19.1 What fats and oils are

Built from three elements

Like sugars and starches, fats and oils are made of carbon, hydrogen and oxygen. In baking books the word lipid usually just means fat, so "the lipid content of eggs" is the fat in eggs.

Fatty acids decide how a fat behaves

Every fat and oil has its own mix of fatty acids, and that mix sets how firm or runny it is. Beef fat is mostly stearic acid. Seed oils are mostly linoleic acid. Fatty acids are also sorted by length into short, medium and long chains.

Each fatty acid has its own melting point. In a fat made of many kinds, they soften one after another as it warms, so the fat melts slowly. In a fat made of one kind, or several very alike, everything melts at nearly the same moment. Coconut fat is the example: solid one second, liquid the next.

Melting points of three common fatty acids.

Saturated and unsaturated

These words describe how fully the carbon atoms in a fatty acid are bonded to hydrogen. One way to sort fatty acids is to count the double bonds between carbons:

• Saturated: no double bonds. Stearic acid is the usual example.
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• Monounsaturated: one double bond. Oleic acid is the usual example.
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• Polyunsaturated: two or more. Linoleic acid is the usual example.

Saturated fat is found in:

• meat: beef, chicken, lamb, pork and veal;
• dairy foods such as butter, cheese and whole milk;
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• lard and shortening;
• coconut, palm and palm kernel oils.

Monounsaturated fat is found in:

• avocados;
• nuts and seeds such as almonds, cashews, pecans and peanuts;
• oils such as olive, canola, sesame, peanut, sunflower and safflower.

Polyunsaturated fat is found in:

• fatty fish such as herring, mackerel, salmon, trout and smelt;
• fish oils;
• the same nuts and seeds;
• canola, corn, flaxseed, soybean and sunflower oils.

On health, the case against saturated fat is not settled, though the usual view is that less is better. Swapping saturated and trans fats for unsaturated ones has been shown to help lower cholesterol, and the dietary guidelines of the United States government give advice on choosing fats. Unsaturated fats also supply omega-3 and omega-6 fatty acids. Even so, too much of any fat means too many calories.

There is a kitchen problem hidden in this. Oils low in saturated fat do not stand up well to the long, high heat of a deep fryer.

Hydrogenation and trans fat

Manufacturers solve that problem with hydrogenation: hydrogen gas is forced into an oil, it bonds to the carbon, and the liquid oil becomes a solid fat. Adding hydrogen changes the melting point. That lets one fat stay workable in a cool bakery and a hot one, or in a cold climate and a warm one.

Partial hydrogenation, which also turns liquid oil into solid fat, creates trans fat. Trans fat raises LDL, the "bad" cholesterol, as saturated fat does. It also lowers HDL, the "good" cholesterol, and low HDL is itself a risk for heart disease. In the American diet, trans fat came mostly from fried foods such as doughnuts, from baked goods and from stick margarine. In the United States, the Food and Drug Administration requires the trans fat content to be shown in the nutrition facts table on a label.

Where fats come from and how they are refined

Edible fat comes from animals and from plants. On the animal side there are fish, sheep, pork and beef, with pork and beef the chief sources across North America. From plants come sunflower, soya bean, safflower, peanut, palm fruit, palm kernel, olive, corn, cotton, coconut and canola.

Refining takes a raw fat through these steps:

• An alkali neutralizes the free fatty acids.
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• The color is taken out.
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• The fat is hydrogenated.
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• Odors are removed (deodorizing).
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• Chilling and beating whitens and softens the fat. This is done in a machine called a votator, which cools and kneads liquid margarine.
• The fat is held in storage so its crystals form correctly. This rest is called tempering.

Emulsifiers

Oil and water do not normally mix. An emulsifier helps them combine into one stable mixture. Egg yolk holds a natural one, lecithin. Makers add mono- and diglycerides to many hydrogenated fats. The fat then forms a finer, more even structure, takes up more sugar and holds more liquid. Such emulsified shortenings suit cakes and icings, but not the deep fryer.

How fat goes bad

Rancid means a fat has spoiled and tastes unpleasant. Air and heat cause it. Unsalted butter left out of the refrigerator goes rancid fast, above all in a hot climate.

The air does its damage through oxidation, and some metals speed it up. Copper is one, which is why doughnuts are never fried in a copper pan. Some oils carry natural antioxidants, for example tocopherols, one of which is vitamin E, but processing often destroys them. Makers then add synthetic ones, commonly BHA and BHT, to slow rancidity.

Frying fat also breaks down by hydrolysis, a reaction with water. The water comes out of the food being fried. It splits fatty acids away from their glycerol, and as these free fatty acids build up in the fryer, the fat starts to taste bad and gives the same taste to everything cooked in it. So between batches of doughnuts, switch the fryer off or set it to standby. Heavy flour on the food, and crumbs that break off it, also wear the fat out.

Smoke point and stability

The smoke point is the temperature at which a fat first gives off smoke. It is not fixed for life: as a fat breaks down, its smoke point falls. A fat's stability is how long it lasts in use, which matters most for frying fats. A smoke point of 220 to 230°C (428 to 446°F) marks a fat as highly stable.