Principal factors affecting the yield of dilute acid pretreatment of lignocellulosic biomass: A critical review

Acid hydrolysis pretreatment for extraction of oligosaccharides derived from spent coffee grounds: valorization of a promising biomass

The coffee industry generates approximately 6 million tons of waste annually, primarily spent coffee grounds (SCGs), whose disposal in landfills poses environmental risks. Therefore, new valorization strategies must be implemented to mitigate their environmental impact. In this sense, the objective of this study was to characterize SCGs and to optimize the dilute sulfuric acid pretreatment process for extracting oligosaccharides (OS). Optimal extraction conditions were determined using response surface methodology (RSM) with a Box-Behnken (BB) 33 design that included five central points for improved accuracy. The factors evaluated were temperature (140-190 °C), solid/liquid (S/L) ratio (1:40-1:4 g/mL), reaction time (20-120 min), and sulfuric acid concentration (0-2% v/v). Hemicellulose was identified as the predominant component, consisting mainly of mannose. OS extraction yields varied from 1.65 to 22.40 g per 100 g dry SCGs, depending on the process conditions. The quadratic model yielded an R2 value of 0.91128, indicating that the S/L ratio was the most influential factor, while reaction time had no significant effect. The optimized conditions-S/L ratio of 1:40 (g/mL), reaction time of 20 min, and H₂SO₄ concentration of 1.43% v/v at 168.57 °C-were experimentally validated and showed a margin of error of less than 9%. MALDI-TOF-MS analysis revealed oligosaccharide structures composed of hexose and pentose chains with up to eight sugar units. This study advances the understanding of OS extraction from SCGs via dilute acid pretreatment and provides valuable insight into waste valorization through process optimization and engineering approaches.

Lignin from Plant-Based Agro-Industrial Biowastes: From Extraction to Sustainable Applications

Lignin, the most abundant aromatic polymer in nature, plays a critical role in lignocellulosic biomasses by providing structural support. However, its presence complicates the industrial exploitation of these materials for biofuels, paper production and other high-value compounds. Annually, the industrial extraction of lignin reaches an estimated 225 million tons, yet only a fraction is recovered for reuse, with most incinerated as low-value fuel. The growing interest in lignin potential has sparked research into sustainable recovery methods from lignocellulosic agro-industrial wastes. This review examines the chemical, physical and physicochemical processes for isolating lignin, focusing on innovative, sustainable technologies that align with the principles of a circular economy. Key challenges include lignin structural complexity and heterogeneity, which hinder its efficient extraction and application. Nonetheless, its properties such as high thermal stability, biodegradability and abundant carbon content place lignin as a promising material for diverse industrial applications, including chemical synthesis and energy generation. A structured analysis of advancements in lignin extraction, characterization and valorization offers insights into transforming this undervalued by-product into a vital resource, reducing reliance on non-renewable materials while addressing environmental sustainability.

Cinnamyl alcohol dehydrogenase downregulation in poplar wood increases saccharification after dilute acid pretreatment: a key role for lignin revealed by a multimodal investigation

This study is the first to apply dilute acid pretreatment (DAP) under different severity conditions to poplar wood genetically modified for the cinnamyl alcohol dehydrogenase (CAD1) gene, which is involved in the lignin biosynthesis pathway. The carefully selected pretreatment conditions resulted in glucose yields that were 15 points higher for the hpCAD poplar line than for the wild-type (WT) wood after 48 h of enzymatic hydrolysis. To explain this higher saccharification rate, the chemical, spectral and structural changes in WT and hpCAD wood were analyzed in relation to the severity of the pretreatment process. Although few differences were found at the chemical level, variations in autofluorescence and cell deformation were more significant: at high severity, the cells of hpCAD wood observed by nanotomography were more easily deformed, but their middle lamella was more resistant than those of WT wood. All these differences are possibly explained by changes in the molecular structure of lignin in hpCAD wood, leading to the formation of more hydrophobic shorter monomer chains with fewer lignin‒carbohydrate interactions.

Substrate-specific responses to mixing conditions in high-solids enzymatic hydrolysis: Insights from microcrystalline cellulose and dilute-acid pretreated corncob

