Preparation of stripped oils
To avoid the influence of the minor compounds in oil such as tocopherols, phospholipids, free fatty acids, and mono- and diacylglycerols, stripped oils were used in this study. Stripped oils were prepared using the previously reported method (Hwang et al., 2019). In brief, a column (5.5 cm i.d., 55 cm length) was prepared with hexane (500 ml) and dry aluminum oxide (333 g). Sand was added to make a thin layer on top of aluminum oxide. Oil (500 g) dissolved in hexane (about 500 ml) was added into the column and eluted to a 2 L round-bottom flask wrapped in aluminum foil under argon blown into the flask. After completion of elution, hexane (800 ml) was added into the column, and the oil/hexane mixture was collected in the flask. Hexane was removed using a rotary evaporator for 2 h at 30-35 ºC followed by drying under high vacuum for 2 days. HPLC was used to confirm the removal of tocopherols and other polar compounds in oil. Stripped oils were stored at -20 ºC under argon, and the storage studies started within a week.