| Levels of atmospheric carbon dioxide have risen significantly since 1800, leading to the prediction that the atmosphere will warm from 1.5 to 4.5{dollar}spcirc{dollar}C in the next century. Renewable fuels such as ethanol produced from herbaceous materials may reduce atmospheric carbon dioxide levels. Sweet sorghum and sorghum-sudangrass may be substrates for fuel alcohol production if storage and spoilage problems associated with the fresh material can be overcome. This work describes efforts to store fermentable and structural carbohydrates in sorghum and to produce ethanol through two fermentation systems: simultaneous saccharification and fermentation (SSF-I) and solid state fermentation (SSF-II).; Structural carbohydrates are preserved by ensiling. Environmental conditions in the silo are suitable for in situ enzymatic hydrolysis of cellulose and hemicellulose. Through SSF-I, ethanol concentrations averaged 28 kg/MT, wet weight, using sweet sorghum ensiled for sixty days. However, problems encountered with SSF-I included loss of fermentable carbohydrates to the lactic acid silage fermentation and product inhibition of cellulase.; A solid state fermentation system (SSF-II) produced a maximum ethanol concentration of 30 kg/MT, wet weight of sorghum, using a modified ensiling process. Montrachet yeast was the inoculum and 0.3% (v/w) formic acid (60%, v/v) was added as a bacterial retardant.; Heat evolution in a pilot-scale SSF-II was monitored over a twenty-eight day period. Core temperatures reached a maximum level of 26.9{dollar}spcirc{dollar}C after 7d of fermentation in a silo having a diameter of 1.2 m and a height of 0.8 m. Silos can be used as bioreactors in SSF-II and as storage facilities for ethanolic silage. As long as anaerobic conditions are maintained, ethanolic silage remains stable for at least 230 days. Yeasts are metabolically active at moisture levels as low as 68%, producing nearly theoretical yields of ethanol (90% of theoretical yields based on the total sugar available for fermentation). SSF-II of sorghum to fuel alcohol makes complete use of readily fermentable sugars without loss to spoilage microorganisms. |
