Materials, Chemicals, and Energy from Forest Biomass - American

Chapter 9

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 12, 2014 | http://pubs.acs.org Publication Date: April 16, 2007 | doi: 10.1021/bk-2007-0954.ch009

Oxygen Barrier Films Based on Xylans Isolated from Biomass Maria

1,2

Gröndahl

and Paul Gatenholm

1

1

Biopolymer Technology, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Current address: Xylophane AB, Stena Center 1B, SE-412 92 Göteborg, Sweden 2

Xylan films were prepared by casting from aqueous solution and their equilibrium moisture content, mechanical properties, morphology and oxygen permeability were measured. The films were transparent and homogeneous and the mechanical properties could be controlled by an addition of plasticizers such as xylitol and sorbitol. Tensile testing showed that barley husk arabinoxylan films had a higher stress at break and strain at break as compared to aspen glucuronoxylan films at corresponding plasticizer contents. It was also shown that water is a good plasticizer for xylans. The water content of the films depends on the chemical structure of the xylan, such as branching and its substituents. The glucuronoxylan films were semicrystalline, whereas the arabinoxylan films were mainly amorphous with small crystalline peaks detected by Wide Angle X-ray Scattering. Both the glucuronoxylan and arabinoxylan films had low oxygen permeability and can thus be used in packaging for oxygen-sensitive products.

© 2007 American Chemical Society

In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

137

138


Introduction Today, the great majority of plastics are based on fossilic raw materials. Their disposal contributes to growing landfills and enhanced greenhouse effects when burned. Furthermore, the earth's fossil resources are limited. Interest in the use of renewable resources for the production of polymeric materials is growing as the problem of rapidly reducing oil resources gains more attention and oil prices increase. Recycling of synthetic polymers has been studied for many years and has become a vital part of the development of a sustainable society (7). However, sustainable development requires greater use of renewable materials. Using plant biopolymers to produce plastic materials has several advantages. In addition to their being a renewable resource, the products can be composted, recycled or incinerated after use with minimal environmental effect (2). One typical application where the life time of the material is rather short is food packaging. The material should have good mechanical properties so that the food remains undamaged during storage, it should be resistant to water and it is important that it does not emit poisonous substances since it is in contact with food (3). In food packaging good barrier properties are necessary to prevent gases such as oxygen from degrading the product. Polysaccharides such as cellulose, starch and hemicelluloses are produced by plants in vast quantities by the conversion of carbon dioxide and water using solar energy, which leads to a better carbon dioxide balance in our ecosystem. Cellulose, which is the reinforcing component of the cell wall, is used in many applications, such as paper, textile fibers, plastics, membranes, food additives and medicines. Starch is produced as an energy reserve in plants and has successfully been converted into plastic materials by conventional thermoplastic processing {4). Hemicelluloses are biosynthesized by the majority of plants and act as a matrix material that is present between the cellulose microfibrils and as a linkage between cellulose and lignin in the cell wall (5). In contrast to cellulose and starch, the commercial utilization of isolated hemicellulose has as yet been very limited. In the pulp and paper industry, retaining the hemicellulose in the pulp has been shown to improve both the mechanical properties of the paper and the yield (tf-7). Hemicellulose has also been used as thickeners and emulsifiers in food (5, 8). There has been interest in the use of hemicellulose as a nutraceutical (9), in chiral separations (70) and as an H I V inhibitor (77). Hemicellulose can also be hydrolyzed to monosaccharides that are converted to chemicals such as furfural and xylitol or can be used as fermentation feedstock for making ethanol or lactic acid (72-73).

Hemicelluloses Hemicelluloses were originally believed to be intermediates of cellulose biosynthesis but later proved to be a group of heterogeneous polysaccharides



139 that are formed through their own biosynthetic routes (14). Hemicelluloses are synthesized in the Golgi apparatus. They are packed into vesicles and targeted to the plasma membrane where they become integrated with the newly synthesised cellulose microfibrils (15-18). A general definition of hemicellulose is polysaccharides that can be extracted by water or aqueous alkali from plant tissue (8, 19). Hemicelluloses are heteropolysaccharides whose composition and amount vary between different plant species. In wood, the hemicellulose comprises between 20 and 35 % of the total material and the hemicellulose content in brans and hulls from annual plants, such as maize, can be as high as 40-50 wt. % (8, 20). The most common monosaccharides are D-xylose, L-arabinose, D-glucose, D-galactose, D-mannose, D-glucuronic acid, 4-0-methyl-D-glucuronic acid and D-galacturonic acid, see Figure 1. In comparison to cellulose, hemicellulose chains are rather short with an average degree of polymerization of only 200 (14).

