2009_Book_FoodChemistry

3.3 Acylglycerols 171
This assertion is only possible when a stereospecific
analysis (cf. 3.3.1.4) provides information
on the fatty acids at positions 1, 2 and 3.
rac-POS = sn-POS and sn-SOP in the molar ratio
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 as α, β ′ and β. 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-
Table 3.12. Triacylglycerols and their polymorphic
forms
Compound Melting point ( ◦ C)
of polymorphic form
α β ′ β
Tristearin 55 63.2 73.5
Tripalmitin 44.7 56.6 66.4
Trimyristin 32.8 45.0 58.5
Trilaurin 15.2 34 46.5
Triolein −32 −12 4.5–5.7
1,2-Dipalmitoolein 18.5 29.8 34.8
1,3-Dipalmitoolein 20.8 33 37.3
1-Palmito-3-stearo-2-olein 18.2 33 39
1-Palmito-2-stearo-3-olein 26.3 40.2
2-Palmito-1-stearo-3-olein 25.3 40.2
1,2-Diacetopalmitin 20.5 21.6 42.3
sen. The α-form has the lowest melting point.
This modification is transformed first into the β ′ -
form upon heating and then into the β-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 yields β-form crystals.
X-ray analysis as well as measurements by Raman
spectroscopy revealed that saturated triglycerols
in their crystalline state exist in a “chair
form” (Fig. 3.6a): The “tuning fork” configuration
for the β ′ -modification was not verified. The
different properties of the three forms are based
on the crystallization in different systems.
Fig. 3.6. Arrangement of the β ′ -andβ-form of saturated triacylglycerols in the crystalline lattice (Cartesian
coordinates a, b, c)