This study investigates the mixing effects on the enzymatic hydrolysis of microcrystalline cellulose (MCC) and dilute-acid pretreated corncob substrates under high-solid conditions. Enzymatic hydrolysis experiments were conducted to assess cellulose conversion rates under varying mixing conditions (0, 50, 150, and 250 rpm) and solids loadings (5 %, 15 %, 25 %, and 35 %, w/v), and distinct physicochemical properties of the substrates were characterized. Additionally, the role of mixing conditions and solid loadings on cellulose hydrolysis kinetics and enzyme adsorption on both substrates and lignin were elucidated. Results demonstrated that both substrates exhibited a decrease in cellulose conversion as solid loading increased, with dilute-acid pretreated corncob consistently showing higher conversion rates compared to MCC. Kinetic analysis revealed that both the rate constant k and the fractal exponent h increased with mixing intensity; however, the increase was more pronounced for dilute-acid pretreated corncob's k and MCC's h. Enzyme adsorption studies indicated dilute-acid pretreated corncob had a higher adsorption capacity qmax and a weaker binding affinity K compared to MCC. Furthermore, increased mixing enhanced enzyme adsorption qmax while decreasing binding affinity K, with more pronounced effects on dilute-acid pretreated corncob's qmax and MCC's K. Lower mixing intensities favored cellulase-lignin binding, with an optimal cellulase-to-lignin binding ratio of approximately 1:1.3, suggesting a more stable interaction. These findings underscore the critical role of substrate disparities in high-solid enzymatic hydrolysis, offering valuable guidance for optimizing mixing strategies for efficient conversion of lignocellulosic biomass.

Revisiting alkali pretreatment to transform lignocellulose fermentation with integration of bioprocessible lignin

This study emphasized the synergistic production of bioprocessible lignin and carbohydrates during a sequential liquid hot water and alkali pretreatment of lignocellulose, facilitating their subsequent individual fermentation. Increasing the dose of alkaline lignin from 0 to 8 g/L inhibited cell growth in anaerobic digestion, with varying levels of inhibition observed in the following order: hydrolytic bacteria < acidogens < acetogens. Alkali pretreatment was adapted to maximize yields of bioprocessible lignin liquor without compromising utilization of the carbohydrates. Increasing the NaOH dose from 50 to 200 mg/g-feedstock monotonically improved lignin yields, but further increases in alkali loading led to a decline in lignin recovery. Volatile fatty acids production from anaerobic digestion of the carbohydrate moiety consistently increased with higher NaOH doses. The optimal conditions for maximizing lignin yields were determined to be 105 °C for 30 min, with NaOH loading in the range of 150-200 mg/g-feedstock, resulting in approximately 80 % lignin recovery, of which 35 % was biologically utilizable. Liquid hot water treatment prior to alkali pretreatment was confirmed as necessary to preserve carbohydrates of 0.1 g/g-feedstock at a low temperature of 70 °C. These findings are crucial for economically producing bioprocessible lignin without carbohydrate loss, a key step towards achieving full lignocellulose valorization.

Integration of lactic acid biorefinery with treatment of red mud from alumina refinery: win-win paradigm for waste valorization

The cost of detoxification and neutralization poses certain challenges to the development of an economically viable lactic acid biorefinery with lignocellulosic biomass as feedstock. Herein, red mud, an alkaline waste, was explored as both a detoxifying agent and a neutralizer. Red mud treatment of lignocellulosic hydrolysate effectively removed the inhibitors generated in dilute acid pretreatment, improving the lactic acid productivity from 1.0 g/L·h-1 to 1.9 g/L·h-1 in later fermentation. In addition, red mud could replace CaCO3 as a neutralizer in lactic acid fermentation, which in turn enabled simultaneous bioleaching of valuable metals (Sc, Y, Nd, and Al) from red mud. The neutralization of alkali in red mud by acids retained in lignocellulosic hydrolysate and lactic acid produced from fermentation led to effective dealkalization, rendering a maximum alkali removal efficiency of 92.2 %. Overall, this study offered a win-win strategy for the valorization of both lignocellulosic biomass and red mud.

ALACEN: A Holistic Herbaceous Biomass Fractionation Process Attaining a Xylose-Rich Stream for Direct Microbial Conversion to Bioplastics

Lignocellulose biorefining is a promising technology for the sustainable production of chemicals and biopolymers. Usually, when one component is focused on, the chemical nature and yield of the others are compromised. Thus, one of the bottlenecks in biomass biorefining is harnessing the maximum value from all of the lignocellulosic components. Here, we describe a mild stepwise process in a flow-through setup leading to separate flow-out streams containing cinnamic acid derivatives, glucose, xylose, and lignin as the main components from different herbaceous sources. The proposed process shows that minimal degradation of the individual components and conservation of their natural structure are possible. Under optimized conditions, the following fractions are produced from wheat straw based on their respective contents in the feed by the ALkaline ACid ENzyme process: (i) 78% ferulic acid from a mild ALkali step, (ii) 51% monomeric xylose free of fermentation inhibitors by mild ACidic treatment, (iii) 82% glucose from ENzymatic degradation of cellulose, and (iv) 55% native-like lignin. The benefits of using the flow-through setup are demonstrated. The retention of the lignin aryl ether structure was confirmed by HSQC NMR, and this allowed monomers to form from hydrogenolysis. More importantly, the crude xylose-rich fraction was shown to be suitable for producing polyhydroxybutyrate bioplastics. The direct use of the xylose-rich fraction by means of the thermophilic bacteria Schlegelella thermodepolymerans matched 91% of the PHA produced with commercial pure xylose, achieving 138.6 mgPHA/gxylose. Overall, the ALACEN fractionation method allows for a holistic valorization of the principal components of herbaceous biomasses.

