The physical characteristics of a fat or oil are dependent upon the degree of unsaturation, the length of the carbon chains, the isomeric forms of the fatty acids, molecular configuration, and processing variables.
Food fats and oils are made up of triglyceride molecules which may contain both saturated and unsaturated fatty acids. The fatty acids that combine to make up triglycerides will vary; therefore, triglycerides can contain all saturated fatty acids, all unsaturated fatty acids or a mixture of both saturated and unsaturated fatty acids. Depending on the type of fatty acids combined in the molecule, triglycerides can be classified as mono- or di-, -saturated (alternatively mono- or di- unsaturated), tri-saturated and tri-unsaturated as illustrated in Figure 3.
Diagrams of Mono-, Di-, Trisaturated and Triunsaturated Triglycerides
Generally speaking, fats that are liquid at room temperature tend to be more unsaturated than those that appear to be solid, but there are exceptions.
For example, coconut oil has a high level of saturates, but many are of low molecular weight, hence this oil melts at or near room temperature. Thus, the physical state of the fat does not necessarily indicate the amount of unsaturation.
The degree of unsaturation of a fat, i.e., the number of double bonds present, normally is expressed in terms of the iodine value (IV) of the fat. IV is the number of grams of iodine which will react with the double bonds in 100 grams of fat and may be calculated from the fatty acid composition. The typical IV for soybean oil is 123-139, for cottonseed oil 98-110, and for butterfat it is 25-42.
The melting properties of triglycerides are related to those of their fatty acids. As the chain length of a saturated fatty acid increases, the melting point also increases (Table II). Thus, a short chain saturated fatty acid such as butyric acid has a lower melting point than saturated fatty acids with longer chains. This explains why coconut oil, which contains almost 90% saturated fatty acids but with a high proportion of relatively short chain low melting fatty acids, is a clear liquid at 80°F while lard, which contains only about 42% saturates, most with longer chains, is semi-solid at 80°F.
For a given fatty acid chain length, saturated fatty acids will have higher melting points than those that are unsaturated. The melting points of unsaturated fatty acids are profoundly affected by the position and conformation of double bonds. For example, the monounsaturated fatty acid oleic acid and its geometric isomer elaidic acid have different melting points (Table III). Oleic acid is liquid at temperatures considerably below room temperature, whereas elaidic acid is solid even at temperatures above room temperature.
The molecular configuration of triglycerides can also affect the properties of fats. Melting points vary in sharpness depending on the number of different chemical entities present. Simple triglycerides have sharp melting points while triglyceride mixtures like lard and most vegetable shortenings have broad melting ranges.
In cocoa butter, palmitic (P), stearic (S), and oleic (O) acids are combined in two predominant triglyceride forms (POS and SOS), giving cocoa butter its sharp melting point just slightly below body temperature. This melting pattern partially accounts for the pleasant eating quality of chocolate.
A mixture of several triglycerides has a lower melting point than would be predicted for the mixture based on the melting points of the individual components and will have a broader melting range than any of its components. Monoglycerides and diglycerides have higher melting points than triglycerides with a similar fatty acid composition.
Solidified fats often exhibit polymorphism, i.e., they can exist in several different crystalline forms, depending on the manner in which the molecules orient themselves in the solid state. The crystal form of the fat has a marked effect on the melting point and the performance of the fat in the various applications in which it is utilized. The crystal forms of fats can transform from lower melting to successively higher melting modifications. The order of this transformation is:
The rate and extent of transformation are governed by the molecular composition and configuration of the fat, crystallization conditions, and the temperature and duration of storage. In general, fats containing diverse assortments of molecules with varying fatty acids or fatty acids locations tend to remain indefinitely in lower melting crystal forms (i.e. Beta Prime), whereas fats containing a relatively limited assortment of these types of molecules transform readily to higher melting crystal forms (i.e. Beta). Mechanical and thermal agitation during processing and storage at elevated temperatures tends to accelerate the rate of crystal transformation. Table IV lists the crystal form, in their most stable condition of many of the fats and oils used in today’s products.
Additional processing steps such as interesterification and fractionation can be used to further enhance and develop these crystal forms.Manufacturers use this knowledge of crystal formation to create many of the shortening products used today. Beta crystals tend to be large course grainy crystal and are desired in products such as frying and liquid shortenings. They are also ideal for creating pie crusts. Beta-prime crystals are small, fine crystals that create a smooth creamy shortening or margarine. Most solid shortenings use Beta-prime crystals because they aerate and cream well and are ideal for most baking and frosting applications.