Optimization of biogas production with bioconversion of organic solid wastes (manure) and food industry wastes

Abstract

The increasing demand for bioethanolworldwide inducesmany efforts for increasing its production, and the metabolic engineering of yeast cells is one alternative for increasing fermentation yield. At industrial conditions, ethanol yield is in the range of 90% of the maximal due to side production of glycerol, biomass and small amounts of organic acids. Thus,metabolic engineering strategies have been focus on the decrease of glycerol, the major batch fermentation by-product, in order to increase ethanol production. Recombinant strains of S. cerevisiae cells for different metabolic engineering strategies were submitted to fermentation assays in different medium carbon/nitrogen ratio, using high biomass and oxygen limitation. Three strategies were tested. The first was based on the modification of ammonia assimilation by changing the NADPH-dependent GDH1 pathway by ATP-consuming NADHdependent GS-GOGAT pathway (Nissen et al 2000). The second strategy attempt to by-pass the yeast NAD-dependent glutaraldehyde 3-P dehydrogenase by a bacterial NADP-dependent enzyme expressing the gapN gene (Bro et al, 2006). And in the third strategy, yeast cells expressed the bacterial gene encoding NADdependent alanine dehydrogenase. Both laboratory and industrial strains were modified. The results corroborated the published metabolic effect under laboratory medium composition. However, when the substrate was changed to composition closer to those found in industrial sugar cane juice thegeneticmodificationshardly produced theexpectedeffect onethanol yield.Only cellswith lower growth rates (over-expressing gapN or deleted for gdh1) produced significantly more ethanol and less glycerol than their parental. However, as we already know, such low growing cell might not be stableunderhardenvironmental conditions (Silva-Filhoet al 2005). Another important question to be raised is the fact that industrial strains are alreadymuch close tomaximal theoretical yield, so that measuring small changesathighcell density andhigh sugar content can be a difficult task.