Whey Protein-Based Packaging Films and Coatings

CHAPTER 11

Whey Protein-Based Packaging Films and Coatings Markus Schmid1 and Kerstin Müller2 1

Albstadt-Sigmaringen University, Sigmaringen, Germany, 2Fraunhofer Institute for Process Engineering and Packaging IVV, Freising, Germany

11.1

INTRODUCTION

Due to rising environmental awareness, the development of bio-based plastics has been gaining more and more attention in recent years. Renewable raw materials used for biopolymers such as proteins, polysaccharides or lipids derive from a variety of crops and even more promisingly from waste streams accruing from processing by the agro-food industry. Proteins have been successfully used for the formation of films and coatings. Well-studied proteins such as whey protein, casein, wheat gluten, soy protein, or zein have been used to develop films that are gained from renewable resources with faster degradability than other polymeric materials (Cinelli et al., 2014; Ramos, Fernandes, Silva, Pintado, & Malcata, 2012). Moreover, films derived from agricultural proteins develop new market opportunities for agricultural products, by-products, and waste streams within the food process chain (Embuscado & Huber, 2009). The ability of globular proteins to unfold and cross-link to new polymeric structures under certain conditions makes them excellent raw materials for films and coatings. Compared to their polysaccharide-based counterparts, cross-linked proteins films are more stable and often depict longer durability (Barone & Schmidt, 2006). In terms of packaging, promising oxygen, nitrogen, and carbon dioxide barriers can result from densely packed network structures developed from a variety of interactions and bonds between the protein chains and general protein hydrophilicity (Schmid, Zillinger, Müller, & Sängerlaub, 2015c). Properties of such protein films and coatings initiated extensive research within the movement towards more sustainable biopolymers for industrial applications. Thus, this chapter includes relevant discussion of the application of protein films and coatings as well as an outlook on their current and future industrial potential. Whey proteins with suitable functionalities as Whey Proteins. DOI: https://doi.org/10.1016/B978-0-12-812124-5.00012-6 © 2019 Elsevier Inc. All rights reserved.

407

408

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

food packaging materials are described together with the different technologies for processing and current state of the art about film-forming formulations for tailored barrier, mechanical, surface, and end-of-life properties. Furthermore, the regulative background about packaging use, end-of-life, and perspectives is covered, especially with focus on the environmental impact of packaging materials. Existing drawbacks with regard to mechanical and barrier properties of whey protein-based packaging films and coatings are discussed, as well as strategies to overcome those limitations in order to match existing solutions. Physical, chemical, and biochemical methods can be applied for this purpose. Therefore, an overview of the effects of various treatments on whey protein-based films and coatings is given, focusing on food packaging applications. However, most of those modifications are still not fully grasped at a fundamental level. Understanding these modifications is an important step towards the industrial implementation of protein-based films and coatings (Zink, Wyrobnik, Prinz, & Schmid, 2016).

11.2

WHEY PROTEIN FILM FORMATION

11.2.1 Protein Structure Related Properties and Preconditions for Film Forming Proteins are macromolecules that naturally display different sequences of amino acids that are combined by peptide bonds. For protein film formation, chain alignment and structure highly depend on the present arrangement of amino acids. Geometrical structures based upon the primary amino acid sequences depict α-helices, β-sheet, or other steric turns and loops stabilized by intramolecular interactions varying in strength. Besides, strong disulfide bridges, hydrogen bonds, van der Waals-, electrostatic-, and hydrophobic interactions take place and are responsible for the final, three-dimensional protein structure. With regard to protein-based films, the initial amino acid sequence is the determining factor for interactions between the protein chains themselves as well as other film components (Belitz, Grosch, & Schieberle, 2008; Cheftel, Cuq, & Lorient, 1985). In terms of whey proteins, especially, the contained thiol groups of cysteine residues are able to form disulfide bridges, both intra- and intermolecular (Barone, Dangaran, & Schmidt, 2006). Whey proteins include different globular proteins; β-lactoglobulin (β-Lg) (B57%), α-lactalbumin (α-La)(B19%), bovine serum albumin (B7%), several immunoglobulins (B13%), and the polypeptides proteose-peptone (B4%) (Lent, Vanasupa, & Tong, 1998) (see Chapter 1 for more details on whey proteins). Being the major whey protein, β-Lg dominates gelation and aggregation behavior in whey protein formulations (Hammann & Schmid, 2014).

11.2 Whey Protein Film Formation

Native β-Lg is a small globular protein with a molecular weight of about 36.6 kDa with defined secondary and tertiary structure. At room temperature in aqueous solution (pH 5 7) it is present mainly as a dimer with a molecular weight of 18.3 kDa of each subunit (Jovanovi´c, Miroljub, & Ognjen, 2005). The molecule is made up of α-helical, β-sheet, and random coil structures. It consists of 162 amino acids, of which the nonpolar amino acids can be found on the inside and the polar residues on the outside of the globular structure. This explains the good water solubility of native β-Lg molecules (De Wit, 2001b). It contains five cysteine residues of which Cys 121 is freely available and decisive for film formation, although buried inside the protein structure under normal conditions (De Wit, 2001a; Morr & Ha, 1993). α-La is the second largest fraction in whey proteins and is a small acidic protein with a molecular weight of 14.2 kDa. The single polypeptide chain is made of 123 amino acids, including eight cysteine residues. Four disulfide bonds and Ca21 ions participate in the tertiary structure stabilization (Belitz, Grosch, & Schieberle, 2007; Jovanovi´c et al., 2005). It consists of a large α-helical domain and a small β-sheet domain, connected by a calcium-binding loop. Due to its disulfide bonds and the absence of free sulfhydryl (-SH) groups, α-La has the greatest thermal stability besides the proteose-peptone fraction (Jovanovi´c et al., 2005). The capabilities of whey proteins to change chain conformations and interact with each other to form modified three-dimensional networks are excellent properties to be used for films and coatings.

