S.H. Imam and G.M. Glenn,     WRRC, USA

E. Chiellini,     WRRC, USA; University of Pisa, Italy

Note: This chapter was originally published as Chapter 2 ’Types, production and assessment of biobased food packaging materials’ by S. Imam, G. Glenn and E. Chiellini in Enviromentally compatible food packaging, ed. E. Chiellini, Woodhead Publishing Limited, 2008, ISBN: 978-1-84569-194-3.

21.1 Introduction: rationale and need for biobased food packaging

Food packaging was initially created to facilitate trade and transportation of commodities over long distances. These commodities include both perishable as well as non-perishable foods. Paper, cardboard and cellulosic fibers – as well as glass, aluminum and tin – were the materials of choice for packages such as cartons, sacks, containers, bottles, etc. Over the last few decades, the packaging industry has transformed into a highly sophisticated and intelligent service industry, particularly for perishable foods; the industry has taken advantage of the state-of-the-art in material science, manufacturing and process engineering along with ever-advancing knowledge of food science (Truong et al., 2001; Ahvenainen, 2003; Robertson, 2005). Today, food packaging has many purposes. It is designed not only to contain and protect food, but also to keep food safe and secure, to retain food quality and freshness, and to increase its shelf-life. In addition, packaging should be affordable to consumers worldwide and, more importantly, it must be naturally biodegradable upon disposal. Undeniably, packaging has become the very core of the thriving businesses of fast-foods, ready meals, on-the-go beverages, snacks and manufactured foods, and is one of the fastest growing sectors of the global economy.

The packaged food industry experienced a tremendous growth in the later half of the twentieth century owing mostly to the advances made by the petrochemical industry offering new and innovative plastics with a wide range of useful properties. Such synthetic plastics not only offer large processing windows, but are physically strong, chemically and biologically inert, produced at a fraction of the cost of earlier plastics, and are adaptable to most plastic processing equipment. Of the petrochemical-based synthetic plastics, high- and low-density polyethylene (LDPE and HDPE), polypropylene (PP), polystyrene (PS), polyvinylchloride (PVC), polyethylene tere-phthalate (PET), polyvinyl alcohol (PVA) and polycaprolactone (PCL) are among the major synthetic polymers routinely utilized by the food packaging industry. Synthetic plastics offer excellent barrier (to moisture and gas) and thermal insulation properties that are considered critical for packaged foods. Some examples of synthetic food-contact articles/packagings include grocery bags, packaging containers for fresh produce, dairy and meat products, clamshells for the food service industry, dinnerware and containers for hot and cold beverages. However, one drawback of synthetic plastics is that they are exceedingly recalcitrant to biodegradation, and for that reason, these plastics have become a challenge for the municipal solid-waste (MSW) management companies, and are posing a real threat to the already rapidly shrinking capacities of landfills in the United States and Europe. Furthermore, in some developing countries, and in most under-developed countries, which lack sound MSW practices, the unregulated disposal of single-use plastic packaging has become a nuisance and is impacting the quality of life of the local populations and the health of the local environment.

21.2 The environmental impact of conventional food packaging

Synthetic plastics are the wonder material of today’s world, and life without them is unimaginable. Unfortunately, these same useful qualities are overshadowed by their steady contribution to litter worldwide and its negative consequences for the environment. The unrestricted volume generated by the single-use consumer packaging made from such plastics accumulates because they do not readily break down in nature. In fact, synthetic plastic disposed of today may still be around for hundreds of years.

Because the cost of the virgin resins is so low, recycling is not an attractive option and is limited to only certain plastic types. The Food and Drug Administration (FDA) of the United States has raised many concerns regarding recycled plastics, particularly for re-use of these materials in food-contact articles. These concerns include: (a) the contaminants from the post-consumer material that may appear in the final food-contact product made from the recycled material, (b) incorporation of recycled postconsumer material that is not regulated for food-contact into food-contact packaging and (c) assimilation of adjuvants/additives in the recycled plastic not approved for food-contact use.

In view of the serious threat posed by synthetic plastics to marine life and the environment, in 1987 the US Congress enacted the Marine Plastic Pollution Research and Control Act. This law prohibits the dumping of plastics in all US waters. In an effort to further save the environment as well as marine life, in 1997 the US Congress signed the International MARPOL treaty (Marine Pollution Treaty) prohibiting all US and foreign vessels, both naval and commercial, from discarding any plastic waste overboard in US territorial waters unless it is shown to be completely biodegradable. It is estimated that about 1 million metric tons of plastics per year are dumped into the oceans and that in certain areas as many as 17 500 pieces of plastic are present per square kilometer (Narayan, 1994).

