A Metabolomics Approach Based on 1H-NMR Fingerprinting and Multivariate Data Analysis for Virgin Olive Oil Stability Assessment

Abstract

The quality of virgin olive oil (VOO) is related to its oxidative stability, storage, and sensory and nutritional properties. VOO oxidation leads to the formation of off-flavour substances, degradation of its bioactive antioxidants and accumulation of degradation compounds, causing the loss of its sensory and health-promoting qualities, its commercial value and consumer acceptance. VOO resistance to oxidation depends on its chemical composition and its exposure to pro-oxidant factors such as oxygen, light and temperature. VOO stability during storage has been studied by an untargeted metabolomics approach based on 1H-NMR fingerprinting and pattern recognition. In a preliminary study, the stability of a VOO was studied under normal storage conditions, i.e., in dark and at room temperature, over a period of 43 months to evaluate the approach for virgin olive oil stability assessment. Principal component analysis (PCA) of the 1H-NMR fingerprints allowed to visualize the evolution of VOO with time. In a further study, shelf life conditions during commercialization of VOOs were simulated, and a representative set of VOOs covering the full range of possible chemical compositions were exposed to these conditions: light (500 lux for 12h/day) at 25ºC for 12 months or stored in the dark at 25 ºC, 30ºC and 35ºC for 24 months. Supervised multivariate data analysis of the 1H-NMR spectra of VOOs provided classification models to evaluate VOO freshness and verify the light exposure of the VOO during storage, and regression models to determine VOO storage time and tentatively the best before date of a fresh VOO. The different pattern recognition techniques disclosed VOO compositional changes due to hydrolytic and oxidative degradation taking place during storage, and confirmed that light and increasing temperature enhance these processes. The presence of characteristic resonances of hydroperoxides (primary oxidation products) and the decrease of 1H signals assigned to phenolic compounds, fatty acids, squalene and native (E)-2-hexenal present in fresh VOO revealed its oxidative degradation. The emergence of low intensity 1H signals of saturated aldehydes with increasing temperature in darkness, as well as in light exposure, meant that the secondary oxidation process has started at a low rate and yield in the stored oils. The decrease of the 1H signals of triacylglycerides and sn-1,2-diacylglycerides, and increase of sn-1,3-diacylglycerides indicated that VOO hydrolytic degradation and diacylglyceride isomerisation was occurring. 1H-NMR fingerprinting of VOO together with pattern recognition techniques afford relevant information to assess VOO quality regarding legal, sensory and health-promoting aspects.