11.2.2

General Steps for Film Formation

The formation of whey protein-based films can be generally separated into different steps. Since whey proteins are globular proteins, the first step is unfolding of the protein’s native state by relieving low-energy intermolecular bonds. Protein unfolding and dissociation can be caused by several treatments such as change in temperature, change in pH, shear forces, or addition of organic solvents or salts. The generated, entangled protein chains expose reactive, mainly hydrophobic, functional groups and can now be newly orientated and arranged. The exposed groups are now able to form a threedimensional chemical network stabilized by new bonds such as disulfide bridges or physical linkages including van der Waals interactions, hydrogen bonding, and electrostatic and hydrophobic interactions (Onwulata & Huth, 2008). Although several processes are possible for film forming, the mainly applied technique to form coherent films is thermal denaturation. The denaturation temperature for whey proteins normally lies at about 78 C, although it depends on the formulation composition (Plackett, 2011). The degree of

409

410

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

denaturation in whey proteins is determined by β-Lg. Not only is it the main fraction of whey proteins, the rate of aggregation also depends on the concentration of free sulfhydryl groups which only occur in β-Lg. Additionally, the free sulfhydryl groups appear to increase denaturation reactions by inducing cleavage of intramolecular disulfide bonds in α-La and other whey proteins (Calvo, Leaver, & Banks, 1993). At pH 6, thermal denaturation takes place in two steps. In the first step, above 40 C, dimers dissociate into monomers. At temperatures around 70 C the globular molecules start unfolding and can interact via sulfhydryl groups, forming small aggregates with either β-Lg molecules or other thiol-containing proteins. Secondly, small aggregates interact forming high-molecular-weight aggregates. At higher temperatures, irreversible degradation of the protein starts. However, the influence of pH, salt, sugar, and protein concentration must be considered concerning thermal behavior of β-Lg. (Dewit & Klarenbeek, 1984; Jovanovi´c et al., 2005). Because the pH directly affects protein conformation, it also influences solubility and structure and thereby denaturation behavior. Dewit and Klarenbeek (1984) showed that β-Lg has an increased thermostability at pH 3 and a decreased thermostability at pH 7.5, which is explained by increased reactivity of thiol groups due to dissociation at alkaline pH. Initial products to form whey protein-based films are whey protein isolate, whey protein concentrate, or β-Lg in aqueous solution with different plasticizers. Final film properties vary with the used proteins and formulation additives such as plasticizer, chemical agents, or the addition of lipids or salts (Coltelli et al., 2016), as well as the applied denaturation process. However, native whey protein formulations are also able to form coherent films. Here, film cohesion is mainly based on low-energy bonding like van der Waals forces, electrostatic interactions, or hydrogen bonding (Pérez-Gago & Krochta, 1999). For further information on whey protein denaturation, also see Chapter 6 in this book.

11.3 TECHNOLOGIES FOR PROCESSING FILMS AND COATINGS 11.3.1

Wet Coating

Wet processing techniques are widely used for the formation of thin layer films. Stand-alone films can be obtained by casting solutions or suspensions while coatings are always associated with different substrates. The common method used for academic research regarding formulation evaluation is casting since it represents a quick and simple method to determine film-forming properties. Especially solvent casting is a cost-efficient method for the

11.3 Technologies for Processing Films and Coatings

development of protein polymer films with equipment available from laboratory to pilot scale (Embuscado & Huber, 2009). Among others, coating techniques can mainly be divided into deposition from solution such as spray-, dip-, roll- or spin-coatings, and physical vapor deposition (Coltelli et al., 2016). For all the coating techniques, of which the mainly applied ones are lacquering or spraying, rheological behavior of the coating formulations is decisive (Tracton, 2005). Furthermore, final film properties are influenced by the applied drying techniques which range from ambient condition drying to conventional hot-air drying, infrared or microwave drying (Embuscado & Huber, 2009). Due to solvent evaporation, the applicable coat weight can be much lower compared to extrusion coatings (Rastogi & Samyn, 2015). When compared to multilayer films, lower material usage is demanded (Mihindukulasuriya & Lim, 2014), bringing both economic and ecological advantages. When packaging applications are concerned, coatings on paper or polymeric substrates are used to attain or improve material properties such as barrier, mechanical, surface, or antimicrobial properties. Protein solutions used for casting or coating are usually denatured by heat to initiate cross-linking for film formation. The step of heat treatment can be performed either before, after, or during the coating process (Schmid, Noller, Wild, & Bugnicourt, 2013b), although the formation of native protein films is possible, too (Perez-Gago & Krochta, 2001a). To prevent protein films from brittleness caused by excessive cross-linking in the protein network, the use of plasticizers is necessary (Plackett, 2011), while compatibility of all film components must be considered since they directly influence final film properties (Lent et al., 1998; Mahmoud & Savello, 1993). When stand-alone films are processed via solvent casting, the film-forming solution is spread out evenly on a chosen surface such as a petri dish or Teflon plate before it can be removed therefrom after drying. Protein-based coatings are either applied on substrates such as paper or polymer films or directly applied on food products as edible coatings. In all cases, solvent removal is required by drying (Gennadios, 2002), limiting the coating thickness. Solvents used to prepare protein-based films are usually based on water, alcohol, or a blend of other solvents. At each film preparation step, the final film properties are influenced by the formulation and method applied. Therefore, materials obtained via wet processing display different functional properties according to the protein concentration used in solution, the pH of the solution, the solvents and additives used, and the drying conditions applied, such as drying rate and temperature (Gennadios, Weller, & Testin, 1993). Another important fact needs to be taken into account, namely the post cross-linking of whey protein-based films and coatings during storage.

411

412

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

Schmid, Reichert, Hammann, and Stäbler (2015b) observed that after film forming and drying further alterations of molecular interactions in whey protein-based films occur. A significant decrease in oxygen permeability was observed during storage caused by changes in covalent and noncovalent bonds between the polypeptide chains. It was concluded that the crosslinking by disulfide bonds as well as hydrogen bonds are the predominant interactions formed during the initial days of storage, accompanied by a fast decrease in oxygen permeability. The contribution of hydrogen bonds in relation to disulfide bonds increases during storage leading to an additional, but slower, decrease in oxygen permeability until 2 3 weeks after initial protein-film production (Schmid et al., 2015b).

11.3.2

Extrusion

Extrusion processing is the conventional method for preparing plastic granules and films and has therefore also been used and adapted for processing protein-based plastics (Verbeek & van den Berg, 2010). Globular proteins such as whey proteins tend to unfold and cross-link under the influence of heat, leading to a closely networked material with thermoset behavior. During continuous extrusion, the raw material mass melts under the supply of heat and energy input caused by friction between the screws, becomes formable, and is pressed through the extruder die into a desired shape (Domininghaus, Eyerer, Elsner, & Hirth, 2008). During the whole process the mass can be compressed, mixed, plasticized, homogenized, chemically transformed, degasified, or gasified (Braun, 2003; Kaiser, 2007). The extrusion process, however, requires thermoplastic properties of the raw material. Since proteins naturally do not have a thermoplastic behavior, formulation modifications such as the use of additives are essential for protein extrusion and haves been widely studied for whey proteins (HernandezIzquierdo & Krochta, 2008, 2009; Hernandez-Izquierdo, Reid, McHugh, Berrios, & Krochta, 2008; Schmid, Müller et al., 2014; Schmid, Sängerlaub, Wege, & Stäbler, 2014, 2016; Verbeek & van den Berg, 2010). Final extrudate properties are influenced by applied process and formulation parameters. Referring to the extruder plant itself, mixing of raw materials is more efficient when using corotating twin-screw extruders. The configuration of the screws can include conveying as well as reconveying elements, dividing the process into different zones to fulfill process requirements such as conveying, compression, plasticization, mixing, or homogenization of the mass. The main physical characteristic of an extruder is, however, the length-to-diameter ratio, or L/D ratio (Giles, Wagner, & Mount, 2013). The specific mechanical energy input (SME) caused by friction of the screws is characteristic for each product and applies regardless of the extruder size. High SME inputs promote extensive cross-linking and should be kept low during extrusion of

