Hydroxytyrosol fatty acid esters as new candidate markers for detecting olive oil inadequate storage conditions by UHPLC-QqQ-MS/MS

Recently, lipophenols (or phenolipids), which are composed of a fatty acid (FA) and a phenolic core such as hydroxytyrosol (HT), have been detected in extra virgin olive oils (EVOO), promoted by the esterification reaction between FA and HT in a unique lipophenolic molecule (HT-FAs). However, the levels of HT-FAs, as well as potential alterations in their concentrations during the storage of EVOO, are still unexplored. Thus, the present study uncovers, by UHPLC-MS/MS triple quadrupole analyzer, the changes of HT-FA levels in extra virgin olive oil (EVOO) during 12 months of storage, mimicking both commercial temperatures (room temperature (RT)), as well as inappropriate conditions of storage at high temperatures (40 degrees C). In addition, levels of HT-FAs in olive oils (blends with refined olive oil) compared to EVOO were assessed to investigate the effect of the refining process on lipophenol generation. Free HT concentration in EVOO was increased, after 12 months, by 121.31% and 236.80% at RT and 40 degrees C, respectively. Additionally, HT-FAs in EVOO samples, mainly, esterified with linoleic acid and oleic acid, enhanced up to 2.6-fold and 5.5-fold, respectively after 12 months at 40 degrees C. Nonetheless, in blends with refined oils, the levels of free HT decreased by 80.2%, whereas esterified HT augmented significantly. The type of FA linked to HT core, as well as the degree of unsaturation of FA seem to influence the lipophenolic derivative generation. Significant concentration differences found during storage time supported their potential use as candidate markers, but these findings so require further assessment to establish the HT-FAs concentration range for authentication purposes. Likewise, the ratio between esterified/free HT could be employed as freshness or aging indices of the EVOO during its shelf life, when used in conjunction with validated olive quality parameters.

Automated summary

The study reveals that levels of HT-FAs in EVOO increase over time under both commercial room temperature and high temperature storage conditions, with the type and degree of unsaturation of fatty acids playing a role. In blends with refined oils, free HT levels decrease while esterified HT significantly increases.