Rancid Olive Oil: Causes and Prevention

1The mechanism of oxidation

Rancidity does not come from "outside" as a contamination: it is born inside the oil, when its fats slowly react with oxygen. EVOO is rich in unsaturated fatty acids (those with double bonds): they are precious, but also highly sensitive to oxidation.

Simply put: at the beginning, the oil is protected and fragrant. Then, over time and under the wrong conditions, molecules form that completely change the aroma. And when the transformation really starts, it tends to accelerate.

The process can be imagined in three steps:

1. 1Initiation: something "triggers" the reaction (light, heat, metals, time) and the first radicals form.
2. 2Propagation: oxygen comes into play and the reaction self-sustains.
3. 3Termination: volatile compounds responsible for the stale smell accumulate (those that make you think of old butter, oxidized nuts, or stale fat).

This is why an oil that "just starts" to turn can degrade very quickly if left in the same conditions.

2Factors that accelerate oxidation

Rancidity is not bad luck: it is almost always due to the environment.

Light is one of the worst accelerants (especially if the oil is kept in clear glass). Heat does the rest: near stoves or in a very hot kitchen, the oil ages much faster. Then there is air: the more oxygen remains in contact with the oil (bottle opened often, mostly "empty," imperfect cap), the more fuel the reaction has.

3How polyphenols prevent it

Here is the beautiful part: EVOO has a natural defense, the polyphenols. They are the ones who "sacrifice themselves" first, blocking the oxidative chain and slowing down aromatic aging.

It is one of the reasons why an oil rich in polyphenols is often bitter and pungent: those sensations are a signal (not a defect) showing that the oil has a structure more resistant over time.

However, when polyphenols are consumed (due to light, heat, and oxygen), the oil remains exposed and rancidity takes over.

4How to prevent it

The goal is simple: protect the oil from its three main enemies — light, heat, and air.

• Choose containers that block light (tins or very dark glass).
• Keep it in a cool, stable place (not above the oven, not in the sun).
• Always close the cap tightly and reduce the time of exposure to air.
• If you consume little, buy smaller sizes: oil should be finished while "alive," not dragged on for months.