Bioethanol offers a promising alternative to fossil fuels, providing renewable and “carbon neutral” energy sources that do not contribute to the green house effect. Amongst other possible sources of bioethanol precursors, lignocellulosic biomass can be enzymatically hydrolysed to provide fermentable sugars. However, because of its complex chemical structure, lignocellulose can only be efficiently hydrolysed by presently known enzyme activities after some pretreatment that renders cellulose fibres accessible to enzyme catalysis. Such pretreatment processes typically involve heating biomass to high temperatures (100-250° C.) or addition of chemicals. Large scale production of bioethanol, or other fermentation products, from lignocellulosic biomass requires large scale pretreatment and processing. Accordingly, an intense interest has arisen in methods of biomass processing that reduce costs or otherwise increase commercial viability of bioethanol on a production scale.
Two factors which heavily influence the overall production costs of lignocellulosic bioethanol are pretreatment and the cost of cellulase enzymes. Accordingly, it is advantageous to provide processing methods which reduce energy costs and hydrolysis methods that improve cellulase efficiency.
Ensiled biomass has recently been reported to provide promising raw material for bioethanol production. Silage is primarily used as a method for preservation of plant material as animal feed. Silage typically comprises a whole harvested crop, including stems, leaves and starch-rich grains, which are cut, compressed and stored anaerobically in e.g. silos. The growth of naturally occurring microorganisms during the early stages of ensiling depletes oxygen and converts soluble sugars into acids, thus lowering the pH. After about approximately 3-5 weeks the pH and the concentrations of lactic acid are constant in the ensiling biomass and the silage can be stored until use. Ensiling inhibits unwanted growth of other microorganisms, which decompose polysaccharides, and degrade holocellulose.
Ensiled biomass comprises both starch content, which can be degraded by comparatively inexpensive amylase enzymes, as well as lignocellulosic content, which can be degraded only by cellulase enzymes.
Here we report, surprisingly, that reasonable ethanol yields can be obtained from ensiled biomass that has been hydrolysed by direct enzymatic treatment without energy-consuming pretreatment. Unhydrolysed components of the ensiled biomass can be recovered from the initial enzymatic hydrolysis and/or SSF and subsequently subject to heat pretreatment.