Packaging technology

Published: 11-12-2011, 10:20

Biobased materials

The prefix ‘‘bio-’’ means life in Greek. Biobased materials indicate the substances obtained from living or dead animals or plants. Biobased materials are a large group of loosely related processing (or engineering) materials, which are mainly derived from substances originally existing in nature, such as in living tissues or organisms, but may also be obtained by synthetic methods. Accordingly, biobased materials include common commodities such as leather and wood, as well as those that have undergone more extensive processing such as pectin, oleic acid and carboxymethyl cellulose. In literature, the term ‘‘biobased material’’ is often used synonymously with the words biomass or biomaterial. The three words differ from each other slightly in definition and mostly in habitual uses. Biomass refers to animal or plant matter grown for uses for production of chemicals and fibers, but does not include their products after processing; and more commonly, biomass is frequently used in literature of energy production, emphasizing its status in the carbon cycle as a renewable fuel. The term ‘‘biomaterial’’ has an additional meaning. As a result of its long use for biomedical applications, it refers to materials that can perform biological functions and are biocompatible when in contact with living tissues. The study of biobased materials is now a large sector in material science and agrobusiness, as are (a) the study of biomaterials in biomedical and pharmaceutical sciences and (b) the study of biomass in renewable energy.

Based on their sources and production, biobased materials can be divided into three major categories (1, 2):

  • Category 1 includes those removed (extraction, exuding, isolation after milling, etc.) from biomass. Examples are (a) polysaccharides such as starch and cellulose and (b) proteins such as collagen and casein. 
  • Category 2 includes polymers produced by a chemical method using renewable biobased monomers. Examples are some polyesters such as poly(lactic acid), which is produced from the polymerization of lactic acid, a fermentation product of carbohydrate feedstock. Category 2 biobased materials also include monomers or low-molecular-weight chemicals (socalled ‘‘building blocks’’) obtained from biobased feedstocks by chemical or biochemical methods, such as rosin, castor oil, and terpene. 
  • Category 3 includes materials produced by microorganisms or genetically modified bacteria. Examples are bacterial cellulose and polyhydroxyalkonoates such as polyhydroxybutyrate. 

Because of their overwhelming presence in the world and the versatility of their chemistry and architecture, biobased materials are the sources of many industrial products, such as medicinal, chemicals, fibers, and paint, as well as plastics, and so on. Although most of them can be used for packaging purpose, the consumption of biobased materials in the packaging industry is only about 1%; a large portion of these products are currently produced from petroleum-derived materials. The limitation of petroleum resources and the awareness of environmental protection have raised a new prospect that biobased materials may be once again the major contributor to the industries. Biobased materials are less dense than metal, and some petroleum-derived thermal plastics, are ideal components for many structural materials. Most biobased polymers perform in a fashion similar to that of conventional polymers. Unlike petroleum-derived materials, most biobased materials are biodegradable. This property enables the end-use products of biobased materials to be disposed of upon completion of their useful life without causing any environmental concerns. This is attractive in the production and applications of packaging materials and has become their main focus. The use of biobased materials also addresses other economic issues: the use of surplus stocks and the production of higher value-added material from agricultural products and byproducts. Therefore, the use of biobased materials promotes agrobusiness development. 

Scientists and engineers are developing new technologies that will provide competitive cost for products from biobased materials, meet the standards of various applications, and optimize their performance (3–9). Presently, important research areas include: (I) Reproducibility and quality of biobased materials. These not only depend on the methods and processing conditions of separation, purification and fabrication, but also dramatically rely on the sources of raw materials, such as where grown, when harvested, and how and how long they are stored. All these variations make product quality control more complicated and difficult. (II) In composites, water absorption can be considered a disadvantage. Migration of water through the polymer can lead to disturbance of the filler/ matrix interface, reducing the overall strength of the composites. Most biopolymers are hydrophilic. It is a challenge to improve the water resistance of biobased materials in order to retain good mechanical properties when the composites are exposed to highly humid conditions. (III) The durability of biobased materials is related to their biodegradability. The degradation of biobased products should be controllable and their properties should be constant during the time of their useful life. (IV) Gas barrier properties have specific significance in packaging materials. In particular, food packaging requires specific atmospheric conditions to sustain food freshness and overall quality during storage. Biobased materials mimic quite well the oxygen permeability of a wide range of the conventional petroleum-derived thermal plastics. As many of the biobased materials are hydrophilic, their gas barrier properties are very dependent on environmental humidity. (V) Thermal stability. Most category 1 biobased materials are not stable at higher temperature, limiting applications and choices of processing methods. (VI) Safety. Biobased materials, particularly those come from biological processes, may support microorganism growth. Concerns have risen over the spread of novel trails in existing populations and the introduction of modified species.

