Strategies to modify physicochemical properties of hemicelluloses from biorefinery and paper industry for packaging material

Hemicelluloses are heteropolysaccharides existing in plant cell wall and seed, and they can be extracted or separated from plants as byproducts during the biomass pretreatment in biorefineries and the pulping in paper industry. The hemicelluloses have many applications such as in biofuels, platform chemicals, and materials. Producing packaging materials (films) is a potential high-value application of the hemicelluloses. However, native hemicelluloses are usually unable to form strong and durable films due to their short chain (low molecular weight), high hydrophilicity, and heterogeneous nature. Chemical and biological modifications could change the physicochemical properties of the hemicelluloses and thereby improve the strength and performance of the hemicellulose-based films. The present review extensively summarized and discussed the recent development and progress in hemicellulose modification strategies and methods for improving the formability and properties of the hemicellulose-based packaging films such as mechanical strength, processability, thermal stability, hydrophobicity, and oxygen and water vapor permeability, which include enzymatic treatment, esterification, etherification, oxidation, coupling, and crosslinking. The challenges and opportunities of hemicellulose as packaging materials were addresses.     

Formaldehyde cross-linking of gliadin films: effects on mechanical and water barrier properties

In this study, pioneering results on specific chemical modifications of wheat gluten gliadins and the corresponding impact on mechanical and water barrier properties of derived films are presented. Films were prepared from gliadins chemically treated with formaldehyde and subsequently mixed with different concentrations of glycerol as a plasticizing agent. Water vapor barrier and mechanical properties of the films were evaluated as a function of relative humidity and glycerol concentration. Formaldehyde treatment led to enhanced mechanical properties and, to a lesser extent, improved water barrier of the films, effects which point to the formation of new intermolecular bonds between monomeric gliadins. The occurrence of cross-linking was supported by SDS-PAGE analysis. Cross-linked films maintained their integrity after immersion in water and had similar optical properties to control films. The effect of glycerol and humidity on water vapor permeability and the mechanical properties of films was less acute when proteins were treated with formaldehyde. Thus, chemical treatment of proteins is shown to be a very effective route for optimizing the use of these films in packaging applications.     

Mechanical properties and water vapor transmission in some blends of cassava starch edible films

The continuous increase of consumer interest in quality, convenience and food quality has encouraged further research into edible films and coatings from natural polymers, such as polysaccharides. Ecoefficient products are the new generation of biobased products prepared with sustainable materials, that agree with ecological and economic requirements including environmentally acceptable disposal of post-user waste. The numerous potential applications of natural polymers such as polysaccharides stimulated the study with edible films based on cassava starch. Blends of glycerol (GLY) and polyethylene glycol (PEG) as plasticizers, and glutaraldehyde (GLU) as crosslinking agent were prepared in order to determine the mechanical properties and water vapor transmission of those films. A response surface methodology was applied on the results to identify the blend with the best mechanical properties and lowest water vapor transmission. The crosslinking effect of glutaraldehyde in the films can be observed. The plasticizing action of polyethylene glycol was restrained by more than 0.5 g of glutataraldehyde. The use of glycerol was less evident for this property even after 284 h of contact time with water vapor.     

High performance edible nanocomposite films containing bacterial cellulose nanocrystals

Bacterial cellulose obtained from Gluconacetobacter xylinus in the form of long fibers were acid hydrolyzed under controlled conditions to obtain cellulose nanocrystals. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) confirmed the formation of rod like cellulose nanocrystals having an average diameter and length of 20 ± 5 nm and 290 ± 130 nm respectively. These nanocrystals were used to prepare gelatin nanocomposite films and characterized for elucidating its performance. The formation of percolated networks of cellulose nanocrystals within gelatin matrix resulted in improving the mechanical properties of nanocomposites. The moisture sorption and water vapor permeability (WVP) studies revealed that the addition of cellulose nanocrystals reduced the moisture affinity of gelatin, which is very favorable for edible packaging applications. Results of this study demonstrated the use of bacterial cellulose nanocrystals (BCNCs) in the fabrication of edible, biodegradable and high-performance nanocomposite films for food packaging applications at relatively low cost.     