The DEP1 Mutation Improves Stem Lodging Resistance and Biomass Saccharification by Affecting Cell Wall Biosynthesis in Rice

BACKGROUND

Plant cell walls have evolved precise plasticity in response to environmental stimuli. The plant heterotrimeric G protein complexes could sense and transmit extracellular signals to intracellular signaling systems, and activate a series of downstream responses. dep1 (Dense and Erect Panicles 1), the gain-of-function mutation of DEP1 encoding a G protein γ subunit, confers rice multiple improved agronomic traits. However, the effects of DEP1 on cell wall biosynthesis and wall-related agronomic traits remain largely unknown.

RESULTS

In this study, we showed that the DEP1 mutation affects cell wall biosynthesis, leading to improved lodging resistance and biomass saccharification. The DEP1 is ubiquitously expressed with a relatively higher expression level in tissues rich in cell walls. The CRISPR/Cas9 editing mutants of DEP1 (dep1-cs) displayed a significant enhancement in stem mechanical properties relative to the wild-type, leading to a substantial improvement in lodging resistance. Cell wall analyses showed that the DEP1 mutation increased the contents of cellulose, hemicelluloses, and pectin, and reduced lignin content and cellulose crystallinity (CrI). Additionally, the dep1-cs seedlings exhibited higher sensitivity to cellulose biosynthesis inhibitors, 2,6-Dichlorobenzonitrile (DCB) and isoxaben, compared with the wild-type, confirming the role of DEP1 in cellulose deposition. Moreover, the DEP1 mutation-mediated alterations of cell walls lead to increased enzymatic saccharification of biomass after the alkali pretreatment. Furthermore, the comparative transcriptome analysis revealed that the DEP1 mutation substantially altered expression of genes involved in carbohydrate metabolism, and cell wall biosynthesis.

CONCLUSIONS

Our findings revealed the roles of DEP1 in cell wall biosynthesis, lodging resistance, and biomass saccharification in rice and suggested genetic modification of DEP1 as a potential strategy to develop energy rice varieties with high lodging resistance.

Highly-efficient lipid production from hydrolysate of Radix paeoniae alba residue by oleaginous yeast Cutaneotrichosporon oleaginosum

Valorization of herbal extraction residues (HERs) into value-added products is pivotal for the sustainability of Chinese medicine industry. Here, seven different enzymatic hydrolysates of dilute acid pretreated HERs were evaluated for lipid production by Cutaneotrichosporon oleaginosum. Among them, the highest sugar yield via hydrolysis and the maximum lipid production were obtained from Radix paeoniae alba residue (RPAR). More interestingly, high proportion of sugar polymers was disintegrated into fermentable sugars during the pretreatment step, allowing a cheap non-enzymatic route for producing sugars from RPAR. A repeated dilute acid pretreatment gained a high sugar concentration of 241.6 g/L through reusing the pretreatment liquor (PL) for four times. Biomass, lipid concentration, and lipid content achieved 49.5 g/L, 35.7 g/L and 72.2 %, respectively, using fed-batch culture of PL. The biodiesel parameters indicated lipids produced from HERs were suitable for biodiesel production. This study offers a cost-effective way to upgrade the HERs waste into micro-biodiesel.

Combined dilute sulfuric acid and Tween 80 pretreatment of corn stover significantly improves the enzyme digestibility: synergistic removal of hemicellulose and lignin

Pretreatment is a prerequisite to tackle the issue of biomass recalcitrance, which is the major hindrance of lignocellulose-to-sugars routes. In the present study, a novel combination of dilute sulfuric acid (dilute-H₂SO₄) with Tween 80 pretreatment of corn stover (CS) was developed to significantly enhance the enzyme digestibility. Strong synergistic effects of H₂SO₄ and Tween 80 for simultaneously eliminating hemicellulose and lignin and significantly promoting saccharification yield were observed. A response surface optimization realized the maximum monomeric sugar yield of 95.06% at 120 °C for 1.4 h with 0.75wt% of H₂SO₄ and 73.92 wt% of Tween 80. The excellent enzyme susceptibility of pretreated CS was explained by their physical and chemical characteristics via SEM, XRD, and FITR. The repeatedly recovered pretreatment liquor exerted highly-effective reusability in the subsequent pretreatments for at least four cycles. This strategy offers a highly-efficient and practical pretreatment strategy, which provides valuable information for the lignocellulose-to-sugars routes.