2009_Book_FoodChemistry

704 15 Cereals and Cereal Products
eals (cf. Table 15.6). The endosperm cells of oats
contain a higher level of lipids (6–8%) than wheat
(1.6%). For this reason, the overall lipid content
of oats is higher than in wheat and in other cereals.
The lipids are preferentially stored in the germ
which, in the case of corn and wheat, serves as
a source for oil production (cf. 14.3.2.2.4). Lipids
are stored to a smaller extent in the aleurone layer.
Cereal lipids do not differ significantly in their
fatty acid composition (Table 15.31). Linoleic
acid always predominates. Close attention has
been given to wheat lipids since they greatly
influence baking quality and they have therefore
been studied thoroughly.
A wheat kernel weighs 30–42 mg and contains
0.92–1.24 µg of lipid. The germ and the
aleurone cells are rich in triglycerides, which are
present as spherosomes, while phospholipids and
glycolipids predominate in the endosperm.
Wheat flour contains 1.5–2.5% lipids, depending
on milling extraction rate. Part of this lipid is nonstarch
lipid. This portion is extracted with a polar
solvent, water-saturated butanol, at room temperature.
Nonstarch lipid comprises about 75% of
the total lipid of flour (Fig. 15.19). The residual
lipids (25%) are bound to starch (cf. 15.2.4.1).
Nonstarch- and starch-bound lipids in wheat
differ in their composition (cf. Table 15.28
and Table 15.32). In nonstarch-bound lipids
the major constituents are the triacylglycerides
and digalactosyl diacylglycerides, while in
starch-bound lipids, the major constituents are
lysophosphatides in which the acyl residue is
located primarily in position 1. A decrease in
amylose content is accompanied by a decrease
in the lipid content (Table 15.28). The ratios
of nonstarch-bound lipid classes are dependent
Fig. 15.19. Differentiation of wheat flour lipids by their
solubility. 1 Flour extraction with water-saturated butanol
(WSB) at room temperature, 2 with WSB at
90–100 ◦ C, 3 with petroleum-ether, and subsequently,
4 with WSB
on the flour extraction grade. An increase in
extraction grade increases the triacylglyceride
content, since more of the germ is transferred
into the flour.
The rheological dough properties are affected by
nonstarch-bound lipids which are separated into
free and bound lipids when extracted with solvents
of different polarity. The free lipid fraction
contains 90% of the total nonpolar lipids and 20%
of the total polar lipids listed in Table 15.32.
By kneading the flour into dough, the glycolipids
become completely bound to gluten, while
other lipids are only 70–80% bound. The extent
of binding of triacylglycerides depends on dough
handling. Intensive oxygen aeration and, particularly,
addition of lipoxygenase (cf. 15.4.1.4.3)
increase the fraction of free lipids.
The increased binding of lipids in the transition
of flour to dough, which is expressed in their decreasing
extractability, is explained by the following
hypothesis.
Table 15.31. Average fatty acid composition of acyl lipids of cereals (weight-%)
14:0 16:0 16:1 18:0 18:1 18:2 18:3
Wheat 20 1.5 1.5 14 55 4
Rye 18 <3 1 25 46 4
Corn 17.7 1.2 29.9 50.0 1.2
Oats 0.6 18.9 1.6 36.4 40.5 1.9
Barley 2 22 <1 <2 11 57 5
Millet 14.3 1.0 2.1 31.0 49.0 2.7
Rice 1 <28 6 2 35 39 3