## 3 Plastics Used for Food Packaging
### Selected Plant-derived Polymers
#### 3.3.2 Protein-Based Polymers  
Research in food packaging has also focused on protein-based films due to their good film-forming properties, low cost, and biodegradable nature [83]. Materials synthesized from proteins exhibit desirable film-forming and barrier properties, which are often comparable to fossil-based products [90]. Proteins from different sources have been investigated for the synthesis of bioplastic films for packaging applications including collagen, gelatin, caseins, soy/whey/quinone protein, egg white protein, myofibrillar protein, corn zein, wheat gluten, and keratin [83; 161; 162].  
Among all the protein sources, soy proteins got great attention as a potential source for bio-based packaging materials. This development is based on excellent film-forming and oxygen barrier properties of films produced from soy protein isolate. However, these materials cannot meet the requirements of a film with mechanical and water barrier properties guaranteed by conventional plastics [83; 163]. Compared to films from other proteins, soy protein-based films are characterized by transparency, flexibility and cost-effectiveness [162]. Furthermore, they show good oxygen barrier properties under low moisture conditions [164]. A disadvantage that limits their use beside low mechanical strength is a lack of heat stability compared to LDPE [165; 166; 167; 168].  
Whey proteins are able to form elastic films [169], which are transparent, flexible, and exhibit good oil and oxygen barrier properties at low humidity. A disadvantage is a moderate moisture permeability. Nevertheless, whey proteins have been intensively studied as raw material for biodegradable packaging [170].  
Collagen and gelatin are proteins originating from animal sources acquired by a controlled hydrolysis reaction. In nature, collagen is the most abundant occurring protein [171]. Collagen-based bioplastic films are characterized by good mechanical properties, [172] and therefore, are suitable for various applications [173]. In contrast to collagen-basedfilms, gelatin films show poor mechanical and barrier properties according to their hydrophilic nature [174]. [PERSON] et al. determined functional properties of gelatin-based films. Compared to synthetic polymers, good gas barrier properties were reached by gelatin films cross-linked with ferulic acid. Gelatin films with poly(ethylene glycol) (PEG) 200 showed high gas barrier properties and high permselectivity towards carbon dioxide and oxygen [80].  
Gelatin has been introduced in the manufacturing of packaging films due to its low cost and abundance [175]. Furthermore, gelatin is used to produce biodegradable packaging materials due to its good properties, such as low melting and gelling points, good capacity of oxygen barrier, biodegradability, and excellent film formation [176]. The use of gelatin-based composite films incorporating other materials like chitosan, sunflower oil, and corn oil to enhance the barrier and mechanical properties of these films was studied by different authors [177]. In addition to gelatin, gluten is used to prepare films of high homogeneity, excellent gas barrier properties, and mechanical strength [178].  
[PERSON] developed a new intelligent-active food packaging film using poly(vinyl alcohol) (PVA) and gelatin incorporated with Amaranthus leaf extract to monitor freshness and increase the shelf life of fish and chicken meat. Incorporation of Amaranthus leaf extract improved its mechanical and water vapor barrier properties next to active functions. The decrease in solubility enables the use for packaging of flesh foods. Samples packed in neat films had a shelf life of 3 days while those in active films spoiled after 12 days. The results of the study suggest the application of Amaranthus leaf extract containing PVA-gelatin films being both active and intelligent ensuring quality and safety of flesh foods [179].  
For comparative purposes, Table 3 shows the discussed barrier as well as mechanical properties of protein-based polymers.