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

•Increase of HT and HT-FAs in EVOO after storage for 12 months.•The rate of increase of HT-FAs was greater at high temperature (40 °C) than at room temperature.•HT-FAs as markers for detection of EVOO aging in inappropriate conditions.•HT-FAs levels were raised in blends with refined olive oils.•The ratio esterified/free HT could be used as aging indices of EVOO during its shelf life.