CATEGORY 1 BIOBASED MATERIALS

There are a large amount of polysaccharides and proteins that are directly isolated from agricultural or marine plants and animals, and they can be used in packaging applications. Examples of category 1 biobased materials include starch, cellulose, pectin, alginate, collagen, soybean flour protein, and zein. Most category 1 biobased materials exhibit useful gas barrier properties; but they are hydrophilic and unstable at higher temperature, causing problems in processing.

Starch

Starch is a widely available raw material suitable for a variety of applications, including packaging (10–12). Starch is a storage polysaccharide found in the cytoplasmic granules of plant cells. Starch is a composite consisting mainly of amylose and amylopectin, and it is primarily derived from corn, wheat, potatoes, and rice. Due to its huge availability and low cost in processing, starch is economically competitive with petroleum-derived materials and is therefore a promising candidate for preparing compostable plastics. Starch is nontoxic, biologically absorbable, resistant to passage of oxygen, and semipermeable to carbon dioxide. On the other hand, starch granules are rigid, and starch blends are brittle. Plasticizers, such as glycerol, low-molecular-weight polyhydroxy compounds, polyethers, or ureas, can be included in starch blends to reduce the intermolecular hydrogen bonding and thus increase the flexibility. Because of the hydrophilic nature of starch, the performance of starch blends changes during and after processing as a result of water content changes with the changes in humidity. Side-chain modification to obtain more hydrophobic starch derivatives is a strategy to overcome this challenge.

Cellulose

Cellulose is the most abundant naturally occurring polymer. Cellulose is a high-molecular-weight 1-4-betalinked polymer of D-glucopyranose, which displays a diverse range of conformations and crystalline packing arrangements, as well as fiber structure. Because of its regular linear structure and array of hydroxyl groups, cellulose-based films tend to be tough and flexible and resistant to fat and oil. Cellulose is hydrophilic; it swells, but does not dissolve in water. To produce cellulose films, an aggressive process is required that involves several strong basic, strong acid and toxic solvents. Although, the resultant films possess good mechanical properties, they are hydrophilic and moisture-sensitive (1, 13).

Cellulose derivatives are used for film forming, coating, and encapsulating applications. Cellulose derivatives, such as ethyl cellulose, methyl cellulose, carboxy methyl cellulose, hydroxyethyl or hydroxypropyl cellulose, and cellulose acetate, are commercially available. Cellulose acetate possesses relatively low gas and moisture barrier properties (1, 10, 14–16).

Pectin

Pectin is a cell wall polysaccharide. The majority of the pectin structure consists of homopolymeric partially methylated poly a-(1-4)-D-galacturonic acid residues (‘‘smooth’’ regions), but there are substantial ‘‘hairy’’ regions of alternating a-(1-2)-L-rhamnosyl-a-(1-4)-Dgalacturonosyl sections containing branch-points with mostly neutral side chains (1–20 residues) of mainly Larabinose and D-galactose (rhamnogalacturonan I). Pectins may also contain rhamnogalacturonan II with side chains containing other residues such as D-xylose, Lfucose, D-glucuronic acid, D-apiose, 3-deoxy-D-manno-2- octulosonic acid, and 3-deoxy-D-lyxo-2-heptulosonic acid attached to poly a-(1-4)-D-galacturonic acid regions. The types and amounts of substructural entities in pectin preparations depend on their source and extraction methodology. Commercial pectin is mainly derived from citrus peels and apple pomace. It can also come from sugar beet pulp and sunflower heads. Commercial extraction causes extensive degradation of the neutral sugar-containing side chains. The pectin molecule does not adopt a straight conformation in solution, but is extended and curved with a large amount of flexibility. The carboxylate groups tend to expand the structure of pectin. Methylation of these carboxylic acid groups forms their methyl esters, which are much more hydrophobic and have a different effect on the structure of surrounding water. Thus, the properties of pectin depend on the degree of esterification (D.E.). High D.E. pectin (W40% esterified) tends to gel through the formation of hydrogen-bonding and hydrophobic interactions at low solution pH (pHB3.0) to reduce electrostatic repulsions, or in the presence of sugars (W70% esterified). Low D.E. pectin (o40% esterified) gels by calcium divalent cations that bridge adjacent twofold helical chains to form the so-called ‘‘egg-box’’ junction zone structures so long as a minimum of 14–20 residues can cooperate (17).

