Oxidative stability of virgin olive oils, produced by organic, integrated or conventional agricultural methods

The relationship between oxidative stability and phenolic content of virgin olive oils (VOOs) produced by organic, integrated or conventional agricultural methods was investigated. The oil samples were obtained from olive orchards located in different Italian regions (Sicily, Umbria and Puglia) during the years 2003 and 2004. The oxidative stability was studied by measuring the primary oxidative products through the peroxide value and the OSI (Oxidative Stability Instrument) time value; the measure of the OSI time value determines indirectly also the secondary oxidative products, extrapolating in hours (h) the end of the first period of oxidation. The phenolic content was determined by a spectrophotometric assay (Total Phenols, TP) and a high-performance liquid chromatographic method, employing a mass spectrometry detector (HPLC-DAD-MSD). That allows identification and quantifications of many single phenolic compound. Belongin to phenyl ethyl alcohol, lignan and secoiridoid classes, are present the main o-diphenolic components, which are molecules provided with the highest antiradical power. To test the scavenging activity of the VOO phenolic extracts the widely known 1,1-diphenyl-2-picrylhydrazyl radical (DPPH_) method was used and the results were expressed as Trolox equivalents (an hydrophilic standard analogue of vitamin E). Moreover the research of some metals was carried out by inductively-coupled plasma optical emission spectroscopy analysis (ICP-OES) with CCD (charge coupled device) detector. In fact, metals, such as copper, are able to catalyse lipid oxidation, causing a deleterious influence for to VOO shelf-life. The difference in terms of trace metals is mainly linked to the use of fertilizers and the olive treatment processes; these practices may differentiate VOOs produced by organic, integrated and conventional agricultural methods. All the data were statistically elaborated to find possible correlations among the oxidative stability of VOOs, the quali-quantitative composition of their phenolic fraction, the presence of metal traces and the agricultural methods applied.