Using a Plackett–Burman design to maximise yield of rosemary essential oil by distillation

Several factors influence the yield of rosemary essential oil obtained by steam distillation (ST) and hydro distillation (HY). This study proposes to identify and analyse them, in order to maximise essential oil yield and reducing consumptions. First, a Plackett–Burman design (PBD) was applied to test the simultaneous effect of several factors involved in the extraction process, and understand their relative influence on yield. The following factors were tested: extraction method (HY and ST), extraction time (30 min and 120 min), cooling water flow rate (1 L/m and 5 L/m), heating power (600 W and 2000 W), ratio of rosemary leaves to deionised water (solid/ liquid ratio 1:2 and 1:6). Two leaf pre-treatments were also considered: grinding and blast freezing. The PBD results identified two significant factors (extraction time and extraction method) that were subsequently validated in a full factorial experiment. Two extraction methods and four extraction times were tested. Analysis of variance results showed that maximum yield was obtained by ST with an extraction time of 120 min. Finally, chemical and physical analyses were conducted to evaluate qualitative aspects of the obtained essential oils.