3-33: Novel process strategies lead towards efficient bioethanol production at high solid loadings

One current research trend for bioethanol production is to increase the solid substrate loading in order to increase the final ethanol concentration, leading to decreased distillation costs and improved water economy. Several challenges have to be addressed for successful fermentation at high substrate loadings, typically 15-30 % WIS. Firstly, at high solids loading,  high viscosity prevail leading to poor mixing and therefore large deviation in local conditions in the fermentor. Secondly, high inhibitor concentrations will typically be found in materials pretreated at high solid loadings leading to poor cellular performance and cell viability problems.

One way of addressing the problems accompanying high solid fermentations is by developing the fermentation technology and optimizing the process conditions, which will be discussed in this presentation. Three different approaches have been taken all resulting in better ethanol yield, productivity and cell viability. The approaches encompass:

• Reducing the amounts of inhibitors present in the lignocellulose hydrolysate through different strategies of detoxification
• Supplying critical nutrients to the nutritionally poor lignocellulosic hydrolysates, which helps the cells to handle the effects of the inhibitors
• Optimising the process by designing an fed-batch SSF process configuration involving feeding of substrate, enzyme and yeast leading to improved mixing and other process conditions. The feed of freshly cultivated yeast throughout the fermentation process ensured active metabolic state of yeast.

 Our approaches led to several fold improved performance at 20-25 % WIS of steam-pretreated spruce, showing that efficient fermentation at high solid loadings is within reach.