Biomedical Engineering Reference
In-Depth Information
2.2.2.2
b -Glucans
b-Glucans are also called lichenins and are present particularly in barley (3-7%)
and oats (3.5-5%), whereas less than 2% b-glucans are found in other cereals. The
chemical structure of these NSP is made up of linear D-glucose chains linked via
mixed b-(1,3)- and b-(1,4)-glycosidic linkages. b-Glucans show a higher water
solubility than AX (38-69% in barley, 65-90% in oats) and form viscous solutions,
which in the case of barley may interfere in wort filtration during the production of
beer.
2.3
Proteins
The average protein content of cereal grains covers a relatively narrow range
(8-11%, Table
2.2
), variations, however, are quite noticeable. Wheat grains, for
instance, may vary from less than 6% to more than 20%. The content depends on the
genotype (cereal, species, variety) and the growing conditions (soil, climate,
fertilization); amount and time of nitrogen fertilization are of particular importance.
Proteins are distributed over the whole grain, their concentration within each
compartment, however, is remarkably different. The germ and aleurone layer of
wheat grains, for instance, contain more than 30% proteins, the starchy endosperm
~13%, and the bran ~7% [
3
]. Regarding the different proportions of these compart-
ments, most proteins of grains are located in the starchy endosperm, which is the
source of white flours obtained by milling the grains and sieving.
White flours are the most important grain products. Therefore, the predominant
part of the literature on cereal proteins deals with white flour proteins. The amino
acid compositions of flour proteins from various cereals are shown in Table
2.3
.
Typical of all flours is the fact that glutamic acid almost entirely occurs in its amidated
form as glutamine [
56
]. This amino acid generally predominates (15-31%), fol-
lowed by proline in the case of wheat, rye, and barley (12-14%). Further major
amino acids are leucine (7-14%) and alanine (4-11%). The nutritionally essential
amino acids tryptophan (0.2-1.0%), methionine (1.3-2.9%), histidine (1.8-2.2%),
and lysine (1.4-3.3%) are present only at very low levels. Through breeding and
genetic engineering, attempts are being made to improve the content of essential
amino acids. These approaches have been successful in the case of high-lysine
barley and corn.
2.3.1
Osborne Fractions
Traditionally, cereal flour proteins have been classi fi ed into four fractions (albumins,
globulins, prolamins, and glutelins) according to their different solubility and based
on the fractionation procedure of Osborne [
57
] . Albumins are soluble in water,
