2009_Book_FoodChemistry

3.8 Unsaponifiable Constituents 233
reaction sequence given in Fig. 3.43. As shown,
the assay is applicable only to sterols containing
a double bond, such as in the B ring of
cholesterol.
Sterols are separated as 3,5-dinitrobenzoic acid
derivatives by thin layer chromatography and,
after reaction with 1,3-diaminopropane, are
determined quantitatively with high sensitivity
in the form of a Meisenheimer adduct. Sterols
and triterpene alcohols are silylated and then
analysed by gas chromatography. One application
of this method is illustrated by the detection
of 5% coberine in cocoa butter (Fig. 3.44). The
compounds α-amyrine and lup-20(29)-en-3β-ol
(Formula 3.113a and 3.116) serve as indicators.
They are present in much higher concentrations
in some cocoa butter substitutes than in cocoa
butter. Coberine is a cocoa butter substitute made
by blending palm oil and shea butter (the shea is
an African tree with seeds that yield a thick white
fat, shea butter).
The content of egg (more accurately, the yolk) in
pasta products or cookies can be calculated after
the cholesterol content has been determined,
usually by gas chromatography or HPLC. Vitamin
D determination requires specific procedures
in which precautions are taken with regard to
the compound’s sensitivity to light. A chemical
method uses thin layer chromatographic separation
of unsaponifiables, elution of vitamin D from
the plate and photometric reading of the color developed
by antimony (III) chloride. An alternative
method recommends the use of HPLC.
3.8.3 Tocopherols and Tocotrienols
3.8.3.1 Structure, Importance
Fig. 3.44. Gas chromatographic separation of the
triterpene alcohol fraction from coberine (A), cocoa
butter (B) and cocoa butter +5% coberine (C) (according
to Gegiou and Staphylakis, 1985). 1, lanosterol;
2, β-amyrine; 3, butyrospermol; 4, 24-methylene
lanostenol; 5, parkeol;6, cycloartenol; 7, α-amyrine;
8, lup-20(29)-en-3β-ol; 9, 24-methylene cycloartenol;
10, ψ-taraxasterol; 11, taraxasterol; 12, cyclobranol
The methyl derivatives of tocol [2-methyl-2(4 ′ ,
8 ′ ,12 ′ -trimethyltridecyl)-chroman-6-ol] are denoted
tocopherols. In addition the corresponding
methyl derivatives of tocotrienol occur in
food.
All four tocopherols and tocotrienols, with the
chemical structures given in Fig. 3.45, are found
primarily in cereals (especially wheat germ oil),
nuts and rapeseed oils. These redox-type lipids
are of nutritional/physiological and analytical interest.
As antioxidants (cf. 3.7.3.2.1), they prolong
the shelf lives of many foods containing fat
or oil. The significance of tocopherols such as vitamin
E is outlined in 6.2.3.
About 60–70% of the tocopherols in oilseeds are
retained during the oil extraction and refining
process (cf. 14.4.1 and Table 3.54). Some oils
with very similar fatty acid compositions can
be distinguished by their distinct tocopherol
spectrum. To illustrate this, two examples are
provided. The amount of β-tocopherol in wheat
germ oil is quite high (Table 3.54), hence it
serves as an indicator of that oil. The blending
of soya oil with sunflower oil is detectable
by an increase in the content of linolenic acid
(cf. 14.5.2.3). However, it is possible to make
a final conclusive decision about the presence
and quantity of soya oil in sunflower oil only
after an analysis of the composition of the
tocopherols.
The tocopherol pattern is also different in almond
and apricot kernel oil (Table 3.54) whose fatty
acid compositions are very similar. Therefore
adulteration of marzipan with persipan can be
detected by the analysis of the tocopherols.