Hemicellulose, starch, chitosan materials

One of the goals of our research is to develop high-value gels with unique absorptive and antimicrobial properties from renewable materials such as hemicellulose (HC), starch (S) and chitosan. Applications include absorbents used in personal health care products such as diapers, hemostatic materials, plastics, disposable towels, animal bedding, disposable food and waste containers. HC, starch and chitin are by-products or waste products of the crop industries, the paper industries and the fishing industries and are available in large quantities.  Preliminary research activities have demonstrated that the pathways to the gels proposed are viable and the proposed research has good potential for success.

This research will improve the sustainability of the US agriculture/forestry system. The US is the largest producer of paper in the world and is endowed with an abundance of trees for this process.  The US paper industry is under stress to remain competitive in the face of increased foreign competition.  One method that the industry can employ is to develop higher valued products.  A strong example is using hemicelluloses (a by-product of wood pulping) as a feedstock for products. Environmental benefits include (1) substitution of petroleum based monomers with renewable polymers from plants and animals decreasing pollution associated with petroleum processing, (2) utilization of renewable plant material with a net negative carbon dioxide emission, (3) utilization of waste agricultural residues and wood residues to produce high value products rather than consuming landfill volume. 

The United States is the largest producer of pulp and paper in the world and is endowed with an abundant amount of trees for this process.  However, countries in Asia and South America recently have developed competitive pulp and paper manufacturing facilities.  These countries benefit from inexpensive labor, less-stringent environmental regulations, easier industrial permitting, and access to large markets.   Often favorable currency exchange rates and government limits on imports also benefit these countries.  The US pulp and paper industry is currently under significant stress to remain competitive in the face of this.  One method that the industry can employ is to develop higher valued products from the pulp and paper process.  A strong example is documented in this proposal, using hemicelluloses (a by-product of wood pulping used to produce paper) as a feedstock for new absorbent, antibacterial gel products. Currently, the hemicellulose present in the wood (approximately 25% of the dry wood weight) is burnt for energy or remains in the paper, both of which are low valued applications. The US will certainly derive great economic and environmental benefit from developing high valued products from the by-products and wastes of pulp and paper manufacturing.

Research and development efforts are underway for isolating hemicelluloses during the pulping of wood as a potential starting material for ethanol generation and for other applications.  For example a significant effort is being carried out  by the US Department of Energy, Industrial Technologies Program, Forests Products Industry of the Future in a research project titled, Hemicellulose Extraction and its Integration in Pulp Production. The benefits of the project include increasing the energy efficiency of the pulp and paper making process and its economics.

Organosolv pulping processes (Abaecherli 2004) are another method to isolate hemicellulose from lignocellulosic biomass.  Of particular interest for this proposal is a process developed at the National Renewable Energy Laboratory that uses a mixture of ethanol/methyl isobutyl ketone/ water, along with an acid catalyst.  The exciting aspect of this process is the ability to easily separate the lignin and hemicelluloses, and recover each component at high yield and purity.  Separation of the lignin and hemicelluloses is achieved by adding water to the system after the cellulose has been recovered, and creating a lignin-rich ethanol/MIBK phase and a hemicellulose-rich aqueous phase.  The lignin and hemicelluloses can be recovered by distillation/steam stripping and spray drying, respectively, in high purity and high yield (greater than 95%).  Depending on the original reaction conditions, and the conditions used for spray-drying the hemicelluloses can be recovered in their polymer form or as oligomers.  

Crop industries may also benefit from this research.  Residual agricultural wastes have serious disposal issues associated with them.  Hemicelluloses are a significant portion of these residues.  Producing  a high-valued product from these residues from the hemicellulose not only provides income of a saleable product  but also avoids disposal costs. Organosolv pulping processes are compatible with agricultural residues.

Starch is an abundant crop and its production is expected to increase if it is further used as feedstock for ethanol generation. There exists an opportunity to utilize the starch in multiple products in the future, providing a flexibility to producers of the starch. 

