Review of Mechanical and Physicochemical Pretreatments in Essential Oil Extraction: Mass-Transfer Enhancement and Yield Optimization

Essential oils are volatile bioactive compounds widely used in pharmaceuticals, food preservation, cosmetics, and aromatherapy. Conventional hydrodistillation and steam distillation remain the primary recovery methods but suffer from long extraction times, moderate yields, and thermal degradation. Mechanical and physicochemical pretreatments address these limits by disrupting secretory structures, shortening diffusion paths, and enhancing mass transfer. From a chemical engineering perspective, this review synthesizes evidence published between 2010 and 2025 on particle-size reduction, ultrasound-assisted hydrodistillation, microwave-assisted hydrodistillation, steam explosion, instant controlled pressure drop, and cold pressing of citrus peels. Outcomes vary by matrix: in seeds such as celery, ultrasound-assisted hydrodistillation increases yield by nearly 50% compared with conventional hydrodistillation; in citrus peels, steam explosion accelerates extraction up to eightfold but reduces composition to limonene, while cold pressing preserves thermolabile aldehydes and esters crucial for fragrance. Instant controlled pressure drop applied to hyssop and Tagetes enhances yield, accelerates kinetics, and improves antioxidant indices through microstructural expansion confirmed by microscopy. In leaves and flowers including rosemary and lavandin, ultrasound- and microwave-based methods consistently shorten cycles while maintaining comparable chemical and sensory profiles. The addition of low-cost modifiers such as sodium chloride and optimized water-to-solid ratios further improves rosemary hydrodistillation without compromising oil quality. These findings highlight trade-offs among rate, yield, and composition. Standardized reporting of particle size, moisture content, and kinetic parameters is recommended to ensure reproducibility and cross-study comparison. Mechanical pretreatments thus provide a flexible framework to optimize essential oil extraction across industrial and bioengineering applications.