P29: Upgrading the hemicellulosic fraction of biomass using various fermentation strategies

Most processes for the biological conversion of ligno-cellulosic biomass (LCB) to bio-products require the use of cellulases. One of the most efficient organisms for the production of these enzymes is the imperfect fungus Trichoderma reesei, mainly thanks to its high secretion capacity. Unfortunately, the cost of cellulases is still high and is one of the main bottlenecks to the development of biological processes using ligno-cellulosic feedstocks.

Of the different possible options for decreasing the enzyme production cost, one of the more promising is to use inexpensive substrates. The possibility of producing cellulases from the hemicellulosic fraction was investigated since T. reesei can use pentoses as carbon sources. Many pretreatment methods of the ligno-cellulosic materials result in the hydrolysis and solubilization of the hemicellulosic fraction. In addition to monomeric pentoses and/or hexoses, the hemicellulosic hydrolysates often contain acetic acid, furfural, 5-HMF and phenolics, which result from the degradation of sugars and lignin.

Upgrading pentose sugars in such a process is not easy, since these degradation compounds are known to inhibit T. reesei. Therefore we determined the kinetics of growth and enzyme production by Trichoderma on different hemicellulosic hydrolysates (wheat straw, etc). Various fermentation strategies have been tested to reduce the effect of the degradation compounds and a successful approach has been designed to produce cellulases at a concentration higher than 100g/L.

These results illustrate how the economic success of using LCB as an industrial fermentation substrate in industry will largely be dependent on microbial capabilities and process considerations.