8Th Tan Sri Dato Seri B Bek-Nielson Foundation Lecture - 2003

Fractional crystallisation: Fat Modification for the 21st Century

Fractional crystallisation-the fat modification process

forthe 21st century

Ralph E. Timms

Consultant – Oils & Fats, Nocton, Lincoln, UK

[Republished with copyright permission of European Journal of Lipid Science and Technology,

original reference: Timms RE (2005) Fractional crystallisation – the fat modification process for the 21st century.

Eur J Lipid Sci Technol 107:48-57. Dol: 10.1002/ejlt.200401075]

Abstract: The historical development of fractionation, from the use of fractionated tallow in Mège-Mouriès’ margarine to the modern dry fractionation process used to produce 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.

Malaysian Oil Science and Technology 2005 Vol. 14 No. 11

Fractional crystallisation: Fat Modification for the 21st Century

Keywords: Fractionation, crystallisation, palm, hydrogenation, separation.

1. Historical background

Fractional crystallisation, usually known simply as fractionation, is undoubtedly the oldest fat modification process and was the foundation of the modern edible-oil and fat-processing industry. On 15 July 1869, Hippolyte Mège-Mouriès applied for the French patent number 86480 entitled: “Demande d’un brevet d’invention de quinze ans pour la production de certains corps gras d’origine animale”, that is: Application for a patent of fifteen years for the production of certain fats of animal origin”. The patent was granted in October 1869. The product was what we know today as margarine. Mège-Mouriès had been working at the Imperial farm at Faisanderie in Vincennes. The French emperor, Napoleon III, had offered a prize to anyone who could produce a cheap substitute for butter. He wanted to improve the diet of his people, and particularly his soldiers, to strengthen France for the impending war with Germany. Having observed that the fasting cows still produced milk, Mège-Mouriès concluded that butter must be derived from cows’ tissues and hence from tallow. He found that tallow itself was too high-melting and had a waxy mouthfeel, but by separating a liquid fraction from it, a suitable fat, oleo-margarine, was obtained.

In July 1870, Napoleon III declared war on Germany, or strictly speaking on Prussia, and a margarine factory was quickly built at Passy, near Paris. However, with the complete defeat of the French army at the Battle of Sedan in September and the entry of the Prussian army into Paris, the emperor was deposed and the factory was closed. Napoleon fled to Britain where he died on 9 January 1873. He is buried in the Imperial Crypt at Saint Michaels’ Abbey, Farnborough.

Now without a patron, Mège-Mouriès sold his invention for 60,000 francs to two Dutchmen, Simon vanden Bergh and Anton Jurgens. They quickly established margarine factories in The Netherlands and in other European countries. Thus, as a result of Napoleon III’s failed imperial ambitions, The Netherlands, not France, became the centre of Europe’s margarine, edible-oil and fat-processing industries, and our modern industry had begun.

Mège-Mouriès’ patent describes most of the steps that are familiar to us in a modern refinery, e.g. bleaching or decolourising of the tallow (by treatment with dilute hydrochloric acid) and deodorisation or removal of off-flavours (by treatment with an acidified infusion of pig’s stomach), to give a fat that “has no longer the odour of animal fat and has all the flavour of the most delicate fat”, although clearly the modern technology for these various steps is quite different from that recommended in the patent. For the present purpose the section on pressing is the most important.

“Pression - Cette opération est destinée a séparer la partie dure qui rend le corps gras grenu, le fait figer rapidement, et se colle au palais. On sait que I’industrie ordinaire fait cette opération très difficilement; elle devient industrielle par les moyens suivants: le corps gras liquide et limpide est versé dans les caisses qui ont une ouverture au bas, et contiennent une couche d’eau tiède; on les couvre, et lorsque, par le refroidissement, la cristallisation est faite, on enlève l’eau par l’ouverture, on renverse la caisse, on laisse tomber la messe sur une table, on la coupe en gâteaux de 1 à 2 centimètres d’épaisseur, on enveloppe ces gâteaux dans une toile et on les met sous presse entre les plaques chaudes; on obtient ainsi environ 60p. % d’un mélange de margarine et d’oléine, d’une composition identique au saindoux, mais d’une saveur bien supérieur; quant à la partie solide, elle reste dans ses toiles.”

Which I translate as:

“Pressing – This operation is meant to separate the hard part that makes the fat grainy, makes it congeal rapidly and stick to the palate. It is known that it is very difficult to do this operation by ordinary industry methods; it must be done by the following means: the clear and liquid fat is poured out into boxes that have an opening at the bottom and contain a layer of lukewarm water; they are covered, and when, by chilling, the crystallisation is done, the water is raised by the opening, the box is turned over, the mass is let fall on a table and cut into cakes of 1 to 2 cm thickness; the cakes are wrapped in a linen cloth and placed under pressure between warm plates; thus one obtains about 60% of a mixture of margarine (note: stearic acid or harder fat/fatty acid) and olein (note: oleic acid or liquid fat/fatty acid) with a composition identical to lard, but with a very superior flavour; as for the solid part, it remains on the cloths”

In more detail added in 1874, the best size for the boxes is given as 20L of rectangular shape and the crystallisation conditions as 25-30°C for about 24h.

