Interesterification permits a rearrangement or redistribution of the fatty acids on the glycerol backbone of the TAG molecules. Interesterification is promoted by an alkaline catalyst or by lipase. The most commonly used alkaline catalysts are sodium methylate and sodium ethylate. The mechanism of interesterification is described in detail in the literature (Rozendaal and Macrae, 1997). Alkaline-catalysed reactions produce a mixture of TAGs where the fatty acids are distributed randomly over all three positions of all TAG molecules.

In contrast to chemical interesterification, lipase catalysed reactions are more gentle. They proceed slowly and at lower temperatures. Temperature limitations are, for example, dictated by the thermal stability of the enzyme used. When applied to high-melting fats, for example, this can be a source of viscosity-related problems. For most enzymes used in fat technology, less than a handful beyond academic applications, only the two terminal positions of the glycerol backbone are randomized. The selectivity of the enzymes in many cases is not only limited to the configuration of the TAG but also to certain fatty acids. These are thus converted at specific reaction rates. Key parameters in the operation of enzyme-catalyzed rearrangement, an industrial application still in an infant state, are water activity, raw material pre-treatment and enzyme utilization. Since the process proceeds at a much lower rate than the chemical interesterifica-tion, it allows an abundance of different fat phases to be created from a given starting mixture. This is so because the process can, through variation of contact time, be managed such that only partial randomization is achieved.

The alkaline (sodium (m)ethylate)-catalysed reaction takes place in pre-refined oil (low in water and in free fatty acids) at elevated temperatures (100-110 °C). The reaction is very fast; full randomization is reached within a few minutes even in factory-scale vessels (10-40 tons oil content). After the reaction, the catalyst is deactivated by water addition; sodium hydroxide and (m)ethyl esters will be formed. Sodium hydroxide will react with fatty acids and oil to form soap, which is subsequently removed by water washing and decanting. (M)ethyl esters are more volatile than TAG molecules and are removed during standard downstream processing, i.e. high-temperature deodorization.

The modification of the fatty acid distribution of the TAG molecules by interesterification will in general lead to a modification of the solid phase line, resulting in a change in the crystallization behavior. This is particularly true because typically a mixture of fats and/or oils with different SFC profiles is subjected to this process.

15.3.3 Fractionation

Fractionation is a process that separates a fat phase into two phases according to the crystallization behavior of its molecular species. This slow, well controlled crystallization process aims at the manufacture of preferentially large crystals to be separated from the surrounding mother liquor. The resulting high-melting fraction, solid crystals plus liquid entrapment after filtration, is referred to as stearin. The remaining liquid fraction, with the same composition as the entrapment, is called olein. It is easily appreciated that unless additional solvent is added to the system (solvent fractionation) only a limited amount of solid material, typically less than 25%, can be separated out of a fat composition.

By far the most important oil fractionated worldwide is palm oil; the main reason being the demand for clear liquid oil (palm olein). More recently there has been a growing interest in the solid product of palm oil fractionation (palm stearin), for production of cocoa butter equivalents, cocoa butter replacers and margarine hardstocks. Besides palm oil, palm kernel oil, partly hydrogenated liquid oils, cottonseed oil and milk fat are also fractionated.

The fractionation process consists of the following steps:

1 Crystal nucleation.

2 Crystal growth.

3 Crystal slurry filtration.

4 Filter cake squeezing/pressing.

There are two defined forms of nucleation: primary and secondary. Primary nuclei are formed when oil is supersaturated or under-cooled; this is the driving force of the fractionation process. Secondary nucleation is the result of 'mechanical' attrition of existing crystals. The presence or addition of secondary crystals shortens the induction time necessary for primary nucle-ation and can initiate a better-controlled crystal growth regime. The aim for fractionation is to grow large, dense crystal agglomerates that can easily be separated from the liquid oil. The level of supersaturation and the presence of growth nuclei essentially drive crystal growth. Crystal slurry is made up of potentially fragile crystal agglomerates. This slurry must not experience high shear stresses during the transfer to the filter and inside the filter. The filtration characteristics of the slurry depend on size of the crystal agglomerates, the separation efficiency of the slurry and the solid phase content. Most modern fractionation plants use membrane filter presses. These enable the filter cake, produced by simple filtration, to be squeezed to both increase the yield of olein and produce a harder stearin.

The combination of process conditions influencing these fractionation steps determines the characteristics and yield of both the olein and stearin. The most important parameters for solid fat production are:

• the type and quality of the feedstock;

• the crystallization temperature;

• the type and size of the crystals;

• the efficiency of the separation process.

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