2009_Book_FoodChemistry

15.2 Individual Constituents 703
from barley β-glucans can interfere in wort
filtration.
15.2.4.2.3 Glucofructans
Wheat flour contains 1% water-soluble, nonreducing
oligosaccharides of molecular weight up
to 2 kdal. They consist of D-glucose and D-fructose.
Glucofructan, which predominates in durum
wheats, probably has the following structure:
Fig. 15.18. Oxidative crosslinking of cereal pentosans
Accordingly, the baking result is positively influenced
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 hydrogen
peroxide/peroxidase. This is due to the presence
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 responsible
for the lack of solubility of most pentosans.
15.2.4.2.4 Cellulose
(15.6)
Cellulose is a minor constituent of the carbohydrate
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 relatively
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 leavening
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-
15.2.4.2.2 β-Glucan
The β-glucan content of cereals varies: barley
3–7%, oats 3.5–4.9%, wheat and rye kernels
only 0.5–2%. These are linear polysaccharides
with D-glucopyranose units joined by β-1,3 and
β-1,4 linkages. Polysaccharides of the β-glucan
type are also called lichenins. At 38 ◦ C, 38–69%
of the β-glucans of barley dissolve in 2 hours and
65–90% of the β-glucans of oats. β-Glucans are
slimy mucous substances which provide a high
viscosity to water solutions. In beer production
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
Oligosaccharides a 1.2–1.3
a Fraction soluble in 80% ethanol.