Seasonal variation in yield and composition of Thymus zygis L. subsp. sylvestris essential oil M. Molda Ä o-Martins, 1 M. G. Bernardo-Gil, 2 * M. L. Beira Äo da Costa 1 and M. Rouzet 3 1 Centre for Microbiological and Agriculture Industries, DAIAT, ISA, Tapada da Ajuda, 1399 Lisboa Codex, Portugal 2 Centre for Biological and Chemical Engineering Ð DEQ, IST, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal 3 Faculte  de Pharmacie, Universite  de Nantes, 1, Rue Gaston Veil, F-44000 Nantes, France Received 12 May 1998 Revised 30 July 1998 Accepted 19 October 1998 ABSTRACT: 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 modi®ed Clevenger apparatus. Analysis was performed by GC and GC±MS. The yield in essential oil showed a maximum at the ¯owering stage (0.9±1.4%) and a minimum during the dormancy period (about 0.15%). The composition also showed dierent patterns at dierent phases of the vegetative cycle. At the ¯owering period the essential oil is rich in thymol and geraniol. p-Cymene showed a variation presenting a maximum at the minimum of thymol (post-¯owering period). Aiming the use of the essential oil as a food ingredient, the most interesting stage is the post-¯owering 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. KEY WORDS: Thymus zygis L.; seasonal variation; essential oil; thymol Introduction Thymus zygis Loe¯. ex L. (Lamiaceae) is found in the countries surrounding the Mediterranean Sea 1 and is widespread over Portugal and Spain. 2,3 This taxon has three subspecies. 1,4 In Portugal two subspecies are found which dier in certain botanical characteristics, chromosome number and ecology: Thymus zygis Loe¯. ex L. subsp. zygis and Thymus zygis Loe¯. ex L. subsp. sylvestris (Homanns. & Link) Brot. ex Coutinho. 3,4 In Spain, instead of subsp. zygis, it is possible to ®nd subsp. gracilis (Boiss.) R. Morales. 5 Thymus zygis essential oil could be interesting for food ¯avouring 6 and in the pharmaceutical, 7±9 cosmetic and perfumery industries. 10 Thymus zygis essential oil shows antimicrobial activity. 10±12 It is a well-known phenomenon over several species that the yield and composition of the volatile oil shows dierent behaviour both quantitatively and qualita- tively at dierent phases of the vegetative cycle. This has been shown, for instance, for Dracocephalum moldavica, 13 Thymus capitatus, 14 Artemisia judaica 15 and Thymus vulgaris. 16 In those papers, higher yields are reported in the ¯owering or post-¯owering period. In Thymus capitatus, 14 carvacrol, the main compound, is present at higher levels before ¯owering until the post-¯owering period. Some other compounds, e.g. p-cymene and g-terpinene, also showed seasonal varia- tions. p-Cymene shows a minimum level before ¯ower- ing and a maximum after the ¯owering period. On the other hand, g-terpinene showed the opposite variation. There is little data about Thymus zygis but it is reported that the content of hydrocarbons decreases with an increase in the size of the leaves, the content of oxygenated hydrocarbons showing the opposite variation. 17 Infraspeci®c variability is another important phenomenon to be considered in Thymus essential oil composition. This phenomenon has been studied by several authors and recently reviewed. 18 Seven dierent chemotypes of Thymus zygis were reported to occur in Spain. 19 Other authors demonstrated the presence of eight chemotypes in Portuguese Thymus zygis subsp. sylvestris (linalol, thymol, carvacrol, geraniol/geranyl acetate, 1,8-cineole/linalol, 1,8-cineole/thymol, linalol/ thymol and 1,8-cineole/linalol/thymol 20 ). In the TraÂs-os-Montes region of Portugal, four chemotypes of Thymus zygis L. were identi®ed with thymol, carvacrol, thymol/carvacrol and geranyl acetate/gera- niol as principal components. 21 Variability was found, re¯ecting environmental conditions. 5 As the essential oil is eventually used as food ¯avouring, its acceptability is important. Panel test FLAVOUR AND FRAGRANCE JOURNAL Flavour Fragr. J., 14, 177±182 (1999) * Correspondence to: M. G. Bernardo-Gil, Centre for Biological and Chemical Engineering Ð DEQ, IST, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal. Copyright # 1999 John Wiley & Sons, Ltd. CCC 0882±5734/99/030177±06$17.50