Vol. V · The Molecules of Food Plate 6 of 11fats, oils, triglycerides, saturated, unsaturated, melting range, butter, cocoa butter, coconut oilMF

Fats and Oils.

A fat is solid and an oil liquid only at a given temperature. Both are mixtures that melt over a range — and that range shapes textures from pastry to chocolate.

Almost all food fat is triglyceride: three fatty-acid chains attached to a small glycerol backbone. Saturated fatty acids are straight and pack neatly into crystals, so fats rich in them (butter, lard, tallow, coconut oil, cocoa butter) are solid at room temperature. Unsaturated fatty acids have kinks where double bonds are, pack badly, and stay liquid (olive, rapeseed, sunflower oils). A "fat" and an "oil" differ only in where room temperature falls on their melting curves.

Because each fat is a mixture of many triglycerides, it melts over a range rather than at a point. Butter softens from the fridge to the counter and melts entirely only near body temperature; cocoa butter melts sharply just below body temperature, which is why good chocolate snaps in the hand and melts in the mouth; coconut oil turns from solid to liquid within a few degrees around 24 °C. The workshop shows the solid fraction of each fat at the temperature you choose.

Below: the solid fraction of seven fats against temperature (teaching curves placed within published melting ranges), with your temperature marked.

What you will learn
Workshop
Solid fraction of each fat against temperature: logistic teaching curves placed within published melting ranges (McGee; Fennema's Food Chemistry). Fridge (4 °C), room (21 °C) and mouth (37 °C) are marked.

Melting ranges make textures

In this model butter is 73% solid straight from the fridge and only 14% at room temperature — a network of crystals holding liquid fat: hard enough to cut into flour for flaky pastry when cold (XXV.5), soft enough to cream with sugar when warm (XXV.2). Cocoa butter is 99% solid at room temperature and only 4% in the mouth — that steep drop just below body temperature is the whole pleasure of chocolate, and the reason it must be tempered to set in the right crystal form (XXV.10).

Saturation also decides stability: unsaturated oils oxidise and go rancid faster (IX.4), saturated fats are more stable. And it shapes cooking fats: a liquid oil coats evenly and flows into every gap of a pan; a solid fat like lard or butter, cut into dough, makes layers and pockets that steam into flakes. "Healthy" choices are a matter for nutrition (XXIII.9); for the cook, melting range and flavour decide.

At the stove

Put a teaspoon each of butter, coconut oil and olive oil on a plate in the fridge and another set on the counter. After an hour compare: olive oil liquid in both, coconut oil hard in the fridge and liquid in a warm room, butter firm then soft.

Margarine was invented in France in 1869 by Hippolyte Mège-Mouriès as a cheap butter substitute for the navy; turning liquid oils into solid spreads later relied on hydrogenating their double bonds.

Terms

Triglyceride
Three fatty acids attached to glycerol: the molecule of almost all food fats.
Saturated fatty acid
A straight chain with no double bonds; packs into crystals.
Unsaturated fatty acid
A chain with one or more double bonds that kink it; stays liquid.
Melting range
The span of temperature over which a fat goes from solid to liquid.
Fats are mixtures that melt over a range; saturated chains make them solid, and the range shapes textures from pastry to chocolate.

Sources

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