Food Lipids: Chemistry, Nutrition, and Biotechnology

however, since monoacylglycerols and diacylglycerols are undesirable by-products
[28]. Glycerolysis is an alcoholysis reaction in which glycerol acts as the alcohol
[31].
Transesterification is the most widely used type of interesterification in the food
industry. Hence, we concentrate on this reaction. Figure 1 shows the effects of interesterification
on the fatty acid distribution of a putative triacylglycerol (1-stearoyl-
2-oleoyl-3-linoleoyl glycerol) (SOL). In sequence, the ester bonds linking fatty acids
to the glycerol backbone are split; then the newly liberated fatty acids are randomly
shuffled within a fatty acid pool and reesterified onto a new position, either on the
same glycerol (intraesterification) or onto another glycerol (interesterification) [1].
For reasons involving thermodynamic considerations, intraesterification occurs at a
faster rate than interesterification [33]. Once the reaction has reached equilibrium, a
complex, random mixture of triacylglycerol species is obtained (Fig. 1).
The extent of the effects of interesterification on the properties of a fat will
depend on the fatty acid and triacylglycerol variety of the starting material. If a
single starting material (e.g., palm stearin) is randomized, the effects will not be as
great as if a hardstock is randomized with a vegetable oil [34]. Furthermore, if a
material has a quasi-random distribution prior to randomization (e.g., tallow), randomization
will not lead to notable modifications.
The interesterification reactions consists of three main steps: catalyst activation,
ester bond cleavage, and fatty acid interchange. We now examine each subject in
detail.
V. INTERESTERIFICATION CATALYSTS
A. Is a Catalyst Necessary for Interesterification?
Interesterification can proceed without catalyst at high temperatures (�300�C); the
desired results are not obtained, however, because equilibrium is slowly attained at
such temperatures, and isomerization, polymerization, and decomposition reactions
can occur [11,28,35]. In fact, polymerization has been shown to occur at 150�C [36].
Addition of catalyst significantly lowers reaction temperature and duration [37].
Other important considerations include the type and concentration of catalyst [38].
B. Available Catalysts for Interesterification
There are three groups of catalysts (acids, bases, and their corresponding salts and
metals), which can be subdivided into high and low temperature groups [27]. High
temperature catalysts include metals salts such chlorides, carbonates, oxides, nitrates,
and acetates of zinc, lead, iron, tin, and cobalt [39]. Others include alkali metal
hydroxides of sodium and lithium [40]. Most commonly used are low temperature
catalysts such as alkylates (methylate and ethylate) of sodium and sodium/potassium
alloys; however, other bases, acids, and metals are also available [41]. Alkylates of
sodium are simple to use and inexpensive, and only small quantities are required.
Furthermore, they are active at low temperatures (