New strategies for assessing virgin olive oil stability under moderate light and temperature conditions using spectroscopic methods

Resumo

The control of virgin olive oil freshness requires new tools for assessing the oil stability that reflect the diverse chemical changes that take place during marketing. The current accelerated methods established for assessing virgin olive oil stability provide information that are not always well correlated with the actual shelf life, since they use conditions that are far from those given in the market. Thus, low intensity of light and mild heating need to be integrated in stability studies to ensure confidence in the olive oil sector when distributing the product. The main objective of this Thesis was to develop a multiparametric strategy based on spectroscopic techniques to assess the stability of virgin olive oil during storage under moderate conditions of light and temperature, emulating those found in supermarkets. Mesh cell-Fourier transform infrared (FTIR) spectroscopy was optimised for this aim, in order to evaluate the stability of the oils under mild conditions and to compare the results with the common methods using high temperature. With the purpose of gaining knowledge about the degradation processes that take place during storage, fresh monocultivar virgin olive oils were stored during 27 months and the spectral properties were studied together with the quality indexes (peroxide value, free fatty acids, K270, K232), the organoleptic characteristics, and the most relevant chemical parameters (phenolic compounds, volatile compounds, α-tocopherol and pigments derived from chlorophyll) during the storage period. The changes of quality category of the stored virgin olive oils were mainly due to the variations of K270, median of the defect, and median of the fruity attribute. Nevertheless, the storage at moderate conditions generated changes in all the chemical and physical-chemical parameters studied, mainly during the first months of the storage experiment. This observation was also verified with fluorescence spectroscopy, which also showed a similar variation of the fluorescent compounds. On the other hand, the variation in the composition of volatile compounds explained the sensory changes detected by the panellists during the storage months. The stored samples were incubated in mesh cells for 576 h under different moderate conditions (dark and mild heating, light, and light combined with mild heating) and they were analysed by FTIR spectroscopy. This strategy allowed to assess the influence of these conditions on the chemical characteristics of virgin olive oil as they aged, considering different chemical species (primary and secondary oxidation products) simultaneously. This technique combined with chemometric tools provided dynamic information about the degradation of the samples and the variations of virgin olive oil stability during storage. Mesh cell-FTIR spectroscopy showed a good ability to classify virgin olive oils of different storage times. A relationship was also found between the spectral differences observed in the samples during storage and the changes in those parameters that led to a downgrading of category.