Corn stover is a lignocellulosic biomass, an agricultural by-product, a possible raw material for xylose production. In this study corn stover was hydrolyzed with sulfuric and hydrochloric acid. In the presented work, hydrochloric acid resulted in the highest, 88.8 % xylose yield of theoretical under the conditions of 2 % (w/w) hydrochloric acid concentration, 40-minute reaction time, 10 % (w/w) dry matter, at 120 °C. Sulfuric acid experiments resulted in 81.9 % xylose yield of theoretical by using 1.5 % (w/w) sulfuric acid, 60-minute reaction time, at 140 °C, 7 % (w/w) dry matter. Acid hydrolysis at low dry matter content resulted in relatively low sugar concentrations. Hydrolyzate recycling concentrated xylose to three-times, while the recycling does not decrease the xylose yields. It is also shown that the pseudo first-order and biphasic kinetic models can be based on total sugar concentrations.

Xylitol is produced by the heterogeneous catalytic hydrogenation of xylose over Raney nickel. The hydrogenation must typically be followed by several purification steps, which makes the chemical production relatively complex and expensive. In this study, activated carbon and bio-purification treatments of corn stover hydrolysates and subsequent nickel-catalyzed hydrogenation of xylose to xylitol were investigated. The activated carbon treatment was used to eliminate inhibitory compounds and increase the efficiency of the bio-purification step. It was found that the glucose could be completely eliminated from the hydrolysate. The hydrogenation reactions of corn stover hydrolysate demonstrated that a high reaction temperature resulted in high sugar alcohol yields and selectivity. At a given temperature, the flow rate had no significant effect on xylitol yield. Figure 1. Hydrogenation of xylose to xylitol over Raney nickel.

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Abstract

The need to introduce promising bioethanol production technologies calls for advanced laboratory techniques to study experiment designs and to obtain their results in a quick and reliable way. Real time monitoring based on general principles of ethanol fermentation, such as effl uent CO 2 volume, avoids time consuming steps, long lasting analyses and delivers information about the process directly. A device based on the above features and capable for real time monitoring on parallel channels was developed by the authors and is described in this paper. Both for calibration and for fermentation, test runs were carried out on different days and channels. Statistical evaluation was based on the obtained data. According to the t-test (P=0.05) and Grubbs analysis, the calibration method is reliable regardless of the date of calibration. When evaluating the fermentation results by ANCOVA acceptable standard derivations were obtained as impact of channel (58.8 ml), date (82.1 ml) and incorporating all impacts (116.2 ml). The fi nal ethanol concentrations calculated based on the gas volume were compared to ones determined by HPLC and an average difference of 10% was found. Thus, the device proved to be advantageous in monitoring fermentation.Keywords: ethanol, fermentation, monitoring, device, CO 2Ethanolic fermentation is an enzymatic disassembly of organic matters. Bioethanol can be produced from a wide range of raw materials built up of either sugar, starch or lignocellulose. Whatever the raw material is, it must be decomposed fi rst to simple sugars of six carbons of which ethanol can be produced by yeast strains, usually by common baker's yeast (Saccharomyces cerevisiae) with the exclusion of oxygen (anaerobic conditions). Based on the stoichiometry of fermentation, theoretically one mole of six carbon sugar delivers 2 moles of ethanol and CO 2 , e.g. same molar amounts of the two compounds are formed (Eq. 1). Of course, this equation describes only the quantity of hexose converted into ethanol, but some of the hexoses are either consumed by the yeast to support growth (under low oxygen conditions this is less than 5% of the sugars) or other metabolites are also produced (glycerol, lactic and succinic acid, etc.) (RUSSELL, 2003). Nevertheless, these other metabolites are small in quantity compared to the amount of the ethanol produced (RUSSELL, 2003). Consequently, glucose conversion is not associated to this equation, but the theoretical ratios of the product may be useful to estimate the amount of one from the amount of the other. Moreover, in the presence of oxygen (at the beginning of the fermentation) respiration also occurs leading to excess CO 2 formation. Although, these minor processes may lead to a deviation from the theoretical 1:1 molar ratio of CO 2 and ethanol.C 6 H 12 O 6 = 2C 2 H 5 OH + 2CO 2 (1)