The situation on land with respect to plastic waste is even worse. According to a US Environmental Protection Agency report published in 2005, roughly 24.2 million metric tons of MSW deposited in US landfills consisted of disposable consumer packaging used for both food and non-food purposes made mostly from non-renewable resources. On average, every American generates approximately 1500 pounds (680 kg) of waste per year, much of which is destined for landfills. Of this waste, single-use food and non-food consumer packaging made from synthetic polymers constitutes about 40–60% of the volume, which roughly represents about 20-30% of the size of a typical landfill. According to some estimates, industrialized nations alone generate quantities of packaging waste in a single day that if stacked-up together would fill up a space equivalent to the Sears Tower, which once was the world’s tallest building. Most of this packaging is made from synthetic plastics. The consequences of such irresponsible behavior will be of enormous proportions; continued production and accumulation of trash is not only detrimental to the environment but is also contributing to the depletion of our precious and finite natural resources.

Currently, one sensible alternative is to produce biobased fuels and chemicals from renewable resources that can compete effectively with petroleum-derived synthetic chemicals in terms of both the overall cost and physical properties (Röpper and Koch, 1990; Swanson et al., 1993; Shogren and Bagley, 1999; Chum and Overend, 2001; Chiellini et al., 2002; Stevens, 2002). This offers an excellent opportunity for biobased/renewable feedstock to be utilized as a raw material substitute for petrochemicals in the manufacturing of food packaging. Despite many challenges, there is a common belief among the scientific community worldwide that inherently biodegradable biopolymers with improved properties are poised to play a positive role in the development of environmentally compatible, single-use consumer packaging, as evidenced by the explosive increase in the number of scientific papers, patents and products that have surfaced in the last decade alone.

21.3 Opportunities for renewable polymers

Heightened fuel prices and the rising cost of petroleum-derived commodity chemicals have provided much of the impetus for the research and development in the field of biobased/renewable polymers. The availability of many renewable polymers in surplus quantities, problems associated with the disposal of recalcitrant plastic products and consumer demand for environmentally compatible, greener products – especially single-use packaged goods – have further helped to build the momentum to seek new uses for agriculturally derived polymers and byproducts.

Generally, agriculturally derived polymers exhibit poor physical-mechanical properties, provide materials of inconsistent purity, present difficulties in material processing and perform poorly under extreme environmental conditions (Luzier, 1992; Swanson et al., 1993; Mayer and Kaplan, 1994). Nevertheless, these materials have an inherent advantage over their petroleum counterparts in that they are susceptible to biodegradation in the environment upon disposal. Renewables such as cellulose, starch, proteins, oils and, to a lesser extent, lignin, are among the most abundant agriculturally derived materials. In order to overcome many of the shortcomings in renewable materials, hybrid blends and composites, particularly made in conjunction with bio-derived and/or biodegradable polymers such as poly (lactic acid) (PLA), poly (hydroxyalcanoates) (PHAs) and PVA, along with other additives, plasticizers and compatibilizers, etc., have shown to be the most promising, and are expected to play a major role in food packaging. Figure 21.1 compares some plastic properties (modulus and elongation at break) of biopolymers with those of synthetic plastic polymers.

21.4 Production of biobased food packaging materials

21.5 Hybrid blends and composites

Renewable polymers are generally sensitive to moisture and do not provide effective gas barrier properties. Hybrid blends and composites, containing renewable polymers in conjunction with other biobased or synthetic polymers and additives, have shown great potential in making up for some of these shortcomings. In fact, the majority of the consumer food packaging currently available commercially worldwide is based on hybrid materials, rendering the desirable properties and functionality for packaging a variety of foods such as fresh meats, dairy products, ready-meals, beverages, fruits and vegetables, snacks, and frozen and dry foods (Table 21.2). For example, multilayer films produced from plasticized wheat starch and various biodegradable aliphatic polyesters via flat film co-extrusion and compression, significantly improved mechanical performance and moisture resistance in melt blended wheat starch films (Martin et al., 2001). In this film, the properties were totally dependent on the compatibility between the respective materials without the use of any additives, compatibilizers or adhesives. Coatings of edible and biodegradable polymers have, in general, been used to achieve an improved moisture barrier and to prolong the shelf-life of perishable food products (Guilbert et al., 1996, 1997; Guilbert, 2000). Improved water permeation barrier properties were observed as a result of an in situ lamination process for baked starch foams with PVA and PVC. These foams had barrier properties similar to the EPS foams (Glenn et al., 2001a).

With the addition of a small amount of cellulose fibers in wheat starch-based extruded films, increases in modulus, strength and temperature stability were observed with concomitant shifts in the glass transition (T_g) temperature. Thermoformed food trays from these hybrid blends showed greatly reduced aging compared with trays without added fibers (Averous et al., 2001). Similarly, products formed from hybrid composite foams prepared by baking granular starch in the presence of 10–30% aqueous PVA showed markedly improved strength, flexibility and water resistance (Shogren et al., 1998). More recently, these investigators have also shown that the addition of softwood fiber and monostearyl citrate in the formulations yielded baked products with sufficient flexibility and water resistance to function as clamshell-type, hot-sandwich food containers (Shogren et al., 2002); aspen fiber also had a similar effect (Lawton et al., 2004). A detailed work has been published recently on extruded and injection-molded hybrid blends and composites containing agriculturally derived fiber and PVA, and their impact on material properties (Chiellini et al., 2004; Cinelli et al., 2006a, 2006b).

