Improved enzymatic saccharification of steam exploded cotton stalk using alkaline extraction and fermentation of cellulosic sugars into ethanol

Xylitol Production by Candida tropicalis from Areca Nut Husk Enzymatic Hydrolysate and Crystallization

Lignocellulosic biomasses are extensively used by researchers to produce a variety of renewable bioproducts. This research described an environment-friendly technique of xylitol production by an adapted strain of Candida tropicalis from areca nut hemicellulosic hydrolysate, produced through enzymatic hydrolysis. To enhance the activity of xylanase enzymes, lime and acid pretreatment was conducted to make biomass more amenable for saccharification. To improve the efficiency of enzymatic hydrolysis, saccharification parameters like xylanase enzyme loading were varied. Results exposed that the highest yield (g/g) of reducing sugar, about 90%, 83%, and 15%, were achieved for acid-treated husk (ATH), lime-treated husk (LTH), and raw husk (RH) at an enzyme loading of 15.0 IU/g. Hydrolysis was conducted at a substrate loading of 2% (w/V) at 30 °C, 100 rpm agitation, for 12 h hydrolysis time at pH 4.5 to 5.0. Subsequently, fermentation of xylose-rich hemicellulose hydrolysate was conducted with pentose utilizing the yeast Candida tropicalis to produce xylitol. The optimum concentration of xylitol was obtained at about 2.47 g/L, 3.83 g/L, and 5.88 g/L, with yields of approximately 71.02%, 76.78%, and 79.68% for raw fermentative hydrolysate (RFH), acid-treated fermentative hydrolysate (ATFH), and lime-treated fermentative gydrolysate (LTFH), respectively. Purification and crystallization were also conducted to separate xylitol crystals, followed by characterization like X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. Results obtained from crystallization were auspicious, and about 85% pure xylitol crystal was obtained.

A comparative cradle-to-gate life cycle assessment of three cotton stalk waste sustainable applications

This paper presents a novel approach to utilizing agricultural waste. It compares three different applications for cotton stalks: fabrication of wood composites, bioethanol production, and biogas cradle-to-gate Life cycle assessment production processes. Cotton cultivation generates a lot of debris, mostly cotton stalks, which are incinerated or landfilled, Sustainable resource management is critical for maintaining the ecosystem, and economic stability, and promoting social fairness since it ensures the long-term availability of resources while minimizing environmental damage. The investigation uses the Ecological Footprint, Impact 2002 +, Global Warming Damage Potential, Greenhouse Gas Protocol, Recipe Midpoint, Ecosystem Damage Potential, and CML IA Baseline-open LCA-enabled environmental sustainability assessments. The analysis showed that bioethanol has a lower carbon footprint and climate change impact than both wood composite and biogas production processes, as a result, this could cause a preference for bioethanol production as an environmentally friendly strategy for cotton stalks utilization. While human toxicity was higher in the biogas production process, it emits less fossil CO2 than biogenic CO2. The total climate change of wood composite, bioethanol, and biogas production processes was 0.01761, 0.011300, and 0.01083 points, respectively. This research helps accomplish wider ecological and economic aims by giving insights into sustainable waste management practices.

Development of engineered Candida tropicalis strain for efficient corncob-based xylitol-ethanol biorefinery

BACKGROUND

Xylitol has a wide range of applications in the pharmaceuticals, cosmetic, food and beverage industry. Microbial xylitol production reduces the risk of contamination and is considered as environment friendly and sustainable compared to the chemical method. In this study, random mutagenesis and genetic engineering approaches were employed to develop Candida tropicalis strains with reduced xylitol dehydrogenase (XDH) activity to eliminate co-substrate requirement for corn cob-based xylitol-ethanol biorefinery.

RESULTS

The results suggest that when pure xylose (10% w/v) was fermented in bioreactor, the Ethyl methane sulfonate (EMS) mutated strain (C. tropicalis K2M) showed 9.2% and XYL2 heterozygous (XYL2/xyl2Δ::FRT) strain (C. tropicalis K21D) showed 16% improvement in xylitol production compared to parental strain (C. tropicalis K2). Furthermore, 1.5-fold improvement (88.62 g/L to 132 g/L) in xylitol production was achieved by C. tropicalis K21D after Response Surface Methodology (RSM) and one factor at a time (OFAT) applied for media component optimization. Finally, corncob hydrolysate was tested for xylitol production in biorefinery mode, which leads to the production of 32.6 g/L xylitol from hemicellulosic fraction, 32.0 g/L ethanol from cellulosic fraction and 13.0 g/L animal feed.

CONCLUSIONS

This work, for the first time, illustrates the potential of C. tropicalis K21D as a microbial cell factory for efficient production of xylitol and ethanol via an integrated biorefinery framework by utilising lignocellulosic biomass with minimum waste generation.