11.4 Protein Modification for Optimized Film Formation and Performance

protein-based plastics, which can be realized with low screw speeds and high mass flow rates. Furthermore, the SME highly depends on the rheological properties of the melt, macromolecular transformations, and interactions between all formulation components (Coltelli et al., 2016). Formulation adjustments are usually made by the use of plasticizers and chemical additives. Modifiers that reduce intermolecular interactions are plasticizers and reducing agents; both generally reduce thermal decomposition temperature and increase the flexibility of thermoplastic protein plastics. The main efficient plasticizers used for whey protein processing are water, glycerol, sorbitol, and sucrose. Conversely, cross-linking agents or other additives that increase intermolecular interactions lead to stronger materials with high strength and stiffness and slightly increased decomposition temperatures. In both cases the melt viscosity is affected, being decreased or increased, respectively. The temperature setting itself is another critical parameter. While slight increases result in increased chain mobility, higher temperatures enable hydrophobic chain interactions due to unfolding and cross-linking of exposed reactive functional groups. A combination of high temperature and low moisture can even lead to protein degradation. Therefore, a suitable balance between melt viscosity and profile temperature is important for thermoplastic protein processing.

11.4 PROTEIN MODIFICATION FOR OPTIMIZED FILM FORMATION AND PERFORMANCE 11.4.1

Physical Modifications

The main applied physical modification for the production of proteinbased films and coatings is heat treatment. Proteins tend to aggregate and form three-dimensional networks when exposed to temperatures above their denaturation temperature (see Chapter 6 of this book for further details on denaturation and aggregation of whey proteins). Thermal denaturation of whey proteins begins at approximately 70 C (Parris, Purcell, & Ptashkin, 1991); however, denaturation conditions in protein formulations are strongly connected to the protein concentration and the additives used as well as the solvents (Renkema, Lakemond, Jongh, Gruppen, & van Vliet, 2000). Besides the extent of heat treatment, the resulting film properties are, furthermore, a function of ionic strength and the presence of other molecules, since they directly influence the developing protein network (Nicolai, Britten, & Schmitt, 2011; Nicorescu et al., 2008). Films processed under increased temperature, which also involves a reduction of the water content, exhibit higher brittleness and strength due to extensive cross-linking and less plasticizing.

413

414

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

Processes such as mixing, coating, and extrusion all transfer shear stress. Under the influence of shear, proteins can aggregate, de-aggregate, and denature (Zink et al., 2016), although intense shear rates are needed for protein denaturation (Thomas & Geer, 2011). Pressure changes occurring due to shearing of a liquid formulation can be followed by deformation and fragmentation of contained protein particles (Taylor & Fryer, 1994). Therewith, eventual aggregates can be redispersed. Conversely, high particle collision rates can also facilitate aggregate formation. Fluid viscosity, also being a function of the protein ratio, directly influences shear-induced aggregation or deaggregation. Higher whey protein ratios, up to 30% w/w, result in smaller, stable aggregates due to higher viscosity and shear stress compared to formulation with lower protein ratios (Wolz, Mersch, & Kulozik, 2016). Since film-forming protein formulations are mainly aqueous, hydrostatic pressure (HP) is another possible treatment for physical modification of whey proteins (see also Chapter 8). However, only a few studies have used HP to form protein-based films until now (Condés, Añón, & Mauri, 2015; Fetzer & Ramachandran, 1979; Molinaro et al., 2015). The influence of high pressure can also lead to disaggregation and unfolding of globular proteins by affecting their conformation. Therewith, hydrophobic groups as well as disulfide groups are exposed and are able to form new inter- and intramolecular bonds (Bouaouina, Desrumaux, Loisel, & Legrand, 2006; Dumay, Kalichevsky, & Cheftel, 1994; Dumay, Kalichevsky, & Cheftel; 1998; Olsen, Ipsen, Otte, & Skibsted, 1999; Tedford & Schaschke, 2000). In comparison to heat-induced protein gels, high-pressure processed β-Lg gels show weaker intermolecular interactions resulting in more porous and less rigid structures with strong water exudation and solubility, and are prone to aggregation during storage (Tedford & Schaschke, 2000). When increased pressure is applied, however, stronger gels with lower solubility can be generated (Famelart, Chapron, Piot, Brulé, & Durier, 1998; Kanno, Mu, Hagiwara, Ametani, & Azuma, 1998). A combination of a suitable heat pretreatment (B55 C) and high pressure also showed improved film-forming properties (Phillips et al., 1990). Another tool for physical protein modification is ultrasound treatment. Ultrasonic cavitation caused by compression and decompression cycles of the sonic waves transfers high amounts of energy, being able to disrupt physical and chemical interactions (See also Chapter 8). Therefore it has been widely used for dispersing, emulsifying, crushing, and activating particles (Xia & Wang, 2003). With regards to proteins, shear forces and energy input arising from cavitation are able to split covalent bonds in aqueous formulations (Brennan, 2006; Gülseren, Güzey, Bruce, & Weiss, 2007). Therefore, high-intensity ultrasound can induce protein denaturation with a combination of shear stress and free radicals arising from water sonolysis

11.4 Protein Modification for Optimized Film Formation and Performance

(Riezs & Kondo, 1992; Suslick, Casadonte, Green, & Thompson, 1987). Especially for the production of whey protein-based nanocomposites, sonication is used to improve final film properties such as mechanical strength and barrier properties by reducing filler aggregation and favoring a homogenous distribution of the nanofiller. However, ultrasound has not been used for the protein-film formation itself so far. Whey protein films, particularly the needed cross-linking for film formation, can also be generated via irradiation. Ionizing irradiation, such as γ-irradiation, is able to induce irreversible conformation changes in the proteins by oxidation of amino acids, formation of protein free radicals, breakage of covalent bonds, and recombination and polymerization reactions (Hammann & Schmid, 2014). Basically, γ-irradiation generates hydroxyl radicals from water that are prone to react with amino acid residues, of which aromatic amino acids are preferred rather than aliphatic amino acids (Sabato et al., 2001). Regarding whey proteins, γ-irradiation forms dityrosine bridges between the protein chains and is able to form insoluble and sterilizable films (Brault, D’Aprano, & Lacroix, 1997). However, dityrosine bridges in whey protein films are naturally limited due to the low number of tyrosine residues in β-Lg (Etzel, 2004; Wong, Camirand, & Pavlath, 1996). Ultraviolet (UV) radiation can be used for film formation by inducing covalent cross-linking in whey proteins. UV exposure of double bonds and aromatic rings leads to free radical formation of amino acid residues which are able to form new cross-links to generate a protein-film network (Gennadios, Rhim, Handa, Weller, & Hanna, 1998; Rhim, Gennadios, Fu, Weller, & Hanna, 1999). Since increased radiation doses lead to increased interactions, UV-irradiated whey protein-based films show increased strength; however, barrier properties are not significantly influenced (Schmid, Held, Hammann, Schlemmer, & Noller, 2015a; Ustunol & Mert, 2004).