Mège-Mouriès described the product as butter in a primitive state. To make “beurre supérieur” or margarine: “The fat is mixed (at animal heat, i.e. about 40°C) with its weight of water in which has been mixed 1/50 of mammary tissue (note: cow’s udder), 1/100 of bicarbonate of soda, 1/50 of fresh milk curds (French: caséum de lait) and a sufficient quantity of yellow colour. It is allowed to digest for at least 3h, while agitating and maintaining it at animal heat; when the transformation is done, it is chilled, preventing the brittle and grainy state…” and making it soft and pliable by a scraping process that he describes as used in soap-making. Apart from the cow’s udder, this process is immediately recognisable as the same as we use to make margarine today, although he does say that it you omit the cow’s udder you get an inferior product. Something for the modern margarine industry to think about?!

At the beginning of the 20th century, cottonseed oil became the most important edible oil in the USA. According to Bailey, writing in 1950 in his classic book ‘Melting and Solidification of Fats’1: “The American trade requires a salad oil that will remain fluid at the temperature of mechanical household refrigerators (40-45°F), and produce a mayonnaise which is likewise stable at low temperatures. For various reasons, cottonseed oil must serve in large measure as a source for salad oils, yet the unprocessed oil solidifies relatively easily. Hence, fractional crystallization is required to obtain a suitable salad oil. Formerly, a sufficient supply of ‘winterised’ oil was produced by allowing the refined oil to stand in outside storage tanks in cold weather, and drawing the liquid portion off from the partially solidified material settling to the bottom of the tanks. Now, however, the oil is generally refrigerated artificially.” Bailey goes on to say: “The fractionation of edible beef fat to produce low melting oleo oil… is an old process… and formerly a highly important one before catalytic hydrogenation provided means of producing comparable products from liquid vegetable oils.” Concluding his short review of fractionation, he says: “there is a considerable quantity of lard fractionally crystallized… Coconut and palm kernel oils are separated into low and high melting fractions with the latter being used as a confectionery fat. In each of these cases the liquid fraction must be separated from a relatively large solid fraction, hence the operations of crystallizing and pressing resemble those employed for the production of oleo oil, rather than the operations of winterizing…”

Rossell2 gives some background on the origins of the fractionation of lauric oils to produce confectionery fats. He says: “The advantages of fractionation were first appreciated by European fat companies who imported coconut oil from Sri Lanka in long wooden barrels called ‘Ceylon Pipes’. The pipes were filled with warm fluid oil which cooled slowly during the sea voyage to cooler European climates. This slow cooling, perhaps coupled with gentle agitation of the ships’ movement, allowed the fat to crystallize and separate into fractions.”

In contrast to fractionation, hydrogenation of oils was not invented until 1901 and patented by WilhelmNormann in 1903 while working in Leprince &Siveke’s factory in Herford, Germany.3 The first production trials took place in 1905 and 1906 at JosephCrosfield’s factory in Warrington, England. Like van den Bergh’s and Jurgens’ factories, it was later to become part of Unilever. Large-scale production of hydrogenated fats began at Crosfield’s in 1908. In 1908, hydrogenated whale oil and, in 1909, hydrogenated cottonseed oil were produced in Herford.

Interesterification was the last of the three fat modification processes to be developed. In the 1920’s it was introduced in the USA and in Europe, like hydrogenation as a means of improving the functional properties of blends for shortenings and margarines and extending the range of raw materials that could be used. Indeed one of the first patents on the topic was taken out by Normann. It was developed further in the USA in the 1940’s and 1950’s by combining it with a fractional crystallisation step to give the so-called directed interesterification process for the improvement of lard as a shortening.

All the fractionation processes mentioned show the same principles and features we use today:

• complete melting of the fat,
• slow cooling,
• gentle or no agitation to encourage the development of large crystals,
• separation into liquid and solid fractions with differing physical and chemical compositions.

As we shall see, there have been tremendous developments in the technology, particularly in the last 25 years, but the principles and the objectives have not changed. Only the use of organic solvents, as patented by Unilever in the late 1950’s for the production of fats to replace cocoa butter, used a process not foreshadowed by the early processes.4

2. Principles of fractionation

2.1Preliminaries

The process of fractionation consists of two steps:5

• crystallisation to produce solid crystals in a liquid matrix,
• separation of the crystals from the liquid matrix.

