The nonvolatile residue of citrus essential oils greatly influences the olfactory properties of these oils. The residue ranges from approximately 1-10% in the different oils and consists, in large part, of many oxygen heterocyclic compounds, particularly the coumarins, psoralens and polymethoxylated flavones.
The qualitative and quantitative compositions of this residue characterize the different citrus oils, and play an important role in identifying the various oils and controlling their quality and genuineness.
Sometimes it is also possible to find a correlation between the quantitative variations and parameters such as the production period, the kind of fruit or the extraction technology. But data in the literature is very limited; usually the quantitative data is poor and often refers only to a limited number of samples,