Effect of calcium ions on ethanol production from molasses by Saccharomyces cerevisiae

Abstract

One of the most widely used feedstocks for potable or biofuel ethanol fermentation is blackstrap molasses, a byproduct of cane sugar production. Inconsistent quality of molasses frequently found in industrial production, however, makes ethanol production much less efficient and cost-ineffective. Besides fermentable sugars (sucrose, glucose and fructose) present in molasses, calcium is also found in the range of 10–20% of the total carbonated ash (15% by weight). In the sugar process, calcium is usually applied as lime, CaO, during defecation, which sometimes results in over-liming if juice quality is poor. In this study, the role of calcium residues (0–0.72% w/v of Ca²⁺ in the form of calcium chloride) on fermentation efficiency of yeast was evaluated using model solutions of sucrose, glucose and fructose (20% w/v). The results suggest a detrimental effect of calcium ions on yeast performance which was concentration dependent. A slight decrease in fermentation rates and ethanol yields was evident when calcium was present at 0.18% w/v Ca²⁺ in all sugar solutions. This effect was more pronounced when calcium ion concentration increased. At 0.72% w/v of Ca²⁺, the rates of fermentation and ethanol yields of all sugars were considerably decreased (the ethanol yields decreased by 14–25% relative to the control sample, i.e. no calcium ion added). At a very high concentration of Ca²⁺ (2.16% w/v), yeast fermentation of sucrose was almost absolutely inhibited. This might be, in part, due to the inhibition effect of invertase enzyme for conversion of sucrose to invert sugars, a limiting step in ethanol fermentation of sucrose by yeast. The pretreatment of molasses by acid prior to fermentation was then introduced to remove calcium which improved the fermentation efficiency.