Microfibrillated cellulose - Its barrier properties and applications in cellulosic materials: A review

Interest in microfibrillated cellulose (MFC) has been increasing exponentially. During the last decade, this bio-based nanomaterial was essentially used in nanocomposites for its reinforcement property. Its nano-scale dimensions and its ability to form a strong entangled nanoporous network, however, have encouraged the emergence of new high-value applications. In previous years, its mode of production has completely changed, as many forms of optimization have been developed. New sources, new mechanical processes, and new pre- and post-treatments are currently under development to reduce the high energy consumption and produce new types of MFC materials on an industrial scale. The nanoscale characterization possibilities of different MFC materials are thus increasing intensively. Therefore, it is critical to review such MFC materials and their properties. Moreover, very recent studies have proved the significant barrier properties of MFC. Hence, it is proposed to focus on the barrier properties of MFC used in films, in nanocomposites, or in paper coating.     

The effects of four different pretreatments on enzymatic hydrolysis of sweet sorghum bagasse

Four pretreatment processes including ionic liquids, steam explosion, lime, and dilute acid were used for enzymatic hydrolysis of sweet sorghum bagasse. Compared with the other three pretreatment approaches, steam-explosion pretreatment showed the greatest improvement on enzymatic hydrolysis of the bagasse. The maximum conversion of cellulose and the concentration of glucose obtained from enzymatic hydrolysis of steam explosion bagasse reached 70% and 25 g/L, respectively, which were both 2.5 times higher than those of the control (27% and 11 g/L). The results based on the analysis of SEM photos, FTIR, XRD and NMR detection suggested that both the reduction of crystallite size of cellulose and cellulose degradation from the Iα and Iβ to the Fibril surface cellulose and amorphous cellulose were critical for enzymatic hydrolysis. These pretreatments disrupted the crystal structure of cellulose and increased the available surface area, which made the cellulose better accessible for enzymatic hydrolysis.     

Rheology of biofilms formed at the surface of NF membranes in a drinking water production unit

In this study, the mechanical properties of biofilms formed at the surface of nano-filtration (NF) membranes from a drinking water plant were analysed. Confocal laser scanning microscopy observations revealed that the NF biofilms formed a dense and heterogeneous structure at the membrane surface, with a mean thickness of 32.5 +/- 17.7 mum. The biofilms were scraped from the membrane surface and analysed in rotation and oscillation experiments with a RheoStress 150 rotating disk rheometer. During rotation analyses, a viscosity decrease with speed of shearing characteristic of rheofluidification was observed (eta = 300 Pa s for y = 0.3 s(-1)). In the oscillation analyses with a sweeping of frequency (1-100 Hz), elasticity (G') ranged from 3000 to 3500 Pa and viscosity (G') from 800 to 1200 Pa. Creep curves obtained with an application of a shear stress of 30 Pa were viscoelastic in nature. The G(0) and eta values were, respectively, 1.4 +/- 0.3 x 10(3) Pa and 3.3 +/- 0.65 x 10(6) Pa s. The relationship between the characteristics of NF biofilms and the flow conditions encountered during NF is discussed.     

Relation between filter paper activity and saccharifying ability of a Trichoderma cellulase system

Summary Tests made to study the relation between filter paper activity and actual saccharifying ability of Trichoderma cellulases show that 30 IU/g of cellulose were sufficient to achieve over 80% hydrolysis of a 25 g/L cellulose suspension in 24 h. With the same enzyme/substrate ratio, but double the concentration of substrate, about 60% hydrolysis was achieved. End- product inhibition is one factor which seriously limits the degree of hydrolysis and therefore the concentration of sugars achievable by enzymatic hydrolysis at high levels of substrate concentration or enzyme/substrate ratio.     

CP/MAS 13C NMR analysis of cellulase treated bleached softwood kraft pulp

Fully bleached softwood kraft pulps were hydrolyzed with cellulase (1,4-(1,3:1,4)-beta-D-glucan 4-glucano-hydrolase, EC 3.2.1.4) from Trichoderma reesei. Supra-molecular structural features of cellulose during enzymatic hydrolysis were examined by using CP/MAS 13C NMR spectra in combination with line-fitting analysis. Different types of cellulose allomorphs (cellulose I(alpha), cellulose I(beta), para-crystalline) and amorphous regions were hydrolyzed to a different extent by the enzyme used. Also observed was a rapid initial phase for hydrolysis of regions followed by a slow hydrolysis phase. Cellulose I(alpha), para-crystalline, and non-crystalline regions of cellulose are more susceptible to enzymatic hydrolysis than cellulose I(beta) during the initial phase. After the initial phase, all the regions are then similarly susceptible to enzymatic hydrolysis.     

