Addition of protease during starch liquefaction affects free amino nitrogen, fusel alcohols and ethanol production of fermented maize and whole and decorticated sorghum mashes

The aim of this research was to study the effect of the dual treatment of sorghum decortication and protease addition during liquefaction with α-amylase, on the concentration of free amino nitrogen (FAN), fusel alcohols and ethanol during yeast fermentation. A bifactorial experiment was designed to revise the differences among grains (maize, whole and 9.7% decorticated sorghum) and the effectiveness of protease addition during liquefaction. The decorticated sorghum was more susceptible to protein hydrolysis compared to the whole kernel sorghum due to its lower fiber content. The protease improved the levels of FAN approximately to 60% and 30% in the maize and sorghum mashes, respectively. The maize mash contained the highest amount of FAN followed by the decorticated and whole sorghum mashes. The protease treatment improved the fusel alcohol concentration in both sorghum beers and did not affect ethanol concentration in the maize mash. Both sorghum decortication and protease addition during liquefaction are therefore recommended treatments to obtain mashes with a higher FAN level. The decorticated protease-treated sorghum mashes yielded higher amounts of ethanol compared to the maize treatments and produced the maximum ethanol after only 20 h of fermentation. This research proved that a positive synergistic effect on FAN concentration, fusel alcohols and bioethanol yields can be achieved through the proposed proceeding of sorghum decortication and protease addition.

► Decortication of sorghum improves protein hydrolysis and FAN contents.
► Protease addition during liquefaction improves FAN contents in sorghum.
► The use of protease improved fusel alcohol concentrations after yeast fermentation.
► Sorghum decortication and protease addition had a synergistic effect on ethanol.
► Decorticated sorghum treated with protease yielded more ethanol compared to maize.