Film forming microbial biopolymers for commercial applications—A review

Abstract

Microorganisms synthesize intracellular, structural and extracellular polymers also referred to as biopolymers for their function and survival. These biopolymers play specific roles as energy reserve materials, protective agents, aid in cell functioning, the establishment of symbiosis, osmotic adaptation and support the microbial genera to function, adapt, multiply and survive efficiently under changing environmental conditions. Viscosifying, gelling and film forming properties of these have been exploited for specific significant applications in food and allied industries. Intensive research activities and recent achievements in relevant and important research fields of global interest regarding film forming microbial biopolymers is the subject of this review. Microbial polymers such as pullulan, kefiran, bacterial cellulose (BC), gellan and levan are placed under the category of exopolysaccharides (EPS) and have several other functional properties including film formation, which can be used for various applications in food and allied industries. In addition to EPS, innumerable bacterial genera are found to synthesis carbon energy reserves in their cells known as polyhydroxyalkanoates (PHAs), microbial polyesters, which can be extruded into films with excellent moisture and oxygen barrier properties. Blow moldable biopolymers like PHA along with polylactic acid (PLA) synthesized chemically in vitro using lactic acid (LA), which is produced by LA bacteria through fermentation, are projected as biodegradable polymers of the future for packaging applications. Designing and creating of new property based on requirements through controlled synthesis can lead to improvement in properties of existing polysaccharides and create novel biopolymers of great commercial interest and value for wider applications. Incorporation of antimicrobials such as bacteriocins or silver and copper nanoparticles can enhance the functionality of polymer films especially in food packaging applications either in the form of coatings or wrappings. Use of EPS in combinations to obtain desired properties can be evaluated to increase the application range. Controlled release of active compounds, bioactive protection and resistance to water can be investigated while developing new technologies to improve the film properties of active packaging and coatings. An holistic approach may be adopted in developing an economical and biodegradable packaging material with acceptable properties. An interdisciplinary approach with new innovations can lead to the development of new composites of these biopolymers to enhance the application range. This current review focuses on linking and consolidation of recent research activities on the production and applications of film forming microbial polymers like EPS, PHA and PLA for commercial applications.     

Hemicellulose biosynthesis

One major component of plant cell walls is a diverse group of polysaccharides, the hemicelluloses. Hemicelluloses constitute roughly one-third of the wall biomass and encompass the heteromannans, xyloglucan, heteroxylans, and mixed-linkage glucan. The fine structure of these polysaccharides, particularly their substitution, varies depending on the plant species and tissue type. The hemicelluloses are used in numerous industrial applications such as food additives as well as in medicinal applications. Their abundance in lignocellulosic feedstocks should not be overlooked, if the utilization of this renewable resource for fuels and other commodity chemicals becomes a reality. Fortunately, our understanding of the biosynthesis of the various hemicelluloses in the plant has increased enormously in recent years mainly through genetic approaches. Taking advantage of this knowledge has led to plant mutants with altered hemicellulosic structures demonstrating the importance of the hemicelluloses in plant growth and development. However, while we are on a solid trajectory in identifying all necessary genes/proteins involved in hemicellulose biosynthesis, future research is required to combine these single components and assemble them to gain a holistic mechanistic understanding of the biosynthesis of this important class of plant cell wall polysaccharides.     

Characterization of coating systems

Polymeric film coatings have been applied to solid substrates for decorative, protective, and functional purposes. Irrespective of the reasons for coating, certain properties of the polymer films may be determined as a method to evaluate coating formulations, substrate variables, and processing conditions. This article describes experimental techniques to assess various properties of both free and applied films, including water vapor and oxygen permeability, as well as thermal, mechanical, and adhesive characteristics. Methods to investigate interfacial interactions are also presented.     

Physical properties of nisin-incorporated gelatin and corn zein films and antimicrobial activity against Listeria monocytogenes

Edible films of gelatin and corn zein were prepared by incorporating nisin to the film-forming solutions. Corn zein film with nisin of 12,000 IU/ml had an increase of 11.6 MPa in tensile strength compared with the control, whereas gelatin film had a slight increase with the increase of nisin concentration added. Water vapor permeability for both corn zein and gelatin films decreased with the increase of nisin concentration, thus providing a better barrier against water. Antimicrobial activity against Listeria monocytogenes increased with the increase of nisin concentration, resulting in 1.4 log cycle reduction for corn zein film and 0.6 log cycle reduction for gelatin film at 12,000 IU/ml. These results suggest that incorporation of nisin into corn zein and gelatin films improve the physical properties of the films as well as antimicrobial activity against pathogenic bacteria during storage, resulting in extension of the shelf life of food products by providing with antimicrobial edible packaging films.     

