Food Lipids: Chemistry, Nutrition, and Biotechnology

catalysts were active below the melting point of the fat and that the reaction reached
equilibrium within 30 minutes.
During directed interesterification, two reactions take place simultaneously. As
the trisaturate is produced by interesterification, it crystallizes and falls from solution.
Then, to regain equilibrium, the reaction equilibrium in the remaining liquid phase
is pushed toward increased production of the crystallizing trisaturate [42,47,56].
Crystallization continues until all triacylglycerols capable of crystallizing have been
eliminated from the reaction phase [43].
Early developments in the area of directed interesterification showed that the
following factors determine the effectiveness of the reaction [47,56]:
Interesterification rate in the liquid phase
Rate of heat removal
Fat crystal nucleation rate
Trisaturate crystallization rate out of liquid phase
The rate of interesterification is an important factor, as the trisaturates will precipitate
out of solution only as quickly as they are formed.
Fat crystallization generates heat. Removal of this heat is hindered by the poor
conductivity of fat and the low convection in viscous or plastic media. Heat removal
directly affects the nucleation rate. Rapid cooling to temperatures much below the
melting point of trisaturate increases the nucleation rate, hence crystallization. Trisaturate
crystallization is also hindered by the viscosity of the lipid phase. Gently
yet thorough agitation is helpful in speeding up crystallization.
For directed interesterification, Na/K alloy is the catalyst of choice, given its
low-temperature activity compared to that of the metal alone or that of the alkylates
[28,47]. Typically, the alloy is continuously metered in by a pump and well dispersed
by means of a high shear agitator to provide the proper catalyst particle size, ensuring
optimal activity. Initially, the fat is at least partially randomized at temperatures above
the melting point of the highest melting triacylglycerol. When the fractional crystallization
approach is used, the fat/catalyst slurry is chilled in conventional scrapedwall
heat exchangers to specific temperatures in a series of steps designed to maintain
the directed fractional crystallization process. Once chilled, the mixture is held for
a period of time under gently agitation to achieve the desired degree of crystal
formation. Enhancements of the procedure include stepwise reduction of temperature
and the use of temperature cycling [57,58].
Directed interesterification can be used to increase the solid fat content without
affecting unsaturated fatty acids. Periodic drops in temperature accelerate the reaction.
Kattenberg [58] applied this knowledge to interesterification of sunflower oil
and lard blends and accelerated the reaction by a factor of 3. In another study, various
oils, after directed interesterification, were chilled at 15�C for various durations (30–
180 min), then subjected to further reaction at 23�C for 12–168 hours. These treatments
influenced solid fat content of the final product [59].
The effects of directed interesterification on cottonseed oil were reported by
Eckey [35]. Cottonseed oil contains 25% saturated fatty acids. With random interesterification,
only 1.5% trisaturates was obtained, whereas directed interesterification
led to the production of 19% trisaturates.
A review by Huyghebaert et al. [60] showed that the directed interesterification
of an SOL mixture resulted in the following proportions:
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