A laminate of chitosan-cellulose and PCL film has been shown to be effective for modified atmospheric packaging of horticultural crops such as lettuce, broccoli, tomatoes and sweet corn within the 10–25 °C temperature range (Makino and Hirata, 1997). PE film containing 6% starch has been recommended for storage of wet and dry low-lipid foods. However, significant loss of elongation was observed in these films due to possible interactions between the film and the free radicals developed during lipid oxidation in foods with higher fat content, such as ground beef, during storage under freezing conditions (Holton et al., 1994). It is interesting to note that PE-starch films neither impaired the heat sealing nor accelerated microbial growth in ground beef. Furthermore, there was no impact on color stability during refrigeration and frozen storage (Strantz and Zottola, 1992). In this regard, an excellent review article has been written on the potential of biobased materials for food packaging (Petersen et al., 1999).

Cellulose, one of the world’s most abundant and inexpensive raw materials, is problematic to use because of its hydrophilic nature, insolubility and crystalline structure. Because of its highly ordered structure, hydroxyl groups and strong hydrogen bonding yield highly crystalline microfibrils and fibers. Such fibers are already being used in numerous commercial packaging products made up of paper and cardboard. Fibers also make excellent biological fillers in many plastic wraps and films. Having biological filler in a plastic matrix is advantageous because the biological additive is readily attacked by microbes, which start the deterioration process leading to eventual composting. Waxed or PE-coated papers are commonly used in the food packaging industry.

While cellulose fiber in blends offers many advantages, it can also have a negative impact on the matrix properties. Some of the advantages and disadvantages of using cellulose fiber are outlined in Table 21.3. The strengthening of the polymer matrix by the discontinuous fiber reinforcements is dependent upon the aspect ratio, geometry and orientation of the fibers, and the interface adhesion between the fiber and the matrix, as well as the microstructural features within the matrix. Dispersed microstructures offer higher elastic properties than equivalent aggregated microstructures due to the more efficient reinforcement of a dispersed system.

Cellulose fibers have poor dimensional stability and offer little or no thermal plasticity. The addition of fiber in higher amounts generally results in processing difficulties and a reduction in viscosity. The strategies used to overcome this challenge are usually to increase the shear rate and to use lubricants or plasticizers to improve the flowability.

21.6 New developments in the production of packaging from recycled lignocellulosic fiber and renewable materials

Paper products – such as corrugated boxes, food wraps, bags and single-ply boxes – constitute the largest percentage of single-use items in the United States, and end up as the largest component in MSW streams. Even with recycling (the United States recycles ~ 27% of its MSW stream), lignocellulosic material (paper) accounts for 37–40% of our landfill materials. The need for using recycled materials and renewable non-wood pulps, rather than virgin wood pulp, is clear.

New twists in the traditional slurry-pulp technology provide an outlet for this recyclable fiber, creating food packaging and wraps from 100% recycled and/or non-wood pulps, such as straw and bagasse. Slurry-pulp processing has been used for years to make egg cartons, drink trays and box in-lays. In slurry-pulping, screens in the shape of the finished product are dipped into a tank holding slurry-pulp, a mixture of water (99%) and recycled fiber (1%). As vacuum is applied to the molded screen, a thin layer of fiber forms on to the screen; upon drying, this thin fiber mat, which is in the shape of the contoured mold, is then separated.

Innovations in slurry-pulp processing have provided tremendous flexibility in the size and shape range of products created by innovators such as Greg Gale and colleagues, as described elsewhere (Orts et al., 2003a, 2003b). In particular, two improvements stand out: (a) use of rapid prototyping to create molds within 2 days and (b) drying of the molded package on the mold, which prevents ’slumping’ or sagging of the piece during drying. Rapid prototyping creates a mold using designs drawn in the latest computer aided design programs: these complex designs are then sent to the rapid prototype instrument. The rapid prototyping method produces the tool, one layer at a time by depositing a thin layer of a dry polymer powder followed by application of a laser beam that fuses that layer into a solid. Once one layer is complete, a second layer of powder is deposited and fused, similar to rastering methods used in a laser printer. This layering and fusing process is repeated until the complex mold takes shape, generally within hours or days.

The second feature of improved slurry-pulp processing methods is the ability to dry the fiber product while it is still on the mold. In the traditional (egg carton) process, the carton is taken off the (metal) mold while wet and passed through a drying oven. Pieces must be relatively small or they will ’slump’ and dry unevenly. Blowing hot air under pressure through the mold dries the fiber package evenly without any change in shape, preventing the piece from sagging during drying. With this innovation, shapes can be more extreme with heights exceeding 18 in. (45.7 cm). One example of the innovative shapes that are attainable is the wine packaging/bottle shipper and some other food packaging produced by Regale, Napa, California that prevents label scuffing during transportation.