Material properties of plasticized hardwood xylans for potential application as oxygen barrier films

Free films based on glucuronoxylan isolated from aspen wood were prepared by casting from aqueous solutions and drying in a controlled environment. Addition of xylitol or sorbitol facilitated film formation and thus examination of the material properties of these films. The mechanical properties of the films were evaluated using tensile testing and dynamic mechanical analysis in a controlled ambient relative humidity. The strain at break increased, and the stress at break and Young's modulus of the films decreased with increasing amounts of xylitol and sorbitol due to plasticization. At high amount of plasticizer, it was found that films with xylitol gave lower extensibility. Wide-angle X-ray scattering analysis showed that xylitol crystallized in a distinct phase, which we believe contributes to the more brittle behavior of these films. The effect of the plasticizers on the glass transition temperature was determined using dynamic mechanical analysis and differential scanning calorimetry. An increased amount of plasticizer shifted the glass transition to lower temperatures. The effect of moisture on the properties of plasticized films was investigated using water vapor sorption isotherms and by humidity scans in dynamic mechanical analysis. Sorption isotherms showed a transition from type II to type III when adding plasticizer. The films showed low oxygen permeability and thus have a potential application in food packaging.     

Effect of alkaline and autohydrolysis processes on the purity of obtained hemicelluloses from corn stalks

A study of the potential of autohydrolysis and alkaline extraction processes from corn stalks was performed for high purity hemicellulose extraction. The influence of process parameters on the purity of obtained hemicelluloses was analyzed. An experimental design was developed for the autohydrolysis treatments to determine the optimal conditions to solubilize the hemicelluloses with lowest content in contaminants. On the other hand, alkaline extraction, including raw material pretreatment (dewaxing and delignification step) was carried out analyzing the effectiveness of this processes for maximum pure hemicellulose recovery. The maximum yield (54% of the raw material hemicelluloses) and the best physicochemical properties (highest hemicellulose content free of lignin) were obtained with these pretreatments in alkaline extraction. Moreover, the effect of lignin removal by sulfuric acid from the autohydrolysis liquors before hemicellulose precipitation was studied. This purification step has allowed to obtain lignin-free autohydrolysis hemicellulose but with the presence of sulfur as predominant contaminant.     

Wood hydrolysate barriers: performance controlled via selective recovery

Films and coatings were produced from a noncellulosic polysaccharide-rich wood hydrolysate (WH), and the resulting oxygen barrier performance was improved by a selective choice of upgrading conditions. The WH was obtained from process water in the hydrothermal treatment of hardwood and subjected to one of three alternative upgrading treatments, resulting in xylan-rich fractions with significant differences in structure, composition, and properties of the recovered WH fractions, which in turn had a major impact on their performance with respect to tensile and oxygen barrier properties. The WH in the least upgraded state, the crudest fraction, produced films with the best performance in terms of oxygen permeability and was superior to corresponding films based on highly purified hemicellulose.     

Production of Hydrophobins from fungi

Hydrophobins (HPs) are industrially important surface active, amphipathic proteins produced by fungi. There are many applications reported for HPs in the literature notably as, agents for enhancing bioavailability of water insoluble drugs, food stabilizers, antifouling agents for biomedical devices like catheters, fusion partner for recombinant proteins for purification, low friction coatings on biomaterials, immobilizing enzymes in biosensors, etc. However, there are limitations for industrial scale production of HPs. Various methods have been reported for their production e.g. use of wild fungi from natural hydrophobic environments, use of modified bioreactors for submerged and solid state fermentation and recombinant homologous as well as heterologous microbes. Knowing the industrial importance of HPs many reviews have been published focusing on technical and medical applications of these proteins; however there is no comprehensive overview of HP production in the literature. This review summarizes the efforts made to improve yields of HPs by various bioprocesses and also highlights the strategies designed to overcome problems of low yield of HPs.     