In addition to its gelling properties, pectin is a wellestablished film-forming material. In isolated form, pectin readily reassociates or aggregates to form networks, and it interacts with proteins, other polysaccharides, and synthetic hydrocolloids via hydrogen bonding, ionic, or hydrophobic interactions. This character has led to applications of pectin in encapsulation, coating, packaging, and wrapping for food and pharmaceutical products (18–20).

Alginate

Alginates are mainly derived from seaweed. The alginic acid family of linear 1-4-linked glycuronans are copolymers composed of beta-D-mannopyranuronic acid (M) residues and alpha-L-gulupyranuronic acid residues (G) that are arranged in homopolymeric blocks (GG and MM) and heteropolymeric (GM) sequences in varying proportions and distribution patterns. Alginates possess good film-forming properties that make them particularly useful in food packaging applications. Divalent cations, such as calcium, magnesium, manganese, and aluminum, are used as gelling reagents in alginate film formation. Calcium ion appears to be more effective in gelling alginate than other divalent ions; calcium propionate provides acceptable flavor. Desirable properties of alginate films include the improved product texture, juiciness, color, odor, and appearance, along with moisture retention and shrinkage reduction. Sodium alginate is water soluble. Sodium alginate coatings are used to extend the shelf life of foods and fruits (21, 22).

Chitin and Chitosan

In its structure, chitin is a cellulose analogue and is comprised of 1-4-beta-linked N-acetyl-D-glucosamine units. Chitin is the second most abundant glycan after cellulose, being present in the exoskelton of invertebrates. Chitin occurs in several crystalline polymorphic forms, of which the alpha-chitin is the most common. Like cellulose, chitin chains adopt a 21 screw axis. All of the hydroxyl groups are hydrogen bonded, and the bonding between sheets accounts for the fact that chitin does not swell in water. The fully or partially N-deacetylated derivative of chitin, chitosan, has received considerable attention. Some desirable properties of chitosan include its film forming properties, its antimicrobial activity, and the ability to absorb heavy metal ions. Chitosan films exhibit good oxygen and carbon dioxide permeability, as well as good mechanical properties. Chitosan films and coatings show activity against bacterial yeasts and molds, and they inhibit the growth of a number of microorganisms. The cationic nature of chitosan allows for electronic interactions with anionic compounds during processing and can lead to the incorporation of specific properties into products (23–25).

Collagen and Gelatin

Collagen is a major structural protein in vertebrates. Most of the ectodermal and mesodermal tissues are composed of collagen. In this sense, collagen is similar to cellulose and pectin, which serve a somewhat analogues role in plants. Collagen has triple helix architecture, thus collagen is insoluble and difficult to process. Commercially available collagens are extracted from animal skin, bone, and tendons. By taking advantages of biotechnology and genetic engineering, collagen can be obtained not only from animal tissues but also from bacterial cultures and even from genetically modified plants. Collagen films have strong mechanical properties and have been proposed for use in food packaging. Edible collagen films can become an integral part of meat products, and thus they function to reduce shrink loss and to increase juiciness and smoke permeability to the meat products (26–28).

Gelatin is a substantially pure product obtained by either partial acid or alkaline hydrolysis of collagen. The denaturation treatments disrupt the tight, helical structure of collagen and release water-soluble fragments that can form stiff gels and films. Gelatin is a highly processable material and is moisture-sensitive. Gelatin gels or films show thermally reversible behavior and melt below body temperature. Gelatin films and coatings have been used to carry flavors or antioxidants in food packaging applications (29, 30).