Ultimately, the economics of slurry-pulp processing depend on reducing drying times to minimize energy costs. Continuing experimentation with alternative fibers has shown that agriculturally derived, non-wood pulps -such as rice straw, wheat straw, grasses, cotton linters, chicken feathers, and fibers recovered from MSW – can be used. Molded fiber packages have been created from processing slurries containing anywhere from 5 to 60% straws and other agriculturally derived fibers, reducing drying times by as much as 22%. A key driving force behind using agriculturally derived fibers in packaging is the need to find novel economically viable uses for crop residues, especially straws and grasses which can no longer be burned in California due to legislation. Regale’s first plant obtains most of its recycling pulp by shredding wine boxes, office waste and brochures from its wine-producing neighbors in the Napa Valley.

21.7 Assessing the biodegradability of renewable materials in food packaging

Biodegradation is a process in which organic material is decomposed via natural biological activity. In this process, biochemical breakdown of an organic compound leads to smaller products (oligomers or monomers) due to the action of microbes (bacteria, fungi, yeast) or their hydrolytic enzymes. Thus, under ideal conditions, biodegradation is generally a two-step process: the polymer is first hydrolyzed into intermediate compounds, where abiotic factors such as ultraviolet light, along with microbial enzymes, may facilitate this process. This step is then followed by further metabolism of such intermediates by microorganisms. Biodegradation can occur under both aerobic and anaerobic conditions. Under aerobic conditions, the final breakdown products are H2O, CO₂ and residual biomass (byproducts). Under anaerobic conditions, biogas such as methane or hydrogen gas is produced in lieu of CO₂.

Eventual biodegradation of renewable polymers and blends is dependent upon a wide variety of factors. Parameters that influence the biodegradation process include environmental conditions like temperature, moisture, salinity and pH, as well as geometry and surface area of the material, inoculum size, type of environment and the availability of microbes. In addition, certain chemical structures are more susceptible to microbial breakdown than others. Availability and accessibility of specific enzymes to hydrolyze a certain polymer are also critical. For example, polymers such as starch, cellulose, protein and polyester will require amylases, cellulases, proteases and esterases, respectively, to hydrolyze these individual polymers. Therefore, in hybrid blends containing two or more polymers, degradation will be influenced by the presence of the right combination of microbes or enzymes in the disposal environment.

In order to assess polymer biodegradation, packaging material is generally exposed to the testing environment (such as soil, compost, seawater, sewage, sludge, etc.) containing appropriate microbes and environmental conditions that are controlled for the duration of the experiment. Sample degradation can be assessed by measuring the deterioration in the physical-mechanical properties such as tensile strength, elongation at break and decrease in molecular weight of the testing material. However, more commonly, polymer degradation is assessed by measuring the production and accumulation of CO₂ (under aerobic conditions) or CH4 (under anaerobic conditions) as a result of enzymatic hydrolysis of the material and further assimilation of intermediate byproducts. Collected data can be useful in determining both the rate and extent of the polymer degradation and, if the exact carbon values in the initial sample are known, theoretical yields can be calculated to determine overall carbon to CO₂ or CH₄ conversion. The reproducibility and reliability of the test method is critical. In order to test the reliability of the system and for comparative purpose, use of a background measurement, as well as positive and negative controls is also required.

There are several international organizations that are actively involved in writing, examining and establishing standards for testing polymeric materials for biodegradability. The International Organization for Standardization (ISO), American Society for Testing Materials (ASTM), European Committee for Standardization (CEN), German Institute for Standardization (DIN), Organic Reclamation and Composting Association (Belgium based) and Institute for Standards Research (ISR) are some of the leading institutions playing a major role in defining and regulating the standards to assess polymer biodegradation. Standards pertaining to biodegradable plastics put forward by these organizations differ somewhat in their definitions and specific requirements, but the ultimate approach/goal is more or less the same. Great efforts have been made by international scientists to reconcile the European and American standards with international standards. The Sturm test (or modified Sturm test), BODIS test, composting test, anaerobic test, enzyme test, soil burial test and toxicity test are some of the most relevant tests pertaining to bioplastics. No attempt will be made to provide details on the standardized tests, and a list of the most commonly used tests is provided in Table 21.4. The only exception is the ASTM D5338-98 test for determining the compostability of the plastic materials. This test is quite important, as most of the biodegradable single-use food packaging in future will be sent to composting facilities. Furthermore, knowing the product’s compostability will be a desirable feature for commercialization and ultimately acceptance of the product.

In order to apply the ASTM D5338-98 (composting) standard, composting materials need to have both the capacity to biodegrade and to physically disintegrate. Disintegration must lead to the physical collapse of the plastic matrix yielding visually indistinguishable fragments, a requirement for composting. To achieve total biodegradability, after disintegration, polymer chains must be first broken down by microbes and their enzymes, followed by their complete mineralization, i.e. polymer conversion into CO₂, H₂O and minerals. Mineralization rate, however, has to be high and compatible with the composting process. Materials having a biodegradation capacity equal to or more than that of cellulose are considered compostable under this testing standard.