Fennel waste-based films suitable for protecting cultivations

Biodegradable, flexible, and moisture-resistant films were obtained by recycling fennel waste and adding to fennel homogenates the bean protein phaseolin that was modified or not modified by the enzyme transglutaminase. All films were analyzed for their morphology, mechanical properties, water vapor permeability, and susceptibility to biodegradation under soil-like conditions. Our experiments showed that transglutaminase treatment of the phaseolin-containing fennel waste homogenates allowed us to obtain films comparable in their mechanical properties and water vapor permeability to the commercial films Ecoflex and Mater-Bi. Furthermore, biodegradability tests demonstrated that the presence of the enzyme in the film-casting sample significantly influences the integrity of such a product that lasts longer than films obtained either with fennel waste alone or with a mixture of fennel waste and phaseolin. These findings indicate the fennel-phaseolin film prepared in the presence of transglutaminase to be a promising candidate for a new environmentally friendly mulching bioplastic.     

Quantification and monosaccharide composition of hemicelluloses from different plant functional types

Hemicelluloses are the second most abundant polysaccharide in nature after cellulose. So far, the chemical heterogeneity of cell-wall hemicelluloses and the relatively large sample-volume required in existing methods represent major obstacles for large-scale, cross-species analyses of this important plant compound. Here, we apply a new micro-extraction method to analyse hemicelluloses and the ratio of ‘cellulose and lignin’ to hemicelluloses in different tissues of 28 plant species comprising four plant functional types (broad-leaved trees, conifers, grasses and herbs). For this study, the fiber analysis after Van Soest was modified to enable the simultaneous quantitative and qualitative measurements of hemicelluloses in small sample volumes. Total hemicellulose concentrations differed markedly among functional types and tissues with highest concentration in sapwood of broad-leaved trees (31% d.m. in Fraxinus excelsior) and lowest concentration between 10 and 15% d.m. in leaves and bark of woody species as well as in roots of herbs. As for total hemicellulose concentrations, plant functional types and tissues exhibited characteristic ratios between the sum of cellulose plus lignin and hemicelluloses, with very high ratios (>4) in bark of trees and low ratios (<2) in all investigated leaves. Additional HPLC analyses of hydrolysed hemicelluloses showed xylose to be the dominant hemicellulose monosaccharide in tissues of broad-leaved trees, grasses and herbs while coniferous species showed higher amounts of arabinose, galactose and mannose. Overall, the micro-extraction method permitted for the simultaneous determination of hemicelluloses of various tissues and plant functional types which exhibited characteristic hemicellulose concentrations and monosaccharide patterns.     

N-(Carboxyacyl)chitosans

Hemicelluloses were isolated from pineapple-leaf fibers under different conditions. Study of the properties of these hemicelluloses gave direct evidence of some ester linkages between the hemicellulose and the lignin in this fiber. An aldobiouronic acid was isolated from this fiber hemicellulose, and characterized as 2-O-(4-O-methyl-α-d-glucopyranosyluronic acid)-d-xylose. This indicates that the general nature of the hemicellulose is similar to those of jute and other fiber hemicelluloses.     

Polypeptide multilayer films

Research on polypeptide multilayer films, coatings, and microcapsules is located at the intersection of several disciplines: synthetic polymer chemistry and physics, biomaterials science, and nanoscale engineering. The past few years have witnessed considerable growth in each of these areas. Unexplored territory has been found at the borders, and new possibilities for technology development are taking form from technological advances in polypeptide production, sequencing of the human genome, and the nature of peptides themselves. Most envisioned applications of polypeptide multilayers have a biomedical bent. Prospects seem no less positive, however, in fields ranging from food technology to environmental science. This review of the present state of polypeptide multilayer film research covers key points of polypeptides as materials, means of polymer production and film preparation, film characterization methods, focal points of current research in basic science, and the outlook for a few specific applications. In addition, it discusses how the study of polypeptide multilayer films could help to clarify the physical basis of assembly and stability of polyelectrolyte multilayers, and mention is made of similarities to protein folding studies.     

Physical, mechanical, and antibacterial properties of chitosan/PEO blend films

Films formed by blending of two polymers usually have modified physical and mechanical properties compared to films made of the individual components. Our preliminary studies indicated that incorporation of chitosan in polyethylene oxide (PEO) films may provide additional functionality to the PEO films and may decrease their tendency to spherulitic crystallization. The objective of this study was to determine the correlation between chitosan/PEO weight ratio and the physical, mechanical, and antibacterial properties of corresponding films. Films with chitosan/PEO weight ratios from 100/0 to 50/50 in 10% increments were characterized by measuring thickness, puncture strength (PS), tensile strength (TS), elongation at break (%E), water vapor permeability (WVP), and water solubility (WS). Additionally, the films were examined by polarized microscopy, wide-angle X-ray diffraction (WAXD), and Fourier transform infrared (FTIR) spectroscopy, and their antibacterial properties were tested against Escherichia coli. The chitosan fraction contributes to antimicrobial effect of the films, decreases tendency to spherulitic crystallization of PEO, and enhances puncture and tensile strength of the films, while addition of the PEO results in thinner films with lower water vapor permeability. Films with 90/10 blend ratio of chitosan/PEO showed the most satisfactory PS, TS, %E, and antibacterial properties of all tested ratios.     

