The demand for materials from renewable resources is increasing. Hemicelluloses are a group of polysaccharides found in plants that are, at present, not fully exploited in a commercial sense. In this article, we describe our work on targetting hemicelluloses from wood for food packaging applications, in particular film wrapping. Nanoscale fillers generated from wood pulp are introduced as reinforcement, resulting in coherent, easy-to-handle films with improved mechanical properties. The oxygen permeability of these novel composite films is similar to that reported for high-barrier materials such as polyvinyl alcohol (PVOH) and ethylene vinyl alcohol (EVOH).
by By Dr Natanya M.L. Hansen & Dr David Plackett
Polymers from renewable resources
As we look to a future in which the oil on which we have depended for the last 100 years starts to run out, sustainable sources of raw materials for industrial and consumer products are increasingly of interest. Bio-derived materials are therefore attracting more attention from a research perspective, as well as from the viewpoint of manufacturers. In this context, there is a considerable history in the field of biopolymers obtained from marine or land-based biomass or indeed from animal sources. Such biopolymers include those that are already on the market, such as polylactide (PLA), produced at present from corn, and the polyhydroxyalkanoates (PHAs) generated in-situ by certain bacteria grown under controlled conditions. In addition to these are starch-based thermoplastics and chitosan, obtained by deacetylation of chitin derived from the shells of marine crustaceans (e.g. shrimps). One can also add the now rapidly developing biomass-derived polyethylene, generated, for example, by dehydration of bioethanol.
Hemicelluloses
Most biomass is composed of cellulose, lignin and hemicellulose sugars and, although there has been a long history of research and development on new chemicals and materials from cellulose and lignin, the hemicelluloses have remained relatively unexplored until quite recently. While cellulose has a unique structure, the hemicelluloses comprise a group of non-crystalline hexose and pentose sugars. Four main groups have been defined according to their primary structure: xyloglycans (xylans), mannoglycans (mannans), b-glucans and xyloglucans. The hemicelluloses in the plant cell wall are bound to cellulose and lignin. Detailed isolation procedures are required in order to separate these components from the plant raw material. A number of methods are used to isolate hemicelluloses from plant sources including extraction with alkali, dimethyl sulfoxide or methanol/water, as well as steam or microwave treatment. The composition of the hemicelluloses varies between different feedstocks, as well as between sources, depending on factors such as origin and growth stage.
Hemicelluloses as a source of new biopolymer films and coatings
Films and coatings from renewable materials have numerous potential applications in the food industry, medicine and related industries. In the food industry, a number of researchers have explored the feasibility of producing new biopolymer films based on hemicelluloses with packaging in mind. Hemicellulose biopolymers are hygroscopic in nature and films, and as a result, do not behave well under conditions of high humidity. Previous research, largely aimed at overcoming such drawbacks, can be broadly broken down into categories based on the approach adopted: 1) formation of composite films or films with additives and hemicellulose as the major component, 2) coating of a hemicellulose film on a support layer, 3) chemical modification of hemicellulose prior to film casting, 4) chemical modification of previously formed hemicellulose films.
The formation of films from hemicellulose acetates was reported as early as 1949 by Smart and Whistler. Since that time there have been many reported studies [1]. Composite films based on xylans from wood have been formed by mixing this compound with other biopolymers such as alginate and carboxymethyl cellulose to form good oxygen barriers. The water vapour barrier properties of xylans extracted from maize bran have also been greatly improved by mixing with emulsified lipids. The addition of plasticisers such as glycerol, sorbitol and xylitol in the correct amounts has been a route adopted to create more practical, flexible films from hemicelluloses. Similarly, edible glucan coatings have been used to form moisture barriers on fruit. In the field of chemical modification, ethers with improved barrier properties have been formed by the reaction of hemicelluloses with benzyl chloride. Grafting of fluorinated moieties to xylan-based films has also been shown to be effective in reducing water uptake and creating a hydrophobic surface. A similar effect has been demonstrated by grafting stearyl methacrylates on to hemicellulose films, which also provided for reduced water permeability.
New directions in improving hemicelluloses as a raw material for food packaging
The work on enhancing hemicelluloses at Risų DTU started with an interest in exploring higher value uses for hemicelluloses generated as by-products from bioethanol production. However, we now know that these by-products are significantly degraded and we have therefore taken a more fundamental approach in which we target developments in the use of films from xylans, which can be derived from biomass by a variety of processes.
One of the research strategies has been to investigate the use of microfibrillated cellulose (MFC) from wood pulp as a way of producing reinforced xylan films. We have focused our attention on commercially available xylan extracted from birch wood and MFC provided by research partners at The Royal Institute of Technology (KTH) and Innventia AB in Stockholm. Our past research formed part of the EU Sixth Framework programme Sustainpack project (http://www.sustainpack.com) and included work on films cast from MFC as well as the potential use of this nanofiller to reinforce and upgrade starch-based films. The scientific literature on MFC utilisation includes a high proportion of studies aimed at MFC in combination with hydrophilic polymers because of obvious compatibility advantages. By using a polysaccharide such as xylan, this potential advantage is maintained and significant difficulties with mixing and dispersion can be avoided.
We have generated films from xylan/MFC mixtures containing up to 50% MFC by casting from alkaline solution followed by drying under controlled conditions. In some cases we also added glycerol plasticiser. Although it was not possible to cast films from pure xylan, the addition of MFC, especially at loadings over 10%, gave films which were at least 20-30 microns in thickness and could be handled and tested for various properties [Image 1]. As an example, the tensile strength of such films as a function of their MFC content is illustrated in Figure 1.
The tests showed that MFC has a positive effect on xylan film tensile properties and, since elongation is also increased, MFC may be acting as a reinforcing plasticiser. The addition of glycerol as a plasticiser had the expected effect of decreasing tensile strength while increasing the measured strain at break. The measured oxygen permeability of unplasticised xylan/MFC films gave promising values of ~ 1 cm3 25µm m-2 24h-1 bar-1. While these oxygen permeability values for unplasticised xylan/MFC films are considered to be quite low (and comparable with literature values cited for polymers such as PVOH and EVOH, which are generally regarded as high barrier materials), there was a clear and expected increase in permeability when plasticiser was added. For example, the permeability of films containing 30 % glycerol was approximately 40 times greater than that of the corresponding unplasticised films.
Future developments
We will be continuing our research on nanofiller-reinforced hemicellulose films with a view to fuller characterisation and a better understanding of properties in relation to potential food packaging applications. The direction described here is complemented by another strategy in which we are synthesising esterified xylans in ionic liquids with the objective of identifying new and more easily processable bioplastics for packaging and other uses.
Reference
1. Hansen NML, Plackett D, Sustainable films and coatings from hemicelluloses: a review. Biomacromolecules 2008; 9 (6): 1493-1505.
The authors
Dr Natanya M.L. Hansen & Dr David Plackett
Risų National Laboratory for Sustainable Energy
Technical University of Denmark
P. O. Box 49
DK-4000 Roskilde, Denmark
Email: nath@risoe.dtu.dk