The elasticity of spectrin-actin gels at high protein concentration

Human erythrocyte spectrin of high purity was studied alone and mixed with rabbit skeletal actin by dynamic rheometry as a function of protein concentration at pH 7.4 and 24 degrees C. Pure spectrin had a very low storage modulus, G', increasing slightly with increase in protein concentration (approximately 3 dynes/cm at 25 mg/ml). In contrast, unpurified cytoskeletal extracts containing spectrin, actin, and band 4.1 showed a marked concentration dependence for G', increasing to 150 dynes/cm at 20 mg/ml. Mixtures of purified spectrin and skeletal actin at a weight ratio of 4:1 also showed G' markedly dependent on concentration (approximately 150-200 dynes/cm at 20 mg/ml). Maximum elasticity of spectrin-actin gels occurred at a molar ratio of actin monomers to spectrin tetramers of 14:1. We conclude that the reconstituted in vitro spectrin-actin network consists of actin fibers cross-linked by spectrin tetramers at regular intervals. The gel is rapidly reformed after mechanical disruption or thermal collapse, indicating that the polymer fibers are in equilibrium with the constituent monomers.     

Effect of particle size on the rate of enzymatic hydrolysis of cellulose

The effect of particle size on enzymatic hydrolysis of cellulose has been investigated. The average size of microcrystalline cotton cellulose has been reduced to submicron scale by using a media mill. The milled products were further subjected to hydrolysis using cellulase. High cellulose concentration (7%) appeared to retard the size reduction and resulted in greater particles and smaller specific surface areas than those at low concentration (3%) with the same milling time. Initial rate method was employed to explore the rate of enzymatic hydrolysis of cellulose. The production rate of cellobiose was increased at least 5-folds due to the size reduction. The yield of glucose was also significantly increased depending upon the ratio of enzyme to substrate. A high glucose yield (60%) was obtained in 10-h hydrolysis when the average particle size was in submicron scale.     

Enzymatically cross-linked tilapia gelatin hydrogels: physical, chemical, and hybrid networks

This Article investigates different types of networks formed from tilapia fish gelatin (10% w/w) in the presence and absence of the enzymatic cross-linker microbial transglutaminase. The influence of the temperature protocol and cross-linker concentration (0-55 U mTGase/g gelatin) was examined in physical, chemical, and hybrid gels, where physical gels arise from the formation of triple helices that act as junction points when the gels are cooled below the gelation point. A combination of rheology and optical rotation was used to study the evolution of the storage modulus (G') over time and the number of triple helices formed for each type of gel. We attempted to separate the final storage modulus of the gels into its chemical and physical contributions to examine the existence or otherwise of synergism between the two types of networks. Our experiments show that the gel characteristics vary widely with the thermal protocol. The final storage modulus in chemical gels increased with enzyme concentration, possibly due to the preferential formation of closed loops at low cross-linker amount. In chemical-physical gels, where the physical network (helices) was formed consecutively to the covalent one, we found that below a critical enzyme concentration the more extensive the chemical network is (as measured by G'), the weaker the final gel is. The storage modulus attributed to the physical network decreased exponentially as a function of G' from the chemical network, but both networks were found to be purely additive. Helices were not thermally stabilized. The simultaneous formation of physical and chemical networks (physical-co-chemical) resulted in G' values higher than the individual networks formed under the same conditions. Two regimes were distinguished: at low enzyme concentration (10-20 U mTGase/g gelatin), the networks were formed in series, but the storage modulus from the chemical network was higher in the presence of helices (compared to pure chemical gels); at higher enzyme concentration (30-40 U mTGase/g gelatin), strong synergistic effects were found as a large part of the covalent network became ineffective upon melting of the helices.     

