The most critical decision and least understood variable in producing fine olive oil is the level of ripeness of the fruit when the olives are harvested, affecting both yield and organoleptic characteristics. Additional factors of regional variations are harvest time, risk of frost and mill schedules. These all affect the quality of the finished product.
Theoretically, there exists an exact moment when ripeness and acidity levels are at their respective optimums. Crushing the fruit before this imaginary "moment" or peak of ripeness will translate to a lower yield and greener tasting oil. "Grassy" or greener tasting oil is the result of higher levels of chlorophyll still held in the fruit. Crushing the fruit before it is ripe does provide one major benefit: the acidity levels are much lower in unripe fruit. Since the primary chemical test for grading olive oil focuses on the acidity level, this early harvest oil is sometimes cynically referred to as the "virgin maker." The lower yield, and bitter tasting aspects resulting from crushing olives before they are ripe can be offset by using this oil as a blending agent that serves to lower the acidity levels of oils that might not otherwise meet the chemical standard. Early harvest olive oil can also provide a semblance or note of freshness to oils.
Crushing olives that are overripe will produce olive oil that is smoother and softer in its inherent intensity and sought after fruity characteristics. The practice of letting the fruit become overripe on the tree has the significant economic benefit to the crusher of increasing the overall ratio and yield of oil to olive by weight. This also lowers the cost of the oil. The acidity level (free fatty acids or FFA’s) rises as the fruit begins to decompose, increasing until it is unfit for human consumption. Until it is refined, which is why there is so much refined olive oil produced.
Farmers who let their olives become overripe on the tree are rewarded economically by a very high yield. The difference in yield from early harvest oil (12% to 16% oil to olives) and late harvest yield (20% to 28%) is significant and increases in yield between 33% and 133% can be achieved. Today the world market price that separates refined olive oil from extra virgin olive oil is less than 12%.
The competing interests of yield, acidity level, and flavor profile created when the olive is crushed become the most important considerations when it comes to producing high quality extra virgin olive oil. If the fruit is crushed before it is ripe it will be excessively expensive and the oil will have a bitter less fruity chlorophyll taste. If the fruit is allowed to become too ripe then it will be unfit for consumption unless it is first refined. When either consideration of higher yield or lower acidity level becomes too dominant the cost and quality of the oil suffer. It seems fitting that a balanced approach is the most rewarding one.