Choosing cooking oils: fat types, heat and reuse

Online charts rank cooking oils by smoke point, as if one number settled the choice. It doesn't. The fats in the oil, the heat and cooking time, reuse and the amount that ends up on the plate all count, and no oil is either completely harmless or forbidden.

Every oil is a blend

Oils and solid fats are almost pure fat, so all of them are energy dense. They differ in the mix of fatty acids, and each is a blend in which one type dominates.

The limits come from WHO's healthy diet guidance, which also keeps total fat at 30% of daily energy or less and advises replacing saturated and trans fats with unsaturated plant fats where possible.

Saturated fat is a matter of balance over a day or a week, not of one dish. It can raise blood cholesterol, so the British Heart Foundation does not rate it as the best everyday cooking fat, but coconut milk in a curry or butter in a mash fits into a varied diet.

Thailand banned the main source of trans fat

WHO links trans fat to a higher risk of heart attacks and of death from heart disease. Industrial trans fat comes from partially hydrogenating vegetable oil. WHO suggests replacing those oils with ones rich in polyunsaturated, then monounsaturated, fats, and asks countries either to cap industrial trans fat at 2 g per 100 g of total fat or to ban partially hydrogenated oils, known as PHO (WHO).

Thailand chose the ban. Ministry of Public Health Notification No. 388 (B.E. 2561) has prohibited producing, importing or selling PHO and foods that contain them since 9 January 2019 (Thai FDA). Because beef, milk, butter and cheese contain small natural amounts, the Thai FDA notes, the rule does not forbid trans fat being detected in food. "No PHO" and "zero trans fat" are different claims.

Smoke point tells you when the smoke starts

Smoke point, the temperature at which an oil starts to visibly smoke, is easy to compare. As the BHF points out, it tells you only when smoke appears; how an oil holds up under heat depends more on the type of fat in it.

Heated fats can break down into lipid oxidation products. These form more readily, the BHF says, during prolonged cooking at very high temperatures (above 260 °C) and when oil is reused, so a quick stir-fry is a different matter from hours in a deep fryer. Monounsaturated oils are more heat-stable than polyunsaturated ones, which are the least stable at high temperatures even though their smoke points are often high. Saturated fats resist heat naturally, but that does not make them the best everyday choice for the heart.

Old frying oil has a legal limit

The BHF advises against reusing oil, because it speeds up the formation of unwanted by-products. For food sold to the public, Thailand sets a limit: Ministry of Public Health Notifications No. 283 (B.E. 2547) and No. 347 (B.E. 2555) cap polar compounds, the standard marker of oil breakdown, at 25% by weight in frying oil. Monitoring by the Department of Medical Sciences in 2015–2017 found 8 of 417 used-oil samples (1.9%) over the limit, mostly from market stalls frying snacks such as patongko (DMSC).

At home, deep-fry less often, use fresh oil for each batch where you can, and don't keep topping up the same oil for weeks.

At the stove

1. Match the oil to the method: refined oils for everyday frying and roasting, unrefined ones such as extra virgin or cold-pressed for dressings, drizzling and gentle cooking.
2. Rotate a few oils instead of crowning one, to spread the fat types across your week.
3. Watch the amount. The BHF's rule of thumb for home cooking is about one tablespoon of oil for four people.
4. Read the label. Where a packaged food in Thailand carries the standard nutrition panel, it lists total fat and saturated fat; our guides to reading a ready meal and to nutrition per pack show how to use them.

We choose cooking fats with the cooking method in mind. It is a working principle and makes no fat harmless: every oil has trade-offs, and the fat in a dish depends on the whole recipe. The sodium line on the same label gets its own guide: salt and sodium.