Thermal-Induced Alterations in Phenolic and Volatile Profiles of Monovarietal Extra Virgin Olive Oils

Abstract

In the present study, the influence of heating on the evolution of oxidative indices, antioxidant activity, phenolic and volatile compounds in monovarietal extra virgin olive oils (EVOOs) obtained from Leccino, Istarska bjelica, and Buža cultivars was investigated. The samples were submitted to heating in an air oven (180 °C and 220 °C), simulating usual roasting conditions typical for Mediterranean cuisine. The decreases in the oxidative indicators, phenolic and volatile compounds were more pronounced at higher heating temperatures, underlining the temperature dependency of the oxidative degradation during heating conditions. Despite this, it must be emphasized that a significant amount of phenolic compounds and antioxidative activity remained preserved after the heating treatment. Each oil cultivar showed some specificity during the course of the thermal degradation. Hydroxytyrosol acetate among phenolic compounds and octanal, (E)-2-octenal, hexanal, 3-pentanone, and 1-penten-3-one among the volatiles were underlined as possible markers of thermal oxidation. Principal component analysis revealed that the content of volatile compounds in monovarietal EVOO samples distinguished samples primarily by the heating temperature, while the changes in the phenolic compounds were cultivar-dependent aside from being influenced by the temperature of heating.

Keywords: PCA; monovarietal extra virgin olive oil; oxidative stability; phenolic compounds; thermal treatment; volatile compounds.

Figure 1

Groups of phenolic compounds in monovarietal olive oil samples unheated and subjected to 1 h of heating at 180 and 220 °C. Data are expressed as means ± standard deviation (n = 3), calculated as a sum of individual phenolic compounds: (a) simple phenols, phenolic acids, flavonoids, lignans, and (b) secoiridoids, and total identified phenolic compounds. Columns labeled with a different letter within the same group of phenols and single monovarietal olive oil (Leccino—L, Istarska bjelica—IB and Buža—B) at different temperature of heating are statistically different (Tukey’s test, p ˂ 0.05).

Figure 2

Radical scavenging activity in monovarietal olive oil samples unheated and subjected to 1 h of heating at 180 and 220 °C. Data are expressed as means ± standard deviation (n = 3). Columns labeled with a different letter within the single oil cultivar (Leccino—L, Istarska bjelica—IB and Buža—B) at different heating treatments are statistically different (Tukey’s test, p ˂ 0.05).

Figure 3

Separation of olive oil samples according to olive variety along principal components PC1 and PC2: (a) Loading plot of the selected variables (PV—peroxide value; K₂₃₂, K₂₆₈—spectrophotometric indices; fatty acids C18:2 and C18:3; and phenolic compounds) of the first two factors (PC1 and PC2). (b) Score plot projecting the monovarietal virgin olive oils Leccino (□), Istarska bjelica (ᴑ) and Buža (◊); unheated (green) and heated in an air oven for 1 h at 180 °C (blue) and 220 °C (red).

Figure 4

Separation of olive oil samples according to heating treatment along principal components PC1 and PC2: (a) Factor loadings of selected variables (PV–peroxide value; K₂₃₂, K₂₆₈ –spectrophotometric indices; fatty acids C18:2 and C18:3; and volatile compounds) on the first two principal components (PC1 and PC2). (b) Score plot projecting the monovarietal virgin olive oils of Leccino (□), Istarska bjelica (ᴑ) and Buža (◊); unheated (green) and heated in an air oven for 1 h at 180 °C (blue) and 220 °C (red).