2009_Book_FoodChemistry

176 3 Lipids
Alternatively, the stereo-specific analysis can be
carried out chemically. The TGs are partially
hydrolyzed in the presence of ethyl magnesium
bromide. The resulting diacylglycerols
are isolated and their OH groups converted to
urethane with (S)-1-(1-naphthyl)ethylisocyanate.
The sn-1,3- and the diastereomeric sn-1,2-
and 2,3-di-acylglycerol urethane derivatives
are separated in a subsequent HPLC step. The
fatty acid analysis of the urethanes show the
distribution of the acyl residues in positions 1, 2
and 3.
Individual TGs or their mixtures can be analyzed
with these procedures. Based on these results
(some are presented in Table 3.16), general rules
for fatty acid distribution in plant oils or fats can
be deduced:
• The primary HO-groups in positions 1 and 3 of
glycerol are preferentially esterified with saturated
acids.
• Oleic and linolenic acids are equally distributed
in all positions, with some exceptions,
such as cocoa butter (cf. Table 3.16).
• The remaining free position, 2, is then filled
with linoleic acid.
Results compiled in Table 3.16 show that for
oil or fat of plant origin, the difference in acyl
residues between positions 1 and 3 is not as great
as for TGs of animal origin (e. g., chicken egg).
Therefore, the 1,3-random-2-random hypothesis
can provide results that agree well with experimental
findings.
The fatty acid pattern in animal fats is strongly
influenced by the fatty acid composition of animal
feed. A steady state is established only after
4–6 months of feeding with the same feed composition.
The example of chicken egg (Table 3.16)
indicates that positions 1 and 3 in triglycerides
of animal origin show much greater variability
than in fats or oils of plant origin. Therefore, any
prediction of TG types in animal fat should be
calculated from three separate fatty acid pools
(1-random-2-random-3-random hypothesis).
The specific distribution of saturated fatty acids
in the triglycerols of fats and oils of plant origin
serves as an evidence of ester oils.
Ester oils are produced by esterification of
glycerol with purified fatty acids obtained from
olive oil residues. In this case the saturated acyl
groups are equally distributed between all three
positions of the glycerol molecule, whereas
Table 3.16. Results of stereospecific analysis of some fats and oils a
Fat/Oil Position 16:0 18:0 18:1 (9) 18:2 (9,12) 18:3 (9,12,15)
Peanut 1 13.6 4.6 59.2 18.5 –
2 1.6 0.3 58.5 38.6 –
3 11.0 5.1 57.3 18.0 –
Soya 1 13.8 5.9 22.9 48.4 9.1
2 0.9 0.3 21.5 69.7 7.1
3 13.1 5.6 28.0 45.2 8.4
Sun- 1 10.6 3.3 16.6 69.5 –
flower 2 1.3 1.1 21.5 76.0 –
3 9.7 9.2 27.6 53.5 –
Olive 1 15.2 2.9 68.6 11.0 –
2 2.5 0.6 81.0 14.6 –
3 19.6 5.2 62.6 9.4 –
Palm 1 60.1 3.4 26.8 9.3 –
2 13.3 0.2 67.9 17.5 –
3 71.9 7.6 14.4 3.2 –
Cocoa 1 34.0 50.4 12.3 1.3 –
2 1.7 2.1 87.4 8.6 –
3 36.5 52.8 8.6 0.4 –
Chicken 1 68.2 6.0 12.4 2.3 –
egg 2 4.8 0.3 60.8 31.3 –
3 8.9 7.7 69.4 5.4 –
a Values in mol%. In order to simplify the Table other fatty acids present in fat/oil are not listed.