Food Lipids: Chemistry, Nutrition, and Biotechnology

D. The ‘‘Real’’ Catalyst
The real catalyst is believed to be a metal derivative of a diacylglycerol, and the
aforementioned catalysts are most likely its precursor [11,42]. Upon catalyst addition
to the lipid, a reddish brown color slowly develops (within a few minutes, depending
on the application and reaction conditions) in the mixture, indicating the activation
of the presumed true catalyst.
Some workers time the interesterification reaction from the appearance of the
reddish brown color; others simply time the reaction from the moment of catalyst
addition. Because it is impossible to predict reaction onset, it is difficult to obtain a
partial interesterification. Most reactions are conducted until an equilibrium has been
reached. Reaction times are longer in industrial settings, because the catalyst must
be totally homogenized within the fat [43]. Preactivation is unnecessary if the catalyst
is predissolved prior to addition to the substrate [35]. Placek and Holman [47] incorrectly
attributed the induction period to the interaction between the catalyst and
impurities. While impurities are sometimes present, the induction period is not
strictly due to their presence; rather, it is due to catalyst activation. As stated by
Coenen [41] and many others, the activation energy for the catalyst is higher than
for the reaction. A preactivation of 15 minutes has been found to accelerate the
reaction itself [45]. Interestingly, Hustedt [29] stated that once the brown intermediate
had appeared in the reaction mixture, interesterification was complete. No basis was
given for this statement.
E. Reaction Termination
The interesterification reaction is allowed to continue for a predetermined time period
and is stopped with addition of water and/or dilute acid. Going [28] described three
patents dealing with catalyst removal techniques for minimizing fat loss. Generally,
most of the catalyst can be washed out with water to a separate salt, or a soap-rich
aqueous phase. Alternatively, reaction with phosphoric acid results in a solid phosphate
salt, which can be filtered out. Both these method result in substantial fat loss.
A technique has been developed that minimizes loss by addition of CO2 along with
water. The system becomes buffered with sodium carbonate at a pH low enough to
not split the fat [48].
VI. REACTION MECHANISMS
The exchange of fatty acids between triacylglycerol hydroxyl sites does not occur
directly but via a series of alcoholysis reactions involving partial acylglycerols [49].
The proposed mechanisms of chemical interesterification depend on the inherent
properties of the triacylglycerol ester carbonyl group (C—O). The carbonyl carbon
is particularly susceptible to nucleophilic attack due to electronic and steric considerations.
The electronegative oxygen pulls electrons away from the carbonyl carbon,
leading to a partial positive charge on the carbon, and also increases the acidity of
hydrogens attached to the carbon at a position � to the carbonyl group (Fig. 2).
Steric considerations also come into play. The carbonyl carbon is joined to
three other groups by � bonds (sp 2 orbitals); hence they lie in a flat plane, 120�
apart. The remaining p orbital from the carbon overlaps with a p orbital from the
oxygen, forming a � bond. This flat plane and the absence of neighboring bulky
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