Which Fat for Which Job? Smoke Points, Flavour and Structure

Fat performs several distinct jobs and choosing one means deciding which of them matters. It transfers heat efficiently and at temperatures far above what water can reach, which is what allows browning and crisping. It dissolves and carries flavour compounds that are not water soluble, which is why many aromatics are cooked in fat at the start of a dish and why a fat-free version of a sauce tastes flat. It coats proteins and starches and interferes with them joining, which is what makes pastry short and cakes tender. It traps air when beaten, which leavens. It prevents sticking. And it contributes texture in the mouth directly, which is difficult to replace with anything else.

A handful of properties explain most of the practical differences:

• •Saturation, which determines whether a fat is solid or liquid at room temperature
• •Melting behaviour, with some fats melting sharply and others over a wide range
• •Smoke point, the temperature at which visible breakdown begins
• •Flavour, which ranges from assertive to essentially neutral
• •Stability against oxidation, which determines shelf life and how a fat behaves on repeated heating
• •Water and solids content, which matters enormously in butter and affects both browning and pastry

Smoke point is quoted as a fixed property of each oil and is nothing of the sort. It varies with how refined the oil is, since free fatty acids and other impurities lower it substantially and refining removes them, so two bottles of the same oil can differ by a large margin. It falls as the oil is used, because frying generates exactly the compounds that lower it, which means a smoke point measured on fresh oil does not describe the oil after an hour of service. Published figures vary widely between sources for the same oil. More useful measures of how an oil behaves under heat concern its oxidative stability rather than the temperature at which it visibly smokes, and by that measure several oils with modest smoke points perform better under prolonged heat than the figures suggest.

Fats deteriorate by two distinct routes and the difference matters for storage. Oxidation is a reaction with oxygen that attacks the double bonds in unsaturated fats, producing the stale cardboard smell of an old oil, and it is accelerated by heat, light, oxygen and traces of metals, which is why oil is sold in dark bottles and why decanting into a clear jug on a sunny shelf ruins it. More unsaturated oils oxidise faster, which is why nut and seed oils keep poorly and why some are refrigerated. Hydrolysis splits the fat with water, releasing free fatty acids, which is what turns butter rancid in a different and soapier way and is accelerated by the water and enzymes butter contains. Antioxidants slow the first route and are added to many commercial oils for that reason.

The practical decisions follow from what the fat is being asked to do. For high-temperature searing and deep frying, stability under heat and a neutral flavour matter, which favours refined oils. For dressing and finishing, flavour is the whole point and heat is irrelevant, which is why an expensive unrefined oil is wasted in a pan. For pastry, the fat must be solid at working temperature and must stay in distinct pieces, which is why butter is chilled and why the water content of butter contributes steam and layers. For cakes, the fat must cream with sugar to trap air. For shallow frying, browning is wanted, which butter provides through its milk solids and which also burns, hence the common practice of combining butter with an oil to raise the effective working temperature.

Fat carries heat above what water reaches, dissolves flavour compounds that water cannot, and interferes with proteins and starches joining, which is what makes pastry short. Smoke point varies with refining and falls as oil is used, so quoted figures describe fresh oil only. Stability under prolonged heat is the more useful measure and does not track smoke point.