Bioethanol production from spent mushroom compost derived from chaff of millet and sorghum

Background: In Uganda, the chaff remaining from threshed panicles of millet and sorghum is a low value, lignocellulose- rich agricultural by-product. Currently, it is used as a substrate for the cultivation of edible Oyster mushrooms (Pleurotus ostreatus). The aim of this study was to assess the potential to exploit the residual post-harvest compost for saccharification and fermentation to produce ethanol. Results: Sorghum and millet chaff-derived spent oyster mushroom composts minus large mycelium particles were assessed at small-scale and low substrate concentrations (5% w/v) for optimal severity hydrothermal pre-treatment, enzyme loading and fermentation with robust yeasts to produce ethanol. These conditions were then used as a basis for larger scale assessments with high substrate concentrations (30% w/v). Millet-based compost had a low cellulose content and, at a high substrate concentration, did not liquefy effectively. The ethanol yield was 63.9 g/kg dry matter (DM) of original material with a low concentration (19.6 g/L). Compost derived from sorghum chaff had a higher cellulose content and could be liquefied at high substrate concentration (30% w/v). This enabled selected furfural-resistant yeasts to produce ethanol at up to 186.9 g/kg DM of original material and a concentration of 45.8 g/L. Conclusions: Spent mushroom compost derived from sorghum chaff has the potential to be an industrially useful substrate for producing second-generation bioethanol. This might be improved further through fractionation and exploitation of hemicellulosic moieties, and possibly the exploitation of the mycelium-containing final residue for animal feed. However, spent compost derived from millet does not provide a suitably high concentration of ethanol to make it industrially attractive. Further research on the difficulty in quantitatively saccharifying cellulose from composted millet chaff and other similar substrates such as rice husk is required.