.

.0

D-xylopyranose

D-glucopyranuronic acid

HOH C

OH L-arabinofuranose

4 -O-methylD-glucopyranuronic acid

2

0

D-glucopyranose

D-galactopyranuronic acid

Figure 1. The most common building blocks in hemicelluloses.

The most abundant hemicelluloses, found mainly in hardwood and annual plants, comprise a l,4-P-D-xylopyranosyl main chain with a varying number of side chains based on L-arabinofuranosyl, 4-O-methyl-D-glucuronopyranosyl, Dgalactopyranosyl or D-glucurono-pyranosyl units. Xylans isolated from hardwood and annual plants differ from one another. In hardwoods such as aspen, beech and birch, the hemicellulose consists chiefly of 0-acetyl-(4-0-methylglucurono)xylan (21), often simply referred to as



140 glucuronoxylan, see Figure 2. The backbone consists of P-(l-»4)-linked Dxylopyranosyl residues substituted with one a-(l->2)-linked 4-0-methyl-Dglucuronic acid per approximately every tenth such residue (22). The xylopyranosyl residues are partially acetylated in the C2 and/or C3 positions (21, 23). The degree of acetylation in native aspen glucuronoxylan has been reported to be between 0.6 and 0.7 (24-25). Xylan is generally considered to be amorphous in the native state but can crystallize after isolation (26).

Figure 2. Schematic structure of hardwood xylan.

The hemicelluloses found in annual plants, such as maize, rice, oats, rye, barley and wheat, are generally more structurally diverse and complex. These plant hemicelluloses have a 1,4-p-D-xylopyranosyl main chain that can be heavily branched with xylopyranosyl, arabinofuranosyl and galactopyranosyl side chains and can also contain 4-0-methyl-D-glucuronopyranosyl or Dglucuronopyranosyl substituents. The two predominant monosaccharides in these annual plant hemicelluloses are generally xylose and arabinose, and they are thus termed arabinoxylans. The degree of side-chain substitution determines the degree of solubility of the xylan; the higher the degree of substitution, the higher the water solubility (27). In addition to substitution with acetyl groups, 4-

10
S

4-*

CO

10

20

30

40

50

60

Sorbitol + Water (wt. % )

Figure 5. Stress at break of arabinoxylan films as a function of the total amount of added plasticizer (0 %, 20 % and 30 % sorbitol) and water.

10000

CO Û.

Έ J3

3 Ό Ο Έ υ) Ό) c ο

>ίο -I 0

.

.

.

.

10

20

30

40

1 50

60

Sorbitol + Water (wt. %)

Figure 6. Young's modulus of arabinoxylan films as a function of the total amount of added plasticizer (0 %, 20 % and 30 % sorbitol) and water.


148 1200

1000

800

c/>

c


βοο

c

400

200

0 5

10

15

20

25

30

2 thêta (°) Figure 7. X-ray diffractograms of aspen glucuronoxylan (top) and arabinoxylan from barley hush (bottom).

The oxygen permeability of xylan films is very low and in the same range as the frequently used barrier plastic ethylene vinyl alcohol (EVOH). The permeability of the xylan films is also compared with that of the plasticized starch polymers (amylose and amylopectin), with poly vinyl alcohol (PVOH) and with low density polyethylene (LDPE), which is not an oxygen barrier material.

Conclusions Xylans are among the most abundant biopolymers on earth, but the commercial utilization of isolated xylan is as yet very limited. There are several separation processes of xylans from biomass on the laboratory and pilot scales. The development of xylan-based products will promote up-scaling of the separation processes. Externally plasticized hardwood xylans and arabinoxylans from agricultural residues were found to form films with good mechanical properties that exhibited excellent oxygen barrier properties. Xylans are potential candidates for oxygen barriers in multilayer packaging.


149 Table III. Oxygen Permeability of Xylan Films, Amylose, Amylopectin, PVOH, E V O H , and LDPE


Material Glucuronoxylan with 35 wt. % sorbitol Arabinoxylan with Xyl/Ara=4.5 Amylose with 40 wt. % glycerol Amylopectin with 40 wt. % glycerol PVOH EVOH LDPE

Oxygen Permeability (cm3 pm)/(m2 d kPa) 0.21

Testing Conditions References 50 % R H

(57)

0.16

50 % R H

7

50 % R H

(59)

14

50 % R H

(59)

0.21 0.1-12

50%RH 70 % V O H , 0-95 % R H 50 % R H

(57) (60)

1870

(60)

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Materials, Chemicals, and Energy from Forest Biomass - American

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