Enzyme catalysed interesterification

A schematic representation of enzyme catalysed interesterification processes and products is shown in Figure 12.6. These processes generally involve hydrolysis and re-synthesis. Under restricted water conditions, interesterification is found to be predominant (Matsuo et aL, 1980, 1981 Coleman and Macrae, 1980). Chemically catalysed interesterification processes lead to randomization of the acyl groups along the glycerol chain. Using lipases with regio-specificity, the acyl transfers are restricted to the fatty acids located at the precise positions specific to the enzyme. 

Ison, A.P., Dunnill, P. and Lilly, M.D. (1988) Effect of solvent concentration on enzyme catalysed interesterification of fats. Enzyme Microb. Technol. 10, 47-51. 

The role of reversed micelles in the manufacture of fine chemicals with enzymes also needs to be assessed and analysed. An outstanding example is lipase catalysed interesterification to produce cocoa butter substitute from readily available cheap materials (Luisi, 1985). This example of reversed micelles is sometimes referred to as a colloidal solution of water in organic systems. A number of water insoluble alkaloids, prostanoids, and steroids have been subjected to useful transformations (Martinek et al., 1987). Peptide synthesis has also been conducted. The advantages of two liquid phases are retained to a very great extent the amount of water can be manipulated to gain advantages from an equilibrium viewpoint. 

For 1,3-spedfic lipase catalysed interesterification reactions, Kyotani et al. (1988a) have provided elaborate kinetic models of the reactions using biphasic and microaqueous conditions. Four models were studied (a) first order kinetics (b) hydrolysis followed by resynthesis (c) reaction via the glyceride-enzyme complex (d) reactions via the acyl-enzyme complex. Details of the models are beyond the scope of this chapter. Interested readers are referred to Kyotani et al. (1988a) for an in-depth treatment of each model. For illustration purposes, the mathematical treatment of the simplest model is presented here. 

Interesterification can also be catalysed by enzymes, many of which show useful specificities. The 1,3-specific lipases, such as those derived from Aspergillus niger, Mucor javanicus, M. miehei, Rhizopus arrhizus, R. delemar, and R. niveus, are particularly useful for interesterification. They are used to effect acyl exchange at the sn- and 3 positions while leaving acyl groups at the sn-2 position unchanged. 

The lipase-catalysed reactions of glyceride hydrolysis and synthesis are reversible. Thus, if the water content of the reaction system is limited, the non-specific enzyme results in random interesterification but the 1,3-specific enzyme and the fatty acid-specific enzyme can be used to introduce desirable fatty acids into the triglyceride molecule and to produce preferred positional fatty acid arrangements in the triglyceride molecule. 

The uses cited by Coleman and Macrae (1980) for the lipase catalysed reaction are examples of acidolysis and transesterification. Acidolysis is illustrated by the increase in l(3)-palmitoyl-3(l)-stearoyl-2-olein (POSt) resulting from the reaction of palm oil mid-fraction with stearic acid in the presence of a 1,3-specific enzyme, and by the increase in the content of linoleic acid in olive oil when that oil is reacted with linoleic acid in the presence of an enzyme with fatty acid specificity. An example of transesterification is shown by the increase in POSt triglycerides when a 1,3-specific enzyme is used to catalyse the interesterification of shea butter and palm oil mid-fraction. 

This set of reactions is catalysed either by enzymes or acidobasic catalysts and is characterised by migration of acyls between the molecules of alcohols or in the glycerol molecule. When using acid-base catalysts, acyl migration is statistically random. There is a growing interest in using enzymes (lipases or phospholipases) in technological applications, where it is possible to perform selective interesterification reactions catalysed by specific enzymes. 

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