Microstructure and kinetic rheological behavior of amidated and nonamidated LM pectin gels

The microstructure, kinetics of gelation, and rheological properties have been investigated for gels of nonamidated pectin (C30), amidated pectin (G), and saponified pectin (sG) at different pH values, both with and without sucrose. The low-methoxyl (LM) pectin gels were characterized in the presence of Ca(2+) by oscillatory measurements and transmission electron microscopy (TEM). The appearance of the gel microstructure varied with the pH, the gel structure being sparse and aggregated at pH 3 but dense and somewhat entangled at pH 7. During gel formation of pectins G and C30 at pH 3 there was a rapid increase in G' initially followed by a small increase with time. At pH 7 G' increased very rapidly at first but then remained constant. The presence of sucrose influenced neither the kinetic behavior nor the microstructure of the gels but strongly increased the storage modulus. Pectins G and C30 showed large variations in G' at pH values 3, 4, 5, and 7 in the presence of sucrose, and the maximum in G' in the samples occurred at different pH values. Due to its high Ca(2+) sensitivity, pectin sG had a storage modulus that was about 50 times higher than that of its mother pectin G at pH 7.     

Effect of Urea on the Enzymatic Hydrolysis of Lignocellulosic Substrate and Its Mechanism

Effect of hydrogen bond breaker (urea) addition on the enzymatic hydrolysis of Avicel and eucalyptus pretreated by dilute acid (Eu-DA) was investigated. Urea enhanced the enzymatic hydrolysis of Eu-DA at 50 or 30 °C when the concentration of urea was below 60 g/L, while it inhibited the hydrolysis of Avicel. Low concentration urea (<?240 g/L) had little effect on the cellulase spatial structure and its activity. But it decreased cellulase binding to cellulose surface to inhibit the cellulose hydrolysis. Meanwhile, urea obviously prevented the adsorption of cellobiohydrolase I (CBHI) on the lignin in spite of little effect on the adsorption of β-glucosidase (BGL) and two endoglucanases (EGIII and EGV) on lignin. It was proposed that urea enhanced the enzymatic efficiency of Eu-DA by decreasing the cellulase adsorption on lignin surface.     

The enzymatic hydrolysis rate of cellulose decreases with irreversible adsorption of cellobiohydrolase I

Protein adsorption onto solid substrates usually takes place in an irreversible fashion and this irreversible adsorption also occurs in some enzymatic reactions. In this work the adsorption behavior of intact β-1, 4-glucan-cellobiohydrolase (CBH I) from Trichoderma reesei onto microcrystalline cellulose was monitored by surface plasmon resonance and UV-spectral method. It was found that there existed reversible binding and irreversible binding of CBH I during its interaction with cellulose substrate. To evaluate the influence of adsorption on cellulose enzymatic hydrolysis, the reaction dynamics on pure cellulose were determined. A plot of the hydrolysis rate against the surface density of irreversibly adsorbed CBH I, revealed an inverse relationship in which an apparent decrease in the hydrolysis rate was observed with increasing surface density. Taken together, results presented here should be useful for modifying the binding characteristics of CBH I and making them more effective in cellulose hydrolysis.     

Arabinoxylan gels: impact of the feruloylation degree on their structure and properties

Arabinoxylan (AX) samples of decreasing ferulic acid (FA) contents were chemically prepared from water-extractable wheat arabinoxylans without affecting their other structural properties. Gels were obtained from these partially feruloylated WEAX (PF-WEAX) by enzymatic covalent cross-linking of FA leading to the formation of diferulic (di-FA) and tri-ferulic acid (tri-FA). WEAX gelling ability was found related to the WEAX FA content whereas the gel structure and properties depended on the density of newly formed covalent cross-links. FA content of WEAX ranging from 1.4 to 2.3 microg/mg AX gave gels with di-FA cross-links contents from 0.20 to 0.43 microg/mg AX and G' values from 5 to 44 Pa. For WEAX gels with initial FA contents from 1.6 to 2.3 microg/mg AX, average mesh size ranging from 331 to 263 nm were calculated from swelling experiments. Cross-linking densities of gels, determined from swelling experiments, were higher than those that could be theoretically estimated from the di-FA and tri-FA content of WEAX gels. This result suggests that, in addition to di-FA and tri-FA, higher ferulate cross-linking and physical entanglements would contribute to the final WEAX gel structure.     

