Vol. IX · Fats and Oils Plate 1 of 10fat, oil, heat transfer, frying, flavour carrier, tenderising, texture, cooking mediumHF

What Fat Does.

A chip fries in two and a half minutes and bakes in thirteen at the same temperature. Fat is a better road for heat than air, and a hotter one than water.

Fat does four jobs in cooking. It carries heat: hot oil touches every part of the food and moves heat into it hundreds of times faster than still air, and, unlike water, it can be far above 100 °C, so the surface dries and browns (Vol. VIII). It carries flavour: many aroma molecules dissolve in fat rather than water, which is why spices are bloomed in oil (XI.8) and why fat-free food tastes thin.

It tenderises: fat coats flour proteins and keeps them from joining into gluten (IX.9), and it lubricates meat fibres in the mouth. And it makes texture: crisp fried crusts, flaky pastry, creamy sauces and the melt of chocolate are all fat at work. The workshop cooks the same 10 mm potato stick by five methods at the temperature you set.

Below: minutes for the centre of a 10 mm potato stick to reach 90 °C in hot oil, boiling water, steam, a fan oven and a still oven, and how brown the surface gets in that time (thermal bench).

What you will learn
Workshop
A 10 mm slab of potato from 20 °C until its centre reaches 90 °C (thermal bench, course heat-transfer coefficients: oil 350, boiling water 800, steam 1500, fan oven 28, still oven 12 W/m² K, the ovens with wall radiation). Water and steam stay at 100 °C whatever the setting.

Four jobs

At 175 °C the stick is cooked through in 2.5 minutes in oil, 8.4 in a fan oven and 13.3 in a still oven — oil is 5 times faster than still air at the same temperature. Boiling water is not far behind (3.3 minutes) because water also touches the food all over, but it holds the surface at 100 °C, so boiled potatoes never brown. Only oil is both quick and hot.

Fat's other jobs are less visible but as important. Fry garlic, ginger and chilli in oil before the main ingredients and their fat-soluble aromas spread through the dish (XI.8); a spoonful of butter or a drizzle of sesame oil at the end adds flavour and a rounder feel; meat with some marbling tastes juicier. Too much fat dulls flavours and feels heavy; balance it with acid (XI.4).

At the stove

Warm a teaspoon of ground cumin in a dry pan and another in a tablespoon of oil, both gently for a minute. Smell each, then stir each into a little plain yoghurt. The oil-bloomed cumin is louder and rounder.

The Chinese kitchen calls hot-oil cooking 油, and distinguishes dozens of ways by oil temperature — 温油 (warm), 热油 (hot) and 旺油 (very hot) — judged by the shimmer of the oil and the bubbles round a chopstick (XXVII.2).

Terms

Cooking medium
What carries heat to the food: air, water, steam, oil or metal.
Heat-transfer coefficient
How fast a medium moves heat into a surface per degree of difference (II.3).
Fat-soluble
Dissolving in fat rather than in water, as many aroma molecules do.
Blooming
Heating spices in fat to release their aromas.
Fat carries heat above 100 °C, carries flavour, tenderises and makes texture.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.