Effects of heating on virgin olive oils and their blends: focus on modifications of phenolic fraction.

The phenolic profiles of two different virgin olive oils and their admixtures in different percentages have been analyzed after heating treatments by microwave or conventional oven. Changes in the phenolic profile upon heating were evaluated by chromatographic and spectroscopic methods, also monitoring the antioxidant activity by ABTS(*+) test. 3,4-DHPEA-EA, p-HPEA-EA, and EA showed the highest decreases after thermal treatments. The only compounds that showed a clear increase with heating, in particular by conventional oven, were the dialdehydic form of elenolic acid (EDA) and p-hydroxyphenylethanol linked to the dialdehydic form of elenolic acid (p-HPEA-EDA). A comparison between the variations after heating of the sum of monoaldehydic and dialdehydic forms of phenolic compounds obtained by using different analytical approaches (HPLC-DAD/MSD and 1D and 2D NMR spectroscopy) was made. The results showed a good agreement of these two high-resolution techniques.