2009_Book_FoodChemistry

15.2 Individual Constituents 705
Table 15.32. Nonstarch lipids in wheat flour
Nonpolar lipids (59%)
Sterol lipids 43
Triacylglycerols (TG) 909
Diacylglycerols (DG) 67
Monoacylglycerols (MG) 53
Free fatty acids (FFA) 64
Glycolipids (26%)
Sterol glycosides 18
Monogalactosyldiacylglycerols 115
(MGDG)
Monogalactosylmonoacylglycerols
(MGMG) 17
Digalactosyldiacylglycerols (DGDG) 322
Digalactosylmonoacylglycerols 52
(DGMG)
[3pt] Phospholipids (15%)
N-Acyl-phosphatidyl ethanolamines 95
N-Acyl-lyso-phosphatidyl 33
ethanolamines
Phosphatidyl ethanolamines
Phosphatidyl glycerols 19
Phosphatidyl cholines 96
Phosphatidyl serines
Phosphatidyl inositols 9
Lyso-phosphatidyl glycerols 5
Lyso-phosphatidyl cholines 29
a Based on dry matter.
mg/100g a
The neutral lipids are present in flour in the form
of spherosomes and their membranes are formed
by a part of the phospholipids. The spherosomes
can be extracted with nonpolar solvents. The
other phospholipids and all the glycolipids
form inverse hexagonal phases (cf. 8.15.2.2),
which are only partly extractable. During dough
making, the water added results in the conversion
of the inverse hexagonal to a laminar phase,
which in turn stabilizes a microemulsion of the
neutral lipids. The microemulsion vesicles are
enclosed by the network of gluten proteins and,
consequently, difficult to extract. If the dough
is suspended in water, the lipids appear in the
aqueous phase that separates on ultracentrifugation
only when the framework of gluten proteins
has been destroyed by reduction, e. g., with
dithiothreitol.
Other hypotheses which explain the decreasing
extractability of free lipids by selective binding,
e. g., of glycolipids to starch and gluten, have not
been confirmed.
The gas-holding capacity of doughs and, after
passing through a minimum, the baking volume
(Fig. 15.20) are positively influenced by polar
lipids. Two effects are assumed in explanation.
The polar lipids get concentrated in the boundary
layer gas/liquid and stabilize the gas bubbles
against coalescence. In addition, the lipid
vesicles seal the pores which are formed in the
protein films on kneading. On the other hand,
the nonpolar lipids generally negatively influence
the backing result with most varieties of wheat
(Fig. 15.20).
Carotenoids and tocopherols belong to the
minor components of the cereal lipid fraction.
Wheat flour has a carotenoid content averaging
5.7mg/kg. In durum wheats, which have
a more intense yellow color, the carotenoids are
7.3mg/kg of flour.
The major carotenoid, lutein (cf. 3.8.4.1.2),
is present in free or esterified form (either
mono or diester) with the fatty acids listed in
Table 15.31). The following carotenoid pigments
are also present: β-carotene, β-apo-carotenal,
cryptoxanthin, zeaxanthin and antheraxanthin
(for structures see 3.8.4.1). Carotenoid content
of corn, depending on the cultivar, is 0.6–
57.9mg/kg, with lutein and zeaxanthin being the
major constituents.
Fig. 15.20. The effect of free nonstarch lipids on
the baking quality of defatted wheat flour (according
to Morrison, 1976). — Lipids (total), −◦–◦– nonpolar
lipids, – • – • – polar lipids