INDUSTRIAL PRODUCTION OF INTER-ESTERIFIED PRODUCTS

Vaibhav P. Nikhar 12OIL1013

Interesterification

It is of common knowledge that natural fats do not have a perfect distribution of fatty acids among the glyceride molecules. The tendency for certain acids to be more concentrated at particular Sn positions varies according to species and their environment and location in the plant or animal. The physical characteristics of a fat are greatly affected not only by the nature of constituent fatty acids (i.e. chain length and unsaturation) but also by their distribution in the triacylglycerol molecules. In effect, the unique fatty acid distribution patterns of some natural fats limits their industrial applications. Interesterification is one of the processes that can be applied to improve the consistency of such fats and to improve their usefulness. This process involves rearranging the fatty acids in such a way that their distribution among the triacylglycerol molecules of the fat becomes random (random interesterification) or conforms to some special pattern (directed interesterification).

1. How does interesterification occur?

The term interesterification refers to the exchange of acyl radicals between an ester and an acid (acidolysis), an ester and an alcohol (alcoholysis) or an ester and an ester (transesterification). It is the latter reaction that is relevant to the industrial interesterification of oils (also referred to as randomization) since it involves ester interchange within a single triacylglycerol molecule (intraesterification) as well as ester exchange among different molecules.

If one considers that two (n) fatty acids (A and B) are available to be esterified to glycerol, eight possible triacylglycerol species can result (n3).

Regardless of the distribution of the two acids in the original fat (e.g. AAA and BBB or ABB, ABA, BBA), interesterification results in the "shuffling" of fatty acids within a single molecule and among triacylglycerol molecules until an equilibrium is achieved in which all possible combinations are formed. Quantitative proportions of the different species depend only on the amount of each acid in the original fat and can be predicted.

2. Industrial interesterification processes

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Batch processes

The most common industrial interesterification process is a batch process using sodium methanolate or ethanolate as the catalyst. It employs a reaction vessel with a conical bottom that can be heated, agitated and evacuated. Because alcoholates react with water and FFA, the oil or fat to be interesterified must be dried to a water content below 0.01% and neutralized to a FFA content below

0.05%. In practice this is achieved by the addition of caustic soda followed by drying and taking a sample to demonstrate alkalinity. Drying can be effectively achieved by spray drying in vacuo, but agitating a batch heated to 80 to 100°C or higher under vacuum and/or sparging with nitrogen is also sufficiently effective. An amount of 0.05% by weight of catalyst generally suffices and the use of preweighed aliquots packaged in sealed plastic bags is recommended. Equilibrium is generally reached within a few minutes at temperatures around 90°C but reaction times of 30 min are common.

After this period of time, the catalyst is inactivated by the addition of water that may have been acidified. This water leads to the formation of soaps that can subsequently be removed by washing with water or by the addition of some silica hydrogel. If acidified water is used for the inactivation, the resulting FFA will be removed during the subsequent deodorization treatment, which will also remove the FAME formed after catalyst addition. The yield loss involved in the formation and subsequent removal of FAME and FFA has been estimated by Kellen’s (2000) to amount to 13 times the weight of the sodium methanolate added; this corresponds to 30% of the variable costs.

According to the enolate mechanism, anhydrous catalyst inactivation would avoid a fatty acid moiety to be split off and lost. Accordingly, the use of gaseous carbon dioxide, but the use of phosphoric acid is also mentioned as a means to minimise process losses; Eckey used glacial acetic acid for catalyst inactivation. Instead of alkali alcoholates, alkali metals can also be used as catalysts in the randomisation process. They can be extruded into the oil to be interesterified, where they melt if the oil is sufficiently warm so that they can be easily dispersed by a homogeniser or high shear agitator.

