The availability and low cost of lignocellulosic biomass has caused tremendous interest …

• Abstract
• Background
• Methods
• Results
• Discussion
• Conclusion
• References
• Table 1
• Figures

Fermentation reactions are important for the production of many valuable products including pharmaceuticals, beverages, and biofuels. In lignocellulosic ethanol production, biomass feedstocks are chemically pretreated, hydrolyzed by cellulases and hemicellulases, and the resulting sugars are fermented by yeast or bacteria to produce ethanol [1]. In large-scale production of biocommodities such as ethanol, feedstocks have a large and often dominant impact on process economics and process development [2]. For example, the amount of sugar available to fermentation reactions is important because in the absence of limiting factors, substrate availability determines product yield; thus methods to measure sugar available to a fermentation reaction are potentially valuable for selecting feedstocks.

One approach is to complete a simultaneous saccharification and fermentation (SSF) process [3] and to assay residual sugars and inhibitory products such as glucose, cellobiose, and acetic acid by high performance liquid chromatography (HPLC), and ethanol concentration by gas chromatograph (GC) or HPLC. The whole procedure takes about 168 hours according to the SSF protocol specified by the National Renewable Energy Laboratory (NREL) [4]. Recently, Weimer et al have developed a higher throughput method to predict the fermentability of cellulosic biomass to ethanol through in vitro gas production [5]. In this procedure, fermentations are carried out in sealed serum bottles, and the gas produced is measured as an indicator of the digestibility of the cellulosic biomass.

In contrast to full SSF processes, many sugar detection and quantitation methods can be employed, including chemical reducing sugar assays and enzymatic assays [6,7]. All of these methods require sampling the fermentation reaction and measuring sugars in the sample. However, because cellulases and hemicellulases are product inhibited, simple approaches that only involve hydrolytic enzymes and a sugar assay (or biosensor) yield low, non-representative estimates of conversion potential. Determining a meaningful sugar yield requires that sugars be removed as they are produced, as is done in SSF processes [3].

Once a sugar scavenger has been added to the mixture of hydrolytic enzymes, the problem of product inhibition has been solved, but another problem replaces it: the sugars produced are consumed as they are produced, making measurements of sugar concentrations a poor predictor of total sugar released. For this reason, simply relying on a sugar assay or a sugar biosensor, such as one described by Lidgren et al [8], will not work. Therefore, our objective was to develop a system that could be used to monitor glucose catabolism as an indicator of feedstock convertibility and to demonstrate its application to monitoring corn stover hydrolysis in a process similar to that used for lignocellulosic ethanol production. Such a system would be useful for rapidly screening varieties for suitability as biomass feedstocks in plant breeding programs and for evaluating different hydrolytic systems. Furthermore, this system could be used to measure sugar production in a wide range of other experiments.