Oleuropein is a naturally occurring phenolic compound found predominantly in olive trees (Olea europaea), including the leaves, fruit, and bark. It is a significant component responsible for the bitter taste of olives.
Oleuropein is most abundant in the leaves and unripe fruit of the olive tree. It serves as a protective agent for the plant, providing resistance against pests, pathogens, and herbivores due to its bitter taste and antimicrobial properties. As olives ripen, the oleuropein content decreases, which is why ripe olives are less bitter than green, unripe ones.
Oleuropein contributes to the distinctive flavor of extra virgin olive oil, which retains more of the natural polyphenols compared to refined olive oil.