Abstract:
In sorghum brewing, obtaining sufficient Free Amino Nitrogen
(FAN) for rapid and complete fermentation remains a problem
due to the high proportions of unmalted sorghum used and the
poor digestibility of wet-heat treated sorghum protein. Sorghum
mutant lines with high protein digestibility have been developed
through breeding. These high protein digestibility sorghums
(HPDS) have protein bodies with villi-like borders that apparently
facilitate protease access. This work investigated FAN
production from HPDS when malted and mashed, to assess their
potential for use in sorghum brewing to improve wort FAN levels.
When malted, HPDS contained substantially higher levels of
FAN than normal protein digestibility sorghums (NPDS), 32
mg/100 g malt more. However, when the HPDS were mashed
either as malt, or as grain or malt plus exogenous proteases,
FAN production during mashing was not substantially higher
than with NPDS subjected to the same treatments, only 6, 6–18
and 9–13 mg/100 g grain or malt, respectively. This is probably
due to wet-heat induced cross-linking of the kafirin proteins
reducing their susceptibility to proteolysis. Notwithstanding this,
HPDS could be very useful for improving FAN levels in sorghum
brewing if they are malted. sorghum this is a particular issue as the grists often comprise
a low proportion of malt relative to unmalted adjunct
or even 100% unmalted sorghum. Recent work on improving
FAN production in sorghum brewing has focused
on the use of exogenous proteases7,10,12. However, it appears
that to achieve sufficient proteolysis of the sorghum
proteins, excessively high levels of exogenous proteases
may be required. This can be attributed to the poor digestibility
of the sorghum kafirin storage protein3, when it
has been subjected to wet-heat treatment, as occurs in the
brewing process.
Weaver and co-workers at Purdue University identified
mutant sorghum lines with improved protein digestibility23.
These high protein digestibility sorghums (HPDS)
were developed by crossing normal sorghum lines with a
high-lysine sorghum mutant. More recently, research at
Texas A&M University has focused on breeding to improve
the agronomic and end-use quality of the HPDS,
and determination of quantitative trait loci (QTLs) for the
HPD trait24.
This work therefore investigated FAN production from
HPDS when malted and mashed, with the objective of
assessing their potential for use in sorghum brewing to
improve wort FAN levels.
