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Ravid et al. (1992) determined that the range of fenchone content of sweet fennel oil (Foeniculum vulgare var. dulce) was 2.2–5.2%, whereas in bitter fennel oil (F. vulgare var. vulgare) it was 7.2–20.4%. The authors further determined that the enantiomeric distribution of fenchone in both sweet and bitter fennel seed oil was (+)-fenchone (100%): (-)-fenchone (0%). As a result, fennel oil would make an excellent source of (+)-fenchone.
Gora et al. (1997) reported that Polish fennel seed oil contained fenchone (19.1%) and (E)-anethole (69.4%). This fenchone level indicates that the oil was produced from bitter fennel. The major constituents of bitter fennel oil produced in the laboratory by Bas¸er et al. (1997) from seeds of Uzbekistani origin were found to be: