Fractional crystallisation – the fat modification process for the 21 st century

Abstract

The historical development of fractionation, from the use of fractionated tallow in Mege-Mouries' margarine to the modern dry fractionation process used to produced steep-melting palm fractions for cocoa butter equivalents, is described. The principles of fractionation by fractional crystallisation are explained. The fractionation process is carried out in two stages: firstly, a crystallisation stage; secondly, a separation stage. Crystallisation may be effected without any solvent (dry fractionation) or in the presence of a solvent. It can be shown that the efficiency of separation of triglycerides is more or less independent of the solvent so that dry fractionation is, in principle, capable of giving as good a fractionation as solvent fractionation. However, separation of the solid phase (crystals) from the liquid phase is easier in the presence of a solvent, which dilutes the oil and lowers the viscosity. It is mainly developments in separation over the last 25 years that have led to the improved effectiveness of dry fractionation so that it can achieve results that rival solvent fractionation. The concept of ‘entrainment’ is explained with reference to the different separation methods and to their different efficiencies. Today, hydrogenation is in decline, due to nutritional concerns about trans fatty acids and to environmental concerns about nickel catalysts and their disposal. Increasingly, oils with reduced linolenic acid (C18:3) can be produced agriculturally so that stable frying oils may be produced without hydrogenation. With the decline in hydrogenation, interesterification has seen a renaissance, although it is only partially able to replace hydrogenation. Additionally, interesterification suffers from the ‘chemical’-process image and environmental drawbacks of hydrogenation. Fractionation is a purely physical process which satisfies today’s increasing environmental and health concerns. It is the main modification process used for palm oil, whose production is still increasing rapidly and which is likely to become the world’s most-produced oil within 10 years. If hydrogenation is to be avoided, then only palm stearins can supply the higher solid fat content components required to produce the margarines and shortenings essential to produce the bread, pastries and cakes we like to eat. Fractionation is therefore set to become the dominant modification process of the 21st century.