Soybean Flour Protein

Soy proteins are commercially available as soy flour (50% protein), soy concentrate (70% protein), and soy isolate (90% protein). Flours are made by grinding and sieving flakes. Concentrates are prepared by extracting and removing the soluble sugars from defatted flakes, by leaching with diluted acid at pH 4.5, or by leaching with aqueous ethanol. Isolate soy proteins are obtained by extracting the soluble proteins with water at pH 8–9, precipitating at pH 4.5, followed by centrifugation and drying. Soy proteins consist of two major protein fractions, 7S (conglycinin, 35%) and 11S (glycinin, 52%). Both fractions are considered as storage proteins and contain cysteine residues leading to disulfide bridge formation. Soy proteins are adhesive- and moisture-sensitive. Soy protein coatings can reduce moisture loss and control lipid oxidation in coated samples. Soy proteins are also used as binders for aqueous inks and as pigmented coatings on paperboard. Furthermore, soy flour proteins are proposed for the use in food coating, encapsulation, and active packaging (18, 19, 31, 32).

Zein

Zein is a class of alcohol-soluble prolamine proteins, obtained from corn gluten meal. Pure zein forms a hard, edible, clear, odorless, tasteless, and water-insoluble material, making it invaluable in processed foods and pharmaceuticals. Zein is now used as a coating for candy, nuts, fruit, pills, and other encapsulated foods and drugs. Zein can be further processed into resins and other bioplastic polymers by extrusion or rolling into a variety of plastic products. Zein-based films can function as water barriers and, thus, have potential use for packaging materials (33, 34).

Casein and Whey

Casein and whey are milk-derived proteins. Both have high nutritional values, excellent mechanical, and emulsion and barrier properties, and they are available in large volume worldwide. They have been used in the manufacture of edible films. Due to its random coil structure, casein is easily processable. By controlling the types of plasticizers, casein films can be made with very different mechanical properties varying from stiff and brittle to flexible and tough materials. Casein and whey films can reduce moisture loss, delay lipid oxidation, and reduce peroxide value of packaged food products (35–37).

CATEGORY 2 BIOBASED MATERIALS

The use of classical chemical methods to produce polymers, monomers, and other chemical ‘‘building block’’ materials from biobased feed stocks have generated a wide spectrum of category 2 products. Theoretically, all the conventional packaging materials currently derived from petroleum can be produced from monomers obtained from biobased materials; however, due to their high cost, the production of those monomers is not economically feasible. Thus, these costs have been an obstacle on the road to broadening the application of biobased monomers. An exception may be poly(lactic acid) (PLA) production. As a result of the increase in both the efficiency of lactic acid fermentation and the market price of raw oil, the cost of making poly(lactic acid) from biobased materials is gradually approaching the price of PLA obtained from fossil fuel.

Poly(Lactic acid)

Poly(lactic acid) or polylactide (PLA) is a thermoplastic, aliphatic polyester derived from fermentation of agricultural products and byproducts such as corn starch and other starch-rich substances like maize, sugar, or wheat. Bacterial fermentation is used to produce lactic acid, which is oligomerized and then catalytically dimerized for ring-opening polymerization. It can be easily produced in a high-molecular-weight form, most commonly using a stannous octoate catalyst. The properties of PLA strongly depend on the ratio of the two mesoforms (L and D) of the lactic acid monomer. Poly L-lactide (PLLA) is the product resulting from polymerization of L-lactide. PLLA has crystallinity around 37%, a glass transition temperature (Tg) of 50–801C, and a melting temperature (Tm) of 173– 1781C. If a mixture of D- and L-PLA is used, a polymer with adjustable Tm and Tg, PDLLA, can be obtained. The physical blends of PDLA and PLLA are useful for producing loose-fill packaging, compost bags, microwavable trays, food packaging and disposable tableware. PLA can also be plasticized by blending it with its monomer or oligomer. The resultant blends possess lower Tm and Tg. PLA resembles conventional petrochemical-based plastics in its characteristics and has good water vapor barrier properties and relatively low gas transmittance. PLA can be processed into fibers, blown films, injected molded objects, and coatings on standard equipment that already exist for the production of conventional thermoplastics. To date, PLA has shown the highest potential for a commercial major-scale production of biobased packaging materials (38–41).