Saccharification Yield through Enzymatic Hydrolysis of the Steam-Exploded Pinewood

Pressure, temperature, and retention time are the most studied parameters in steam explosion pretreatment. However, this work aimed to fix these parameters and to evaluate the influences of several less investigated steam explosion parameters on the saccharification yield in hydrolysis. In this study, firstly, pinewood samples smaller than 200 µm were treated with steam explosion at 190 °C for 10 min. The variable parameters were biomass loading, N2 pressure, and release time. Steam-exploded samples were hydrolyzed with the Trichoderma reesei enzyme for saccharification for 72 h. The sugar content of the resultant products was analyzed to estimate the yield of sugars (such as glucose, xylose, galactose, mannose, and arabinose). The best glucose yield in the pulp was achieved with 4 g of sample, N2 pressure of 0.44 MPa, and short release time (22 s). These conditions gave a glucose yield of 97.72% in the pulp, and the xylose, mannose, galactose, and arabinose yields in the liquid fraction were found to be 85.59%, 87.76%, 86.43%, and 90.3%, respectively.

Evaluation and characterization of novel sources of sustainable lignocellulosic residues for bioethanol production using ultrasound-assisted alkaline pre-treatment

In recent years, research is focused on finding a sustainable and eco-friendly lignocellulosic biomass for the effective production of bioethanol to meet the world's energy demand. The present study investigates the bioethanol production potential of four different lignocellulosic biomass residues viz., Saccharum arundinaceum (hardy sugar cane), Arundo donax (giant reed), Typha angustifolia (narrow-leaved cattail), and Ipomoea carnea (pink morning glory). The maximum reducing sugar release showed 185.00 ± 1.57, 213.73 ± 3.47, 187.57 ± 2.14, 294.08 ± 3.98 mg/g and fermentation efficiency of 72.60 ± 8.17%, 82.59 ± 7.42%, 77.45 ± 7.35%, and 85.04 ± 8.37% which was analyzed by estimating the percentage of bioethanol yield were achieved for Saccharum arundinaceum, Arundo donax, Typha angustifolia, and Ipomoea carnea, respectively. The chemical composition of biomass was characterized using National Renewable Energy Limited (NREL) protocol. The effect of ultrasound (US)-assisted alkaline pre-treatment on the four biomasses was characterized by different techniques. The cavitation phenomena of US-assisted alkaline pre-treatment was evident from the decreased value of lignin percentage, increased surface porosity and area, changes in crystallinity index (CrI) values and in the functional groups of biomass. The results revealed that all the four lignocellulosic biomass residues could be utilized as an effective and sustainable source for the production of bioethanol using US-assisted sodium hydroxide as a pre-treatment tool.

Lignin monomer in steam explosion assist chemical treated cotton stalk affects sugar release

In this study, the fermentable sugar released from cotton stalk (CS), which were pretreated by instant catapult steam explosion (SE) combined with different concentrations of strong monobasic acid (HCl), weak monobasic acid (CH₃COOH), strong monobasic alkali (NaOH) and weak monobasic alkali (NH₃·H₂O), followed by hydrolysis in cellulase/xylanase mixed enzyme solutions, were comparably investigated. The highest yield of 73.22% of fermentable sugar yield was obtained in SE-2.4 MPa-5%NH₃·H₂O treated CS substrates, which was 5.14 times higher than that from enzymatic hydrolysis (EH) of raw CS. Furthermore, evaluation of monolignins content (H, G, S) in different CS samples suggested that substrates rich in guaiacyl (G) and syringyl (S) would generate a higher efficiency of enzymatic saccharification. Therefore, the slight genetic modification of monolignins for cotton stalk might be a potential way to enhance biomass degradation and transformation.

Development of co-immobilized tri-enzyme biocatalytic system for one-pot pretreatment of four different perennial lignocellulosic biomass and evaluation of their bioethanol production potential

Today, many researchers are focusing on research for alternative promising energy sources and sustainable technology for bioethanol production to meet the increasing global energy demand. Here, we develop a novel one-pot pretreatment technology by co-immobilizing laccase, cellulase and β-glucosidase to act as a tri-enzyme biocatalyst for evaluating the bioethanol production potential of four sustainable lignocellulosic biomasses viz., Typha angustifolia, Arundo donax, Saccharum arundinaceum, and Ipomoea carnea. The co-immobilized enzyme system was more stable at different temperatures and at longer storage, compared to free enzyme. During enzymatic saccharification, Saccharum arundinaceum showed higher total reducing sugar of 205 ± 3.73 mg/g when compared to other biomass. The highest percentage of bioethanol yield of 63.43 ± 9.35% was obtained with Ipomoea carnea. The effects of co-immobilized tri-enzyme biocatalyst on the biomasses were evaluated. The results revealed that the co-immobilized tri-enzyme biocatalyst could act as effective one-pot pretreatment for the production of bioethanol from lignocellulosic biomass.

Improving enzymatic hydrolysis efficiency of wheat straw through sequential autohydrolysis and alkaline post-extraction

In this work, a two-step pretreatment process of wheat straw was established by combining autohydrolysis pretreatment and alkaline post-extraction. The results showed that employing alkaline post-extraction to autohydrolyzed wheat straw could significantly improve its enzymatic hydrolysis efficiency from 36.0% to 83.7%. Alkaline post-extraction lead to the changes of the structure characteristics of autohydrolyzed wheat straw. Associations between enzymatic hydrolysis efficiency and structure characteristics were also studied. The results showed that the factors of structure characteristics such as delignification, xylan removal yield, crystallinity, accessibility and hydrophobicity are positively related to enzymatic hydrolysis efficiency within a certain range for alkaline post-extracted wheat straw. The results demonstrated that autohydrolysis coupled with alkaline post-extraction is an effective and promising method to gain fermentable sugars from biomass.

