3.1. Effect of extraction conditions on yield
The solvent type had a significant (P < 0.05) effect on the extraction yield of oleoresins (Fig. 1). Overall, ILs increased (P < 0.05 ) the extraction yield (25.11 ± 0.84 wt% for IL1 as compared to 18.37 ± 1.34 wt% for ethanol) via traditional extraction (i.e., without ball milling) (Fig.1). The higher extraction efficiencies with the imidazolium based ILs were as expected due to the combined extractive power of the composite systems. In a similar way, other researchers showed that imidazolium based ILs can increase the extraction yield of bioactive phytochemicals, such as flavonoids, alkaloids, caffein, etc., from plant tissue (Bogdanov & Svinyarov, 2013; Claudio et. al, 2013). The increase in the extraction efficiencies can be explained by the combination of dipole, ionic, and hydrophobic interactions of the IL components. In parallel to the extraction efficiency, lipid, protein, and phenolic compositions were also significantly (P < 0.05) affected by the extraction conditions as discussed in the following sections (Table 1). The efficiency of traditional solvent extraction with ethanol and dichloromethane was the same (P > 0.05); however, the compositions of their oleoresin extracts were different. There was no significant difference (P > 0.05 ) in the extraction yield of two ILs as well (Fig. 1). Thus, the difference between the type of charged groups in ILs was not responsible for breaking the solid enclosure required for extraction. In a recent study, Cheng et. al (2020) used different ILs with 11 anions and 8 imidazolium-based cations including the ILs in our study for extraction of isoflavone compounds from Puerariae lobatae. They showed that the extraction efficiencies of the ethyl and hexyl substituted imidazolium ILs were the same, yet changed with ion concentration in their solution. Therefore, we can conclude that imidazolium ILs regardless of the cation and anion composition were more effective than traditional solvents due to their ability to disrupt cellular structure that determine extraction capacity. On the other hand, the chemical difference between the two ILs affected the oleoresin composition (Table 1) as discussed later.
The wet milling process with a ball mill significantly (P < 0.05) increased the extraction yield as compared to traditional solvent extraction. For example, the extraction yield increased more than 50% when IL2 was used with ball mill (31.36 ± 1.01 wt%) as compared to traditional batch extraction (18.92 ± 0.68 wt%) (Fig. 1). This can be explained by the ability of ball mill to break down the integrity of the hard enclosure trapping the oleoresins via omnidirectional impact of the beads and associated high shear.