Brewer’s spent grain (BSG) is the main by-product of the beer brewing process. It has a huge potential as a feedstock for bio-based manufacturing processes to produce high-value bio-products, biofuels, and platform chemicals. For the valorisation of BSG in a biorefinery process, efficient fractionation and bio-conversion processes are required. The aim of our study was to develop a novel fractionation of BSG for the production of arabinose, arabino-xylooligomers, xylose, and bioethanol. A fractionation process including two-step acidic and enzymatic hydrolysis steps was investigated and optimised by a response surface methodology and a desirability function approach to fractionate the carbohydrate content of BSG. In the first acidic hydrolysis, high arabinose yield (76%) was achieved under the optimised conditions (90 °C, 1.85 w/w% sulphuric acid, 19.5 min) and an arabinose- and arabino-xylooligomer-rich supernatant was obtained. In the second acidic hydrolysis, the remaining xylan was solubilised (90% xylose yield) resulting in a xylose-rich hydrolysate. The last, enzymatic hydrolysis step resulted in a glucose-rich supernatant (46 g/L) under optimised conditions (15 w/w% solids loading, 0.04 g/g enzyme dosage). The glucose-rich fraction was successfully used for bioethanol production (72% ethanol yield by commercial baker’s yeast). The developed and optimised process offers an efficient way for the value-added utilisation of BSG. Based on the validated models, the amounts of the produced sugars, the composition of the sugar streams and solubilised oligo-saccharides are predictable and variable by changing the reaction conditions of the process.

Crop residues can serve as low-cost feedstocks for microbial production of xylitol, which offers many advantages over the commonly used chemical process. However, enhancing the efficiency of xylitol fermentation is still a barrier to industrial implementation. In this study, the effects of oxygen transfer rate (OTR) (1.1, 2.1, 3.1 mmol O2/(L × h)) and initial xylose concentration (30, 55, 80 g/L) on xylitol production of Candida boidinii NCAIM Y.01308 on xylose medium were investigated and optimised by response surface methodology, and xylitol fermentations were performed on xylose-rich hydrolysates of wheat bran and rice straw. High values of maximum xylitol yields (58–63%) were achieved at low initial xylose concentration (20–30 g/L) and OTR values (1.1–1.5 mmol O2/(L × h)). The highest value for maximum xylitol productivity (0.96 g/(L × h)) was predicted at 71 g/L initial xylose and 2.7 mmol O2/(L × h) OTR. Maximum xylitol yield and productivity obtained on wheat bran hydrolysate were 60% and 0.58 g/(L × h), respectively. On detoxified and supplemented hydrolysate of rice straw, maximum xylitol yield and productivity of 30% and 0.19 g/(L × h) were achieved. This study revealed the terms affecting the xylitol production by C. boidinii and provided validated models to predict the achievable xylitol yields and productivities under different conditions. Efficient pre-treatments for xylose-rich hydrolysates from rice straw and wheat bran were selected. Fermentation using wheat bran hydrolysate and C. boidinii under optimized condition is proved as a promising method for biotechnological xylitol production.

One of the main distinguishing features of bacteria belonging to the Cellulomonas genus is their ability to secrete multiple polysaccharide degrading enzymes. However, their application in biomass deconstruction still constitutes a challenge. We addressed the optimisation of the xylanolytic activities in extracellular enzymatic extracts of Cellulomonas sp. B6 and Cellulomonas fimi B-402 for their subsequent application in lignocellulosic biomass hydrolysis by culture in several substrates. As demonstrated by secretomic profiling, wheat bran and waste paper resulted to be suitable inducers for the secretion of xylanases of Cellulomonas sp. B6 and C. fimi B-402, respectively. Both strains showed high xylanolytic activity in culture supernatant although Cellulomonas sp. B6 was the most efficient xylanolytic strain. Upscaling from flasks to fermentation in a bench scale bioreactor resulted in equivalent production of extracellular xylanolytic enzymatic extracts and freeze drying was a successful method for concentration and conservation of the extracellular enzymes, retaining 80% activity. Moreover, enzymatic cocktails composed of combined extra and intracellular extracts effectively hydrolysed the hemicellulose fraction of extruded barley straw into xylose and xylooligosaccharides. Key points • Secreted xylanase activity of Cellulomonas sp. B6 and C. fimi was maximised. • Biomass-induced extracellular enzymes were identified by proteomic profiling. • Combinations of extra and intracellular extracts were used for barley straw hydrolysis.