Olive Oil

Olive oil keeps about 18–24 months unopened and about 3–6 months after opening at good quality. Store it in a cool, dark cupboard — not on the counter next to the stove, which is where most people keep it and the worst place for it.

Heat, light, and air are the three things that degrade olive oil, and a clear bottle in a sunny kitchen gets all three. Buy oil in dark glass or tin, and decant into a small cruet for daily use rather than opening the main bottle constantly.

Do not refrigerate olive oil. It turns cloudy and solidifies, and while this does no harm and reverses at room temperature, it offers no real benefit and makes the oil inconvenient.

Keep the cap on tightly. Each opening introduces oxygen.

Olive oil does not spoil microbially — there is no water in it for anything to grow in. It fails chemically, through oxidative rancidity.

Oxygen reacts with the unsaturated fatty acids in the oil, producing peroxides and then aldehydes and ketones — the compounds responsible for the stale, waxy, crayon-like smell of rancid oil. Light and heat accelerate the reaction substantially, which is why storage conditions matter more for olive oil than for almost anything else in a cupboard.

Extra virgin olive oil is more delicate than refined oil in one sense and better protected in another. It contains polyphenols that act as natural antioxidants and slow rancidity, but it also retains the volatile aromatic compounds that make it worth buying, and those fade first. An extra virgin oil loses its peppery, grassy character long before it becomes obviously rancid.

Olive oil does not improve with age. Unlike wine, it is best fresh, and the harvest date on a bottle is more informative than the expiration date.

Rancidity is a quality failure rather than a safety hazard — rancid oil tastes bad and its beneficial compounds are degraded, but it will not make you ill.

Cloudiness in cold oil is natural crystallization of waxes and is not a defect.