2009_Book_FoodChemistry

3.3 Acylglycerols 173
a) A single-phase interesterification where the
acyl residues are randomly distributed:
(3.22)
b) A directed interesterification in which the reaction
temperature is lowered until the higher
melting and least soluble TG molecules in the
mixture crystallize. These molecules cease to
participate in further reactions, thus the equilibrium
is continuously changed. Hence, a fat
(oil) can be divided into high and low melting
point fractions, e. g.:
Table 3.14. Average fatty acid and triacylglycerol composition
(weight-%) of cocoa butter, tallow and Borneo
tallow (a cocoa butter substitute)
Cocoa Edible beef Borneo
butter tallow tallow a
16:0 25 36 20
18:0 37 25 42
20:0 1 1
18:1 (9) 34 37 36
18:2 (9,12) 3 2 1
SSS b 2 29 4
SUS 81 33 80
SSU 1 16 1
SUU 15 18 14
USU 2
UUU 1 2 1
a cf. 14.3.2.2.3
b S: Saturated, and U: unsaturated fatty acids.
3.3.1.4 Structural Determination
(3.23)
Apart from identifying a fat or oil from an unknown
source (cf. 14.5.2), TG structural analysis
is important for the clarification of the relationship
existing between the chemical structure and
the melting or crystallization properties, i. e. the
consistency.
An introductory example: cocoa butter and beef
tallow, the latter used during the past century
for adulteration of cocoa butter, have very similar
fatty acid compositions, especially when the
two main saturated fatty acids, 16:0 and 18:0, are
considered together (Table 3.14). In spite of their
compositions, the two fats differ significantly in
their melting properties. Cocoa butter is hard and
brittle and melts in a narrow temperature range
(28–36 ◦ C). Edible beef tallow, on the other hand,
melts at a higher temperature (approx. 45 ◦ C) and
over a wider range and has a substantially better
plasticity. The melting property of cocoa butter
is controlled by the presence of a different pattern
of triglycerols: SSS, SUS and SSU (cf. Table
3.14). The chemical composition of Borneo
tallow (Tenkawang fat) is so close to that of cocoa
butter that the TG distribution patterns shown in
Table 3.14 are practically indistinguishable. Also,
the melting properties of the two fats are similar,
consequently, Borneo tallow is currently used as
an important substitute for cocoa butter. Analysis
of the TGs present in fat (oil) could be a tedious
task, when numerous TG compounds have to be
separated. The composition of milk fat is particularly
complex. It contains more than 150 types of
TG molecules.
The separation by HPLC using reverse phases
is the first step in TG analysis. It is afforded
by the chain length and the degree of unsaturation
of the TGs. As shown in Fig. 3.7
the oils from different plant sources yield
characteristic patterns in which distinct TGs
predominate.
TGs differing only in the positions of the acyl
residues are not separated. However, in some
cases it is possible to separate positional isomeric
triglycerols after bromination of the double
bonds because triglycerols with a brominated
acyl group in β-position are more polar compared
to those in α-position.
The separation capacity of the HPLC does not
suffice for mixtures of plant oils with complex
triglycerol composition. Therefore it is advisiable
to perform a preseparation of the triglycerols according
to their number of double bonds by “argentation
chromatography” (cf. 3.2.3.2.3).