(257f) Production of Yeast Biomass From Wood Molasses Using Non-Recombinant Xylose-Utilizing Saccharomyces Cerevisiae

The yeast Saccharomyces cerevisiae has been used for over 200 years in industrial applications for production of bread, beer, wine, fuel ethanol, protein-rich animal feeds, flavors, amino acids, vitamins, enzymes and various other products. The world consumes over 3 million tonnes of S.cerevisiae per annum, just for the manufacture of leavened bread alone. This yeast therefore represents a very important part of the human food chain. The major cost component for yeast manufacture is the substrate molasses or syrups that are rich in easily metabolized six carbon sugars such as glucose and fructose. The cost and availability of such substrates for yeast biomass production are under increasing pressure as more of these are used for fuel ethanol production. Therefore the yeast manufacturing industry seeks alternative substrates for manufacture of the yeast biomass.

Hemicellulose is present as 20 to 40% dry weight of all wood and grass derived materials. Hence, agricultural residues, forestry materials, paper/pulp byproducts and even organic fractions from municipal wastes could all be hydrolyzed to produce xylose-rich concentrates known as ?wood molasses? and these could represent substrates for manufacture of yeast biomass. The problem with wood molasses as an alternative substrate though is that native S. cerevisiae is generally regarded as being unable to utilize the five carbon sugar xylose, which is the predominant sugar within hemicellulose. However, we have used natural genetic procedures, without the use of any genetic engineering, to obtain non-recombinant strains of S. cerevisiae that grow aerobically on xylose as well as other carbon present in wood molasses.

Wood molasses was produced from sugarcane bagasse using a dilute acid, low temperature pretreatment to disrupt the crystalline structure. This allowed for the hydrolyzed hemicellulose components to be separated as a liquid fraction from the still solid lignin-cellulose fibre. The xylose-rich hemicellulose fraction was concentrated to produce a wood molasses that was also high in acetate, furans and other compounds. This wood molasses was used to grow S. cerevisiae strain MBG3248, using an aerobic fed-batch process similar to that currently employed in the bakers yeast manufacturing industry. Yeast doubling times, yield efficiencies on substrate fed and productivity rates approached those achieved industrially when molasses is used as the substrate, although the source of the wood molasses did impact on these parameters.

The ability to grow non-recombinant S. cerevisiae on wood molasses opens up several opportunities in lignocellulosic bioconversion. Most obviously it will be possible to make cellulosic ethanol and yeast biomass from the same feedstock, which means that we are addressing issues of food versus fuel and making it possible to vastly improve the productivity of land currently used for biofuel crops. It will even be possible to manufacture protein-rich feed yeast from land used for energy crops such as switch grass. The value of the non-recombinant S. cerevisiae can range between 20 cents and $7 per kg, depending on its application, and this would be beneficial to a cost effective cellulosic ethanol biorefinery.