Cereals processing technology

Biotechnology, cereal and cereal products quality 67
The nucleotide sequence of the hybrid Bacillus -glucanase was extensively
modified, without altering the amino acid sequence, so that the codon usage was
more like the endogenous -glucanase. This was necessary since some codons
are very rarely used in barley aleurone and would therefore limit expression
levels. One of the -amylase promoters was used to drive the expression of the
heat-stable -glucanase. The result was that the transformant indeed produced a
heat-stable -glucanase during germination that was absent in the control plants.
The effect of the heat-stable -glucanase on malting, however, has to be
determined.
The fungal -glucanase was used in unmodified form and driven by the -
amylase promoter as well. The protein was expressed and functional during
germination through molecular weight and iso-electric points were different
from the protein isolated from the fungus. The enzyme was thermotolerant
which was revealed by -glucanase assays at 65ºC. The transgenic barley has
not been used in malting studies yet but the enzyme has been exogenously
applied during mashing and has proven to keep soluble glucans low and to
improve filterability of the wort.
The mutagenised barley -amylase with higher heat stability was driven by
the endogenous -amylase promoter. Several transformants were obtained that
expressed a heat-stable -amylase which was absent in the untransformed
plants. The effects on malting and how the heat-stable -amylase would alter the
sugar spectra of worts remains to be investigated.
4.7.4 Improved baking quality
Considerable amounts of fundamental research into the function of glutenins in
relation to dough properties have accumulated. The processing characteristics of
wheat dough are thought to be closely related to the number of active highmolecular-weight
glutenin genes. High gluten doughs are in general more elastic
and more suitable for making bread. The function and formation of the glutenin
polymer and how these could be targeted by genetic engineering are reviewed by
Vasil and Anderson (1997). Several groups have now taken transformation
approaches to introduce extra glutenins into wheat. Blechl and Anderson (1996)
and Altpeter et al. (1996) introduced various constructs containing highmolecular-weight
glutenin subunits (HMW-GS) into the wheat variety
Bobwhite. The expression of the HMW-GS under control of a glutenin
promoter was clearly demonstrated but no data on the elasticity of the dough
were presented. More recently, Barro et al. (1997) showed that indeed dough
elasticity increased with an increase in copies of HMW-GS. They transformed a
wheat line containing less endogenous HMW-GS copies and the challenge now
is to transform current cultivars which are already selected for bread-making
quality to see whether the dough elasticity in these cultivars can also be
improved.