Impact of aconitic acid on ethanol fermentation

In this work, the fermentation of the sweet sorghum sugars sucrose, glucose, and fructose to ethanol was studied in the presence of aconitic acid.  Aconitic acid is naturally present in the juice at concentrations up to 5.5 g/L.  At elevated levels, this acid is one of the major inhibitors to ethanol fermentation from sweet sorghum juice and syrups at low pH (~ pH 2.5).  An industrial strain of distiller’s yeast was used to produce ethanol from 100 g/L (83 g/L after inoculum) of total sugars.  In experiments where the pH was initially set to pH 4.5, but not controlled, very little (if any) impact was noted in the presence of up to 13 g/L aconitic acid.  In fact, the impact on fermentation rate was positive compared to acid-free controls.  This was attributed to the pH-buffering effect of aconitic acid.  However, when the initial pH was lowered and controlled, a significant impact of aconitic acid was noted when compared to acid-free controls.  Mathematical modeling of the acid-base distribution for the three protonated forms of aconitate suggest that the fully protonated form is responsible for the decrease in fermentation rate.