DES/Extraction and Process Optimization of Callistemon citrinus (Curtis) Skeels Essential Oil

The essential oil of Callistemon citrinus (Curtis) Skeels is a valuable natural product with a complex volatile composition. In this study, an ultrasonic-assisted hydrodistillation process was systematically optimized using a combination of the steepest ascent method and response surface methodology (RSM) to refine key extraction parameters and identify the central point of the experimental design. Heating time, heating power, and ultrasonic treatment time were selected as critical variables influencing extraction efficiency. The optimized conditions were determined to be a heating time of 57 min, a heating power of 563 W, and an ultrasonic treatment time of 23 min, under which an essential oil yield of 0.8 mL/kg (dry weight basis) was achieved. Gas chromatography–mass spectrometry (GC–MS) analysis indicated that pretreatment with different metal ions resulted in pronounced differences in the chemical profiles of the extracted essential oils, particularly in major constituents such as cineole and α-terpineol. Among the tested ions, Sr2+ and Ca2+ pretreatments were associated with a higher relative abundance of monoterpenoid compounds, which suggests a selective influence of divalent metal ions on the extraction behavior of volatile components. Principal component analysis (PCA) further revealed clear discrimination among the essential oils obtained under different ion pretreatment conditions, confirming the compositional variability induced by ion-assisted extraction.