Original papersThe generation of fermentation inhibitors during dilute acid hydrolysis of softwood
Introduction
Due to the need for environmentally sustainable energy sources, lignocellulosic materials are considered as raw material in the production of ethanol for use as liquid fuel.1, 2 Lignocellulosic materials can be hydrolyzed by acids3, 4, 5, 6, 7, 8 and enzymes.9, 10 Hydrolysis with dilute sulfuric acid is a simple and fast method,8, 11 and it has been suggested that there is potential for improvement.6 Higher ethanol yields have been achieved with enzymatic hydrolysis;12 however, the current use of this method will be highly dependent upon the monetary cost of enzymes required.13
Ethanol production from wood has been investigated since the beginning of 19th century (for a review, see Parisi14) whereas the commercial production of wood sugars for ethanol production has been considered only since the beginning of the 20th century.3 Some of the earliest processes using dilute sulfuric acid produced 72 l ethanol (100%) per 1 t dry matter3 which is equivalent to 24% of the theoretical yield. Through the use of percolation-type reactors (Scholler process),15 the yield of ethanol was increased to between 55–64%.4, 11 The “Scholler” process was economically optimized in the U.S. (“Madison process”) where ethanol yields of 60% of the theoretical yield were reported.11 One of the major problems associated with dilute acid hydrolysis of lignocellulosic biomass is the poor fermentability of the produced hydrolysates.2, 10 In lignocellulosic hydrolysates, the concentration of sugars as well as the concentration of hydrolysis by-products depend on hydrolysis conditions.8 Sugars can be degraded to furfural which is formed from pentoses and 5-hydroxymethylfurfural (5-HMF) which is formed from hexoses (Figure 1). 6, 8, 16 5-HMF can be further degraded, forming levulinic acid and formic acid (Figure 1).17, 18, 19 In addition, formic acid can be formed from furfural under acidic conditions at elevated temperatures.20 Acetate is liberated from hemicellulose during hydrolysis.9
During acid hydrolysis, a minor part of lignin is degraded, resulting in a range of aromatic compounds.9, 19, 21, 22 Of these compounds, the low molecular weight phenolics have been suggested to be the most inhibitory.19, 22 In addition, fermentation products such as ethanol and acetic acid contribute to the inhibition.23
To combine the residence time and temperature during hydrolysis into a single reaction ordinate, Overend and Chornet24 defined a severity factor according to the following equation: where t is residence time (min), Tr is the reaction temperature (°C), and Tb is a reference temperature (100°C).24
Chum et al.25 introduced a third parameter, the environmental pH, into the above equation to describe the combined severity (CS): where the pH is calculated from the amount of sulfuric acid added. CS has previously been used to evaluate the sulfuric acid pretreatment of spruce prior to enzymatic hydrolysis in the production of ethanol.10
In the present study, the influence of the CS during acid hydrolysis on the formation of the lignocellulose breakdown products furfural, 5-HMF, acetic acid, formic acid, and levulinic acid and on ethanolic fermentation with Baker’s yeast was studied. The effect of acetic acid, formic acid, levulinic acid, furfural, and 5-HMF was evaluated in model fermentations.
Section snippets
Preparation of hydrolysates
Freshly chipped spruce, Picea abies, was provided by the Höörsågen AB, sawmill (Höör, Sweden). The composition of raw material (w/w % of the dry weight) was: 43.4, glucan; 28.1, lignin; 12.0, mannan; 4.9, xylan; 1.8, galactan; 1.1, arabinan; and 1.0 extractives. The equivalent of 200 g dry weight (DW) wood chips (moisture content 57%), less than 30 mm in size were mixed with sulfuric acid to a final concentration of 0.5%, 2.4%, or 4.4% (w/w liquid) and incubated at room temperature overnight.10
Results
Seventy-six acid hydrolysates treated at temperatures 150, 180, 200, 210, 225, and 240°C for 1, 5, 10, 15, 20, and 30 min and with 0.5%; 2.4%, and 4.4% (w/v) sulfuric acid were produced (Table 1). CS was between 1.4–5.4. The highest concentrations of sugars were 35.9 g l−1 glucose, 22.4 g l−1 mannose, and 10.2 g l−1 xylose which was obtained in hydrolysates No. 28 and 10 (Table 1). The highest concentrations of galactose and arabinose were 4.7 g l−1 and 2.6 g l−1, obtained in hydrolysate No. 10
Discussion
Early studies on hydrolysis of lignocellulosic materials mainly concentrated on obtaining maximum yield of sugars rather than efficient fermentation.3, 4 In later studies, the influence of fermentation inhibitors found in hydrolysates versus their fermentability has been considered to a large extent.5, 19, 22, 30, 31, 32, 33 In a recent study where the temperature was varied during dilute acid hydrolysis, both the release of fermentable sugars and fermentability of the hydrolysates were
Acknowledgements
The Swedish National Board for Industrial and Technical Development (NUTEK) is acknowledged for financial support. Simona Larsson acknowledges a scholarship within TEMPUS JEP-09723-95 project. Anders Reimann and Niklas Garoff are gratefully acknowledged for CE and GC analysis of samples. Nina Fälstrand-Larsson is gratefully acknowledged for the preparation of hydrolysates. Dr. Scott Pedler is gratefully acknowledged for linguistic advise.
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