Biomedical Engineering Reference
In-Depth Information
dough elasticity. A cysteine residue, likely to become involved in intra- or intermo-
lecular linkages, may also be in a different configuration in the two subunits. There
are also other protein components, such as low molecular weight glutenin
subunits
80,81
and gliadin proteins that have been found to be associated with qual-
ity.
82,83
Branlard and Dardevet
84
used the French wheat cultivar Darius, which has
very good bread-making quality, even though it possesses the HMW glutenin subunit
combination 2, 7 and 12, associated with poor quality, and a null allele at the
Gli-
D1
locus. The absence of the
Gli-D1
encoded
-gliadins was associated significantly
with higher dough tenacity and strength. These results demonstrated that using only
one locus breeders can improve particular quality traits.
84
In this way, genetic
engineering could be used to aid in the construction of wheat and other cereal
varieties with predetermined functional properties designed for a precise processing
purpose, such as bread-making, breakfast foods, meat analogs, and hydrolyzed
products, as well as for nonfood applications.
63
ω
Malting and Brewing
The technology of producing beer involves the processes of malting and brewing.
In malting, the barley grain is germinated under conditions leading to enzymatic
hydrolysis or modification of starch and protein reserves and the production of flavor
compounds, whereas brewing involves the fermentation of sugars to produce alcohol
by yeast.
85
Different varieties differ in regard to their suitability for malting, and in
general barley suitable for malting should have a low protein content.
86
It is thought
that storage proteins released from protein bodies during germination surround starch
grains, and also reduce access to amylolytic enzymes and delay sugar release, and
this leads to poor fermentation.
63
Malting quality may be affected by the hordein
fractions,
63,87
particularly B hordeins and especially disulfide-linked aggregates that
may be less easily degraded when adhering to starch. However, malting quality is
a complex character, and hordeins may affect other stages of brewing, such as
filterability, foaming, and haze formation,
88
but with further knowledge it should be
possible to identify specific proteins (e.g., disulfide-linked components) that could
be manipulated or their expression reduced such that malting quality will be
improved. Other aspects that could be manipulated include identification and removal
of genes involved in polyphenol (proanthyocyanidins) production to prevent the haze
formed by interaction of polyphenols and protein.
86,89
The copy numbers of
α
-amy-
lase genes and those of other hydrolytic enzymes (e.g.,
-glucanases) can be
increased to speed up breakdown of seed reserves during malting. The wheat
β
-amy-
lase gene, the major enzyme of starch degradation, has already been cloned, and it
has been expressed in modified yeast cells. The
α
α
-amylase has also been used for
production of low-calorie beer.
90
I
MPROVING
N
UTRITIONAL
P
ROPERTIES
Seed Storage Proteins
The nutritional quality of grains very much depends on the amino acid composition
of the storage proteins, and being macromolecules directly coded by specific genes,
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