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University of Ioannina (2015)

Changes in olive oil composition during storage and thermal treatments

Kotsiou, Kali

Titre : Changes in olive oil composition during storage and thermal treatments

Μεταβολές των συστατικών του ελαιολάδου κατά την αποθήκευση και το μαγείρεμα

Auteur : Kotsiou, Kali

Université de soutenance : University of Ioannina

Grade : Doctor of Philosophy (PhD) 2015


In the present study, the changes in the phenolic fraction of virgin olive oil (VOO) during domestic thermal treatments, were studied. Moreover, results obtained during VOO storage, focusing on volatile and phenolic compounds as well as organoleptic attributes, were evaluated.VOO samples were subjected to thermal treatments, simulating common domestic processing, including boiling and frying. These processes can affect the phenolic compounds content of VOO to a certain degree, depending on each treatment. Thermal oxidation of oils at 180 °C (frying) caused a significant decrease (p<0.05) in hydroxytyrosol derivatives (reduction of 60% after 30 min and 90% after 60 min) and, to a lower degree, in tyrosol derivatives and lignans. On the other hand, thermal oxidation of oils at 100 °C (boiling) for 2 hours caused a decrease of less than 20% in all classes of phenolic compounds. During thermal treatment, the evolution of oxidized phenolic compounds was observed. The structures of these oxidized products were characterized by HPLC-MS.Furthermore, VOO samples, of five Greek olive varieties, were stored in dark glass bottles (headspace 0.5%) in a basement without central heating. Even after 24 months of storage, the basic quality characteristics of the samples, such as free acidity, K232, K270 and peroxide values did not exceed the upper limits set by European Community Regulation 2568/91 for VOO.Changes in the volatile composition of VOO were monitored during 18 months of storage. Regarding the Lipoxygenase (LOX) pathway products, responsible for the positive sensory notes, it was shown that their total concentration remained stable during the entire storage period. Even though the physicochemical characteristics did not exceed the upper limits, high rates of formation of volatile oxidation products, which are responsible for the off-flavours, were observed. Among the volatile oxidation products, the highest rates of formation were recorded for heptanal, (Ε)-2-heptenal and pentanal followed by hexanal and nonanal. The formation rate of volatile oxidation products was higher in VOO from Lianolia and Asprolia compared to VOO from Koroneiki, Native from Zakynthos and Thiaki varieties. Finally, stepwise linear regression analysis (SLRA) selected linoleic acid and total phenol/ortho-diphenols ratio as having the maximum correlation with the formation rate of (Ε)-2-heptenal.Quantitative variations of the phenolic compounds and their degradation products were monitored over 24 months of storage using high-performance liquid chromatography (HPLC). The differences observed in the initial total phenolic compounds concentration (ranging between 250-930 mg/kg) confirmed the influence of extraction system, olive variety, and maturity stage. VOO were found to contain higher phenolic concentration when obtained in a two-phase decanter compared to a three-phase. Under the selected storage conditions, even after 24 months, the degree of reduction in total phenolic compounds did not exceed 31%. The reduction was more pronounced in dialdehydic form of oleuropein aglycone (DAFOA) and dialdehydic form of ligstroside aglycone (DAFLA), indicating a more active participation in the hydrolysis and oxidation processes of the more polar secoiridoids. The initial total phenolic content was the main factor correlated to the degradation rate of the phenolic compounds, while initial oxidation status and free acidity where correlated to a much smaller extent. The decrease in secoiridoid derivatives, gave rise to the simple phenols (hydroxytyrosol and tyrosol) content and to the formation of four oxidized products.Finally, changes in the organoleptic attributes of VOO during storage, were evaluated. The selected samples belonged to Lianolia and Koroneiki varieties and were all of extra virgin olive oil (EVOO) quality. As shown by the organoleptic evaluation, some of the samples developed organoleptic defects, thus, by the end of storage, not all of the samples were able to meet the requirements of the “extra virgin olive oil” category. Samples from Lianolia variety were more susceptible to degradation in relation to samples from Koroneiki variety. More specifically, after 12 months of storage 1 out of 6 samples from Lianolia variety developed a defect. After 18 months of storage 2 out of 6 samples from Lianolia variety and 1 out of 6 from Koroneiki variety developed the rancid defect and were classified as virgin olive oils. Moreover, principal component analysis (PCA) was applied in order to classify the samples in virgin and extra virgin olive oil categories according to their volatile profile. PCA extracted two principal components which explained 74.5% of the variance which were highly correlated to the percentage of unsaturated and saturated aldehydes (C7-C10), the percentage of saturated alcohols (C7-C9), the total concentration of the lipoxygenase pathway products as well as the ratio of nonanal/hexanal

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