Seasonal variation in yield and composition of Thymus zygis L. subsp. sylvestris essential oil

The seasonal variation in the yield and composition of the essential oil from collective samples of Thymus zygis L. collected in North Portugal is reported. Essential oil was obtained by steam distillation and by distillation in a modified Clevenger apparatus. Analysis was performed by GC and GC–MS. The yield in essential oil showed a maximum at the flowering stage (0.9–1.4%) and a minimum during the dormancy period (about 0.15%). The composition also showed different patterns at different phases of the vegetative cycle. At the flowering period the essential oil is rich in thymol and geraniol. p-Cymene showed a variation presenting a maximum at the minimum of thymol (post-flowering period). Aiming the use of the essential oil as a food ingredient, the most interesting stage is the post-flowering period, the essential oil at this time being rich in thymol (about 21%), geranyl acetate (about 17%) and geraniol (about 13%), with p-cymene presenting lower levels. Copyright © 1999 John Wiley & Sons, Ltd.