Specifically, the ASTM D5338-98 standard measures compostability of plastic materials. The test method determines the aerobic biodegradation of plastic materials under controlled composting conditions. In this method, plastic is mixed with stabilized and mature compost. The CO₂ evolution is compared with unsupplemented compost. Biodegradation is determined by the rate and extent of material conversion into CO₂ over time. The conversion should be accompanied by weight loss in the plastic material, visible disintegration and high biological activity in the compost. According to the standard, 90% of the disintegrated material must not have any adverse effect on the quality of the compost. In particular, it must not be toxic to other plants. ISO CD 14855 and the CEN test procedures are quite similar to ASTM D5338-92. The only difference is that both ISO and CEN protocols require that the temperature profile of the compost should be continuously at 58 °C, whereas the ASTM procedure follows a temperature profile of 35-58-50-35 °C. In addition, for packaging to turn into visibly indistinguishable fragments in compost, different standards have put forward different requirements regarding time limitations (Table 21.5).

Scores of studies have been conducted to investigate the biodegradabil-ity of starch polymer and its hybrid blends containing both synthetic and/ or renewable polymers and additives (Imam et al., 1992, 1995a, 1995b, 1999a, 1999b; Shogren, 1992; Ramsay et al., 1993; Lawton, 1997; Avévous et al., 2001; Lawton et al., 2004). In the 1970s, investigators from the United Kingdom (Griffin, 1971, 1977) and the United States (Otey et al., 1976, 1987; Doane et al., 1998), for the first time reported on the production of starch-PE blown films where starch was totally accessible to microbial attack, leaving behind a decomposed matrix comprising mostly recalcitrant PE; hence, the birth of the bioplastics. Since that time, scientists worldwide have developed a variety of novel hybrid plastics containing both biodegradable as well as non-biodegradable polymers and additives. They offer novel properties and are useful for single-use packaging applications. Starch, cellulose, PLA, PHBV, PVA, PE, PP, PET and PCL are among the most prominent materials of choice for the production of these hybrid plastics and plasticizers, compatibilizers and coating materials are also used as additives to improve properties. Most of the focus of these developments has been to achieve bioplastics that are mostly biodegradable or compostable. Several excellent articles have been written on this subject (Yasin et al., 1989; Luzier, 1992; Shogren, 1993; Mayer and Kaplan, 1994; Koenig and Huang, 1995; Cutter, 2000; Avérous and Fringant, 2001; Chiellini et al., 2002, 2004).

Hybrid plastics are a multi-component polymeric system. It is possible that, when polymers are mixed and compounded, compatibilized, plasti-cized or surface modified to make a hybrid blend, the properties of the individual polymer(s) may also change. As a result, blends may have biodegradation properties distinct from their individual parent polymers. For example, in PE-based blown thin films containing up to 40% thermoplastic starch (dry weight basis), most of the starch readily degraded when films were disposed of in the environment (Imam et al., 1992, 1996). However, when a similar formulation was used to produce injection-molded articles, most of the starch was found to be encapsulated in the PE matrix, severely compromising the ability of microbes and/or their hydrolytic enzymes to access the starch substrate (Imam et al., 1995a, 1995b). This clearly indicated the influence of processing technique on the biodegradability of the polymer matrix. Packaging films where starch or cellulose fiber were used as fillers in a slow-degrading polymeric matrix, showed quick degradation of these fillers, which accelerated the deterioration of the otherwise slow-degrading polymer and allowed rapid compostability (Bastioli, 2001). Due to the presence of starch in a starch-PHBV blend, a significant enhancement was observed in both the rate and extent of PHBV degradation in a compost environment (Imam et al., 1998). In similar blends, where starch was pre-coated with polyethylene oxide to increase the compatibility between starch and PHBV, starch degradation was negatively impacted. Many biodegradation studies of starch-PHBV hybrid blends in a variety of environments have shown that PHBV and starch both degraded, albeit at different rates and to different extents (Ramsay et al., 1993; Imam et al., 1995a, 1995b, 1999a, 1999b). Blends of the aliphatic polyester PCL or aliphatic-aromatic copolyesters with starch are another important group of biodegradable plastics suitable for food packaging; it has been found that thermal behavior is dominated mainly by PCL and mechanical properties are improved by blending with starch (Rosa et al., 2004b, 2007; Dean et al., 2007). Biodegradability of this blend is heavily influenced by the complex interaction between starch and the polyester not only at the molecular level, but also in the surface properties. A good example of this is seen in modified natural polymers such as starch and cellulose acetates used to improve polymer properties via esterification of hydroxyl groups of sugar residues. The increased degree of substitution (esterification) improves the properties of polymers, but greatly reduces their biodegradability. Starch and cellulose acetates containing large amounts of plasticizers are available commercially and are claimed to be biodegradable. PLA is biodegradable in compost; however, information on its biodegradation in other environments is limited. Several studies have confirmed the degradation of PLA by a variety of microorganisms (Agarwal et al., 1998; Jarerat and Tokiwa, 2001, 2003; Jarerat et al., 2003; Masaki et al., 2005). Changes in environmental factors such as humidity and temperature have been shown to influence PLA degradation (Ho et al., 1999). In one study (Shogren et al., 2003), little or no degradation was observed in injection-molded PLA samples buried in soil for a 1-year period in the Midwestern United States. The reason for the slow breakdown of PLA and other polyester-based plastics may be that the environmental degradation of PLA requires a two-step process. First, high molecular weight polyester chains need to be hydrolyzed into low molecular weight oligomers. This is a rather slow step, but the reaction can be accelerated by acids or bases and is also affected by both temperature and moisture levels of the compost. In a second step, microbes and enzymes convert the low molecular weight components into CO₂, H₂O and residual biomass.

