## 3 Extraction Techniques
### Emerging Extraction Technologies
#### 3.2.1 Ultrasound-Assisted Extraction (UAE)  
Ultrasound has been identified for its potential use in the phytopharmaceutical extraction industry [45]. The UAE system is an uncomplicated, economical, and efficient alternative to the extraction methods used traditionally that can improve extraction yield, extraction rates, or the recovery of heat-sensitive compounds [46]. This kind of technique uses high-frequency waves that promote the formation of bubbles in the medium, leading to the formation of the cavitation phenomenon [47]. The implosion of these bubbles disrupts the cellular structure and facilitates the diffusion of the solvent into the cellular plant tissue, which increases mass transfer and consequently improves the extraction [47]. The efficiency of the UAE is influenced by various operating parameters such as the type and concentration of the solvent, sonication time, temperature, LSR, ultrasonic power, and frequency, which must be optimized to reach high extraction yields [48]. To give an example, [PERSON] et al. [49] applied a Central Composite Rotational Design (CCRD) to optimize the recovery of phytochemicals with antioxidant capacity from Brazilian hops. The authors evaluated the effect of three extraction parameters: temperature (33-67 \({}^{\circ}\)C), EtOH concentration (43-77%), and LSR (17-33 mL/g) on the total -phenolic content (TPC). According to the results of regression analysis, the LSR was the parameter that showed a greater influence on TPC of the extracts, while the temperature and the concentration of ethanol had a lesser effect. Under optimized extraction conditions (52 \({}^{\circ}\)C, 49% ethylic alcohol and LSR of 34 mL/g), a TPC of 33.93 mg GAE/g for Brazilian hops was obtained. Moreover, the authors compared Brazilian hop extracts with those obtained with the same variety of hops grown in the USA. The results showed that the Brazilian hops showed a higher content of phenols and flavonoids and better antioxidant potential analyzed by the ABTS (2,2'-azino-di(3-ethylbenzothazoline-6-suslfonic acid) and DPPH (\(\alpha\),\(\alpha\)-Diphenyl-\(\beta\)-picrylhydrazyl) tests compared to the USA hops.  
In another study, the UAE has been applied with success for the extraction of diverse types of biocompounds from hops. For example, [PERSON] et al. [3] used this technique with different extractants (methyl, ethyl, and isopropyl alcohol) under three different concentrations to assess the total phenolic content and the antioxidant activity. This research was carried out using fresh hop leaves from different harvested years (2017 and 2018). Process parameters were fixed at 40 kHz during different times (15, 30, and 60 min) at room temperature and under different solvent concentrations (40%, 70% and 96-99.5% (_v/v_)). The authors reported that the extracts (from young hop leaves harvested at the starting vegetation) presented a high antioxidant activity, and this antioxidant potential was influenced by different factors (including the solvent type or extraction time) and concluded that the most effective process was using undiluted methanol during one hour of ultrasound-assisted extraction. The antioxidants content depends on the harvesting time and can be also influenced by climatic conditions, among other environmental elements [3].