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Continuous processes

Continuous processes only lend themselves to standard grades with a steady and sufficiently large demand. Failing this, the necessary product changes would make the continuous process more expensive than the batch process. Directed interesterification processes as used for lard in the U.S. were continuous or semicontinuous. One of the processes used for sunflower seed oil in Belgium commenced continuously with drying the oil and dispersing the catalyst into the oil, activating the catalyst, then chilling the interesterifying stream while feeding this stream into one of several crystallisation vessels, but subsequently, the contents of these vessels were treated as separate batches. Their temperature was maintained and controlled by circulating the vessel contents over a scraped surface heat exchanger and crystallisation was also promoted by slowly agitating the vessels. When a batch had reacted sufficiently, the catalyst was inactivated by the addition of water that had been acidulated with phosphoric acid while the vessel contents were being transferred to a washing vessel. There the reaction mixture was heated and fully melted, the acid water was drained away, and the oil was washed with water before being dried and sent to intermediate storage. The review by Going (1967) shows flow diagrams of continuous randomisation processes using sodium hydroxide catalyst and sodium methanolate as catalysts. These diagrams have been redrawn by Rozendaal (1997). Both diagrams show an oil heater to raise the oil temperature to 150 to 180°C and a two-stage vacuum dryer, but if sodium hydroxide is used, this catalyst is added before the oil is dried. The hot oil is then given a residence time of 2 min before being cooled, water-washed, and centrifuged to eliminate the soaps. If sodium methanolate is used as the catalyst, the oil is dried and cooled to 50°C before catalyst addition. The catalyst is slurried in a small amount of dry oil and metered into the main stream of dry oil, which is then given some residence time before the catalyst is inactivated by water addition. The latter process is still being used. In this process, a solution of caustic soda in glycerol is metered into a stream of preheated oil and the mixture is sprayed into a vacuum dryer. From this vessel, the reaction mixture is pumped through a plug flow reactor to the refining stage.\

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Process control

In industrial practice, control of the interesterification process amounts to verifying if the reaction has taken place. If this is not the case, the batch concerned has to be reprocessed, which means that the control method has to be fast and preferably suitable for immediate use on the shop floor. If the melting point of the interesterified product is sufficiently different from the melting point of the starting mixture, determining the melting point is a quick and easy way of establishing interesterification. If the difference is fairly small, a fast determination of the solid fat content may have to be used. In both cases, samples should be taken before catalyst inactivation so that the reaction can be completed by the addition of extra catalyst. A quite different approach has been described by Liu and Lampert (1999; 2001). It makes use of the reddish-brown colour that appears on initiation of the interesterification reaction, since they claim that “there is a direct correlation between the absorbance of the reaction mixture and the degree of interesterification.” Accordingly, they also claim partially interesterified mixtures, which they say have “unique functional properties compared to the physical and randomized blends.” However, they are not that unique since it was shown on mathematical grounds that partially interesterified products can be regarded as mixtures of the noninteresterified starting material and the fully randomised end-product.

3. Mechanisms

There are two proposed mechanisms for interesterification, both important.

Enolate Ion Formation: - This mechanism supports that an enolate ion (II), typical of the action of a base on an ester, is formed. The enolate ion reacts with another ester group in the triacylglycerol molecule to produce a  -keto ester (III) which in turn reacts further with other esters to give other  -keto esters. In this way all ester groups may react and in the triacylglycerol will thus move around from their initial positions.

The same mode of action applies to ester interchange between two or more triacylglycerol molecules. The intra-ester ester interchange is believed to predominate in the initial stages of the reaction.

Carbonyl Addition: - Here, the alkylate ion adds on to a polarized ester carboxyl producing a diglycerinate intermediate.

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This intermediate reacts with another glyceride by abstracting a fatty acid, thus forming a new triacylglycerol and regenerating a diglycerinate for further reaction. Ester interchange between fully saturated S3 and unsaturated U3 molecules is shown below, as a model for the randomisation which occurs:

The K values presented are the statistical values of equilibrium constants.

4. Directed Interesterification

A random distribution is not always the most desirable. Interesterification can be directed away from randomness if the oil is maintained at a temperature below the melting point of some triacylglycerol’s which might result. This results in selective crystallization of the trisaturated glycerides, with the effect of removing them from the reaction mixture as they crystallize upon formation and the fatty acid composition of the liquid phase keeps changing. Interesterification thus proceeds with the formation of more trisaturated glycerides than would have otherwise occurred. The process continues until most of the saturated fatty acids in the oil have precipitated. If the original oil is a liquid one containing a substantial amount of saturated acids, it is possible, by this method, to convert the oil into a mixture of products, one of them a very unsaturated oil, the other a solid fat with the consistency of shortening, obtained without resorting to hydrogenation or blending with a hard fat. The procedure is relatively slow due to the low temperature used, the time required for crystallization, and the tendency of the catalyst to become coated. A dispersion of liquid sodium-potassium alloy is commonly used to slough off the coating as it forms.