The synthesis and assembly of the supra-macromolecular structure of biopolymers proceeds through distinct biosynthetic pathways, requiring specific biological building blocks joined together via specific linkages or chemical bonds. Thus, specific enzymes are required to disassemble or decompose each polymer. From a biodegradation standpoint, blends con- taining two biopolymers are interesting materials. In starch-PHBV blends, both polymers require different enzymes to degrade effectively. Starch needs microbial amylases that can attack both α-1→4 and α-1→6 linkages to completely break down the polymer. On the other hand, PHBV requires esterases to break down the ester linkages in the polymer to achieve degradation. Similarly, cellulose and lignocellulose would require cellulases and lignases and protein polymer would need proteases to attack polypeptide linkages.

Certain chemical modifications might improve the properties of a polymer, but it would be challenging for natural microbes as they are programmed to degrade naturally occurring polymers. Assessing the degradation of individual polymers in a hybrid blend is challenging. Fourier transform infrared (FTIR) spectroscopy has shown to be a very powerful technique for this purpose in some blends because certain chemical group(s) in each polymer have a characteristic infrared absorption, and the decrease of these absorption peaks or changes in their peak ratios with time can provide useful information on the extent of polymer biodegradation. For example, a hybrid blend made up of PE-starch-protein will show distinguishable peaks that are characteristic of starch, such as the hydroxyl and the fingerprint region. Similarly proteins show the amide I and amide II peaks, and PE shows characteristic C-H stretching bands and a weaker C-H bending absorbance (Imam et al., 1992; Gordon et al., 1996). The CO₂ evolution, loss of polymer weight and decrease in molecular weight and tensile properties in polymers all correlated well and were in excellent agreement with the FTIR data (Imam et al., 1992; Gordon et al., 1996).

Polymer degradation occurs mainly through scission of the main chains or side chains of macromolecules. In nature, in addition to biological activity (enzymes), polymer degradation is also induced by several other processes, including thermal activation, hydrolysis, oxidation, photolysis and radiolysis. Although there is little evidence that PE can be attacked directly by micro-bial enzymes, there are many PE-based products available in the market place that are being sold as ’biodegradable’ materials. The primary step in the degradation consists of a chemically or physically induced reduction of the polymer chain length. Chain length reduction in biodegradable PE has also been attributed to special additives, which trigger the thermal and/or photo-oxidation causing embrittlement of the plastic, followed by enzymatic degradation. Addition of biodegradable fillers like starch or cellulose can further help in the rapid defragmentation of the PE matrix. Such PE-based plastics are termed ’oxobiodegradable plastics’. EPI Environmental Technologies Inc., based in Vancouver, Canada is the supplier of the product TDPATM (Totally Degradable Plastic Additive, a pro-oxidant). An extruded sample of LDPE containing TDPA additive, when first thermally degraded and subjected to mature compost in respirometric studies, showed that these samples were biodegraded by microorganisms and the mineralization rates exceeded 60%, a level typical of several natural polymers. Moreover, the rate of biodegradation was comparatively slower (Chiellini et al., 2003). This additive can also be used with other thermoplastic polymers such as PP, PVC, etc. Other investigators have also proposed that thermally degraded polyolefins can be mineralized by microorganisms in soil (Volke-Sepulveda et al., 1999; Scott and Wiles, 2001). However, further research is needed; in particular, respirometric studies to confirm biodegradability and toxicity evaluations are required to ascertain the safety of the breakdown products. If the claims are confirmed, then this might be the breakthrough technology that would certainly benefit the packaging industry on a wider scale.

21.8 Biodegradable packaging life cycle assessment

A product’s life cycle starts from the moment when raw materials are harvested and processed, followed by the product’s manufacturing, transport, usage and disposal. At every stage of the life cycle, there are emissions of greenhouse gases and consumption of resources/energy. Life cycle assessment (LCA) documents the environmental profile over the life of the product, also known as ’cradle to grave’ analyses of the environmental impact or the product’s ’environmental footprint’. This information helps to evaluate the product’s (such as food packaging) overall sustainability and the entire environmental economy. LCA identifies and quantifies the environmental loads involved at every stage – e.g. the energy and raw materials consumed, including the emissions and wastes generated – evaluates the potential environmental impacts of these loads and assesses available options for reducing these environmental impacts.