Different factors are important at each stage, as shown in Fig. 1. At this point, it is useful to note that the ‘quality’ of the liquid fraction depends only on the crystallisation step, whereas the ‘quality’ of the hard fraction depends on both the crystallisation and the separation step. By quality, the degree of concentration of the desired triglycerides in the separated fraction is meant. Quality is usually assessed by physical criteria, such as cloud point or solid fat content.

When a liquid fat is cooled, a solid phase separates, whose composition and amount depend principally on the temperature. The situation is illustrated in Fig. 2, which shows a schematic phase diagram of a binary mixture of triglycerides A and B which form a continuous solid solution, i.e. they are completely miscible in the solid state. Holding the mixture at temperature T1 results in the formation of a solid phase (crystals) of composition c in a liquid of composition a. The fraction of solid phase = (ab/ac).

Figure 1. Schematic diagram of the fractionation process, indicating factors that are important at each stage (from reference 5)

To obtain crystallisation, it is necessary to increase the concentration of the triglycerides to be crystallised above the saturated-solution concentration at a given temperature. In practice, this is not sufficient to cause crystallisation, and solutions can exist indefinitely with concentrations above the saturation level without forming any crystals. Such solutions are called ‘supersaturated’.

Figure 2. Simple schematic phase diagram illustrating the principles of fractionation.

For any system, we can draw a saturation-supersaturation diagram as shown schematically in Figure 3 for the crystallisation of partially hardened soybean oil.6 The continuous line is the normal solubility or saturation curve. Below this line, crystallisation is impossible because the solution is not saturated and the situation is stable indefinitely. The dashed lines divide a metastable zone from an unstable or crystallisation zone. In the metastable zone crystallisation is possible, but will not occur spontaneously or immediately without assistance, such as stirring or seeding. Crystallisation will occur spontaneously and immediately in the unstable zone. It can be seen that the position of the dashed-line boundary between the metastable and the unstable zones is variable and depends on process variables such as cooling rate and agitation.

Figure 3. Saturation-supersaturation diagram for crystallisation of partially hardened soybean oil. Effect of cooling rate on metastable/unstable boundary (dashed line). Numbers are rates of cooling [°F/min] at an agitation speed of 120 rpm. (Adapted from reference 6).

The reason for the existence of the metastable zone can be understood if crystallisation is treated as a two-step process: Nucleation followed by crystal growth.

2.2 Nucleation

A crystal nucleus is the smallest crystal that can exist in a solution of a certain concentration and temperature. Aggregates of molecules smaller than a nucleus are called embryos and will redissolve if formed.

When molecules come together to form a crystal, there are two opposing forces. Firstly, energy is evolved due to the heat of crystallisation, which favours the process. Secondly, the surface of the crystal increases as the molecules aggregate together. Just as when a balloon is blown up, increasing the surface requires energy to overcome the surface tension or pressure. A stable crystal will form only when the energy due to the heat of crystallisation exceeds that required to overcome the surface energy. Since the surface energy is proportional to the surface area, and hence to size (here: linear dimension of the crystal, e.g. the diameter for spherical crystals) to the second power (size2), and the heat of crystallisation to the volume, and hence to size to the third power (size3), it is clear that the solubility must depend on the size of the crystal. Using data for a typical triglyceride, we can calculate the effect of crystal size on solubility, as shown in Tab. 1. Supercooling is the decrease in temperature below the solubility temperature required to get small crystals (in a supersaturated solution) to crystallise. The ‘critical size’ is the minimum size of a crystal that is stable at the prevailing temperature. We can see that small crystals have an enormously increased solubility and require a lot of supercooling to make them crystallise.

Table 1. Variation of solubility and supercooling with radius of crystals of a triglyceride (from ref.15).

In practice, such spontaneous or homogeneous nucleation rarely occurs in fats. Instead, heterogeneous nucleation takes place on solid particles, such as already existing seed crystals, dust, walls of the container or foreign molecules.

Once crystals have formed due to primary nucleation, secondary nucleation can also occur. Secondary nuclei form whenever small pieces of crystal are removed from the growing crystal surface. If the pieces are smaller than the critical size, they redissolve; if larger, they act as nuclei and grow to become crystals. Secondary nucleation is undesirable in fractionation. Agitation or stirring is the primary cause, and therefore agitation is usually kept to the minimum required to facilitate heat transfer.

2.3 Growth

Once a crystal nucleus has formed, it will start growing by the incorporation of other molecules. These molecules are taken from the adjacent liquid layer, which is replenished continuously from the surrounding, supersaturated liquid by diffusion. The growth rate is proportional to the amount of supercooling and inversely proportional to the viscosity, which affects the rate of diffusion.