Waning quality

Mechanical tree shaker harvesting Arbequina olives.
Kathy Coatney/Corbis

Studies show that as days, weeks and months go by after harvest, the polyphenol content and health benefits of the oil diminish.

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"Think of olive oil as olive juice with a maximum two-year shelf life," said Selina Wang, research director at the UC-Davis Olive Center.

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Several factors are responsible for the polyphenol content of olive oil, according to the experts:

  • Harvesting method: Rougher treatment and exposure to the elements reduces polyphenols.
  • The age of the trees: Older trees contain significantly higher polyphenol content.
  • Olive maturation: Green olives contain more polyphenols than ripe olives, though it's easier to extract more oil from riper olives.
  • Processing: The less processing, the better. "Extra virgin" olive oil, which is cold-pressed only once, has the highest polyphenol levels. Two presses ("virgin" olive oil), reduces polyphenol content further, and oil with three extractions contains only about half the value of "virgin" olive oil. Highly refined or "light" olive oils, which use heat or chemicals in the refining process, have significantly lower polyphenol levels.
  • Storage: Any exposure of the harvested olives or the oil to heat, light or air will reduce polyphenol content. (If you're using extreme heat in cooking, you'll most likely lose the polyphenols anyway, so you might as well use canola oil, which contains more heart-healthy omega-3 fatty acids.)
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