Food Chemistry

702 15 Cereals and Cereal Products

are rich in amylose (cf. Table 4.20). Waxy corn
starch swells considerably on heating, while gran-
ules with amylose swell only slightly (cf. Ta-
ble 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 be-
havior 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 glu-
cofructans. 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.

Table 15.29.Distribution of carbohydrates in wheat
(%)

Endosperm Germ Bran

Pentosans and
hemicelluloses 2. 415. 343. 1
Cellulose 0. 316. 835. 2
Starch 95. 831. 514. 1
Sugars 1. 536. 47. 6

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 water-

soluble.

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 sol-

uble highly branched arabinogalactan peptide.

A chain ofD-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 treat-

ment with anα-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 ofβ( 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-transhydroxy-

proline.

The Ws-AX cause up to 25% of the water bind-

ing 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.

Fig. 15.17.A section of the structure of a water soluble

arabinoxylan from wheat. A xylose in the (1→4)-β-

xylan section is linked in position 3 with a 5-O-trans-

feruloyl-α-L-arabinofuranose