Vol. IX · Fats and Oils Plate 4 of 10rancidity, oxidation, hydrolysis, storage, shelf life, nuts, wholemeal flour, walnut oil, light, coldMS

Rancidity.

A bottle of walnut oil on a sunny shelf can turn in weeks; the same oil sealed in the fridge keeps for many months.

Rancid fat smells of old paint, cardboard or crayons, and tastes bitter. Two chemistries cause it. Oxidation: oxygen attacks the double bonds of unsaturated fatty acids in a chain reaction that, once started, speeds itself up; light and traces of metal start it, heat speeds it, and the more double bonds a fat has — polyunsaturated walnut, flax and fish oils — the faster it goes. Hydrolysis: water and enzymes (lipases) split fatty acids off the triglycerides; in butter these include butyric acid, the smell of sour, cheesy rancid butter.

So storage is chemistry. Keep fats sealed (less oxygen), dark (no light-started chains), cool (slower reactions, V.10), and away from the stove. Antioxidants help: vitamin E in nuts and seeds, the polyphenols of good olive oil, and the rosemary extract added to some oils. Wholemeal flour and brown rice keep less well than white because their germ holds oil. The workshop estimates how long a fat keeps under the conditions you choose.

Below: months before each product turns rancid against storage temperature, and the estimate for the product and conditions you choose (typical storage guidance scaled by the Arrhenius law, a teaching model).

What you will learn
Workshop
Teaching model: months at 20 °C, sealed and dark, from typical storage guidance (olive oil 18, refined rapeseed 12, walnut oil 4, shelled walnuts 6, wholemeal flour 3), scaled by the Arrhenius law with an activation energy of 80 kJ/mol; an opened container halves it and a clear bottle in daylight divides it by 2.5.

Sealed, dark and cold

Opened walnut oil keeps about 2.0 months in a 20 °C kitchen in this model and about 13 in the fridge — 6.7 times longer, because the oxidation reaction slows steeply with cold; sealed it would keep 4 months at room temperature. The freezer extends it so far that other changes take over first. Polyunsaturated oils, nuts, seeds and wholemeal flour benefit most from cold storage; refined oils and olive oil keep well in a dark cupboard.

Smell is the test. Fresh nuts and oils smell of themselves; rancid ones smell of paint, cardboard or putty, and a rancid batch spoils a whole cake. Buy polyunsaturated oils and wholemeal flour in amounts you will use within a few months; keep the bottle capped and away from the stove's heat; choose dark glass or tins; and store nuts and seeds in the fridge or freezer. Rancid fat is not a food-poisoning risk in the way spoiled meat is, but it tastes bad and its oxidation products are best avoided.

At the stove

Smell a spoonful of fresh walnuts against a few that have sat open in a warm cupboard for months. The old ones smell faintly of paint or cardboard: that is oxidation you can taste.

Fish oils, with five and six double bonds per fatty acid, oxidise so readily that cod liver oil was long notorious for its taste; modern supplements are sealed in capsules under nitrogen with added vitamin E.

Terms

Oxidative rancidity
Oxygen attacking the double bonds of unsaturated fats in a chain reaction.
Hydrolytic rancidity
Water and enzymes splitting fatty acids off fats, as in rancid butter.
Antioxidant
A substance, such as vitamin E, that interrupts oxidation.
Polyunsaturated
Having several double bonds; oxidises fastest.
Fats go rancid by oxidation and hydrolysis. Keep them sealed, dark and cold; polyunsaturated fats first.

Sources

  1. Typical storage guidance for oils, nuts and wholemeal flour at room temperature, sealed and dark: refined oils about a year, olive oil 18 months to two years, walnut and other polyunsaturated oils a few months, shelled nuts and wholemeal flour a few months; cold storage extends it several-fold (USDA FoodKeeper). The course scales these with an Arrhenius activation energy of 80 kJ/mol, a teaching value.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.