Storing oils and fats: preventing rancidity

Rancidity is mainly a chemical reaction: unsaturated fatty acids react with oxygen, which is called oxidation. This produces substances that smell and taste unpleasant. Light, heat and traces of metals speed up this reaction. In addition, fats are broken down by moisture and natural enzymes, for example in the germ of wholegrain cereals.

Not all fats are equally sensitive. The more polyunsaturated fatty acids an oil contains, the faster it goes rancid – linseed and walnut oil are among the most sensitive. Saturated fats that are solid at room temperature, such as lard or coconut fat, are considerably more stable. Ghee keeps longer than butter without refrigeration, because water and milk protein have been removed from it.

The reaction can be slowed down, but not stopped. Keeping out air, light and heat buys time. That is also why there are no fixed shelf-life periods for all fats: the manufacturer's date and a sensory check are the most reliable guides.

Stable oils and fats that you also use in everyday cooking are the best choice for the stock. Buy sensitive oils only in small amounts for current use. Several small bottles are better than one large can: an opened bottle is finished quickly, while the others stay sealed and protected until then.

The storage place matters a lot. Next to the cooker, a heater or on a windowsill it is warm and bright – exactly what oil does not like. A cool, dark cupboard or cellar is better. Dark glass bottles or metal tins protect against light. Opened containers are closed tightly again after each use.

Also think of “hidden fats”: nuts, seeds, wholemeal flour, rolled oats and muesli contain fat and go rancid too. Store them tightly sealed and cool and include them in the rotation. Where a fridge or freezer is available, nuts and wholemeal flour keep considerably longer there.

Home-made oil with garlic, fresh herbs or vegetables is a risk of its own. Under the layer of oil there is little oxygen, and at room temperature this gives botulism bacteria good conditions. Botulism is a rare but very dangerous poisoning, and you cannot taste it. Such oils do not belong in the stock: if at all, they are made only according to tested instructions, kept refrigerated and used quickly.

Rancid fats smell and taste sharp, bitter or like paint. They are no longer suitable for eating and are thrown away. So briefly check smell and taste before use, especially with older containers.

When cooking, oil is never heated unattended. If fat in a pan catches fire, no water may go on it: it turns to steam instantly and throws burning fat out. Instead, cover the pan with a lid and turn off the heat.

While tidying up, you find a half-full bottle of oil at the back of the cupboard that has stood open next to the cooker for months. It smells sharp and like paint – it is thrown away. The lesson: in future, smaller bottles stand in the cool store cupboard, and only one is open at a time.

The pack of walnuts from last winter tastes bitter too. So you prefer to buy nuts in small packs, eat them regularly and replace them. If you label every container with its purchase date and use the oldest first, this hardly ever happens.