Pretreatment of wheat straw leads to structural changes and improved enzymatic hydrolysis

Wheat straw (WS) is a potential biomass for production of monomeric sugars. However, the enzymatic hydrolysis ratio of cellulose in WS is relatively low due to the presence of lignin and hemicellulose. To enhance the enzymatic conversion of WS, we tested the impact of three different pretreatments, e.g. sulfuric acid (H₂SO₄), sodium hydroxide (NaOH), and hot water pretreatments to the enzymatic digestions. Among the three pretreatments, the highest cellulose conversion rate was obtained with the 4% NaOH pretreatment at 121 °C (87.2%). In addition, NaOH pretreatment was mainly effective in removing lignin, whereas the H₂SO₄ pretreatment efficiently removed hemicellulose. To investigate results of pretreated process for enhancement of enzyme-hydolysis to the WS, we used scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy to analyze structural changes of raw and treated materials. The structural analysis indicated that after H₂SO₄ and NaOH pretreatments, most of the amorphous cellulose and partial crystalline cellulose were hydrolyzed during enzymatic hydrolysis. The findings of the present study indicate that WS could be ideal materials for production of monomeric sugars with proper pretreatments and effective enzymatic base hydrolysis.

Physico-chemical pretreatment and enzymatic hydrolysis of cotton stalk for ethanol production by Saccharomyces cerevisiae

The aim of this work was to study the physico-chemical pretreatment and enzymatic hydrolysis of cotton stalk for ethanol production by Saccharomyces cerevisiae. Firstly, factors affecting pretreatment were screened out by Plackett-Burman design (PBD) and most significant factors were further optimized by Box-Behnken design (BBD). As shown by experimental study, most significant factors were FeCl₃ concentration (FC), irradiation time (IT) and substrate concentration (SC) affecting pretreatment of cotton stalk among all studied factors. Under optimum conditions of pretreatment FC 0.15mol/l, IT 20min and SC 55g/l, the release of reducing sugar was 6.6g/l. Hydrolysis of pretreated cotton stalk was done by crude on-site produced enzymes and hydrolysate was concentrated. Ethanol production by Saccharomyces cerevisiae using concentrated cotton stalk hydrolysate was 9.8g_p/l, with ethanol yield 0.37g_p/g_s on consumed sugars. The data indicated that microwave FeCl₃ pretreated cotton stalk hydrolyses by crude unprocessed enzyme cocktail was good, and ethanol can be produced by fermentation of hydrolysate.

Improved physicochemical pretreatment and enzymatic hydrolysis of rice straw for bioethanol production by yeast fermentation

Lignocellulosic biomass such as agricultural and forest residues are considered as an alternative, inexpensive, renewable, and abundant source for fuel ethanol production. In the present study, three different pretreatment methods for rice straw were carried out to investigate the maximum lignin removal for subsequent bioethanol fermentation. The chemical pretreatments of rice straw were optimized under different pretreatment severity conditions in the range of 1.79-2.26. Steam explosion of rice straw at 170 °C for 10 min, sequentially treated with 2% (w/v) KOH (SEKOH) in autoclave at 121 °C for 30 min, resulted in 85 ± 2% delignification with minimum sugar loss. Combined pretreatment of steam explosion and KOH at severity factor (SF 3.10) showed improved cellulose fraction of biomass. Furthermore, enzymatic hydrolysis at 30 FPU/g enzyme loading resulted in 664.0 ± 5.39 mg/g sugar yield with 82.60 ± 1.7% saccharification efficiency. Consequently, the hydrolysate of SEKOH with 58.70 ± 1.52 g/L sugars when fermented with Saccharomyces cerevisiae OBC14 showed 26.12 ± 1.24 g/L ethanol, 0.44 g/g ethanol yield with 87.03 ± 1.6% fermentation efficiency.

Intensification of steam explosion and structural intricacies impacting sugar recovery

Dilute acid (DA) pretreatment at pilot level failed for cotton stalk (CS) due to the technical issues posed by its inherent nature. Reasonable glucan conversion has been reported via two-stage pretreatment but adds on to the process cost. Proposed herewith is a single-stage steam explosion (SE) process preceded by water extraction resulting in high sugar recovery from CS. Raising the extraction temperature to 80°C increased the glucan conversion from 37.9 to 52.4%. Further improvement up to 68.4% was achieved when DA was incorporated during the room temperature extraction. LC-MS revealed the formation of xylo-oligomers limiting the glucan conversion in proportion to the length of xylo-oligomers. Varying extraction conditions induced structural alterations in biomass after SE evident by compositional analysis, Infrared Spectroscopy, X-Ray Diffraction and Scanning Electron Microscopy. Overall glucose recovery, i.e. 75.8-76.7% with and without DA extraction respectively was achieved.