The cellulosome and cellulose degradation by anaerobic bacteria

Despite its simple chemical composition, cellulose exists in a number of crystalline and amorphous topologies. Its insolubility and heterogeneity makes native cellulose a recalcitrant substrate for enzymatic hydrolysis. Microorganisms meet this challenge with the aid of a multi-enzyme system. Aerobic bacteria produce numerous individual, extra-cellular enzymes with binding modules for different cellulose conformations. Specific enzymes act in synergy to elicit effective hydrolysis. In contrast, anaerobic bacteria possess a unique extracellular multi-enzyme complex, called cellulosome. Up to 11 different enzymes are aligned on the non-catalytic scaffolding protein and thus ensure a high local concentration, together with the correct ratio and order of the components. These multi-enzyme complexes attach both to the cell envelope and to the substrate, mediating the proximity of the cells to the cellulose. Binding to the scaffolding stimulates the activity of each individual component towards the crystalline substrate. The most complex and best investigated cellulosome is that of the thermophilic bacterium Clostridium thermocellum, but a scheme for the cellulosomes of the mesophilic clostridia and the ruminococci emerges. Many crucial details of cellulose hydrolysis are still to be uncovered. Yet, a mechanistic model for the action of enzyme complexes on the surface of insoluble substrates becomes apparent and the application of enzymatic hydrolysis of cellulosic biomass can now be addressed.     

Reactor design for minimizing product inhibition during enzymatic lignocellulose hydrolysis: I. Significance and mechanism of cellobiose and glucose inhibition on cellulolytic enzymes

Achievement of efficient enzymatic degradation of cellulose to glucose is one of the main prerequisites and one of the main challenges in the biological conversion of lignocellulosic biomass to liquid fuels and other valuable products. The specific inhibitory interferences by cellobiose and glucose on enzyme-catalyzed cellulose hydrolysis reactions impose significant limitations on the efficiency of lignocellulose conversion — especially at high-biomass dry matter conditions. To provide the base for selecting the optimal reactor conditions, this paper reviews the reaction kinetics, mechanisms, and significance of this product inhibition, notably the cellobiose and glucose inhibition, on enzymatic cellulose hydrolysis. Particular emphasis is put on the distinct complexity of cellulose as a substrate, the multi-enzymatic nature of the cellulolytic degradation, and the particular features of cellulase inhibition mechanisms and kinetics. The data show that new strategies that place the bioreactor design at the center stage are required to alleviate the product inhibition and in turn to enhance the efficiency of enzymatic cellulose hydrolysis. Accomplishment of the enzymatic hydrolysis at medium substrate concentration in separate hydrolysis reactors that allow continuous glucose removal is proposed to be the way forward for obtaining feasible enzymatic degradation in lignocellulose processing.     

Influence of solid loading on enzymatic hydrolysis of steam exploded or liquid hot water pretreated olive tree biomass

Olive tree pruning biomass, pretreated by either liquid hot water or steam explosion under selected conditions, was used as a substrate for enzymatic hydrolysis. The pretreated material was further submitted to alkaline delignification, the objective being to improve hydrolysis yields as well as increasing cellulose content in the pretreated feedstock. The enzymatic hydrolysis of pretreated residues was performed using a commercial cellulase mixture supplemented with β-glucosidase, using a solid loading range from 2 to 30% (w/v). The influence of substrate concentration on the enzymatic hydrolysis yield and on glucose concentration was studied. Comparative results with and without a delignification step are presented. Enzymatic hydrolysis at high substrate concentration (≥20%) is possible, yielding a concentrated glucose solution (>50 g/L). Nevertheless, a cellulose fraction of the pretreated residue remains unaltered.     

Toward an aggregated understanding of enzymatic hydrolysis of cellulose: noncomplexed cellulase systems

Information pertaining to enzymatic hydrolysis of cellulose by noncomplexed cellulase enzyme systems is reviewed with a particular emphasis on development of aggregated understanding incorporating substrate features in addition to concentration and multiple cellulase components. Topics considered include properties of cellulose, adsorption, cellulose hydrolysis, and quantitative models. A classification scheme is proposed for quantitative models for enzymatic hydrolysis of cellulose based on the number of solubilizing activities and substrate state variables included. We suggest that it is timely to revisit and reinvigorate functional modeling of cellulose hydrolysis, and that this would be highly beneficial if not necessary in order to bring to bear the large volume of information available on cellulase components on the primary applications that motivate interest in the subject.     

