packaging because it is a renewable raw material, available in large quantities, fully biodegradable, and inexpensive. It has properties suitable for applications in edible films and biodegradable packaging. The methods used for processing WG‐based bioplastics are casting, extrusion, and compression molding [129, 130]. WG‐based films have exceptional oxygen and carbon dioxide barrier properties. However, they have lower water vapor barrier, mechanical strength, and thermal properties in comparison with conventional plastic films [131, 132]. Lignocellulosic reinforcement fillers have been used to improve the properties of WG since they can interact with proteins and lead to the formation of protein–polyphenol complexes, allowing a higher flexural strength, tensile strength, and modulus [133, 134]. Also, WG/montmorillonite (MMT) films were prepared by melt mixing and thermoforming [135]. The introduction of up to 5 wt% of MMT increased the mechanical properties in ways that were not possible by just the variation of the glycerol content and the processing temperature.
Figure 1.8 Scheme of the gelatin manufacturing from denaturation of collagen for film formation.
1.3.11.3 Soy Protein
Soy protein (SP) is an inexpensive renewable resource, sustainable, abundant, and functional, constituted by different globulins with mainly polar amino acids including acidic and basic amino acids and nonpolar amino acids fractions such as 2S, 7S, 11S, and 15S. The major components of SP are β‐conglycinin (7S, nearly 35%) and glycinin (11S, nearly 52%) [136]. The protein with higher 11S fractions produces stronger films with lower water uptake than those richer in 7S, which is attributed to the presence of different sets of amino acids in 7S and 11S fractions [137]. Likewise, different chemical treatments and plasticizers have been explored to improve the intrinsic brittleness and low water resistance of SP‐based films. Among them, glycerol, ethylene glycol, and propylene glycol have been found to outperform 1,3‐propanediol. Glycerol and water can significantly increase the flexibility of films made of SP, but greatly decrease the tensile strength [138].
1.3.11.4 Corn Zein
Zein is a group of alcohol‐soluble prolamin proteins found in the endosperm of corn. It is constituted by hydrophobic and neutral amino acids as well as some sulfur‐containing amino acid [139]. Corn zein has a Mw ranging from 18 to 45 kDa and it is soluble in ethanol solutions in water at 60–70 wt% [140, 141]. Indeed, corn zein is produced commercially by extraction with aqueous alcohol and dried to a granular powder. The high proportion of nonpolar amino acid residues confers highly hydrophobic properties and solubility characteristics to zein, allowing its use in food packaging materials [142]. Zein films are formed by dissolving the protein into aqueous ethanol or isopropanol, heating to 65–85 °C, cooling down to 40–50 °C, and casting them by solvent evaporation. The resultant films are, however, translucid and present an intense yellow color (see Figure 1.9). Glycerol is often used to reduce the film brittleness though it tends to easily migrate through the film matrix because of the weak interaction between protein and plasticizer molecules. Migration of glycerol results in loss of flexibility in the film. Zein films have good oxygen barrier properties and are greaseproof, which have been attributed to the helical conformation of the protein, but their mechanical properties and water resistance are low, similar to those of other protein films [143]. In order to overcome these deficiencies, blended films of zein with other biodegradable biopolymers have been widely studied [144–146].
Figure 1.9 A zein film obtained from corn.
1.3.11.5 Milk Proteins
Biodegradable films can also be formed from milk proteins. The two most important types in the packaging field are casein and whey protein [147]. Casein, comprising 80% of total milk protein, consists of three main components, α, β, and γ with Mws in the 19–25 kDa range. It forms colloidal micelles in milk and is stabilized by calcium phosphate bridging. Casein precipitates when milk is acidified to the isoelectric point (pH = 4.6). Acidified casein is converted to functional soluble caseinates, that is, sodium and calcium caseinates, by neutralization through addition of alkali [148]. Biodegradable films based on caseinates can be obtained by solubilization in water followed by casting and drying. Film formation in water is feasible due to its emulsification capability [149]. Films from caseinates are transparent, with good mechanical and oxygen barrier properties but poor water vapor permeability in the range of WG‐ and SP‐based films [150].
Whey protein is the milk protein that remains soluble in milk serum after casein is coagulated during cheese or casein production. It comprises around 20% of the total milk proteins, being based on a mixture of proteins β‐lactoglobulin (approximately 57%, Mw of 18 kDa), α‐lactalbumin (approximately 20%, Mw of 14 kDa), bovine serum albumin, and immunoglobulins, among others. Formation of whey protein films involves heat denaturation in aqueous solutions, breaks existing disulfide bonds, and forms new intermolecular disulfide and hydrophobic bonds [151]. Films based on whey protein isolate (WPI) have shown promising mechanical properties as well as moderate water vapor permeability and good oxygen barrier properties [152, 153]. Nevertheless, the properties of WPI films are highly affected by relative humidity (RH) and the type and concentration of plasticizer [154, 155].
1.4 Concluding Remarks
Due to their low cost and high‐performance characteristics, fossil derived polymers have remained as the most preferable materials for packaging applications. However, as mentioned throughout this chapter, serious environmental issues along with the limited natural resources have led to an increase in the development and use of biopolymers as potential strategies to generate novel packaging items. Currently, there are still challenges to overcome, since the material performance of biopolymers in terms of mechanical, thermal, and vapor and gas barrier properties still needs to be further improved to be able to compete with the current petrochemical plastics commonly found in the food packaging industry. Furthermore, ethical issues frequently arise when attempts are made to use biomass for industrial purposes.
According to the European Bioplastics, the global production capacities of bio‐based plastics, biodegradable or not, amounted to 1.7 million tons in 2014. This translates to approximately 680 000 ha of land. Consequently, the surface required to grow sufficient feedstock for current bioplastics production is only about 0.01% of the global agricultural area of a total of 5 billion hectares. In any case, the bioplastics industry is also researching the use of nonfood crops and biomass derived from algae, that is, the “second and third generation feedstock”, respectively, with a view to its further use. Innovative technologies are currently being focused on nonedible by‐products or waste materials as the source for bioplastics. In this regard, the trend for the development of the next generation of bioplastics is currently led by the emergence of conventional polymers made from renewable and nonfood sources. This generation feedstock is based on the production of plastics from cellulosic materials derived from food crop by‐products such as straw, corn stover, or bagasse, which are usually left on the field where they biodegrade at a quantity far higher than necessary to restore the soil carbon pool. At best they are used to produce the energy for the conversion process to feedstock. This leaves significant potential for using biotechnological processes to create a platform to generate chemicals for industrial purposes, the so‐called biorefinery concept, which includes the production of bioplastics. Indeed, the word “sustainable” means to maintain or keep going continuously and the word has been used in connection