Monomers and Chemical ‘‘Building Blocks’’

A number of monomers and chemical ‘‘building blocks,’’ low-molecular-weight organic compounds or chemical precursors for preparing polymeric materials, can be obtained from biobased materials (1, 2, 42, 43). Examples include castor oil, oleic acid, molasses, furfural, multifunctional alcohols, multifunctional acids, and terpenes.

Castor oil is used for the preparation of polyurethane. The resultant polyurethane is water-resistant and widely used in the electronics industry. Unsaturated fatty acids and oils, such as oleic acid, linoleic acid, and ricinoleic acid, can be recovered from seed crops, castor beans, coconut, flax, and other agricultural origins and have found applications in water-proof coatings and multilayered packaging materials. Furthermore, oleic acid can be chemically transformed to multi-functional alcohols and acids, amines and esters. The resultant azelaic acid and azelaic diacid are used in polyamide synthesis.

Furfural can be produced from woody biomass and molasses. Furfural can be transformed to furfuryl alcohol and to furan resin and a wide range of furan chemicals. Levulinic acid also can be produced from woody materials. Levulinic acid is a precursor for the synthesis of various lactones, furans, and other functional building blocks, which are used in the production of packaging materials.

Succinic acid and 1,3-propanediol are two examples of chemicals, which are prepared by fermentation of carbohydrate- rich materials using selected microorganisms. Succinic acid and 1,3-propanediol can be used to make polyesters, which, in turn, are used for preparation of packaging materials. Terpene chemicals are isolated from pine trees and have resulted in a number of terpene-based products, which are used to prepare resin materials, or as fragrances incorporated into resin materials for active packaging (1, 42).

At present, biobased monomers and ‘‘building blocks’’ may not be commercially attractive. However, with the progress in biotechnology and genetic engineering, these represent promising alternatives to petroleum-derived materials.

CATEGORY 3 BIOBASED MATERIALS

This group includes polymers produced directly from biomass by natural or modified organisms.

Bacterial Cellulose

Cellulose is an important starting material in many industries. Plants are the main source of cellulose. A harsh chemical treatment is required to isolate plant cellulose from lignin, hemicellulose, and pectins. The treatment severely impairs the chemical and physical characteristics of plain cellulose, such as molecular depolymerization and changes in crystal structure. Bacterial strains of Acetobacter xylinum and A. pasteurianus are able to produce pure cellulose that originally formed in plants (homo-beta-1,4-glucan) under ambient conditions. Bacterial cellulose is highly crystalline (70% in cellulose I form) and has a molecular weight 15 times higher than that isolated from wood pulp. Although the bacterial cellulose shows outstanding mechanical properties, the production cost of bacterial cellulose is high and hampers its application in current packaging industries (44, 45).

Poly(hydroxyalkanoates)

Poly(hydroxyalkanoates) (PHAs) are a family of polyesters that are produced by a large number of bacteria, in the form of intracellular particles, functioning as energy and carbon reserve. The properties of PHAs are dependent on their monomer composition. The specific monomer composition of PHAs depends on the nature of carbon sources used and the bacterial strains selected, providing a tool to control the properties of the resultant final products. PHAs of medium-chain length are elastomers with low melting point and low crystallinity. Poly(hydroxybutyrate) (PHB) is an important member of PHAs. PHB is a highly crystalline thermoplastic. PHB mimics the mechanical behavior of poly(isopropylene). The incorporation of hydroxyvalerate can remarkably change the mechanical properties of PHB, such as a decrease in stiffness and tensile strength and an increase in toughness. PHAs possess a low water vapor permeability, which is close to LDPE. The functional groups in the side chains of PHAs make it possible to chemically modify the polymers. PHA also can be produced from genetically modified crops, such as switchgrass. All these provide an enormous potential of the polymer for packaging applications (46, 47). A commercially available PHA product, MirelTM, can be obtained from Metabolix (Cambridge, MA, USA).