Dry biodetoxification of acid pretreated wheat straw for cellulosic ethanol fermentation
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The efficient removal of toxic inhibitors from pretreated lignocellulose biomass is crucially important for consequent cellulosic ethanol fermentation. A. resinae ZN1 biodetoxifies all toxic inhibitors at the neutral pH of 4–6, and the neutralization of acid catalyst in the pretreated lignocellulose is required. However, aqueous alkaline solutions such as sodium hydroxide solution and calcium hydroxide slurry are used which generate several difficulties.
In this study, a dry biodetoxification method was investigated using dry calcium carbonate (CaCO3) powder as an acid-neutralizing reagent to avoid the use of an aqueous alkaline solution. Dry biodetoxification provides a mild and stable pH without the generation of phenolic compounds. The acid pretreated and dry biodetoxified wheat straw was used as the feedstock of ethanol fermentation and the same performance with the wet biodetoxification using aqueous Ca(OH)2 slurry. The 72 g/L or 9.1% (v/v) of ethanol produced from wheat straw was very close to that of ethanol from corn starch.
Dry biodetoxification provided a practical method to simplify the process of conventional wet biodetoxification to reduce the time, cost and labor.
KeywordsBiodetoxification Acid pretreatment Ethanol Calcium carbonate (CaCO3) powder Lignocellulose
- A. resinae ZN1
Amorphotheca resinae ZN1
National Renewable Energy Laboratory
magnesium sulfate heptahydrate
potato dextrose agar
Pretreatment is an essential step to overcome the recalcitrance of lignocellulosic biomass to facilitate enzymatic hydrolysis. Pretreatment of lignocellulosic biomass generates various toxic inhibitors such as furfural, 5-hydroxymethylfurfural (HMF), acetic acid and phenolic aldehydes, which required to be removed before bioconversion of enzymatic hydrolysis and fermentation process (Klinke et al. 2014; Jing et al. 2009; Palmqvist and Hahn-Hagerdal 2000; Dong and Bao 2010). A kerosene fungus Amorphotheca resinae ZN1 was isolated for complete removal of inhibitors from acid pretreated lignocellulose biomass in our previous studies (Zhang et al. 2010). The biodetoxified biomass has been applied to produce ethanol (Liu et al. 2018), chiral lactic acid (Qiu et al. 2017, 2018), glutamic acid (Wen et al. 2018), citric acid (Zhou et al. 2017), and gluconic acid (Zhang et al. 2016).
Amorphotheca resinae ZN1 grows and metabolizes at the neutral pH range of 4–6, thus the acid catalyst in the pretreated biomass has to be neutralized before biodetoxification (Zhang et al. 2010). Generally, aqueous calcium hydroxide slurry was used to neutralize the acid to pH 5 before biodetoxification. However, the use of calcium oxide (CaO) and calcium hydroxide slurry and mixing of small portion of them with a large amount of solid biomass generate several difficulties including: (1) the reduced solid contents of lignocellulosic biomass; (2) incomplete mixing between the solid particles and liquid slurry; (3) generation of phenolic compounds; (4) expensive and unsafe to handle. Furthermore, a high amount of carbon dioxide is generated and the high energy consumption is required during calcium oxide (CaO) manufacture. Here, we proposed a complete dry biodetoxification using inert calcium carbonate (CaCO3) powder to neutralize the acid catalyst in lignocellulose biomass to avoid the use of the aqueous alkaline solution. In the dry biodetoxification, pH changes slowly and complete mixing could be realized during the mild neutralization period. CaCO3 neutralized acid pretreated biomass can be kept in an open environment because of its stable nature and convenient onsite biodetoxification can be performed after pretreatment. This study tested the reaction balance of the biodetoxification, metabolism, and neutralization, as well as cellulosic ethanol fermentation performance.
Lignocellulosic biomass and reagents
Wheat straw was obtained from Nanyang, Henan, China, in the year 2018. The collected wheat straw was washed, dried and milled using a hammer crusher to pass through the 10 mm (diameter) apertures, and stored in plastic bags at room temperature in dry condition. The raw wheat straw contained 34.59% of cellulose, 24.26% of xylan, 18% of lignin, and 9.6% of ash by weight percentage measured according to NREL protocols (Sluiter et al. 2008, 2012).
Cellulase enzyme Cellic CTec 2.0 was purchased from Novozymes (China), Beijing, China. Yeast extract and peptone were purchased from Oxoid, Basingstoke, Hampshire, UK. CaCO3 and Ca(OH)2 were purchased from Shanghai Titanchem Co. Other chemicals and reagents included glucose, xylose, KH2PO4, MgSO4·7H2O, NaOH, and H2SO4 were purchased from Lingfeng Chemical Reagent Co.
Strains and media
Biodetoxification fungus Amorphotheca resinae ZN1 was isolated in our previous studies and stored in China General Microorganism Collection Center (CGMCC), Beijing, China. The potato dextrose agar (PDA) medium for A. resinae ZN1 included 200 g/L potato extract juice, 20 g/L glucose, and 20 g/L agar.
Ethanol fermentation strain Saccharomyces cerevisiae XH7 was kindly provided by Prof. XM Bao from Shandong University, Jinan, Shandong, China (Li et al. 2015). The medium used for S. cerevisiae XH7 included: (i) medium for strain activation; YPD included, 20 g/L glucose, 20 g/L peptone and 10 g/L yeast extract; (ii) for seed culture, 5% (w/w) of biodetoxified wheat straw, 2 g/L KH2PO4, 2 g/L (NH4)2SO4, 1 g/L MgSO4 and 10 g/L yeast extract; (iii) for adaptation, 10% (w/w) of biodetoxified wheat straw, 2 g/L KH2PO4, 2 g/L (NH4)2SO4, 1 g/L MgSO4 and 10 g/L yeast extract; (iv) for SSCF, 2 g/L KH2PO4, 2 g/L (NH4)2SO4, 1 g/L MgSO4 and 10 g/L yeast extract.
