Food Chemistry

15.2 Individual Constituents 703

Fig. 15.18.Oxidative crosslinking of cereal pentosans

Accordingly, the baking result is positively in-
fluenced by endoxylanases which preferentially
hydrolyze Wi-AX. Since inhibitors are present
in wheat which inhibit the activity of added
endoxylanases, attempts are being made with
the help of molecular engineering techniques to
produce microbial enzymes which do not react
with these inhibitors.
The insoluble portion of pentosans from rye
swells extensively in water. This portion is
responsible for the rheological properties of
dough and the baking behaviour of rye, and
increases the crumb juiciness and chewability

of baked products. An optimum starch-pentosan
ratio is 16:1 (by weight) for rye flour.
Pentosan solutions gel when treated with hydro-
gen peroxide/peroxidase. This is due to the pres-
ence of low levels of ferulic acid (ca. 0.2%). An
enzymic phenol oxidation occurs (cf. Fig. 15.18),
which causes polymerization. This results in
build-up of a network which, along with the low
content of branched arabinofuranose, is respon-
sible for the lack of solubility of most pentosans.

15.2.4.2.2β-Glucan

The β-glucan content of cereals varies: bar-
ley 3–7%, oats 3.5–4.9%, wheat and rye kernels
only 0.5–2%. These are linear polysaccharides
with D-glucopyranose units joined byβ-1,3 and
β-1,4 linkages. Polysaccharides of theβ-glucan
type are also called lichenins. At 38◦C, 38–69%
of theβ-glucans of barley dissolve in 2 hours and
65–90% of theβ-glucans of oats.β-Glucans are
slimy mucous substances which provide a high
viscosity to water solutions. In beer production

from barley β-glucans can interfere in wort

filtration.

15.2.4.2.3 Glucofructans

Wheat flour contains 1% water-soluble, nonre-

ducing oligosaccharides of molecular weight up

to 2 kdal. They consist ofD-glucose andD-fruc-

tose. Glucofructan, which predominates in durum

wheats, probably has the following structure:

(15.6)

15.2.4.2.4 Cellulose

Cellulose is a minor constituent of the carbohy-

drate fraction obtained from starchy endosperm

cells (cf. Table 15.29).

15.2.4.3 Sugars

Mono-, di- and trisaccharides, as well as other

low molecular weight degradation products of

starch, occur in wheat and other cereals in rela-

tively low concentrations (Table 15.30). When

starch degradation occurs during dough making,

their levels increase (cf. 15.4.2.5). Mono-, di- and

trisaccharides are of importance for dough leav-

ening in the presence of yeast (cf. 15.4.1.6.1).

15.2.5 Lipids

Cereal kernels contain relatively low levels of

lipids; nevertheless, differences occur among cer-

Table 15.30.Mono- and oligosaccharides in wheat

flour

Compound (%)

Raffinose 0 .05–0. 17

Glucodifructose 0 .20–0. 30

Maltose 0 .05–0. 10

Saccharose 0 .10–0. 40

Glucose 0 .01–0. 09

Fructose 0 .02–0. 08

Oligosaccharidesa 1 .2–1. 3

aFraction soluble in 80% ethanol.