When crustaceans, such as crabs, shrimps and lobsters, are processed for food, the shells of such creatures are a significant waste stream.   Chitin is present at high levels in this waste.  Again, there would be great benefit in finding a high-volume, high-value product to produce from such a waste stream. This would greatly benefit the US fishing industry.

Hemicellulose             

Hemicelluloses (HC) are one of the most common polysaccharides next to cellulose and chitin, representing about 20-35 % of lignocellulosic biomass, and have not yet found broad industrial applications as does cellulose (Xaio-Feng 2004).   The development of new, high-valued products based on hemicelluloses would be a great breakthrough in the effective utilization of lignocellulosic biomass. The most commonly existing sugars that constitute hemicelluloses are D-glucose, D-mannose, D-xylose, D-glucuronic acid, 4-O-methyl-D-glucuronic acid, and D-galacturonic acid (Gabrielii 1998). The xylan is estimated to account for one third of all renewable organic carbon available on earth (Prade 1996; Sjöstrom 1981).  Recently, increased attention has been paid to the utilization of hemicelluloses as biopolymer resources because hemicelluloses are available in very large amounts in organic wastes from renewable forest and agricultural residues (Ebringerova 1999; Badal 2003). The chemical composition of hemicelluloses is related to cellulose, but its morphological structure is significantly different. Due to the heterogeneity of its chemical constituents, hemicelluloses in their natural state are generally considered non-crystalline.  They are branched polymers of low molecular weight of a degree of polymerization in the range of 80-200 (Sjöström 1981). The chemical modification of hemicelluloses presents a promising method for the preparation of new materials. This enables one to introduce special properties and enlarge the field of potential applications for these biopolymers of abundance.

Starch

Starch (S)  is considered to be one of the most abundant biopolymers worldwide. Starch typically occurs as  semi-crystalline granules composed of amylopectin (branched polymer, ~ 70 %, 4000 glucose units) and amylose (linear polymer, ~ 30 %, 1000 glucose units). Both of amylose and amylopectin are composed of α-glucosidic units connected to each other through a 1,4-oxygen ring atom. The low cost and availability of starch in the market attracts researchers  for developing new functional starch derivatives for industrial applications. The industrial applications of starch derivatives depends on the type and degree of substitution of functional groups that are introduced to the main backbone of starch, its properties (gelatinization, crystallization, retrogradation, gel formation), and amylose / amylopectin ratios that depend on the source of extraction.

             

Chitin

Chitin is an abundant naturally occurring polysaccharide with annual production very near the levels of cellulose, of which it is structurally related. At least 10⁹ tons of chitin are estimated to be synthesized each year (Kumar 2000). Chitin is found in the cell walls of many lower plants such as yeast, mushrooms and other fungi. It also forms a substantial part of the exoskeleton shells of crustaceans, such as crabs, shrimps and lobsters, as well as in the exoskeletons of marine zoo-plankton, including coepods, and in corals and jellyfish. Insects, such as butterflies and ladybird beetles, have chitin in their wings. Chitin consists mainly of b-(1-4)- 2 -acetamido-2-deoxy-D-glucose units. Despite much recent research into its utilization, its strong intermolecular hydrogen bonding and poor solubility in common organic solvents have so far prevented widespread utilization of chitin  (George  1992). Chitosan is the N- deacetylated form of chitin that is obtained by alkaline treatment (NaOH) of chitin at high temperature. Chitosan and its derivatives have become useful polysaccharides in the biomedical area because of their biocompatible, biodegradable, and non-toxic properties (Lee 1997). The anti-microbial and antifungal activities of chitosan and chitosan derivatives (El-Tahlawy 2005; Lim 2004) have previously been described. Chitosan has been found to inhibit the growth of a wide variety of bacteria and fungi. Moreover, chitosan has several advantages over other types of disinfectants, that is, it possesses a higher antibacterial activity, broader spectra of activity, a higher killing rate, and lower toxicity toward mammalian cells. Several mechanisms were proposed for the antimicrobial activity of chitosan: (1) cell wall leakage (Vaara 1983; Nikaido 1996; Helander 2001; Lim 2004) (2) cell wall blockage (Wang 2004; Zheng 2000; Liu 2004) (3) binding to DNA (Liu 2001) and (4) flocculation of cell components (Zheng 2003).

