An Investigation to Determine the Difference in Composition of Monoterpene Constituents Obtained from Cold-Pressed and Steam-Distilled Lime Oils

This investigation has been carried out to determine the differences in composition of the monoterpene constituents obtained from cold-pressed (expressed or écuelled) and steam distilled lime oils. Gas chromatographic techniques were used extensively, and some spectroscopic techniques (infra-red and nuclear magnetic resonance) were also used in the
investigation.
The monoterpene constituents of a number of commercially prepared expressed and distilled lime oils was determined. The differences found between the two types of lime oil were further investigated. A laboratory experiment was devised to simulate the commercial steam distillation process. The procedure involved heating a sample of expressed lime oil, or individual monoterpene, under reflux for a period of time followed by distillation for fifteen minutes. The oil was heated in the presence of citric acid and Kieselguhr.
Gas-chromatographic and spectroscopic (infra-red) analyses showed that this laboratory procedure was valid in producing a distilled oil of comparable composition to that of commercially prepared distilled oils.
Isomerisation, mainly involving protonation and migration of olefinic bonds of the monoterpenes and a number of reactions were shown to occur readily during the distillation of expressed lime oil at low pH's resulting in the production of a distilled lime oil. Three main pathways of reactions were shown to be involved in the changes occuring when expressed lime oil was distilled.
Citral was readily destroyed and p-cymene produced in the distillation. The menthadienes were found to change readily under distillation conditions to produce a number of different products e.g.1;8-cineole,<— and & ~terpinene,.
Acyclic monoterpenes e.g. Linalool and bicyclic monoterpenes eg p-pinene were found to produce menthadienes which fed into the menthadiene pathway thus regenerating compounds as they were used up in reactions or isomerisations, e.g. limonene. Limonene, found to be the main monoterpene of both expressed and distilled oils, played a prominent role in the changes occuring during distillation in being changed into other compounds and also by being regenerated by other reactions.
Thus, the organoleptic changes produced when expressed lime oil was distilled to produce distilled lime oil, with its characteristic lime flavour, were at least found to be the result of, at least three complex pathways involving many changes in monoterpene, constituents.