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Fats And Oils

Fats and oils are chemically the same. Conventionally the word ‘fat’ is used for something that is solid at normal temperatures, and ‘oil’ for a liquid. Coconut and palm oils, however, are liquid in the hot climate of their lands of origin, but generally solid in temperate climates, their melting point being between 21 and 26 °C (70–9 °F). Furthermore, the melting and solidification of fats and oils is a gradual process; they are not fully liquid above or fully solid below a given temperature.

Further details of individual fats and oils are given in the articles on the various nuts and other sources from which they are produced.

Are fats essential?

Fats and oils are to some extent a luxury both in cooking and in diet. Fat contributes to the palatability of food, making it more agreeable by increasing the smoothness and tenderness of the texture. Fat also carries much flavour, though in a pure state it has no flavour of its own. Many flavouring substances are soluble in fat but not in water. The flavour of meat, for example, is largely in its fat. If every trace of fat is excised from a piece of beef and a piece of mutton, it is almost impossible to distinguish between them. Fat also increases the satisfying quality of food. It takes a certain time to digest, so it increases the time that food remains in the stomach, giving a feeling of comfortable satiety.

In terms of the strict necessities of diet, hardly any fat is needed. The body requires fat as part of its own composition, but it can make fat from other foods. (This is in contrast to protein, which can only be had from protein—or its component substances—in the diet.)

However, a few grams of fat are necessary every day to carry the necessary vitamins A, D, E, and K, which are soluble in fat but not in water; and to provide certain substances found only in fat, the essential fatty acids. At least one of these is certainly needed: linoleic acid. Two others, linolenic and arachidonic acid, can probably be made by the body; although if they are eaten there is no need to make them.

Vegetable oils are much richer sources of these essential fatty acids than are animal fats, but in practice this is not very important because of the small quantities involved. Most diets, especially western ones, contain far more fat than the basic requirement. The traditional Japanese diet, for example, has only a fraction of the fat of a western one, and the Japanese remain perfectly healthy on it. Dietitians generally consider that the amount of fat eaten in western countries is excessive and unhealthy. Part of the reason for this is that fat is much higher in energy value than any other food, twice as much as sugar; 9 calories per gram or 255 calories per ounce. A diet rich in fat is therefore concentrated and calorific, and tends to lead to obesity.

Even if it were desirable, a completely fat-free diet would be impossibly bulky. If an active, grown man lived entirely on potatoes, which contain very little fat, he would have to eat about 4 kg or 9 lb of them every day to supply the 3,000 or so calories he needed.

Dietitians also pay attention to the nature of the fat which is eaten. In order to understand the statements they make, for example about the undesirability of consuming much ‘saturated’ fat, it is necessary to consider the chemical nature of fat and the various ways in which fats can be classified.

Lipids and esters

Fat is one of a class of substances called lipids. These include not only fats but also waxes (naturally found in plants); phospholipids, such as lecithin in egg yolk; and steroids, such as cholesterol, also found in egg yolk, and ergosterol, the chemical name of vitamin D.

Lipids belong to a larger class of chemical substances: esters. Esters are also important in food as flavouring compounds, some of which are oils and some are not. Esters which act as flavourings have an aromatic quality (in common language, not in the restricted chemical sense of ‘aromatic’). An example of ester fragrance is ethyl acetate (now called ethanoate), the chief component of the smell of strawberries. Esters with small, light molecules are fragrant because the molecules are light enough to drift about in a vapour. But the esters which make up the bulk of fats have rather large molecules, so that they form liquids or solids with no smell.

Fatty acids

An ester molecule is a combination of several smaller molecules linked together: an acid or acids with an alcohol. In fats the alcohol is glycerol, more familiar under its common name of glycerine. The glycerol molecule has three sites for connecting to fatty acids, and in a fat all three are filled. The fat molecule is therefore called a triglyceride molecule.

