Impact of Potential Fermentation Inhibitors Present in Sweet Sorghum Sugar Solutions

In this work, the fermentation of the sweet sorghum sugars such as sucrose, glucose, and fructose to ethanol was studied in the presence of acetic acid, lactic acid, and aconitic acid, which are present in the juice or produced by microorganisms during prolonged storage of harvested materials or juice. An industrial strain of distiller’s yeast was used to produce ethanol from 100 g/L (83 g/L after inoculum) of total sugars. The fermentation time ranged from 12 to 140 h, with the longer fermentation time corresponding to clear inhibition of yeast growth and product accumulation in the presence of 8 g/L of initial acetic acid. Among the acids, only acetic acid showed a negative impact on the fermentation rates and only at levels greater than 2 g/L. Lower levels of acetic acid and all levels of lactic acid and aconitic acid (1–5 g/L) either showed an improvement in fermentation rates or in final ethanol concentration. The acidity was not controlled during the fermentation but was initially adjusted, and it is presumed that the pH buffering effect on the organic acids contributed to the higher fermentation rates and prevented the pH from naturally dropping as the fermentation progressed.