Agriculture Reference
In-Depth Information
NL (60-72%)
GL (12-22%)
PL (14-26%)
Wheat total lipids
(3-4% grain wt.)
Fig. 21.3
Anatomical distri-
bution of lipid classes in wheat
grain. The distribution of total
lipid is shown under the
names of the three main ana-
tomical parts of the grain,
namely, the germ, bran-aleu-
rone, and endosperm (NL,
nonpolar lipids; GL, glycolip-
ids; PL, polar lipids).
Germ
(25-35%)
Bran/Aleurone
(25-29%)
Endosperm
(35-45%)
NL
(80-90%)
GL
(~5%)
PL
(5-15%)
Non-starch
(~65%)
Starch
(~35%)
NL
(80-85%)
GL
(0-3.5%)
PL
(14-17%)
NL
(33-46%)
NL
GL
(30-38%)
GL
PL
(24-34%)
NL
(~5%)
GL
(0%)
PL
(~95%)
nonpolar lipids are mainly di- and mono-
glycerides, free fatty acids, and sterol esters.
The main components in the polar lipids are
glycolipids and phospholipids found in all mem-
branes, including the amyloplast membrane.
They are organized in a reversed hexagonal liquid
cystalline phase located in a protein matrix closely
bound to starch granules in the endosperm of
wheat kernels (Al Saleh et al., 1986). The princi-
ple glycolipids in whole kernels and starchy endo-
sperm are monogalactosyldiglyceride (MGDG)
and digalactosyldiglyceride (DGDG), with
smaller amounts of the corresponding monoacyl
lipids. The major phospholipids are phosphati-
dylcholine (PC), phosphatidylethanolamine (PE),
and phosphatidylinositol (PI). Monoacyl-
phosphoglycerides, or lysophospholipids, are
usually regarded as degradation products of
phospholipids.
In milled endosperm fl our, the amount of gly-
colipids is often greater than the amount of phos-
pholipids. Apart from those major acyl lipids,
other lipids in wheat include sterols, and lipid-
associated compounds such as carotenoids and
tocopherols. Although carotenoids are very minor
constituents, color contributed by carotenoids is
an important factor in the use of cereal grains in
food production, particularly in the use of durum
wheat for pastamaking.
The nonpolar lipids are concentrated in the
outer layers of the kernel, or the bran and aleu-
rone layers (Hargin and Morrison 1980). The
endosperm, the major fraction of wheat grain, has
signifi cantly lower lipid content than the other
fractions (Fig. 21.3). Hence, the lipid composi-
tion and content in the fl our can differ, depending
on the milling process and the fl our yield
(Morrison and Hargin 1981). It is also important
to note that lipid composition and content are
highly dependent on combined genetic and
agroenvironmental factors. Lipid hydrolysis can
occur during storage of wheat grain and wheat
fl our due to the presence of enzymes, such as
lipases and phospholipases. The consequent
increase of free fatty acids can have deleterious
effects on the quality of wheat fl our (e.g., increased
rancidity) and on the fi nal product.
Lipids in fl our (not wholemeal) can be subdi-
vided into nonstarch and starch lipids. Starch
lipids are predominantly of phospholipids (Fig.
21.3), and almost exclusively lysophospholipids,
in particular lysophosphatidylcholine or lysoleci-
thin. They are tightly bound as an inclusion
compound with amylose. These amylose-lipid
complexes may affect the properties of starch to
some extent, but are not available for interacting
with gluten protein during dough mixing and
gluten formation. Consequently, starch lipids
play little or no part in dough rheological proper-
ties during processing. The nonstarch lipids,
which consist of all the endosperm lipids exclud-
ing those inside starch granules, comprise approx-
imately two-thirds of the total wheat fl our lipids,
consisting of nonpolar lipids, predominantly tri-
glycerides, and relatively high amounts of glyco-
lipids and phospholipids (Fig. 21.3). It is the
