Proteolysis of Sorghum Endosperm Proteins when Mashing with Raw Grain Plus Exogenous Protease and Potassium Metabisulphite

With the aim of improving free amino nitrogen (FAN) production when mashing with raw sorghum grain and exogenous enzymes, the effect of mashing with the addition of the reducing agent potassium metabisulphite (KMS) on the sorghum endosperm proteins was studied. When mashing was conducted at low temperature (40°C) over an extended period (7 h) with 0.1% KMS (sorghum basis) in addition to exogenous protease, FAN increased by approx. 6 fold to approx. 91 mg/100 g sorghum, as opposed to 5 fold to approx. 75 mg/100 g sorghum with the exogenous enzyme only. Confocal laser scanning microscopy revealed that the exogenous protease caused the endosperm protein matrix that surrounds the starch granules to break up on cooking. Transmission electron microscopy showed that the exogenous protease predominantly hydrolysed the glutelin matrix protein surrounding the kafirin protein bodies. In the presence of KMS there was also substantial breakdown of the protein bodies. Sodium dodecyl sulphate‐polyacrylamide gel electrophoresis indicated that KMS had the effect of reducing kafirin polymers and oligomers into monomers. It appears that the addition of KMS in a sorghum grain mashing system significantly improves the rate of sorghum protein hydrolysis because of the reduction of intermolecular disulphide bonds in the kafirin protein, which allows better access of the protease, resulting in improved FAN production.