11.4.2

Chemical Modifications

Chemical modification of proteins generally involves reactions with chemical agents or pH alteration (Zink et al., 2016). Chemical reactions include alkylation, acylation, acetylation, and succinylation. However, they are not suitable for significant functionality improvements of whey protein-based films and coatings, due to the used chemical agents lacking food-safety approval. Yet, chemical grafting, meaning the incorporation of fatty acid chlorides by an acylation reaction, could gain importance for whey protein modification. Here, long alkyl chains are integrated to the protein chains and can act as internal plasticizers. Just like external plasticizers, intermolecular interactions between the protein side chains are reduced, resulting in changed thermal properties and folding of the proteins

415

416

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

(Winkler, Vorwerg, & Schmid, 2015; Zink et al., 2016). Beside the number of available functional groups, the solvent also influences the number of bonded fatty acid chlorides. Incorporation rates for β-Lg, e.g., were higher when organic solvents were used instead of aqueous solutions (Creuzenet et al., 1992). This way of chemical grafting results in higher protein hydrophobicity, giving improved water resistance and lower water absorption and, with regards to films and coatings, improved moisture barriers (Wihodo & Moraru, 2013). Additionally, modified proteins show melting behavior at temperatures between 150 and 200 C and therefore further process and application possibilities (Bräuer, Meister, Gottlöber, & Nechwatal, 2007). Nevertheless, just like other reactions with chemical reagents, alkylated films used for chemical grafting often require toxic agents leading to nonedible films (Ghorpade, Li, Gennadios, & Hanna, 1995). Other chemical modifications can be obtained by pH alteration. Nonenzymatic protein hydrolysis can be acidic as well as alkaline. Protein hydrolysis generally reduces chain length and molecular weight by splitting the initial protein into smaller peptide structures. Another important factor influenced by pH is the protein conformation depending on the net charge when pH differs from the isoelectric point. When pH is lower than the isoelectric point, a positive net charge is present and vice versa (Wihodo & Moraru, 2013). While at pH lower than 3.5 and higher than 8 a monomer is present, octamers are formed between pH 3.5 and 5.2 and dimers occur in the pH range between 5.2 and 8 (Onwulata & Huth, 2008; Verheul, Roefs, & de Kruif, 1998). Consequently, whey protein films derived from formulations with different pH also have different functional properties. While Young’s modulus and stress at break depicted a maximum at pH 7 and 8, the elongation at break increased with increasing pH from 7 to 9 (Anker, Stading, & Hermansson, 1999). Barrier properties are only slightly affected by pH. However, maximum values for both oxygen and water vapor barrier properties depict their maximum at the isoelectric point of around pH 5 (Pérez-Gago & Krochta, 1999; Zink et al., 2016).

11.4.3

Biochemical Modifications

Enzymatic hydrolysis cleaves the peptide bonds in proteins, generating smaller peptide chains or even single amino acids. Peptidases are generally divided into exopeptidases and endopeptidases, depending on their cleavage site within the protein chain. A consequent reduction of the molecular weight results in reduced intermolecular forces along the protein chains, giving increased free volume and higher flexibility of the protein chains (Sothornvit & Krochta, 2000b; Verbeek & van den Berg, 2009). The key parameter to determine protein hydrolysis is the degree of hydrolysis

11.4 Protein Modification for Optimized Film Formation and Performance

(DH), which is defined as the percentage of cleaved peptide bonds in relation to the total number of peptide bonds per protein (Nielsen, Petersen, & Dambmann, 2001). Studies show increased film flexibility with the use of whey protein hydrolysates at constant oxygen permeability (Schmid, Hinz, Wild, & Noller, 2013a; Sothornvit & Krochta, 2000b). As a consequence, the utilization of hydrolyzed whey protein may reduce the amount of technologically necessary plasticizer (Sothornvit & Krochta, 2000b). Other enzymatic modifications for whey protein-based films are made with the use of transglutaminases. Transglutaminases are able to form a ε-(γ-glutamyl) lysine bond by catalyzing the acyl transfer between the γ-carboxyl group of a glutamine side chain and the ε-amino group of a lysine side chain (Ichinose, Bottenus, & Davie, 1990). In this way, peptide chains of the same or of different proteins can be cross-linked, giving more mechanical stability with increased strength to protein-based films. Most transglutaminase cross-linked films also show improved barrier performance against water vapor (DeJong & Koppelman, 2002; Eissa, Puhl, Kadla, & Khan, 2006; Schmid, Müller et al., 2014; Schmid, Sängerlaub et al., 2014; Truong, Clare, Catignani, & Swaisgood, 2004) due to the dense protein network. However, some studies also showed diverse behavior, which could be explained by different formations of the cross-linked network. Further information about protein hydrolysates is given in Chapter 14 of this book.

11.4.4

Film Modifications: Blends and Composites

A common technique used to improve limited properties of polymers such as protein-based films is blending with other polymers, either natural or synthetic. The resulting combination shows mechanical as well as barrier enhancements (Gupta & Nayak, 2015; Plackett, 2011; Schmid, Müller et al., 2014; Schmid, Sängerlaub et al., 2014). Additionally, polymer blending can also offer improved processability, plant flexibility, and quick formulation changes (Utracki, 2003). In addition to conventional methods such as blending with other polymers, the incorporation of nanoparticles or clays into biopolymeric matrices such as whey protein films has become a common modification method to overcome drawbacks (Hassannia-Kolaee, Khodaiyan, Pourahmad, & ShahabiGhahfarrokhi, 2016; Oymaci & Altinkaya, 2016; Zolfi, Khodaiyan, Mousavi, & Hashemi, 2014). Many studies, summarized by Müller et al. (2017), reveal an improvement in film properties by addition of inorganic nanoparticles to whey protein isolate (WPI) based matrices. Just like conventional fillers, nanoparticles give improved mechanical strength to the final films. Additionally, barrier improvements for gases are possible by creating a so-called “tortuous path”

