Seed oils were sold as paint for a century before they were sold as food, and they do in you what they do on a window frame. Linseed oil hardens on a fence post in a few days. Chemists call it a drying oil. Soybean oil hardens too. So does sunflower, so does safflower. They were sold to the paint trade before they were sold to the kitchen. Linoleum was invented in 1860 and it is named after linseed oil, because that is what it is, oxidised and pressed into a floor. The oil does not dry by evaporating. It dries by oxygen attacking the double bonds in its fats and welding them into a hard skin. The oil in your chip pan and the oil in your body carry the same double bonds, so the same attack runs there, with aldehydes thrown off as it goes. A painter calls that curing. A biochemist calls it lipid peroxidation. A doctor calls it inflammation and prescribes something for it. Once the oil is in your fat, it stays about two years, drying from the inside, every time it meets oxygen or sunlight. Butter never dried a fence post. Tallow never varnished a boat. Saturated fat has no double bond for oxygen to attack, which is why it sits in the pan for a week and why it sat in your great-grandmother's cells for a lifetime. A fat that sets hard in the air was never a candidate for the inside of an artery. The flooring industry understood the chemistry in 1860. The heart charity recommended it in April.

The chef who won more Michelin stars than anyone in history made his name with a plate of mashed potato. The secret was that he put in a pound of butter for every two pounds of potato. Joël Robuchon was born in Poitiers in 1945, the son of a bricklayer. He went into a seminary at 12 to become a priest, and found he was happier helping the nuns in the kitchen. At 15 he was an apprentice cook. In 1981 he opened a small restaurant in Paris called Jamin. It won a Michelin star in its first year, a second the year after and a third the year after that. Nobody had ever done it so fast. The critics came for the lobster and the truffles, and went away talking about the mash. He used a small, waxy potato, boiled in its skin and pushed through a sieve. Then he stood over the pan and beat in cold butter, a cube at a time, until the potato could hold no more. At home the ratio was a quarter butter. In the restaurant it was nearer half. It went on the menu of every restaurant he ever opened, in Paris, Tokyo, Las Vegas and London. By 2016 they held 32 stars between them. He was named chef of the century, and for 13 years he was the face of French cooking on television. Robuchon could do anything with food. When he was asked what a cook should aim for, he said the simplest dish is usually the best. The modern mashed potato is also simple. It comes in a plastic tub, or as a packet of flakes you stir into hot water. The label lists rapeseed oil, skimmed milk powder and an emulsifier. The healthy recipe on the back suggests a splash of semi-skimmed and a scrape of low fat spread. It tastes of the tub. So there are two ways to treat the humblest vegetable in the kitchen. One of them won 32 Michelin stars, and it was the one with the butter in.