Bioethanol from Soybean Molasses

Abstract

Bioethanol is an important biofuel produced by the fermentation of sugars that, together with biodiesel, biobutanol, biomethane, and biohydrogen, composes the range of alternatives to the fossil fuels. Soybean molasses is the main by-product generated at the industrial processing of soybean to produce the soy protein concentrate. It is a rich source of carbohydrates, proteins, lipids, and minerals that could be used as a fermentation medium for microbial growth. The aim of this chapter is to describe studies on the development of processes to produce bioethanol from soybean molasses, focusing on the use of different microorganisms, fermentation scales, and pretreatment strategies. Saccharomyces cerevisiae and Zymomonas mobilis presented interesting ethanol yields and productivities at laboratory scale; however, considering the adequateness to industrial facilities, only the yeast-based process was scaled-up. At pilot scale, an average ethanol yield of 44.13 % over the total initial sugars was achieved, which represented an ethanol yield of 129.2 kg, or 163.6 L of absolute ethanol per ton of dry molasses. The average productivity for the pilot scale fed-batch process was 7.882 g/Lh. After scale-up to an industrial plant, one ton of soybean molasses (dry basis) yielded 162.7 L of absolute ethanol and 3.729 tons of vinasse, a by-product containing 19.5 % solids that had to be concentrated to be employed as an energy source. The pretreatment of soybean molasses by acid and enzymatic hydrolyses provided increases in the ethanol yield over total initial sugars to 62 and 68 %, respectively, and reduced the concentration of residual sugars.