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Food Lipids: Chemistry, Nutrition, and Biotechnology

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uted first intramolecularly, then intermolecularly until a r<strong>and</strong>om distribution is<br />

achieved. With enzymatic interesterification, more control of final product composition<br />

is possible, <strong>and</strong> glyceride mixtures that cannot be obtained using chemical<br />

interesterification can be produced (9,10). At present, r<strong>and</strong>omization of acyl group<br />

distribution using chemical interesterification is used to produce changes in crystal<br />

structure, solid fat content, <strong>and</strong> melting point of fats. Interesterification using lipase<br />

with particular specificities is used to produce high-value specialty fats, such as cocoa<br />

butter substitutes <strong>and</strong> confectionary fats (5).<br />

Enzymatic interesterification is accomplished using lipases, which are enzymes<br />

obtained predominantly from bacterial yeast, <strong>and</strong> fungal sources. Extracellular microbial<br />

lipases are produced by microorganisms <strong>and</strong> released into their growth environment<br />

to digest lipid materials (9). Lipases are defined as glycerol ester hydrolases<br />

(EC 3.1.1.3) because they catalyze the hydrolysis of carboxyl ester bonds in<br />

acylglycerols. Depending on the degree of hydrolysis, free fatty acids, monoacylglycerols,<br />

diacylglycerols, <strong>and</strong> glycerol are produced. Lipases are differentiated from<br />

esterases in that they act only on insoluble substrates. Long chain triacylglycerols,<br />

the natural substates of lipases, are insoluble in water, forming aggregates or dispersions<br />

in aqueous media. Lipases have a high affinity for hydrophobic surfaces<br />

<strong>and</strong> can be completely adsorbed from aqueous solution by emulsified long chain<br />

triacylglycerols (11). In the presence of excess water, lipases catalyze the hydrolysis<br />

of long chain triacylglycerols, but under water-limiting conditions, the reverse reaction,<br />

ester synthesis, can be achieved (8,12). Enzymatic interesterification systems<br />

are composed of a continuous water-immiscible phase, containing the lipid substrate,<br />

<strong>and</strong> an aqueous phase containing the lipase. Lipase-catalyzed interesterifications have<br />

been extensively studied in systems using organic solvents. However, if such a process<br />

is to be used in the food industry, solvent-free systems must be developed.<br />

Hence, the emphasis of this chapter will be on enzymatic interesterification performed<br />

in solvent-free systems.<br />

A. Transesterification<br />

As previously defined, transesterification is the exchange of acyl groups between two<br />

esters, namely, two triacylglycerols (Fig. 1). Transesterification is used predominantly<br />

to alter the physical properties of individual fats <strong>and</strong> oils or fat–oil blends by altering<br />

the positional distribution of fatty acids in the triacylglycerols. Transesterification of<br />

butter using a nonspecific lipase has been reported to improve the plasticity of<br />

the fat (13). Kalo et al. (14) found that transesterification of butterfat with a positionally<br />

nonspecific lipase at 40�C increased the level of saturated C48 to C54 triacylglycerols,<br />

monoene C38 <strong>and</strong> C46 to C52 triacylglycerols, <strong>and</strong> diene C40 to C54<br />

triacylglycerols. These authors also found that the diacylglycerol content increased<br />

by 45% whereas the free fatty acid content doubled. Overall, lipase-catalyzed trans-<br />

Figure 1 Lipase-catalyzed transesterification between two different triacylglycerols.<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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