**2.1 Chemicals and plant material**

Deuterated chloroform for NMR analysis (99.8 atom %D) was provided by Sigma-Aldrich Chemie (Steinheim, Germany).

Virgin olive oils (963 samples) from seven countries of the Mediterranean basin, namely Italy (661 VOOs), Spain (144 VOOs), Greece (97 VOOs), France (39 VOOs), Turkey (14 VOOs), Cyprus (6 VOOs) and Syria (2 VOOs), were collected directly from the producers (olive oil mills) from most of the main producing regions of these countries during three harvests (2004/05, 2005/06 and 2006/2007). The sample collection was carried out with the collaboration of Dipartimento di Chimica e Technologie Farmaceutiche ed Alimentari - Università degli Studi di Genova (Italia), Laboratorio Arbitral Agroalimentario (Ministry of Agriculture and Fishery, Spain), General Chemical State Laboratory D'xy Athinon (Greece), General State Laboratory (Ministry of Health, Cyprus), Departamento de Química Orgánica - Universidad de Córdoba (Spain), Istituto di Metodologie Chimiche - Laboratorio di Risonanza Magnetica Annalaura Segre – CNR (Italy), Fondazione Edmund Mach - Istituto San Michele all'Adige (Italy), and Eurofins Scientific Analytics (France), in the framework of the EU TRACE project. The true type (virgin or extra virgin) and origin of the olive oils at the national, regional and PDO level were assured. The Italian samples were representative of the olive oil producing areas, which are markedly influenced by pedoclimatic factors from the North to the South of the country.

For the study of VOO stability, about a liter of VOO was divided into aliquots contained in dark glass 40mL-vials completely filled and kept at -20ºC in a freezer. Each month, over a period of more than 3 and half years (samples for the months 20th, 32nd, 38th and 42nd are missing), one vial was taken from the freezer and stored at room temperature (r.t.) in a closed box. A preliminary supposition was made; this considered that the degradation of VOO at -20ºC is not significant and thus the last aliquot taken out of the freezer was time 0. All aliquots were analyzed by 1H-NMR once the last sample was taken from the freezer.