417

418

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

for the permeating molecules. However, the improvement of film properties highly depends on the particle ratio and geometry, distribution, and orientation within the matrix. Another possibility to enhance the naturally poor water vapor barrier of whey protein-based films is the addition of hydrophobic materials such as lipids and waxes. However, some lipids negatively affect mechanical and optical properties of the films by decreasing film strength and increasing opacity. Studies showed that low lipid or wax contents and smaller particle sizes give the best overall improvements in the protein films (Janjarasskul, Rauch, McCarthy, & Krochta, 2014; Pérez-Gago & Krochta, 2001b; Talens & Krochta, 2005). Apart from approaches for further mechanical and barrier property enhancements, the incorporation of bioactive compounds such as antioxidants or antimicrobial agents for active and intelligent packaging is gaining attention in recent years. Active packaging systems use materials that interact with the packaged goods leading to extended shelf life, improved security, and preserved quality (Koshy, Mary, Thomas, & Pothan, 2015). For example, the use of essential oils, tocopherols, or ascorbic acid showed antioxidant effects in packaged foods, extending their shelf life (Lee & Krochta, 2002; Oussalah, Caillet, Salmiéri, Saucier, & Lacroix, 2004; Pérez-Gago, Serra, & Del Rio, 2006). Edible films with encapsulated antimicrobial agents showed effective growth reduction of inoculated bacteria (Joerger, 2007) and could therefore be able to control growth of pathogenic bacteria in foods.

11.5 11.5.1

PACKAGING RELEVANT PROPERTIES Barrier Properties

The ability of packaging films and coatings to protect packaged goods from negative environmental factors can be determined by barrier property characterization. Water vapor, oxygen, or other gases as well as fluids or flavorings can have a significant impact on the product shelf life when coming into contact with the packaged goods. Depending on the product, high or low barrier properties for certain substances are required. Barrier performance of packaging materials can be determined by permeation measurements. Permeation describes the mass transport of fluids or gases through the packaging material. Permeation is based on the physical process of sorption and/or adsorption of the transported substances in/on the packaging material and the contents, and diffusion through interfaces between packaging and content, or packaging and atmosphere (Piringer, 1993) and can be mathematically described by Henry’s law and Fick’s law.

11.5 Packaging Relevant Properties

Barrier properties of whey protein films or coatings mainly depend on the coating composition, thickness, and the different layers used, such as the substrate. Various studies revealed relatively low oxygen permeability of whey protein-based films, making them potentially useful for coatings or other film material used for oxygen sensitive products (Bugnicourt, Schmid, McNerney, & Wild, 2010; Mate & Krochta, 1996; McHugh & Krochta, 1994; Schmid et al., 2012; Sothornvit & Krochta, 2000a, 2000b). However, poor humidity barriers due to the general intrinsic protein hydrophilicity limit their application. Additionally, induced interaction with water is followed by swelling (Barrer, 1941) and apparent thickness effects (McHugh, Avenabustillos, & Krochta, 1993) leading to a deviation from Henry’s law and Fick’s law (Avena-Bustillos & Krochta, 1993; Barrer, 1941; Schmid, Müller et al., 2014; Schmid, Sängerlaub et al., 2014).

11.5.1.1 Water Vapor Permeability Sensory quality and food shelf life are critically affected by water activity. Not only the extent of microbial growth and chemical and enzymatic reactions that can take place during storage depend on present water activity, but also other sensory factors such as textural appearance. Thus, water vapor permeability is an important property for packaging characteristics. Films and coatings derived from whey proteins display relatively high permeability for water vapor owing to the hydrophilic character of the proteins. Generally, denser film networks, depending on the degree of cross-linking or film modification, result in lower water permeability. Relative humidity of films and coatings as well as the chosen plasticizer loosen the network by generating free volume and reducing protein chain interactions and therefore significantly influence the moisture permeation properties (Khwaldia, Perez, Banon, Desobry, & Hardy, 2004; Pérez-Gago & Krochta, 2002). To reduce water vapor permeability, hydrophobic compounds can be included in the film formulation. This can take place either by the addition of lipids or waxes to the film-forming solution itself, or by lamination of the film with a lipid layer (Ramos et al., 2012). However, for products such as fresh fruit and vegetables, where respiration takes place during storage, water vapor condensation, being the main factor for microbial spoilage, can be prevented with nonmodified whey proteinbased packaging.

11.5.1.2 Oxygen Permeability Many decomposition reactions in food and other products are oxygenrelated. Fat rancidity, microbial growth, enzymatic browning, and vitamin loss are just some examples. Hence, many products require oxygen-protective packaging. For fresh fruit and vegetable products, however, oxygen and

419

420

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

carbon dioxide are essential for respiration during storage and demand moderate barrier packaging. Protein-based films usually display lower oxygen permeability than conventional synthetic polymers such as polyethylene under similar conditions (Pérez-Gago & Krochta, 2002). Compared to other proteins such as soy or wheat gluten, whey protein-based films and coatings present higher oxygen-barrier performance (McHugh & Krochta, 1994). Further enhancements can be realized by protein modifications and formulation-related optimizations such as plasticizer type and content, since the affected polymer free volume directly influences oxygen permeability (Ramos et al., 2012).

11.5.2

Mechanical Properties

Packaging materials have to withstand mechanical stress due to transportation, storage, and consumers themselves. This can lead to diminished containment protection. Since protection is the first and most important purpose of the packaging material, determination of its mechanical properties is indispensable. To prevent breaks or cracks, a material with high tensile properties, such as tensile strength and elongation at break, is demanded. Mechanical properties of whey protein-based films and coatings mainly depend on film-forming conditions, meaning the formulation itself and applied processes. Furthermore, film thicknesses as well as the testing conditions directly influence measured values. As for conventional packaging materials, tensile tests are mainly used to determine characteristic properties such as tensile strength, elongation, or Young’s modulus (Ramos et al., 2012). Tensile strength is defined as the first stress maximum during a tensile test, while maximum sample elongation is specified as the percentage of original length before breaking. The modulus gives information about the stiffness of a material and is measured at low strains in a linear part of the stress strain curve. Two main factors highly influence mechanical properties of whey proteinbased films. First of all, the three-dimensional protein network is formed during film processing. An intensive cross-linked network leads to stronger and stiffer materials, resulting in higher modulus and strength. However, this is often accompanied by lower elongations since the material also becomes less flexible. Using cross-linkers, such formaldehyde or glutaraldehyde, mechanical strength of glycerol-plasticized WPI films could be significantly enhanced (Galietta, Di Gioia, Guilbert, & Cuq, 1998; Ustunol & Mert, 2004). However, those agents are often not suitable for food packaging due to lacking food safety approval.

