Quercus infectoria is a plant that has been known to produce gall, where gall is a special product resulting from an interaction between plants and insects known as gallflies or Cynips gallae-tinctoriae. The gall of Quercus infectoria is called Quercus infectoria gall (Q. infectoria gall). Generally, Q infectoria gall is popular in Southeast Asia, East Asia, and South Asia as a medicinal plant, for example, to treat inflammatory pain, stomach pain, diarrhea, postpartum care, etc. (Adi, 2016; Jamal et al., 2011). Apart from Southeast Asia, East Asia, and South Asia, this plant is also found in Turkey, Syria, Persia, Cyprus, and Greece (Barudin et al., 2015; Rahman et al., 2011). Then, further development of the use of Q. infectoria Gall revealed that this plant has anti-bacterial, anti-inflammatory, and strong antioxidant properties (Do et al., 2014; Dash et al., 2016).
Initially, Q. infectoria gall was consumed directly by the villagers with being soaked in hot water and drunk (Koddami et al., 2013). However, in its development, the use of extraction is carried out first to increase the concentration of the active substance and selective compounds (Hafizah et al., 2014; Arina and Harisun, 2019). In general, the extraction used conventional solvent extraction techniques (CSE) and increased the bioactive compound of phenolics with the main compounds of tannic acid (TA), gallic acid (GA), and ellagic acid (EA), along with the flavonoid compound (Askari et al., 2020; Ajilah et al., 2019; Radzali et al., 2014). The weakness of the Q. infectoria Gall extraction process is that the composition of the extract depends on the solvent used, the extraction temperature, and several other factors that also affect it (Nadia et al., 2018; Khemakhem et al., 2017). As a result of the extraction's weakness, bioactive components can degrade due to the use of high temperatures, reducing the efficacy of extract results if used as a drug (Manna et al., 2015). Furthermore, the resulting extract contains the solvent that was used, which can increase the toxicity of the extract and cause environmental problems (Hartanto et al., 2016; Hasim et al., 2015). Environmental issues due to the waste of the solution are not a simple matter because the solvents used are generally organic solvents, which are toxic and can also be carcinogenic (Markom et al., 2013; Hamad et al., 2017; Wang and Weller, 2006).
Green extraction techniques have been developed for a long time, one of which is the supercritical fluid extraction (SCFE) technique (Zi et al., 2016; Sukor et al., 2020); in addition to that, there has also been the development of other extraction techniques (Singh et al., 2016; Kamarudin et al., 2021). In SCFE-CO2, some researchers postulate that SCFE-CO2 offers extraction packages that have high effectiveness (Shi et al., 2022; Yulianti et al., 2014), but since CO2 is non-polar, it does not have excellent results for the extraction of polar compounds. This is due to the low solubility of CO2. Therefore, the extraction efficiency and selectivity of SCFE-CO2 can still be improved by modifying the addition of the corresponding joint solvent. In addition, SCFE-CO2 uses very high pressure, so the combination of high pressure and polar co-solvent accretion is expected to improve the efficiency of a good extract (Wyepkowski et al., 2014; Hasimi et al., 2017; Fatima et al., 2008; Bouhafsoun et al., 2018; Hartanto et al., 2016).