Unlocking the Aromatic Profile of Wild-Grown Croatian Fennel: A Comparative Study of Essential Oils and Hydrolates

Foeniculum vulgare Mill. (fennel) is an edible and medicinal plant cultivated worldwide. Owing to its distinctive aroma and diverse biological activities, its essential oils (EOs) have been widely investigated. However, available data predominantly focus on cultivated fennel or commercial EOs, while comprehensive investigations of wild-growing Mediterranean populations—particularly comparisons among different plant parts—remain scarce. In this study, EOs obtained by hydrodistillation from stems, leaves, flowers, and fruits of native Croatian fennel, were chemically characterised using gas chromatography-mass spectrometry (GC-MS), while antimicrobial activity was evaluated using the disc diffusion method against four bacterial strains. Additionally, the volatile profiles of fennel hydrolates were determined by headspace solid-phase microextraction (HS-SPME) and GC-MS analysis. Fennel flowers gave the highest EO yield (1.95%), followed by mature fruits (1.43%), whereas significantly lower yields were obtained from leaves (0.69%) and stems (0.58%). Trans-anethole was identified as the dominant constituent (from 40.96% in stems to 80.71% in fruits), while α-phellandrene predominated in stem EO (42.77%). Hydrolate volatile profiles were more complex—particularly leaf hydrolate, where 29 compounds were identified. The principal constituents were trans-anethole (39.58–57.40%) and fenchone (16.01–28.80%), while the highest content of estragole was observed in fruit hydrolate (6.56%). The EOs demonstrated moderate antimicrobial activity, showing effectiveness exclusively against Escherichia coli, likely attributable to high phenylpropanoid (primarily trans-anethole) and fenchone contents.