The Inhibitory Effects of Aconitic Acid on Bioethanol Production

Abstract

The fermentation of the sweet sorghum sugars, glucose, fructose, and sucrose to ethanol was studied in the presence of aconitic acid. In the past, aconitic acid has been identified as potential fermentation inhibitor, but very limited information exists about its inhibitory effects. As aconitic acid is naturally present in sweet sorghum (and its juice) and the fact that this plant has been proposed as a bioenergy crop, it was necessary to quantify the inhibition. Distiller’s yeast was used to produce ethanol from 83 g/L of total sugars in the presence of 0 to 13 g/L of aconitic acid. In some experiments, the pH of the fermentation was initially adjusted and allowed to drop as the fermentation progressed. In other experiments, the level of aconitic acid was held constant, while the pH was controlled at different set points between 2 and 4.5. In a final set of experiments, the pH was controlled to 2.9 and the initial concentration of aconitic acid was varied. It was conclusively shown that aconitic acid negatively impacts fermentation rate of distiller’s yeast and that the impact was pH dependent. At below pH 3.5, the impact was clearly observed and it became more influential at lower pH. The impact of aconitic acid on the fermentation rate was linked to the presence of undissociated aconitic acid which occurs at below pH 4.5. The level of undissociated aconitic acid that can be tolerated by the yeast depends on the pH. Thus, the most reasonable approach to improve fermentation rates in the presence of aconitic acid is to increase the pH of the fermentation. Both the ethanol yield (on sugar) and the final ethanol concentration (titer) were higher in the presence of aconitic acid but at a very small level (i.e., 4 and 3%, respectively). Thus, if the pH of the fermentation is increased, the presence of aconitic acid can be seen as advantageous.