LCA is becoming so crucial that ISO has standardized this framework within the ISO 14040 series on LCA. In the future, all biodegradable packaging manufacturers will be required to conduct LCAs on their products. In this regard, Novamont in Italy has applied LCA to evaluate their product Mater-Bi bags, used for the collection of organic waste, and to compare it with paper bags and PE bags (Bastioli, 2001). Interestingly, paper bags, due to their weight, consumed much more energy in their production compared with Mater-Bi or PE bags. However, the Mater-Bi bags had a four times lower greenhouse effect than PE bags and a five times lower effect than paper bags. This is attributed to the presence of natural fillers in the Mater-Bi bags.

Preliminary studies carried out under the European Climate Change Program indicated a primary CO₂ savings potential equivalent to approximately 4 million metric tons of CO₂ as a greenhouse gas. This figure is based on the assumption that the bioplastics market, given the appropriate supportive framework conditions, will have grown to around 1 million metric tons (www.european-bioplastics.org). Bioplastics, particularly for food packaging, are at a very early stage of their development, and therefore the information available on the LCA of biobased products is scarce. A comprehensive article on this subject has been published by Patel et al. (2003).

21.9 Food safety concerns, applications and adoption by the industry

The safety of biobased food packaging has to be examined from a variety of perspectives in view of their overall LCA. Raw materials for bioplastics are derived from renewable crops and their monomers are naturally biodegradable and eventually get recycled back to the earth. Material handling, processing and product manufacturing is routine and does not raise any issues concerning workers’ health or environmental safety. The biggest concern, however, is public health, and safety and security of the packaged food. Petroleum-based packagings have contributed tremendously in this regard, improving the stability and safety of packaged foods. No less is expected from biobased packaging. With consumers demanding more environmentally friendly packaging, the question remains, can biobased polymers provide packaging products that can match the properties of petrochemical-based packaging, by delivering food safely to consumers? In this regard, some earlier developments in the biobased food packaging have already provided results that are quite encouraging. For example, with hybrid biobased packaging, improvements have been observed with regard to the handling of food, prevention of moisture loss, reduction in lipid oxidation, improvement in flavor, stabilization of microbial growth and retention of color in foods ranging from fresh fruits, vegetables, dairy products and meats, to processed food requiring modified atmospheric packaging (Petersen et al., 1999; Marron et al., 2000; van Tuil et al., 2000; Haugaard et al., 2001a, 2001b; Weber et al., 2002).

Numerous investigators have observed that biobased packaging technology improved the quality and safety of fresh processed muscle food (Cutter, 2000), enhanced color and storage life of fresh beef (Ayers, 1959; Baker et al., 1994) and provided enhanced barrier and antimicrobial properties to dairy products where stability of microbial environment and storage capacity is critical (Ahvenainen, 2003). Prolongation of the shelf-life of perishable foods using biodegradable films and coatings has also been reported (Ayers, 1959; Baker et al., 1994; Baldwin et al., 1995; Guilbert et al., 1996). Many biobased packagings have also been reported to offer fire-retardant capabilities. Interestingly, very few or no reports exist to indicate any negative impact on food quality resulting from biobased packaging. For example, concern has been raised with respect to petroleum-based plastics as containers (packaging) for foods that are cooked in microwave or conventional ovens. Plasticizers, unreacted monomers, mold release agents or other contaminants found in plastics may leach upon heating, and contaminants may get absorbed by foods. This may produce changes in food flavor and raise safety concerns (Brooker and Friese, 1989; Castle et al., 1992; McNeal and Hollifield, 1993). In contrast, such concerns have not been raised for the biobased packaging made of mostly renewable natural polymers.

Biobased packaging has already been adapted and is in wide use by nonfood industries worldwide. Europe, North America and Asia have taken the lead in this regard. The items manufactured include garbage bags, shopping bags, laundry bags, agricultural mulch films, single-use consumer packaging and corrugated (KTM Industry, Lansing, MI) and loose-fill foams. KTM and Michigan State University are jointly developing ’green’ technology-based novel industrial materials to provide innovative solutions for global packaging applications. Biodegradable starch foam packaging and insulation materials are now available commercially. Some automotive manufacturers (Toyota) as well as giant computer manufacturers (NEC) and other consumer electronics producers (HP and Dell) are using biobased packaging for their products. In particular, renewable polymers have found some useful applications in the field of biomedicine. Implants, prostheses, bone substitutes, sutures and drug delivery vehicles are examples of their applications. This is quite encouraging, as these materials have been shown to be quite compatible with human tissues and blood, and no rejection of these materials or adverse effects of their use have been reported in a mammalian system.

