Implications of organic acids in wet storage and bioconversion of corn stover to ethanol

Narrow harvest windows and contamination concerns with field drying suggest that wet storage will likely be the preferred storage method for biofuel feedstocks in humid regions of the U.S. During wet storage for biofuel production, feedstock is stored under anaerobic and moist (≥ 25% wet basis) conditions, which result in the production of organic acids by natural acidogenic microorganisms. The organic acids have the potential to alter the feedstock structure and provide partial pretreatment, but can also inhibit subsequent biofuel fermentation. Pretreatment is necessary for lignocellulosic feedstocks since it allows plant cell wall degrading enzymes to have access to structural sugars (cellulose and hemicelluloses) and convert them to glucose and other simple sugars. Microbial inhibition results in reduced specific ethanol productivity (the amount of ethanol produced from the feedstock within a given time). In this study, wet storage of corn stover at seven different moisture contents (25-75%) was incubated under anaerobic conditions at two temperature levels (22°C and 37°C) for O, 21, and 220 days. Dry matter loss ranged from less than 1% for 21-day storage to less than 4% for 220-day storage. The results also showed that the various conditions can be cluster into three groups based on organic acid profile using Ward's hierarchical method. Fiber reactivity test was performed on unensiled stover and stover stored at 25% and 75% moisture without prior pretreatment to determine if organic acids produced during ensilage have any significant pretreatment effect on fiber structure. In general, the sugar yield from fiber reactivity was low but the results could be related to the three cluster groups and were indicative that ensilage does have a beneficial effect on fiber structure.