11.5 Packaging Relevant Properties

A second aspect is the plasticizers present, including water. Therefore, relative humidity of the film as well as storage conditions are factors that cannot be discounted and should be controlled. Relative humidity considerably affects mechanical properties by causing reduced modulus and strength and increased elongation values of the films and coatings when the water content is increased (Guilbert, Cuq, & Gontard, 1997; Ramos et al., 2012; Wu, Weller, Hamouz, Cuppett, & Schnepf, 2002). The same effect can be seen with other plasticizers. With increasing content, materials become less rigid resulting in lower tensile strength and hence more flexibility giving extended elongation values (Wihodo & Moraru, 2013). This effect can be related to fewer intermolecular interactions between the protein chains due to plasticizer incorporation. Not only the ratio, but also the type of plasticizer has a major impact on the mechanical performance. Using different plasticizers for β-Lg films, it was shown that glycerol gave the highest elongation increase and tensile strength decrease, followed by polyethylene glycol (200), sucrose and polyethylene glycol (400) (Sothornvit & Krochta, 2001). Films with other plasticizers like sorbitol and xylitol also showed significantly lower tensile strength in comparison to glycerol-plasticized WPI-based films (Shaw, Monahan, O’Riordan, & O’Sullivan, 2002). Schmid et al. (2013a) showed that hydrolyzed whey protein isolate (HWPI) can act as an internal plasticizer. Increasing the HWPI concentration in whey protein-based films at constant glycerol concentrations increased film flexibility significantly while barrier properties were maintained. Schmid (2013) considered these facts and developed formulations with increased barrier performance while maintaining film flexibility. Glycerol was partially replaced by HWPI as the internal plasticizer in WPI-based cast films resulting in improved oxygen and water vapor barrier properties, while the maintaining the mechanical properties. With this approach film flexibility was maintained, even though the external plasticizer concentration was decreased (Schmid et al., 2013a; Schmid, 2013).

11.5.3

Surface Properties

Especially for coatings but also for multilayer systems, cohesion and adhesion on surfaces are decisive. Cohesion of the polymer itself results from strong interactions and formation of bonds between the polymer chains, and the associated prevention of separation. This mainly depends on the polymeric structure itself including its geometry, molecular weight distribution, type and position of lateral functional groups, and general molecular strength (Guilbert, Gontard, & Gorris, 1996). Like most hydrophilic edible coatings, the adhesion of whey protein films is relatively low per se. Therefore, surfactants that reduce surface tension can be used to increase

421

422

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

compatibility between substrate and coating (Lin & Krochta, 2005; Ramos et al., 2012). Additionally, hydrophobicity of whey protein films was also increased by using microbial transglutaminase, indicating an exposure of hydrophobic groups due to moderate cross-linking (Tang & Jiang, 2007). Whey protein-based coatings adhere very well to polyesters such as PET, PBAT, or PLA but also to corona pretreated polyolefins such as PP or PE (Cinelli et al., 2014; Schmid et al., 2012; Schmid et al., 2015b; 2015c).

11.5.4

Optical Properties

Optical properties of a material such as transparency, color, and even ultraviolet- or light-barrier properties are important features, especially when packaging applications are concerned. Especially polymers based on natural raw materials, such as whey proteins, often contain organic colored molecules. Since human color perception is not a physical quantity, it is not measurable by normal engineering methods. However, colors can be described in an objective manner by quantifying them with three distinctive color values in a three-dimensional color space (Klein, 2010) such as the CIE L*a*b* system, Hunter Lab, CIE LCH, or CIE XYZ. Colorimeters using such systems can calculate color differences ΔE* between two color points in the color space. Compared to conventionally used synthetic polymers, whey protein-based films and coatings exhibit comparable transparency (Ramos et al., 2012; Schmid et al., 2012) and slight color differences due to contained yellow metabolites (Schmid et al., 2013a). However, the final properties are also a function of film thickness.

11.6 POTENTIAL APPLICATIONS OF WHEY PROTEINBASED FILMS AND COATINGS IN FOOD PACKAGING 11.6.1

Packaging Requirements of Food Products

For maintaining quality and shelf life of products, especially in the food industry, packaging materials have to meet high standards. The primary responsibility of food packaging is to protect the packaged food from environmental impact along its whole product life cycle. Therefore, packaging does not only have to withstand mechanical stress during transport and consumer handling but also environmental effects such as product contamination and factors leading to deteriorative reactions. Besides sufficient mechanical stability, materials have to display specific barrier properties with respect to light, moisture, and gases such as water vapor or oxygen, depending on the type of packaged food. Packaging atmosphere including reasonable oxygen and carbon dioxide levels as well as respiration rates have to be considered for optimal food preservation and prevention of fat oxidation,

11.6 Potential Applications of Whey Protein-Based Films and Coatings in Food Packaging

microbial deterioration, and deviation of taste or color (Petersen et al., 1999; Schmid et al., 2012). Fig. 11.1 shows gas barrier requirements of different pharmaceutical and food products. Generally, dry products have a long shelf life due to their low water activity. Therefore, dry conditions can be maintained with good moisture barrier packaging. Oxygen-sensitive products such as fat- and oil-containing foods require light- and oxygen-excluding packaging to prevent oxidative rancidity. Fresh fruits and vegetables continue respiration during storage demanding varying packaging systems to control atmosphere. Fresh meat products require either high or low oxygen levels to assure red color, which can be obtained with modified atmosphere packaging (MAP) or vacuum packaging, both demanding high oxygen-barrier materials. Gas compositions vary depending on the type of meat. Seafood, egg, and dairy products also require high oxygen-barrier materials to prevent harmful oxidation processes and microbial growth (Petersen et al., 1999).

Ketchup, sauces Nuts, snacks

Tablets (blister)

100

(at 23°C, 50% r.h.)

Oxygen permeability / (cm3d–1m–2bar–1)

1000

Cooking oil

10

Vakuum-Kaffee Instant coffee Meat/MAP

1

UHT milk Vacuum coffee 0.1

Special, infusion, baby food

Beer

0.01 0.01

0.1

1

10

100

Water vapor transmission rate / (gd–1m–2) at 23°C, 85% r.h.

FIGURE 11.1 Gas barrier requirements of selected pharmaceutical and food products. Adapted from Schmid, M., Dallmann, K., Bugnicourt, E., Cordoni, D., Wild, F., Lazzeri, A., & Noller, K. (2012). Properties of whey protein coated films and laminates as novel recyclable food packaging materials with excellent barrier properties. International Journal of Polymer Science, 2012, Article 562381.