Adoption of these products by the food industry has been steady but slow due to obvious health and food safety concerns as well as regulatory hurdles. In the United States, for example, any food contact item has to pass through a stringent and lengthy process of evaluation before it can be approved by the US FDA for public use. The European Community also has similar protocols in place. For example, the EU Framework Directive 90/128/EEC requires that any biobased packaging for food contact must ensure food quality and safety. Another reason for the slow progress in incorporation of biobased packaging by the food industry concerns the fulfillment of unique and characteristic functionality requirements demanded by packaged food to provide a stable, healthy and safe food to consumers. Packaging biologically active materials in a space made up of mostly biologically active polymers is in itself a challenge. Factors such as modified atmospheres, provision of gas and water vapor barriers, microbial and thermal stability, retention of color, texture and flavor, as well as timecontrolled performance, are not trivial issues and need to be considered in the design of the food package. In the last decade, the science of biobased materials with respect to food packaging has advanced to the highest level and some great strides have been made by the food industry to embrace this new thinking on biobased food packaging. In addition, biobased packaging for foods has been reconsidered as a more environmentally responsible alternative compared with petrochemical-based counterparts. This factor, and the demand for environmentally friendly packaging from consumers and advocacy groups, concern for accumulating recalcitrant plastic waste in landfills and ever-increasing oil prices have all served as catalysts to bring about this change. Many companies worldwide are positioning themselves as leaders in developing biodegradable plastic resins as they foresee a bright future for applications of these materials (Table 21.1); this is evidenced by the food packaging products that are already in the market place. More details on this subject are provided elsewhere (Haugaard et al., 2001a, 2001b).

There are several critically important determinants that will guide the success of renewables and biopolymers as raw materials for food packaging. First of all, the availability of material with consistent properties will be critical for the industry. Factors such as flooding, extended drought, frost, harvesting pattern and crop infestation could potentially have an impact on on both the quality and the quantity of the available raw materials. For example, harvesting a corn crop prematurely would certainly affect the quality of the starch. One consistent fear will be the presence of any chemical contaminants, or their byproducts, from the fertilizers, pesticides or herbicides used in industrialized countries. Although there seems to be no indication of this at the present time, this is one of the aspects that will be closely examined and monitored by regulatory agencies such as the FDA. Currently, most of the renewable crops are produced in surplus, and this seems to be the global trend. However, environmental factors, weather, climate change due to global warming and scarcity of water resources could change this scenario, and will threaten the abundant availability of raw materials. Yet another important aspect of renewable-based plastics is the fact that many renewable crops, for example corn and soybean, are also consumed as food. Many impoverished nations – particularly in Africa, Asia and South America – are dependent on these crops to provide food to their people. There is a fear that, when demand exceeds the supply, market forces will drive the food prices high and will place economic pressure on these countries. The world’s population is expected to double by the year 2050 and a big portion of agricultural land will presumably be lost to the urban development required to accommodate the population. This will, inevitably, provoke competition for renewable crops, i.e. crops grown for food and crops grown for use as raw materials for industrial products. One of the unintended consequences of this competition has already occurred. Many of the small family farms, once considered the backbone of American agriculture, have been replaced by corporate farming organizations. Large corporations with business interests and with intellectual property rights on many commercially important, genetically modiied germplasms are positioning themselves to be the market players in the near future. The effort to produce biodegradable microbial polyesters in plants by Nature Works™ is one such example.

21.10 Future trends

Judging from the R&D achievements of the past 10–15 years with respect to biopolymer-based plastics, the future of biobased polymers in food packaging looks quite promising. Many of the challenges posed by renewable materials have been and are being resolved, and the food industry in general is supporting the concept by moving towards the adoption of biobased packaging. Consumers have shown a strong acceptance on their part for biobased products and are even willing to pay higher prices for the sake of the environment. These products will also be good for MSW management companies and composting facilities, which will generate extra revenues by converting these compostable materials into rich soil additives. This would also reduce pressure on the ever-shrinking landfill spaces. One obvious benefit will be the minimization of litter on land and sea: the litter is not only a nuisance but is also compromising the natural habitats of many animals, including marine life. Many countries and regions have already introduced legislation on the management of plastic waste and have placed incentives to promote biodegradable plastics. Likewise, many international organizations such as ASTM, ISO, CEN, DIN and OECD are coordinating testing standards, criteria and definitions for biodegradable packaging. Composting councils are active in defining what is acceptable for composting and what is not. The LCAs of several renewable and biobased products have shown very encouraging trends indicating a strong environmental benefit of such packaging. A recent EU study estimates a considerable reduction in the production of greenhouse gases as a result of the usage of biodegradable plastics (Patel, 2004).

In the end, a combination of the desired properties and functionalities of the packaging materials and the commodity market price will dictate the successful adoption of renewable polymers in food packaging. If these packages do not perform at the level consumers are currently accustomed to, they will not be accepted or supported. In addition, raw materials have to be cost competitive, otherwise the industry will naturally revert to the petroleum-based chemicals. This is particularly critical for the PHB and PLA polymers, because despite having good properties, the current market price for these resins is not attractive and is keeping many manufacturers away. It is encouraging to note that consumer surveys in industrialized countries have repeatedly shown the willingness of consumers to pay a fractionally higher price for an environmentally friendly packaging derived from biodegradable and/or renewable polymers. For now, there are strong factors in favor of renewables: an astonishing amount of per capita garbage generation in the industrialized world, rising oil prices and the production of crops in surplus quantities. This momentum needs to be sustained with commitment for the design and production of products with useful functionality, i.e., products that perform under a variety of storage conditions and that retain the ability to degrade after use. More efforts are needed globally to take advantage of the changing market trends with respect to biobased packaging.

21.12 References

21.11 Sources of further information and advice