In order to reach higher ethanol concentration, solids content in the SSF broth should be as high as possible. However, using too high dry matter contents can cause difficulties in stirring the viscous medium and inhibition of yeast metabolism due to the increase of toxic compounds derived from sugar or lignin during pretreatment. Both disadvantages together with the end-product inhibition due to glucose accumulation in the broth could be overcome by performing a fed-batch configuration. By the addition of slurry to the SSF broth from time to time yeast improves its ability to detoxify some inhibitors and the viscosity is diminished because the substrate is continuously hydrolysed.

Ethanol production by Kluyveromyces marxianus in a fed-batch SSF at 42 ºC and 15 FPU/g of cellulose of commercial cellulase is tested. Starting substrate loading is 10% (w/v) and it will be increased gradually up to 14% (w/v). Different addition times and different substrate pulses are going to be also studied in order to figure out the best configuration process.