There is a wide assortment of fatty acids, and all fats and oils contain a mixture of them. Fatty acids can exist on their own, and may be detached from the glycerol which holds them. Their molecules vary considerably in length and weight. Those with the shortest molecules are volatile and have strong smells, noticeable when a fat breaks down and releases them, as happens when the fat becomes rancid. The shortest are butyric acid, one of the main smells of rancid butter; caproic, caprylic, and capric acids, which all smell ‘goaty’; and lauric and myristic acids, which are components of the smell of bay and nutmeg. Longer, heavier molecules are not volatile and have no smell. A fat with a majority of short and medium length acids is liquid at normal temperatures; one with predominantly long acids is solid.

Saturation

Another distinction between fatty acids is whether their molecules are saturated, monounsaturated, or polyunsaturated. A fatty acid molecule consists of carbon, oxygen, and hydrogen atoms arranged (at least in most of the fatty acids in foodstuffs) in a straight chain. Each carbon atom, except those at the ends of the chain, may have two hydrogen atoms linked to it. If two adjacent carbon atoms lack these attached hydrogen atoms, they link to each other in a ‘double bond’, which is less stable than a normal single bond. A double bond is therefore a weak link in the chain, which may easily be undone in a chemical reaction.

A fatty acid with no double bonds is said to be ‘saturated’—totally filled with hydrogen. If it has one double bond it is ‘monounsaturated’; if it has two or more it is ‘polyunsaturated’. The more double bonds in the chain, the less stable the fatty acid. This effect is noticeable in the stability of the fat as a whole.

Generally, vegetable oils contain a higher proportion of monounsaturated and polyunsaturated acids than do animal fats, but there are exceptions: coconut and palm oils are highly saturated, and fish oils largely unsaturated.

Most of the longest fatty acid molecules are saturated, while unsaturated acids tend towards the shorter end of the range (although the very shortest acids are saturated). Thus increasing the proportion of saturated acids in a liquid oil tends to turn it into a solid fat. No fat is completely saturated, but full saturation has been produced experimentally. It results in a solid as brittle as china.

An industrial process, hydrogenation or ‘hardening’, is used in making hard margarine from liquid vegetable or fish oils. The oil is heated with hydrogen. The hydrogen atoms attach themselves to carbon atoms at the weak double bonds, turning these into strong single bonds. Hardening a fat not only makes it solid, but also more stable and less likely to become rancid. It is sometimes done for this reason alone, for example in the making of commercial peanut butter.

Commonly used edible oils

Soft margarine described as ‘high in polyunsaturates’ is made by a different process, from oils which would normally be liquid. These are made into an emulsion which depends for its stability on chemical emulsifiers.

The proportion of saturated acids in a fat can be found by testing with iodine, which reacts with the double bonds. The result is expressed as the number of grams of iodine that are taken up by 100 grams of fat. This ‘iodine number’ or ‘iodine value’ runs from 0 for total saturation to nearly 200 in the edible oils highest in polyunsaturates. Coconut oil has an iodine number of 7, butterfat 32, olive oil 84, and soya oil 140.

Extraction and refining

Extracting fats and oils from their sources may be a fairly simple process or a complex one involving several stages.

The best oils from a gastronomic point of view are those extracted simply. The simplest process is cold pressing, as used for ‘virgin’ olive oil and for the best nut oils. The olives or nuts are pressed, and the oil left in the state in which it emerges, with many plant substances present which give it flavour and colour. Unfortunately, these substances include enzymes which attack the oil, so these natural oils do not keep well. However, they are valued for their flavour. Another example of a fat prepared by a natural, simple process which leaves it full of flavour but prone to spoilage is butter.

Cold pressing alone cannot remove all the oil from any fruit, nut, or seed. After the first pressing the crushed mass is heated to make the oil more liquid and readier to emerge. After this, solvents are sometimes used which wash the oil out of the plant tissue in the same way as dry cleaning removes the grease from clothes (in fact similar solvents are used). The solvents are then evaporated and recovered for reuse, leaving the oil. One other method, used for soya beans, is a centrifugal expeller which removes oil in the same way as a salad spinner removes water.

Animal fats and fish oils are extracted from meat trimmings or fish by ‘rendering’ them with steam to melt out the fats. For example, beef yields oleo oil for margarine and oleostearin (tallow) for solid cooking fat.

