Food Chemistry

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 re-

action 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 equi-

librium is continuously changed. Hence, a fat

(oil) can be divided into high and low melting

point fractions, e. g.:

(3.23)

3.3.1.4 StructuralDetermination

Apart from identifying a fat or oil from an un-
known source (cf. 14.5.2), TG structural analysis
is important for the clarification of the relation-
ship 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 simi-
lar 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 pat-
tern of triglycerols: SSS, SUS and SSU (cf. Ta-
ble 3.14). The chemical composition of Borneo

Table 3.14.Average fatty acid and triacylglycerol com-

position (weight-%) of cocoa butter, tallow and Borneo

tallow (a cocoa butter substitute)

Cocoa Edible beef Borneo

butter tallow tallowa

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

SSSb 229 4

SUS 81 33 80

SSU 1 16 1

SUU 15 18 14

USU 2

UUU 1 2 1

acf. 14.3.2.2.3

bS: Saturated, and U: unsaturated fatty acids.

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 particu-

larly 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 un-

saturation 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 inβ-position are more polar compared

to those inα-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 ac-

cording to their number of double bonds by “ar-

gentation chromatography” (cf. 3.2.3.2.3).