 

Gels From Bio-based Materials

Recently new hemicellulose based superabsorbent materials have been synthesized by several scientists.   Lindblad et al (2004) has synthesized a new hydrogel by graft copolymerization of 2-hydroxyethyl methacrylate (HEMA) or poly(ethylene glycol) dimethacrylate (PEGDMA, a crosslinking agent) with oligomeric hydrosoluble hemicellulose modified with well-defined amounts of methacrylic functions. The grafted copolymer was elastic, soft, and easily swellable in water. The viscoelastic and solution rheological properties of the grafted copolymer were characterized. Also, a comparison of hemicellulose-based hydrogels with pure poly(2-hydroxyethyl methacrylate) hydrogels showed that their behaviors were similar, demonstrating the potential of hemicellulose-based gels to compete with gels derived from petroleum based resources.

However, the swelling properties of the hemicellulose/poly(hydroxyethyl methacrylate)  was not adequate enough for many applications.  In this proposal, it is hypothesized and experiments are planned to verify that the introduction of hydroxypropyl/carboxymethyl, polyglycerol, and polycarboxylic groups on the hemicellulose backbone will enhance the swellability greatly. This hypothesis is based on the observation that the suggested functional monomers have a higher hydrophilicity than that of hydroxyethyl methacrylate.   

Gabrielii and Gatenholm (Gabrielii 2000) have separated hemicellulose (xylan) aspen wood with an alkali extraction method combined with ultrafiltration. The hemicellulose was sparingly soluble in cold water but soluble in hot water. Solutions of the hemicellulose did not exhibit good film forming properties. When mixed with chitosan, however, a gel was formed and  films could be produced at compositions of 5% chitosan and above in acidic conditions. Ionic complexes between glucuronic acid functionalities of the hemicellulose and amino groups of chitosan were  suggested to be responsible for network formation (interpolyelectrolyte complex).   The morphologies of these films were examined with WAXS, and a pure xylan film proved to be crystalline. The crystallinities were found to decrease with an increasing amount of chitosan, and the film of pure chitosan had virtually no crystallinity. Films of mixtures of xylan with chitosan displayed slightly higher degrees of crystallinities than would be predicted from the weighted averages of the pure xylan and the pure chitosan films. When immersed in water, films with 5–20% chitosan formed hydrogels, and the degree of swelling of the hydrogels was shown to increase as the films contained more chitosan. Films with more than 20% chitosan dissolved in water. The film and hydrogel forming properties were attributed to crystalline domains of xylan interacting with the chitosan chains, as well as to electrostatic interactions between the acidic groups in the hemicellulose and the amino groups in the chitosan.

In that study, the use of unmodified hemicellulose provided only a  single, low carboxyl content of the hemicellulose which  limited the ionic complexation between the carboxyl groups of hemicellulose and the amino group of chitosan (interpolyelectrolyte complex). (Work in this lab determined the carboxyl content of xylan from aspen wood as about ~ 32 mequivalent / 100g HC.)   The concept of producing a hemicellulose/chitosan gel via ionic complexation is a good one and the concept deserves further study.   

 

The studies above have shown that bio-based materials have strong potential to produce advanced gel materials and indicate that this is a fruitful area of research.  Dr. Venditti and collaborator Dr. Joel Pawlak and coworkers continue to develop new materials and develop an understanding of their behavior. For more information contact Dr. Venditti.