Ingredient

Rapeseed oil

Most of the "vegetable oil" sold in Britain is refined rapeseed. Cold-pressed rapeseed oil, sold in a bottle with a farm name on it, is a completely different product — golden, grassy, faintly cabbagey — and the two are not interchangeable.

Refined against cold-pressed

Refined rapeseed

Pale, neutral, almost odourless. Heated, filtered and deodorised. Cheap, and the standard frying and baking oil.

Cold-pressed rapeseed

Deep gold, grassy and nutty with a distinct brassica note. Pressed without heat, so the flavour survives. For dressings and finishing.

Colour

Refined is nearly colourless; cold-pressed ranges from butter-yellow to deep amber depending on the seed and the season.

Heat

Refined tolerates high heat well and is a sensible frying oil. Cold-pressed loses its character when heated hard and is wasted on it.

Canola

The same plant. The name is used in North America and Australia and refers to varieties bred for low erucic acid, which is essentially all modern rapeseed.

Why refining changes it so much

Cold pressing simply squeezes the seed. Everything that was in it — pigments, aromatic compounds, small amounts of other plant material — comes through into the oil, which is why it is coloured and why it tastes of something.

Refining removes almost all of that. The oil is heated, treated, bleached and deodorised, and what comes out is a neutral fat that will not flavour whatever is cooked in it, keeps for a long time, and copes with high heat.

Neither is a lesser product; they are made for different jobs. A neutral oil is exactly what you want for frying a fish or making mayonnaise. A cold-pressed oil is exactly what you want on a salad or over soup.

Where each belongs

• Refined: deep-frying and shallow frying, where a neutral flavour and heat tolerance matter and the oil is a medium rather than an ingredient.
• Refined: mayonnaise and emulsions, where olive oil would be too assertive and can turn bitter when blitzed.
• Refined: baking, in cakes and batters where the oil should not be noticeable.
• Refined: roasting vegetables, where you want the vegetable to taste of itself.
• Cold-pressed: dressings, where its grassy quality is the point.
• Cold-pressed: over finished food — soup, grilled vegetables, fish, bread.
• Cold-pressed: in mash or in a purée, stirred in at the end.

Buying and keeping it

1. For cold-pressed, buy small bottles. It is a flavour ingredient and it does not improve with keeping.
2. Check the bottle is dark glass or in a tin. Light degrades oil faster than almost anything else.
3. Store both away from the hob. Heat and light together are what turn oil rancid.
4. Smell before use. Rancid oil smells of old crayons or putty and there is no way to rescue it.
5. Do not decant frying oil back into a cold-pressed bottle. It is worth keeping the two clearly separate.
6. Refined oil used for deep-frying can be strained and reused a few times if it has not been overheated or used for fish, though it degrades with each use.

Common questions