423

424

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

11.6.2 Merging Food Packaging Requirements and Properties of Whey Protein-Based Films and Coatings To match those requirements, a combination of different materials is necessary to optimize techno-functional properties, usually performed by multilayer films. However, multilayers obtained from coextrusion or lamination processes are often expensive and use fossil-fuel-based raw materials. Especially regarding oxygen-sensitive products, ethylene vinyl alcohol copolymers (EVOH) are widely used. Furthermore, the combination of many different layers complicates recycling processes that mainly rely on monomaterials of high purity (Endres & Siebert-Raths, 2011). Thus, sustainable packaging materials suitable to replace present conventionally used plastics are part and parcel of today’s research and development. Among others, whey proteinbased films and coatings displaying excellent oxygen barriers can provide a bio-based and biodegradable solution for such composite films (Schmid et al., 2012). One example to name is the project WHEYLAYER (http://www. wheylayer.eu/) which developed whey protein-coated films and laminates with high oxygen-barrier performance and enhanced recyclability due to the biodegradable whey protein-based layer (Schmid, Bischur, Wild, & Noller, 2009). Fig. 11.2 shows the barrier performance of commonly used plastic materials for packaging as well as the WPI-based Wheylayers material. Bugnicourt et al. (2013) presented a study which demonstrated the scalability of the production of a whey protein-based coating. This was a preliminary requirement for its commercialization. The new bio-based coating solution was formulated using WPI and plasticizers to prevent brittleness. The most promising formulations among those evaluated by Schmid et al. (2012) at pilot scale were selected for scale-up. The coating was performed on PET at semi-industrial rates with optimized processing parameters for minimized energy consumption in order to reduce the environmental impact of this manufacturing stage as much as possible. This patented process also allows the correct denaturation of the protein-based coating on PET substrate films for improved barrier properties (Schmid et al., 2013b). After lamination with PE as a sealing and potential food contact layer, a full laminate structure suitable for packing sensitive food products was ready to be characterized. Whey protein-coated films and laminates achieved much better barrier properties compared to other bioplastics. The results also indicate that the oxygen barrier values of whey protein-based coatings approached those of EVOH with high ethylene content and were higher than the oxygen barrier properties of polyamide (PA). The produced laminates were used for food packaging and storage validation with very positive results. Several studies (Bugnicourt et al., 2013; Schmid et al., 2012) have proved the suitability of the whey protein-based coatings for packaging applications and show the potential of substituting other synthetic barrier layers used for food

11.7 Food Safety and Regulatory Aspect

10000

PS

1000

PE-HD

PP

PVC-P

PC

BOPP COC

(at 23°C, 50% r.h.)

3 –1 –2 –1 Oxygen permeability/ (cm d m bar )

PE-LD

PLA

100 PVC-U PET

10

PA 6

PAN PEN

1

Wheylayer PVDC

EVOH, 38%

0.1

Cellulose

EVOH, 44% EVOH, 32% EVOH, 27%

(LCP)

0.01 0.01

0.1

1

10 –1 –2 Water vapor transmission rate / (gd m )

100

1000

at 23°C, 85% r.h.

FIGURE 11.2 Barrier performance of selected packaging materials as well as the Wheylayers material. Adapted from Schmid, M., Dallmann, K., Bugnicourt, E., Cordoni, D., Wild, F., Lazzeri, A., & Noller, K. (2012). Properties of whey protein coated films and laminates as novel recyclable food packaging materials with excellent barrier properties. International Journal of Polymer Science, 2012, Article 562381.

packaging. In terms of validation of the obtained material for food packaging, laminates derived from whey protein-coated films were tested for storing different sensitive food products. The storage tests showed similar results regarding shelf life and sensory quality when compared to conventional reference packaging (Bugnicourt et al., 2013). Additional tests also showed those laminates fulfilled food contact compliance regulations according to the EU regulation 10/2011 in terms of global migration.

11.7

FOOD SAFETY AND REGULATORY ASPECT

Using whey protein films and coatings, applications include direct, edible coatings, coatings on substrates, and stand-alone films. Therefore, they match different categories such as food contact materials, ingredients, food additives, or even food products, resulting in different regulation aspects

425

426

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

(Debeaufort, Quezada-Gallo, & Voilley, 1998). Therefore, different legislation must be applied depending on the respective case. When used as edible coating, the components of the coating formulation are usually food grade and are regulated as additives by legislation such as Regulation (EC) No. 1333/2008. In terms of other regulatory requirements, it must be considered that milk proteins used for films and coatings are possible food allergens. Therefore, especially regarding edible coatings, allergens must be clearly labeled (Gennadios, Hanna, & Kurth, 1997). In the case of coatings on other substrates or when used for flexible films, protein-based packaging is regarded as food-contact material. Within the European Union (EU) those materials are primarily covered by Framework Regulation 1935/2004 (EC). Following good manufacturing practice, components that could endanger human health, cause intolerable food composition changes or cause a decline in organoleptic properties should not exceed respective quantities when transferred to the food. Regarding plastic packaging, materials and additives approved for food contact and eventual migration limits are given for the EU in EC 10/2011. However, many substances used for protein-based films and coatings are not listed. (Coltelli et al., 2016) In summary, respective regulatory aspects always depend on formulations and application and need to be considered separately.

11.8 END-OF-LIFE OPTIONS FOR WHEY PROTEINBASED FILMS AND MULTILAYER LAMINATES According to Badia, Gil-Castell, and Ribes-Greus (2017) long-term properties and end-of-life of polymers are not antagonist issues. They are linked by durability and degradation. The combination of appropriate valorization techniques, i.e., material, energetic, and/or biological at the most suitable stage, should be targeted. The consideration of the end-of-life of a material for a specific application should be the fundamental focus (Badia et al., 2017). This chapter covers two key end-of-life options for whey protein-based films and multilayer laminates, namely biodegradation and recycling.

11.8.1 Biodegradation of Whey Protein-Based Films and Laminates This section deals with the valorization of whey protein-based coatings on biodegradable plastics from the point of view of the reincorporation of polymers into the carbon cycle under biotic conditions. According to Badia et al. (2017), the design of plastic materials for consumer applications such as

11.8 End-of-Life Options for Whey Protein-Based Films and Multilayer Laminates

packaging is moving towards the design of polymers with controlled degradability and enhanced bioreintegration. Biodegradation of plastics can occur through different stages, which erode and disintegrate the polymeric segments by depolymerization followed by reintegration into the carbon cycle by assimilation and mineralization in the media. Three main stages can be distinguished: 1. Deterioration 2. Fragmentation 3. Assimilation These stages are described in detail by Badia et al. (2017). Biodegradation and environmental degradation aspects of proteins in general were summarized by Coltelli et al. (2016) and Cinelli et al. (2014) specifically studied how a whey protein-based coating can improve the oxygen-barrier properties of commercial compostable plastic film while not hindering the biodegradation nor harming the quality of the compost. They applied a whey protein-based coating on to a biodegradable commercial film which was certified to meet the requirements of the composting standard DIN 13432. They found that the oxygen barrier was significantly improved by the WPI-based coating. This is of specific interest since biodegradable packaging films generally do not retain their barrier properties and the application of nondegradable materials to improve gas barriers compromises the composting of the final packaging concepts. They also assessed the biodegradability of the whey protein layer itself as natural polymers may become nonbiodegradable when cross-linked or bonded with nondegradable additives. They found that the material based on denatured WPI and plasticizer presented very fast biodegradability, also in combination with a commercial film. It can be concluded that these results show the huge potential of whey protein-based films and laminates as part of new ecological food packaging concepts (Cinelli et al., 2014). Li and Chen (2000) investigated the biodegradability of WPI and whey protein concentrate (WPC) films from glycerol-plasticized aqueous solutions. They used Pseudomonas aeruginosa under commercial composting conditions. WPC-based films degraded faster than those obtained from WPI, which might show dependence on film composition and extent of cross-linking. Under commercial composting conditions, WPI-based films lost more than 80% of total solids within 7 days (Coltelli et al., 2016; Li & Chen, 2000).

