Beef fat, butter, lard and hard margarine should be avoided because they contain a high level of triglycerides made from saturated fatty acids. Fats arising from polyunsaturated fatty acids, such as soft margarine or fish oils, are more acceptable.

The structure of a triglyceride molecule shows that it is made from three fatty acids, and the figure below shows that for a particular fat or oil, there is a large range of fatty acids used to make the triglycerides. Thus, rather than being a pure compound, a fat or oil is made up of a mixture of different triglyceride molecules, each derived from different combinations of three fatty acids. Although some of the triglycerides will be made from three identical fatty acids, the majority of triglyceride molecules are derived from two or three different fatty acids.

Even if a fat were made from just five of the fatty acids then it would be possible to get up to 75 different triglyceride molecules, each containing a different permutation of three fatty acids from the five available. However, the way the fatty acids are combined to make the triglycerides is not random, and the proportion of each triglyceride molecule is controlled by the organism to give the fat the required properties.

In lard, there is a definite tendency for unsaturated fatty acids to occupy the outer positions; however, in cocoa butter they tend to occupy the central position. This type of structural difference and distribution affects such properties as the melting range: lard softens 20°C lower than cocoa butter. Notice we referred to a melting range rather than a melting temperature. Pure compounds, such as salt and sugar, have distinct melting temperatures. If the compound contains some impurities then the melting temperature is lowered and the substance melts over a range of temperature. Substances such as lard and cocoa butter, which contain a mixture of different molecules, melt over a wide range of temperature. At the bottom of the range the compounds with a low melting temperature melt, and as the temperature is raised so more of the compounds melt.

Most fats are soft and can spread, but do not flow – they have properties intermediate between those of solids and liquids. This again arises because fats are a mixture of a number of different triglycerides. A large proportion of these triglycerides have a melting temperature above room temperature and will tend to solidify. Hence, the mixture consists of solid particles in a matrix of viscous liquid oils made up of triglycerides with lower melting temperatures. A solid material is thus produced that is not rigid because the crystals can slide over each other. This gives rise to the spreading properties associated with a fat.

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