2009_Book_FoodChemistry

172 3 Lipids
Table 3.13. Crystallization patterns of edible fats or oils
β-Type β ′ -Type β-Type β ′ -Type
Coconut Cottonseed Peanut
oil oil
Corn germ
Sunflower
oil Butter oil Whale oil
Olive oil Palm oil Lard
Palm seed Rapeseed
oil oil
α-form: hexagonal system; the melting point is
relatively low, since areas of the methyl ends are
freely arranged as in liquid crystals.
β ′ -form: (Fig. 3.6b): orthorhombic system; the
carbon chains are perpendicular to each other.
β-form: (Fig. 3.6c): triclinic system; parallel arrangement
of the carbon chains.
Unsaturated fatty acids interfere with the orderly
packing of molecules in the crystalline lattice,
thereby causing a decrease in the melting point
of the crystals.
TG such as 1,3-diaceto-palmitin, i. e. a triglyceride
with one long and two short-chain fatty
acids, exists in the exceptionally stable α-form.
Since films of such TGs can expand by 200 to
300 times their normal length, they are of interest
for application as protective coating for fatcontaining
foods. In edible fats and oils, more
than the three mentioned polymorphic forms can
be present, e. g., 4–6 forms are being discussed
for cocoa butter. In order to classify fats and oils,
that form is used that is predominant after solidification
(Table 3.13).
After acidification and extraction, the free fatty
acids are recovered as alkali salts (commonly
called soaps). This procedure is of interest for
analysis of fat or oil samples. Commercially,
the free fatty acids are produced by cleaving
triglycerides with steam under elevated pressure
and temperature and by increasing the reaction
rate in the presence of an alkaline catalyst (ZnO,
MgO or CaO) or an acidic catalyst (aromatic
sulfonic acid).
Methanolysis. The fatty acids in TG are usually
analyzed by gas liquid chromatography, not as
free acids, but as methyl esters. The required
transesterification is most often achieved by
Na-methylate (sodium methoxide) in methanol
and in the presence of 2,2-dimethoxypropane to
bind the released glycerol. Thus, the reaction
proceeds rapidly and quantitatively even at room
temperature.
Interesterification. This reaction is of industrial
importance (cf. 14.4.3) since it can change the
physical properties of fats or oils or their mixtures
without altering the chemical structure of
the fatty acids. Both intra- and inter-molecular
acyl residue exchanges occur in the reaction until
an equilibrium is reached which depends on the
structure and composition of the TG molecules.
The usual catalyst for interesterification is Namethylate.
The principle of the reaction will be elucidated
by using a mixture of tristearin (SSS) and triolein
(OOO) or stearodiolein (OSO). Two types of interesterification
are recognized:
3.3.1.3 Chemical Properties
Hydrolysis, methanolysis and interesterification
are the most important chemical reactions
for TGs.
Hydrolysis. The fat or oil is cleaved or saponified
by treatment with alkali (e. g. alcoholic
KOH):
(3.20)
(3.21)