Influence of high protein digestibility sorghums on free amino nitrogen (FAN) production during malting and mashing

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.