Determination of by-products formed during the ethanolic fermentation, using batch and immobilized cell systems of Zymomonas mobilis and Saccharomyces bayanus

Summary

Recent research has indicated that the bacterium Zymomonas mobilis might be an interesting alternative for the production of ethanol from sugar, when compared to Saccharomyces. Especially in immobilized cell systems the bacterium showed a higher productivity. One of the factors affecting productivity might be a lower efficiency of glucose to ethanol conversion. Therefore the amounts and the nature of the by-products of the ethanolic yeast and bacterial fermentations were studied using batch systems and immobilized cell systems. Glycerol, succinate, acetate, lactate, acetoin, and butanediol syntheses were studied. It was found that Zymomonas produced fewer by-products than Saccharomyces and that the by-product profile was different. Glycerol was a characteristic yeast product while lactate was typical for the bacterium. There results were verified for the batch systems as well as for the immobilized systems. Mixed cultures of yeast and bacterium resulted in the highest productivity using immobilized systems. The system using small stones coated with agar and gelatin was proved to be superior to a previous system using carrageenan gel beads. The higher productivity shown by the bacterium cannot be explained, however, only by the shifting of less sugar to by-product synthesis.