Compound composition table of Taihang Chrysanthemum hydrosol with different pretreatments and different open states

To investigate the differences in volatile components of pure hydrosol from Taihang Chrysanthemum （Opisthopappus taihangensis） at different stages of flowering，as well as the effects of freeze-drying pretreatment on its aromatic components，this study employed ultrasonic-assisted steam distillation to extract essential oils from fresh flowers and freeze-dried flowers of Taihang Chrysanthemum at the budding stage，initial blooming stage，and full blooming stage. Volatile components were analyzed using headspace two-dimensional gas chromatography coupled with quadrupole tandem time-of-flight high-resolution mass spectrometry. By calculating the relative aroma activity value （ROAV），an OPLS-DA model was used to screen for key differential aromatic components in different Taihang chrysanthemum pure essences （ROAV > 1，VIP > 1，P < 0.05）. The results indicated that a total of 217 compounds were identified in Taihang chrysanthemum pure essences，with 187 compounds in pure essences extracted from fresh Taihang chrysanthemum flowers （THC-F），primarily consisting of alcohols，ethers，and ketones，with the main volatile components being eucalyptol （4.63–17.57 mg/kg） and cis-chrysanthemol （2.04–16.14 mg/kg），exhibiting minty，fruity sweet，and pine resin-like aromas; The pure extract （THC-FD） obtained from Taihang chrysanthemum flowers after freeze-drying contains 90 compounds，primarily esters and ethers. The main volatile components are eucalyptol （0.16–14.90 mg/kg） and (Z)-chrysanthemum alcohol acetate （2.04–7.84 mg/kg），exhibiting scents of mint，lemon，and pine resin. OPLS-DA model analysis （ROAV > 1，VIP > 1，P < 0.05） indicated that Eucalyptol and p-cymene are the key aromatic components distinguishing pure essences from Taihang chrysanthemum flowers in different open states; Eucalyptol，borneol acetate，and citral are the key aromatic components distinguishing freeze-dried Taihang chrysanthemum flower pure extract from fresh flower pure extract. This study indicates that the full bloom period，which is rich in volatile components and has a more intense aroma，is the optimal time for extracting pure essence from Taihang chrysanthemum. Additionally，freeze-drying treatment can produce relatively more ester compounds and lemon aldehyde，which are key aromatic components，providing valuable reference for the extraction of pure essence with unique aromas.