Enzymatic hydrolysis of cellulose in a membrane bioreactor: assessment of operating conditions

The optimization of operating conditions for cellulose hydrolysis was systemically undertaken using an ultra-scaled down membrane bioreactor based on the parameter scanning ultrafiltration apparatus. The bioconversion of cellulose saccharification was carried out with freely suspended cellulase from Aspergillus niger as the biocatalyst. The polyethersulfone ultrafiltration membranes with a molecular weight cutoff of 10 kDa were used to construct the enzymatic membrane bioreactor, with the membrane showing a complete retaining of cellulase and cellobiase. The influence of solution pH, temperature, salt (NaCl) concentration, presence of cellobiase, cellulose-to-enzyme ratio and stirring speed on reducing sugar production was examined. The results showed that the addition of an appropriate amount of NaCl or cellobiase had a positive effect on reducing sugar formation. Under the identified optimal conditions, cellulose hydrolysis in the enzymatic membrane bioreactor was tested for a long period of time up to 75 h, and both enzymes and operation conditions demonstrated good stability. Also, the activation energy (E _a) of the enzymatic hydrolysis, with a value of 34.11 ± 1.03 kJ mol⁻¹, was estimated in this study. The operational and physicochemical conditions identified can help guide the design and operation of enzymatic membrane bioreactors at the industrial scale for cellulose hydrolysis.     

Detrimental effect of cellulose oxidation on cellulose hydrolysis by cellulase

To effectively convert complex and recalcitrant biomass carbohydrates to simple platform sugars useful for fuel and chemicals production, mechanical or chemical pre-treatments are often required to make the carbohydrates more accessible for enzymatic hydrolysis. Due to their harsh conditions, some pre-treatments might negatively affect enzymatic hydrolysis because of events such as cellulose oxidation. To study how oxidative modification may impact cellulose's reactivity toward hydrolysis by cellulases, we prepared three cellulose substrates by cupric ion and hypochlorite oxidations, and subjected the derived celluloses to hydrolysis by various cellobiohydrolases from glycoside hydrolase families 6 and 7, and one cellulolytic Hypocrea jecorina extracellular enzyme mixture. We observed a profound decrease of enzymatic hydrolysis on the oxidized celluloses. The effect was attributed to the interference, from oxidized functional groups in cellulose, on its binding/activation in the active pocket/tunnel of cellobiohydrolases. Potential implication of the observed effect from cellulose oxidation on pre-treatment optimization and cellulase improvement was discussed.     

Enhanced rates of enzymatic saccharification and catalytic synthesis of biofuel substrates in gelatinized cellulose generated by trifluoroacetic acid

Background

The crystallinity of cellulose is a principal factor limiting the efficient hydrolysis of biomass to fermentable sugars or direct catalytic conversion to biofuel components. We evaluated the impact of TFA-induced gelatinization of crystalline cellulose on enhancement of enzymatic digestion and catalytic conversion to biofuel substrates.

Results

Low-temperature swelling of cotton linter cellulose in TFA at subzero temperatures followed by gentle heating to 55 °C dissolves the microfibril structure and forms composites of crystalline and amorphous gels upon addition of ethanol. The extent of gelatinization of crystalline cellulose was determined by reduction of birefringence in darkfield microscopy, loss of X-ray diffractability, and loss of resistance to acid hydrolysis. Upon freeze-drying, an additional degree of crystallinity returned as mostly cellulose II. Both enzymatic digestion with a commercial cellulase cocktail and maleic acid/AlCl₃-catalyzed conversion to 5-hydroxymethylfurfural and levulinic acid were markedly enhanced with the low-temperature swollen cellulose. Only small improvements in rates and extent of hydrolysis and catalytic conversion were achieved upon heating to fully dissolve cellulose.

Conclusions

Low-temperature swelling of cellulose in TFA substantially reduces recalcitrance of crystalline cellulose to both enzymatic digestion and catalytic conversion. In a closed system to prevent loss of fluorohydrocarbons, the relative ease of recovery and regeneration of TFA by distillation makes it a potentially useful agent in large-scale deconstruction of biomass, not only for enzymatic depolymerization but also for enhancing rates of catalytic conversion to biofuel components and useful bio-products.