2009_Book_FoodChemistry

15.2 Individual Constituents 701
in Formula 15.4. It was found that GSH reacts
very specifically in dough making because the
intermolecular disulfide bridges involving C b∗
and C x in LMW (Fig. 15.10) are reduced to
an extent of 47% in each case. Intramolecular
disulfide bonds are hardly attacked. Since only
one cleavage of intermolecular disulfide bonds
can weaken the gluten and the dough, the strong
rheological effect of GSH is understandable.
The specificity of GSH is very remarkable since
per g of flour, 50–100 nmol of GSH are faced
with about 9000 nmol/g PSSP, which contain
intermolecular disulfide bonds only to an extent
of 10%.
15.2.4 Carbohydrates
15.2.4.1 Starch
The major carbohydrate storage form of cereals,
starch (cf. Table 15.6) occurs only in the en-
dosperm cells. The size and form of the starch
granules are specific for different cereals. The
polysaccharide molecules in starch granules are
radially organized. Due to the presence of alternating
amorphous (mainly amylose) and semicrystalline
layers (amylopectin), differences in indices
of refraction can be observed under a microscope.
Starch granules swell when heated in water
suspension. At the end of swelling, they lose
their form; i. e. they gelatinize. The temperature
range in which these changes occur and
also the extent of swelling at a given temperature
are characteristic (cf. 4.4.4.14.1) and
may be used for starch source identification.
Starch absorbs ca. 45% of water in the baking
process.
Cereal starches consist of about 25% amylose
and 75% amylopectin (cf. Table 4.20). The chemical
structures of these polysaccharides are presented
in 4.4.4.14.3 and 4.4.4.14.4. Starch granules
in some cultivars, for instance waxy corns,
contain only amylopectin, while some cultivars
Table 15.28. Lipids in various cereal starches
Wheat Corn a (maize) Amylomaize a Waxy maize a
(% or mg/100g) b
Nonpolar lipids 6% 60% 73% 88%
Sterol esters 2 3 9 7
Triacylglycerols 15 5 16 12
Diacylglycerols 7 3 16 6
Monoacylglycerols 8 12 13 5
Free fatty acids 27 380 650 105
Glycolipids 5% 1% 5% 6%
⎫
Sterol glycosides 3 7 13 ⎬
3
Monogalactosyldiacylglycerols 4 1
Monogalactosylmonoacylglycerols 10
⎭
18
Digalactosyldiacylglycerols 11 2
Digalactosylmonoglycerols 24 17 3
Phospholipids 89% 39% 22% 6%
Lyso-phosphatidyl ethanolamines 104 17 16 1
Lyso-phosphatidyl glycerols 23 6 7 trace
Lyso-phosphatidyl cholines 783 226 183 8
Lyso-phosphatidyl serines 26 8 6 trace
Lyso-phosphatidyl inositols
Total lipids 1,047 667 964 153
a Amylose content in starch amounts to 23% (corn), 70% (amylomaize) and 5% (waxy maize cultivars).
b Results for lipid classes are expressed as % of total lipids present in starch, and for individual lipid compounds
as mg/100g starch dry matter.