Green olives are sour because they are unripe and contain a high concentration of a bitter compound called oleuropein.
Here's a more detailed explanation:
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Oleuropein's Role: Oleuropein is a phenolic compound naturally present in olive trees, particularly in the fruit. It serves as a defense mechanism against pests and diseases.
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Ripening Process: As olives ripen and transition from green to black, the concentration of oleuropein decreases significantly. This is why ripe, black olives are much less bitter than green olives.
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Inedibility of Raw Green Olives: The high oleuropein content makes raw green olives intensely bitter and generally inedible without processing.
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Curing Process: To make green olives palatable, they undergo a curing process to reduce the oleuropein levels. This can involve soaking in lye solutions, brining, or water curing. The specific method affects the final flavor profile.
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Comparison to Black Olives: Black olives, being ripe, have naturally lower levels of oleuropein and therefore require less processing to be edible, although they are still typically cured to improve their flavor and texture.
In short, the sourness of green olives is directly attributed to the presence of high concentrations of oleuropein, a bitter compound that diminishes as the olive ripens. This is why they require curing before consumption.
