Cereals processing technology

66 Cereals processing technology
experiments with dicots. The experiments with dicots showed that overexpression
of the viral coat protein could result in viral protection. The
mechanism of this protection is not completely understood though it is thought
to act via silencing of the activity of the viral genome. Several of the transgenic
barley lines were resistant to the barley yellow dwarf virus. However, no field
trials have been conducted yet.
4.7.2 Improved nutritional properties
Barley grains have low contents of lysine and threonine and have therefore poor
nutritional value for animals. The biochemistry of the biosynthetic pathways for
threonine and lysine are well understood and it appears that there are two major
regulatory enzymes: aspartate kinase (AK) and dihydrodipicolinate synthase
(DHPS). Both enzymes in barley are subject to feedback inhibition by the end
products, lysine and threonine. Mutant barley varieties have been identified in
which AK lacks feedback inhibition but these barley mutants have not found any
application for commercialisation. Brinch-Pedersen et al. (1996) have taken a
transgenic approach and have expressed feedback-insensitive AK and DHPS
from E.coli in barley. Both E.coli genes were under control of the constitutive
35S promoter. Analysis of the transgenic plants showed that the leaves of the
transgenic barley lines contained a fourteenfold increase in free lysine and
eightfold increase in free methionine. Moreover, there was a twofold increase in
lysine, arginine and asparagine in the mature seeds while free proline was
reduced by 50%. No differences were observed in the composition of total free
amino acids in the seeds. These results suggest that this transgenic barley would
be of higher nutritional value than the non-transformed. The next step would be
to introduce these genes into a current malting variety and to determine what
effect the transgenes have on malting quality. If there is no effect on the malting
quality then this barley would be of no particular interest to farmers. It quite
often occurs that the malting barley variety in the field is not really up to malting
quality. This barley will be used for feeding animals and the farmers might get a
better price for high-lysine/threonine barley.
4.7.3 Improved malting quality
Since barley is used for malting purposes to serve the brewing and distilling
industry, a lot of effort has gone into transforming barley with malting-related
genes. Barley has been transformed with a heat-stable 1,3-1,4- -glucanase
hybrid from Bacillus (Jensen et al. 1996), a heat-stable -glucanase from the
fungus Trichoderma reesei (Mannonen et al. 1997) and with mutagenised barley
-amylase (Kihara et al. 1997) with higher heat stability. The corresponding
endogenous barley enzymes are heat labile and their activities are destroyed
either during kilning or mashing. In the case of 1,3-1,4- -glucanase this might
result in an extract with a high glucan content which is prone to give a beer with
a haze.