Rearrangement can also be selectively controlled during interesterification by adding excess fatty acids and continuously distilling out the liberated acids that are highly volatile. This impoverishes the fat of its acids of lower molecular weight. The content of certain acids in fat also can be reduced by using suitable solvents to extract appropriate acids during the interesterification process.

5. Interesterification products

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Several interesterification products have already been mentioned, such as the interesterified lard produced in the U.S. by randomisation or by directed interesterification, health margarines produced by directed interesterification of high linoleic acid oils and the products made by enzymatic interesterification, such as cocoa butter equivalents, fat bloom inhibitor, and infant. Accordingly, this section will be limited to interesterification products obtained by chemical randomisation and used, for example, as hard stock in fat blends for margarines and shortenings. One such hardstock is based upon palm oil and/or palm stearin and a lauric oil. Palm stearin itself is not suitable as a hardstock because of its high content of high melting, fully saturated triglycerides that would give the resulting spread a sticky feel in the mouth. Accordingly, the melting point of the high melting triglycerides is reduced by the introduction of shorter chain fatty acids originating from the lauric oil. Such randomised hardstocks have the additional advantage that they have not been subjected to the chemical hydrogenation process, which in health stores tends to be regarded as a positive sales argument. Their disadvantage is that their use leads to margarines with a relatively high content of saturated fatty acids. This content has been somewhat reduced by interesterifying a very hard palm stearin and a palm kernel stearin. The disadvantage has been more effectively overcome by the use of fully hydrogenated and randomised blends of a long-chain oil like soya bean oil and a lauric oil, as had already been described by Gooding (1943). Although the hardstocks themselves are fully hydrogenated, only small amounts (8 to 15% by weight) are required to provide tub margarines with the consistency required. Blending a relatively small amount of such fully hydrogenated hardstocks with, for instance, liquid sunflower seed oil, allows health margarines to be produced with a fat blend that has a low (around 25%) content of saturated fatty acids and a relatively high (about 55%) linoleic acid content. These figures can be improved upon by using the midfraction of such interesterification. The SFC values of such products have been tabulated by Allen (1996). In Europe, sunflower seed oil has a health connotation, so spreads based solely on sunflower seed oil tend to command a premium. However, partially hydrogenated sunflower seed oil is β-tending, so blending this hardstock with liquid sunflower seed oil will lead to sandiness resulting from the large and high melting β-crystals. This can be effectively overcome by interesterifying the partially hydrogenated sunflower seed oil with some liquid oil and using the interesterification product as hardstock to be blended with liquid sunflower seed

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oil. Another possibility is to interesterify the partially hydrogenated sunflower seed oil with oil that is relatively rich in Palmitic acid so that the interesterification product contains more than 10% of this acid. However, since the hydrogenation involved is partial, the hardstock and, thus, the final product will contain some trans isomers, which nowadays are considered to be a disadvantage. They can be produced by interesterifying fully hydrogenated oil with liquid oil and using the resulting interesterification product as hardstock. However, when processed, such fat blends tend to be α-persistent, causing the resulting margarine to be somewhat more brittle and less unctuous. This problem can be alleviated by the addition of crystallisation accelerators, such as triglycerides having fatty acid carbon chains of different chain lengths. Finally, fat substitutes, such as olestra, sucrose esterified with six to eight fatty acids, caprenin (caprocaproylbehenin), salatrim (acronym for short- and long chain acyl triglyceride molecules), and EPG (esterified propoxylated glycerin) should also be mentioned as interesterification products. It does not contribute to the energy content of crisps fried in this fat substitute. Sorbestrin is the sorbitol equivalent of olestra and is also synthesised by interesterifying a sugar (sorbitol) with FAME. The EPG fat substitute is also manufactured by interesterification with FAME. The interesterification substrate is prepared by allowing propylene oxide to react with glycerol in the presence of an alkali metal alcoholates catalyst, and the same catalyst is used for the interesterification with FAME.

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