Solvent-Dependent Selective Retention of Essential-Oil Markers and Modification of the Thymol-Type Chemical Fingerprint in Post-Extraction Thymus vulgaris L. Biomass

This study evaluated whether short-term extraction of Thymus vulgaris L. with solvents of different polarity selectively modifies the essential-oil (EO) fingerprint of post-extraction biomass. The initial material contained 1.85% v/w EO, dominated by thymol (53.65%), p-cymene (21.53%), and γ-terpinene (5.28%). Water, 65% glycerol–water, rapeseed oil, and 50% ethanol–water were compared under different time and temperature conditions. Selective retention of six markers was assessed using an apparent marker retention index (AMRI), combining residual EO content with relative GC-FID composition, together with SEF, thymol-profile ratios, and CLR-PCA. Under solvent-specific reference conditions, thymol AMRI was 59.2% for water, 39.3% for glycerol, 18.4% for oil, and 27.2% for ethanol. The strongest relative enrichment occurred for (E)-caryophyllene after glycerol extraction (AMRI 149.8%; SEF 3.26). CLR-PCA explained 90.2% of compositional variability in the first two components and revealed solvent-dependent reorganization of the marker profile. Water and nanowater produced the highest integrated polar-functional response based on TPC, TFC, and DPPH. These results show that extraction can selectively modify the chemical fingerprint of the residual volatile fraction without implying a change in the biologically determined chemotype, supporting targeted fractionation and sequential use of post-extraction thyme biomass.