2009_Book_FoodChemistry

702 15 Cereals and Cereal Products
are rich in amylose (cf. Table 4.20). Waxy corn
starch swells considerably on heating, while granules
with amylose swell only slightly (cf. Table
4.20 and Fig. 4.31).
Lipids (Table 15.28) and proteins (about 0.5%)
are among the heterogeneous constituents of
starch granules. Lipids are enclosed within the
amylose helices. In wheat starch, they consist
predominantly of lysolecithins (Table 15.28).
They are extractable from partially gelatinized
starch by using hot water-saturated butanol.
During extraction, the lipid in the amylose helix
is replaced by butanol.
The lipids complexed within the starch granules
retard swelling and increase their gelatinization
temperatures; thus they influence the baking behavior
of cereals and the properties of the baked
products.
15.2.4.2 Polysaccharides Other than Starch
Cereals also contain polysaccharides other than
starch. In endosperm cells their content is much
less than that of starch (cf. Table 15.29). They
include pentosans, cellulose, β-glucans and glucofructans.
These polysaccharides are primarily
constituents of cell walls, and are more abundant
in the outer portions than the inner portions of the
kernel. Therefore, their content in flour increases
as the degree of fineness increases (cf. rye as an
example in Table 15.36).
From a nutritional and physiological viewpoint,
soluble and insoluble polysaccharides other than
starch and lignin (cf. 18.1.2.5.1) are also called
dietary fiber. The most important fiber sources are
cereals and legumes, while their content in fruits
and vegetables is relatively low.
15.2.4.2.1 Pentosans
The pentosan content of cereals varies. Rye flour
is exceptionally rich (6–8%) in comparison to
wheat flour (1.5–2.5%). A portion of pentosans,
25–33% in wheat and 15–25% in rye, is watersoluble.
Unlike the water-soluble proteins of cereals, the
soluble pentosans are able to absorb 15–20 times
more water and thus can form highly viscous
solutions. This soluble fraction consists mainly
(ca. 85%) of a linear arabinoxylan and a soluble
highly branched arabinogalactan peptide.
A chain of D-xylopyranose units is typical of
the structure of arabinoxylan (Ws-AX), which
is extractable with water. The OH groups in the
2- and 3-position of this chain are glycosidically
linked to L-arabinofuranose (e. g. 3-position
in Fig. 15.17). The arabinose residues can
be cleaved by mild acid hydrolysis or treatment
with an α-L-arabinofuranosidase, giving
water-insoluble xylan. Although a part of the
arabinoxylan is insoluble in water (Wi-AX)
as a result of cross-linking of the chains, it
can become soluble by means of alkaline or
enzymatic hydrolysis. The backbone of the
arabinogalactan peptide is made of β(1 → 3)
and β(1 → 6) linked galactopyranose units.
It is α-glycosidically bound and contains, in
addition, arabinofuranose residues. The bonding
to the peptide is achieved via 4-transhydroxyproline.
The Ws-AX cause up to 25% of the water binding
in dough. They increase the viscosity and,
consequently, the stability of the gas bubbles.
In contrast, the action of Wi-AX is considered
to be unfavourable. They form physical barriers
against the gluten and destabilize the gas bubbles.
Table 15.29. Distribution of carbohydrates in wheat
(%)
Endosperm Germ Bran
Pentosans and
hemicelluloses 2.4 15.3 43.1
Cellulose 0.3 16.8 35.2
Starch 95.8 31.5 14.1
Sugars 1.5 36.4 7.6
Fig. 15.17. A section of the structure of a water soluble
arabinoxylan from wheat. A xylose in the (1 → 4)-βxylan
section is linked in position 3 with a 5-O-transferuloyl-α-L-arabinofuranose