Impact of Extraction Technique on the Volatile Oil Contents and Composition of four Ocimum Species; Microwave Assisted Extraction versus Distillation Study

Objectives: The aim of this study is to unravel the variabilities posed by alteration of the extraction technique employed on the contents and composition of essential oils  derived from the same plant species Methods: Volatile oils of four different Ocimum species (Ocimum basilicum L., O. africanum Lour., O. americanum L. and O. minimum L. family Lamiaceae) were individually extracted from their fresh aerial parts using green microwave assisted extraction (MAE) method and conventional hydrodistillation (HD) and steam distillation (SD) methods. Extracted volatile oil samples were further analysed by GC-MS. Results: Qualitatively, distillation of the Ocimum samples resulted in higher yields of volatile oil than MAE (0.16-0.42%, 0.16-0.44% and 0.1-0.25% ml/g fresh weight for HD, SD and MAE, respectively). However, MAE technique was accomplished in a fraction of time (8 minutes) compared to distillation procedures (2 - 4 hours). GC-MS analysis of the Ocimum oils extracted  using MAE method revealed higher enrichment of marker ingredients, viz. β-linalool and eucalyptol, over the distillation methods. Relative percentage of β-linalool in oil of O. basilicum and O. africanum was 76.9 & 72.2% versus 31.2 & 42.9% and 24.7 & 57.2%, whereas that of eucalyptol was 11.1 & 9.4% versus 6.2 & 4.5% and 4.8 & 4.2%,  by MAE, SD and HD, respectively. Estragole, a natural volatile having safety concerns, was detected with appreciable amounts in the oil samples obtained by distillation. MAE extraction resulted in less than third the estragole content in oil of O.basilicum when compared to (HD) and (SD) methods (10.2%, 36.7% and 33.2%, respectively).Conclusions: MAE provides a rapid, power saving and green technique for extraction and preserving the valuable constituents of Ocimum essential oils. (MAE) produced an exceptionally β-linalool and eucalyptol enriched oil of sweet basil, much suitable for commercial and medicinal uses. Estragole contents were much reduced in (MAE) prepared oil samples comparable to distillation methods, a fact that prioritize selecting this technique for preparing Ocimum oils intended for systemic and/or pediatric applications.