Material properties of films from enzymatically tailored arabinoxylans

Rye arabinoxylan, with an initial arabinose to xylose (Ara/Xyl) ratio of 0.50, was enzymatically modified with alpha-L-arabinofuranosidase. Different enzyme dosages were used to prepare arabinoxylan samples with a gradient of arabinose content varying from Ara/Xyl ratio 0.50 to 0.20. The degree of polymerization of the arabinoxylans was not affected by the enzymatic treatment, as detected with SEC-MALLS. Arabinoxylan samples with an Ara/Xyl ratio of 0.30 and below agglomerated in a water solution as seen by changes in light scattering. All samples, however, formed cohesive films upon drying, without addition of external plasticizers. The film from untreated arabinoxylan was completely amorphous; whereas films of the enzyme-treated arabinoxylans were semicrystalline with an increasing degree of crystallinity with decreasing arabinose content as determined by WAXS. Oxygen permeability measurements of the films showed that decreased arabinose content also resulted in lower oxygen permeability of the films. All films were strong and relatively stiff, but showed variations in strain at break. The moderately debranched film with an Ara/Xyl ratio of 0.37 had highest strain at break among all the films tested, yet was stiff and strong. This material also exhibited yielding and had stress/strain behavior similar to synthetic semicrystalline polymers, with a tendency to strain-induced crystallization. Such a combination of mechanical properties combined with oxygen barrier properties is very attractive for packaging applications.     

Transparent films based on PLA and montmorillonite with tunable oxygen barrier properties

Polylactide (PLA) is viewed as a potential material to replace synthetic plastics (e.g., poly(ethylene terephthalate) (PET)) in food packaging, and there have been a number of developments in this direction. However, for PLA to be competitive in more demanding uses such as the packaging of oxygen-sensitive foods, the oxygen permeability coefficient (OP) needs to be reduced by a factor of ~10. To achieve this, a layer-by-layer (Lbl) approach was used to assemble alternating layers of montmorillonite clay and chitosan on extruded PLA film surfaces. When 70 bilayers were applied, the OP was reduced by 99 and 96%, respectively, at 20 and 50% RH. These are, to our knowledge, the best improvements in oxygen barrier properties ever reported for a PLA/clay-based film. The process of assembling such multilayer structures was characterized using a quartz crystal microbalance with dissipation monitoring. Transmission electron microscopy revealed a well-ordered laminar structure in the deposited multilayer coatings, and light transmittance results demonstrated the high optical clarity of the coated PLA films.     

Physical-Mechanical and Antifungal Properties of Pectin Nanocomposites / Neem Oil Nanoemulsion for Seed Coating

Biodegradable polymers, when reinforced with nanostructures, are considered good sustainable coatings and viable alternatives to replace conventional coatings. In addition, biopesticides are also considered safe, biodegradable and environmentally friendly; therefore there is a growing interest in nanoemulsions based on phytochemical mixtures. In this context, the aim of this study is to aggregate Neem oil nanoemulsions and pectin matrices to produce nanocomposite films, as well as evaluate the nanoemulsions effect on the film properties for coating soybean seeds. Nanoemulsions were characterized assessing their average diameter and stability, while the nanocomposite antifungal, morphology, mechanical and barrier properties were analyzed. In general, the nanoemulsions had an average diameter close to 59 ± 0.61 nm, showed good stability and its addition improved film mechanical properties: reduced stiffness, resistance, and water vapor permeability, and increased extensibility. In addition, Neem oil provided antifungal properties against Aspergillus Flavus and Penicillium Citrinum. The seed coatings promoted a positive effect on the germination process of soybean seeds. Thus, antifungal nanocomposite films from renewable sources were successfully produced. The fungicidal inhibition of Neem oil as a nanoemulsion makes these new materials promising for the production of seed coatings.