-HCOOH (99:1, _v/v_) led to the best result in terms of hop components recovery, specifically prenylflavonoids and bitter acids, in comparison with other methods including heat-reflux extraction, microwave, or ultrasound.  
Although the application of organic solvents is widespread in industrial extraction processes, their employment presents some inconveniences related to health risk and environmental pollution, limiting their use in the recovery of bioactive compounds [27]. Due to this, a new method of solvents generation known as deep eutectic solvents (DESs) has been applied in the last decade as a promising green alternative substitution for conventional organic solvents in the recovery of biomolecules [27]. In this line, [PERSON] et al. [23] tested the ability of a eutectic mixture composed of glycerol and L-alanine to extract polyphenols from hops. In this study, the authors applied a response surface methodology (RSM) based on a Box-Behnken design for the optimization of process parameters (DES concentration in aqueous mixtures (C\({}_{\text{DES}}\): 55-85% _w/w_), liquid-to-solid ratio (LSR: 20-60 mL/g), and speed of stirring (S\({}_{\text{S}}\): 200-800 rpm)). Under the optimized extraction conditions (C\({}_{\text{DES}}\) = 85% (_w/w_), LSR = 59 mL/g and S\({}_{\text{S}}\) = 688 rpm), a theoretically optimal yield of total polyphenols of 118.97 mg gallic acid equivalents (GAE)/g dm was obtained [23].