Some oils, such as cottonseed and rapeseed oils, are highly impure and inedible in their original state, whatever method of extraction is used, and any oil or fat extracted by any method other than cold pressing will contain impurities which are nearly always unwelcome. So the next stage is refining.

The oil is treated with steam to remove gum, and then with alkali to neutralize free fatty acids which might contribute unpleasant flavours. The alkali causes slight saponification: that is, it splits up some of the triglyceride molecules into glycerol and soaps. (Ordinary soap is made from fats and alkalis.) The glycerol and soap are washed out with water and the oil left for a while so that the water settles to the bottom and can be drained off. Next it is bleached with fuller's earth and given a final deodorization with steam. What emerges is a nearly pure oil containing a blend of saturated and unsaturated fatty acids. It has almost no taste or smell. Ordinary cooking oil is of this kind.

Keeping fats and oils

Even refined oils, devoid of enzymes, are subject to deterioration. Highly unsaturated ones are the least stable, especially those which contain appreciable amounts of linolenic acid (e.g. soya oil), which is the most troublesome of the common fatty acids.

Oils react with oxygen from the air. This can cause oxidative rancidity: a series of chemical reactions which releases compounds with an unpleasant smell and taste. Fish oils, which are highly unsaturated, and again soya oil, are especially prone to this spoilage.

Professional bakers and other food processors add antioxidants—substances which retard rancidity—to their goods. Sugar is a good antioxidant: biscuits with sugar remain fresh over four times as long as those without. Some herbs and spices are effective, including allspice, cloves, oregano, rosemary, sage, and thyme. The essential oils in these are often mildly antiseptic, restraining the growth of micro-organisms which cause rancidity. Many of the spices used in curry powders contain such oils.

Effects of heat

Fats and oils are not much changed by heating to moderate temperatures; they do not become ‘cooked’ in the same way as proteins or carbohydrates. (Butter is changed by heating because it includes water, which boils away, and protein, which cooks.) If an oil is heated excessively it begins to break down; a sign of this is that it gives off smoke. Soya oil, one of the most heatproof oils, has a ‘smoke point’ of 256 °C (492 °F). It does not actually catch fire until it reaches about 350 °C (662 °F). If flour particles fall off food which is being deep fried and float in the oil, they lower the smoke point considerably.

Oils used for deep-frying tend to deteriorate from repeated heating to near their smoke point. Some of the molecules break up and release free acids and other compounds which give the oil a rancid taste. Others polymerize—clump together—which causes the oil to become heavy and sticky.

Oil can also be affected by low temperature. If it is kept in a cold place, some of the heavier molecules may fall below their solidification temperature. The oil clouds and a solid deposit forms at the bottom of the bottle. This does no harm, and the oil will reliquefy if warmed.

Solid fats melt gradually, slowly becoming softer until finally they are fully liquid: this is different from, say, water melting to ice. Even apparently solid fats are not entirely solid. They consist of crystals of solid fat made up of the heavier molecules surrounded by liquid fat (or oil) made up of the lighter ones, which have a lower melting point. As the temperature changes, increasing numbers of molecules pass from one state to the other, according to their individual melting points. It is a curious feature of fats that once melted, they have to be cooled to well below their melting point to resolidify them. Butter, for example, melts at about 35 °C (96 °F) but has to be cooled to about 23 °C (73 °F) to solidify it.

Texture of foods

The presence of crystals in solid fats accounts for the pleasant, crumbly texture of pastry. The crystals force the starch particles in the flour apart and retard the formation of elastic gluten (as in bread), so the pastry stays ‘short’. If the pastry becomes warm in making, the crystals melt, gluten forms, and the pastry becomes tough. Using too little fat also allows gluten to form. Fats with large crystals make good pastry. One such fat is lard, whose molecules tend to have a particularly large acid in the centre of each triglyceride trio, which helps large crystals to form. Small crystals, such as those in vegetable cooking fats, make better creamed cake mixes. The fat spreads evenly throughout the flour and lubricates the surfaces of starch and gluten particles, so that they slide together when eaten and the cake has a tender texture.

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