Figure 5. Cross-sectional structures of long-chain compounds. Source: Ref. 18, courtesy of Journal of the American Oil Chemists' Society.

cell in the crystal. X-ray diffraction analysis enables measurement of the short spacings and long spacings of the unit cell. Principal short spacings of the polymorphic forms are: a, 4.15 Á; p', 4.2 and 3.8 Á; y?, 4.6, 3.85, and 3.7 Á (20). Fat crystals of different polymorphs have different crystal habit. That is, they appear to have different shapes when seen in the microscope: a crystals are fragile, transparent platelets of about 5-/um size; ft' crystals are tiny needles about 1 ¡urn long; /? crystals are large and coarse, averaging 25 to 50 ¡Xm in size (21). These crystals can now be made visible by scanning electron microscopy (22).

For the production of margarines and most shortenings, the fat should be present in the /?' form. Some fats have a tendency to convert slowly from the ft' to the /? form and this results in a coarsening of the crystal structure, making the product unacceptable. Fats which contain a very high level of fatty acids with identical chain lengths are especially prone to this effect. Canola and sunflower oils are notorious in this respect and have well over 90% of C18 fatty acids (Table 5).

The melting point of a fat is basically determined by the melting points of its constituent fatty acids. When we speak of the melting point of a fat we mean the end of the melting range, since only simple substances have a sharp melting point. The end of the melting range of a fat is dependent on the method used. Chain length and unsaturation of fatty acids determine their melting point. In addition, the configuration around the double bond is important, as is the arrangement of fatty acids at the different positions of the glycerides. Trans-unsaturated fatty acids have much higher melting points than their cis counterparts, making them more comparable with saturated fatty acids. For example, oleic acid (cis) has a melting point of 13°C and elaidic acid (trans) has a melting point of 44°C.

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