Dry acid pretreatment, dry biodetoxification and ethanol fermentation
Dry acid pretreated wheat straw was prepared according to Zhang et al. (2011) and He et al. (2014). The pretreated wheat straw contained 26.7 mg/g DM of glucose and 153.3 mg/g DM of xylose, 3.14 mg/g DM of furfural, 1.46 mg/g DM of HMF, 12.52 mg/g DM of acetic acid, 0.43 mg/g DM of vanillin, 0.51 mg/g DM of syringaldehyde and 0.36 mg/g DM of 4-HBA, determined according to NREL protocols (Sluiter et al. 2008, 2012).
The dry acid pretreated wheat straw was neutralized to pH 5.5 with dry CaCO3 powder and 20% of Ca(OH)2 slurry, respectively, and disk milled to remove the long cellulosic fibers. Then, inoculated with 10% (v/v) solid seed of A. resinae ZN1 for biodetoxification at ambient temperature for 60 h. Ethanol fermentation was performed in 5 L bioreactor. The biodetoxified wheat straw was pre-hydrolyzed into liquid hydrolysate slurry at 50 °C, pH 5 for 12 h. Then, the simultaneous saccharification and co-fermentation (SSCF) was performed by inoculating shortly adapted fermentation yeast Saccharomyces cerevisiae XH7 into the hydrolysate at 10% (v/v) in the same bioreactor. The nutrients addition included 2 g/L of KH2PO4, 2 g/L of (NH4)2SO4, 1 g/L of MgSO4 and 10 g/L of yeast extract. The SSCF was carried out at 30 °C, pH 5.5 for 120 h.
Glucose, xylose, ethanol, and inhibitors such as acetic acid furfural and HMF were analyzed using HPLC (LC-20AD, refractive index detector RID-10A, Shimadzu, Japan) with Bio-Rad Aminex HPX-87H column at 65 °C. The mobile phase was 5 mM H2SO4 at the flow rate of 0.6 mL/min. Syringaldehyde, vanillin, and hydroxybenzaldehyde (4-HBA) were analyzed using the reversed-phase HPLC (LC-20AT, UV/VIS detector SPD-20A, Shimadzu, Japan) with a YMC-Pack ODS-A column at ambient temperature (Liu et al. 2018). The mobile phase was 100% acetonitrile with 0.1% formic acid at the flow rate of 1.0 mL/min. The yeast cell viability during the simultaneous saccharification and co-fermentation (SSCF) was assayed on the YPD medium Petri dish by counting colony-forming units (CFU), when the 100 μL of the 105 diluted fermentation broth sample withdrawn after every 12 h was stretched and cultured at 30 °C for 48 h (Gu et al. 2015).
Results and discussion
Dry neutralization of acid catalyst using calcium carbonate and the biodetoxification assay
Evaluation of ethanol fermentation using dry biodetoxified wheat straw
Amorphotheca resinae ZN1 is a biodetoxification strain and used on the pretreated wheat straw to remove the inhibitors. After the biodetoxification, wheat straw was enzymatically hydrolyzed into fermentable sugars such as glucose and xylose. The fermentation microorganism S. cerevisiae XH7 was used to convert the glucose and xylose sugars to ethanol. The aerobic fungus Amorphotheca resinae ZN1 was no long alive because of the anaerobic condition in hydrolysis and ethanol fermentation.
The dry biodetoxification method performs excellent cellulosic ethanol production and provides mild and stable pH with no reduction of fermentable sugars and solid contents and without the generation of phenolic compounds. Dry biodetoxification is a practical method to improve biodetoxification efficiency for cellulosic ethanol production from acid pretreated lignocellulose feedstock.
Dry biodetoxification in this study realized complete mixing of solid–solid particles without loss of solid content and generation of phenolic derivatives which is more safe and costly effective than that of conventional wet biodetoxification. Consequently, use of dry biodetoxified biomass achieved high cellulosic ethanol production [72 g/L or 9.1% (v/v)] and all advantages of this method make it possible for further industrial applications.
The authors are grateful for the financial support received from the National Natural Science Foundation of China (31961133006).
FA, YZ and JB designed and wrote the experiment; FA and YZ conducted the experiment; JB conceived the research. All authors read and approved the final manuscript.
This work was supported by National Natural Science Foundation of China (31961133006).
Ethics approval and consent to participate
All the authors have read and agreed the ethics for publishing the manuscript.
Consent for publication
The authors approved the consent for publishing the manuscript.
The authors declare that they have no competing interests.
- Li H, Wu M, Xu L, Jin H, Guo T, Bao X (2015) Evaluation of industrial Saccharomyces cerevisiae strains as the chassis cell for second-generation bioethanol production. Microb Biotechnol 82:6Google Scholar
- Sluiter A, Hames B, Ruiz R, Scarlata C, Sluiter J, Templeton D (2008) Determination of sugars, byproducts and degradation products in liquid fraction process samples. NREL/TP-510-42623. National Renewable Energy Laboratory, GoldenGoogle Scholar
- Sluiter A, Hames B, Ruiz R, Scarlata C, Sluiter J, Templeton D, Crocker D (2012) Determination of structural carbohydrates and lignin in biomass. NREL/TP-510-42618. National Renewable Energy Laboratory, GoldenGoogle Scholar
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