The best olives for oil come from low growing trees, which need a lot of room and sun so that the fruit can mature correctly.
If it is a traditional olive grove, there are only about 60 trees per acre, so that each tree may receive sufficient moisture and nutrients from the soil.
Traditional as well as organic growers typically use only horse manure as a fertilizer. Watering is carried out sparingly, sometimes with a drip system which delivers the necessary water directly to the roots.
If this procedure is used, more quality trees may be planted per acre.
Volume producers cram many more trees per acre for high production and ease in mechanical harvesting. They use chemical fertilizers. Only hand-picked, fully ripe, undamaged olives are used for the finest olive oils. Rejected are unripe or over-ripe olives because they reduce the fruitiness of the oil and contribute to its acidity.
Windfall olives are usually over-ripe or bruised and only used for bulk oil.
The harvested olives need to be processed promptly within 48 hours of being picked. Most of the Spanish olive oils that are sold by Tienda are processed the same day. The goal is to extract the oil as gently as possible without generating heat that would reduce the flavor and nutrient content of the oil.
First, large blowers remove leaves and twigs. Then, the olives with their pits intact are ground to a paste by granite rollers, or by a worm gear mechanism. The traditional manner which is virtually unchanged since medieval times, is to press the ground olive mash between circular mats.
The resulting juice is a mixture of oil and natural fruit water. After sitting for an hour the oil rises to the top and is siphoned to another container where any particles that came from the skins and fruit will eventually settle to the bottom. The olive oil is decanted into bottles from the top. No filtration is involved; the oil is slightly cloudy due to microscopic fiber.
The modern process of making olive oil used by virtually all of olive mills in a country like Spain employs centrifuges to separate the oil. The mash is put into a small centrifuge in a container with small holes, with the ground olive mulch yielding and oil and natural fruit water mixture. This mixture is then put in an additional centrifuge where the oil is separated from the natural fruit water.
Often, the slightly cloudy oil is often passed through a series of large settling tanks where it is clarified by gravity, and eventually placed in containers that are maintained at a constant cool temperature.
A few producers pass the oil through diatomaceous sand-like filter, instead of the gravity process. The result is a sparkling bright oil, which is beautiful to behold, but has lost much of its natural flavor.
Click on related links to learn more how to make olive oil.