Food Chemistry

3.3 Acylglycerols 171

This assertion is only possible when a stereo-
specific analysis (cf. 3.3.1.4) provides informa-
tion on the fatty acids at positions 1, 2 and 3.
rac-POS = sn-POS and sn-SOP in the molar ra-
tio 1:1, i. e. the fatty acid in position 2 is fixed
while the other two acids are equally distributed
at positions 1 and 3.

POS=mixture of sn-POS, sn-OPS, sn-SOP,

sn-PSO, sn-OSP and sn-SPO

The physiological calorific value of TGs depends

on the fatty acid composition. In the case of TGs
with fatty acids of medium chain length (6–10 C
atoms), the calorific value decreases from 9
to 7 kcal/g and in the case of asymmetric TGs,
e. g., a combination of 2:0, 3:0 or 4:0 with 18:0, it
decreases to 5 kcal/g. These special TGs, which
are available only synthetically, are classified as
fat substitutes (cf. 8.16).

3.3.1.2 Melting Properties.

TG melting properties are affected by fatty acid
composition and their distribution within the
glyceride molecule (Table 3.12).
Mono-, di- and triglycerides are polymorphic,
i. e. they crystallize in different modifications,
denoted asα,β′andβ. These forms differ in their
melting points (Table 3.12) and crystallographic
properties.
During the cooling of melted acylglycerols, one
of the three polymorphic forms is yielded. This
depends also on the temperature gradient cho-

Fig. 3.6.Arrangement of theβ′-andβ-form of saturated triacylglycerols in the crystalline lattice (Cartesian
coordinates a, b, c)

Table 3.12.Triacylglycerols and their polymorphic

forms

Compound Melting point (◦C)

of polymorphic form

αβ′ β

Tristearin 55 63. 273. 5

Tripalmitin 44. 756. 666. 4

Trimyristin 32. 845. 058. 5

Trilaurin 15. 23446. 5

Triolein − 32 − 12 4 .5–5. 7

1,2-Dipalmitoolein 18. 529. 834. 8

1,3-Dipalmitoolein 20. 83337. 3

1-Palmito-3-stearo-2-olein 18. 23339

1-Palmito-2-stearo-3-olein 26. 340. 2

2-Palmito-1-stearo-3-olein 25. 340. 2

1,2-Diacetopalmitin 20. 521. 642. 3

sen. Theα-form has the lowest melting point.

This modification is transformed first into theβ′-

form upon heating and then into theβ-form. The

β-form is the most stable and, hence, also has the

highest melting point (Table 3.12). These changes

are typically monotropic, i. e. they proceed in the

order of lower to higher stability.

Crystallization of triglycerides from a solvent

system generally yieldsβ-form crystals.

X-ray analysis as well as measurements by Ra-

man spectroscopy revealed that saturated triglyc-

erols in their crystalline state exist in a “chair

form” (Fig. 3.6a): The “tuning fork” configura-

tion for theβ′-modification was not verified. The

different properties of the three forms are based

on the crystallization in different systems.