11.8.2 Recyclability of Whey Protein-Based Multilayer Laminates Polymer-based multilayer packaging materials are commonly used in order to combine the respective properties of different polymers. With this

427

428

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

approach, tailored functionality of packaging concepts is created to sufficiently protect sensitive food products and to obtain extended shelf life. Due to their poor recyclability, most multilayers are incinerated or landfilled, counteracting efforts towards a circular economy. In the waste hierarchy, mechanical recycling is number three, after waste avoidance and reuse but should be favored to energy recovery and disposal. According to Lazarevic, Aoustin, Buclet, and Brandt (2010) recycling generally is the environmentally preferred treatment option when compared to municipal solid waste incineration. Even though multilayer plastic films provide a range of properties, their recyclability is still an open issue and should be improved. As already reported in detail in this chapter, the possibility exists of using whey protein-based coatings as an excellent barrier against oxygen. Whey protein-based coating is capable of replacing petrochemical nonrecyclable materials such as EVOH in multilayer laminates. Cinelli, Schmid, Bugnicourt, Coltelli, and Lazzeri (2016) followed an innovative approach in their study in order to achieve recyclability of the substrate films in multilayer laminates by separating them. They used a simple process compatible with industrial procedures, in order to promote recycling processes leading to high-value products. This could potentially beneficially impact the packaging and food industries. They prepared polyethylene terephthalate (PET)/polyethylene (PE) multilayer laminate films based on PET coated with a whey protein layer. This structure was then laminated with PE. As whey proteins constituting the coatings can be degraded by enzymes, the coatings can be washed off the plastic substrate layer. The enzyme types, dosage, time, and temperature optima used, are compatible with procedures in industrial waste recycling. The separation of the respective films of the samples based on PET/whey and PET/whey/PE were efficient when performed with an enzymatic detergent containing proteases. Different types of enzymatic detergents presented positive results in removing the whey protein-based layer from the PET substrate and from the PET/whey/PE multilayer films. These results showed the possibility of separating the whey-based multilayer film by washing with different available commercial enzymatic detergents. This allows a better recycling of the two different polymers. Mechanical properties of the plastic substrates (e.g., stress at yield, stress, and elongation at break), did not significantly change by the washing and separation process with enzymatic detergents (Cinelli et al., 2016). This enables an efficient multilayer recycling process in the near future. Certainly, it will take time until enough whey protein-based plastic laminate materials are available in the market at quantities sufficiently high to be worthwhile to sort and separate from waste streams. However, if protein- and in particular whey protein-based, multilayer laminates are available they potentially could be recycled according to the process mentioned above.

11.9 Conclusions and Industrial Perspectives

11.9

CONCLUSIONS AND INDUSTRIAL PERSPECTIVES

In contrast to standard plastics derived from fossil-fuel sources, bio-based polymers are from renewable biomass sources. The use of this class of materials is not new. In fact, e.g., back in the 1940s, Ford introduced a motor car body fully made of cellulose fibre and resin extended with by-products of the soybean oil extraction process. However, later the interest in this class of materials was reduced with the development of more durable and resistant fossil-fuel based plastics. Now, research has been intensified in the field of biopolymers revealing a large range of possible resources and extending the spectra of applications. Thanks to the improvement in their properties, biopolymers are more competitive with their synthetic counterparts. The sources for naturally occurring biopolymers range from proteins (animal- or plant-sourced) to lipids and polysaccharides (e.g., starch- and cellulose-based biopolymers). Some biobased polyesters such as polyhydroxyalkanoates (PHAs) are naturally produced in microbial cultures. Other biopolymers, such as polylactic acid (PLA), are produced from bio-derived monomers (e.g., lactic acid from corn starch) which are then polymerized. The same principle applies to the newly commercially available bio-based polyethylene terephthalate (Bio-PET) or polyethylene (Bio-PE). The so-called “drop-in solutions” are now the biggest segment in terms of production volume among biopolymers. Packaging is the biggest market for the plastics industry with a share of approximately 40% of all plastic produced in Europe. New developments in that sector are driven by legislation and market requirements such as weight reduction, recyclability, waste reduction, and utilization of sustainable/ renewable raw materials. When providing similar properties as their synthetic counterparts, biopolymers could provide a solution for conserving depleting fossil-fuel resources, and reducing CO2 emissions and environmental pollution in the case of biodegradability as an end-of-life option. However, research into the development of tailor-made solutions for this sector is a key because most biopolymers do not meet the requirements of packaging for sensitive food products due to their low barrier properties and thus do not guarantee product quality throughout its shelf life. Requirements in terms of barriers against light and gases are specific to the type of food packed. To obtain the optimal combination of final properties, multilayer laminates are commonly used. However, due to the impossibility of separating the materials during recycling operations, the recyclability of multilayer packaging is often economically not possible. As described in this chapter, the development of a biopolymer-based coating for plastic films based on whey protein is able to replace current synthetic

429

430

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

oxygen-barrier layers used in food packaging such as EVOH. These developments are not far from reaching the market. Since most whey is a by-product of cheese manufacturing, it is not in direct food competition. In summary, it seems to be obvious that, especially in the field of packaging, the ecological advantages of biopolymers, such as whey protein-based films and coatings, are significant over traditional plastics. Bioplastics are produced from an increasing range of renewable resources including wastes and/or by-products which are not in competition with food. The research on bioplastics is very dynamic and there is still a lot of progress to be made. For example, the whey protein-based films and coatings described in this chapter exhibited excellent optical and barrier properties outperforming existing biopolymers. Whey protein-based films and coatings seem to be very promising as a replacement for synthetic barrier polymers used in food packaging concepts. Whey protein-based multilayer laminates were positively validated for storing various food products. This novel whey protein-based coating can be removed to allow multilayer films to become recyclable. Whey protein-based packaging concepts could provide a valuable contribution to sustainability due to the possibility of recycling materials as opposed to incinerating, as is done for laminates containing EVOH or PA, but also due to the utilization of bio-based raw materials which are a by-product from the agro-food industry.

References Anker, M., Stading, M., & Hermansson, A.-M. (1999). Effects of pH and the gel state on the mechanical properties, moisture contents, and glass transition temperatures of whey protein films. Journal of Agricultural & Food Chemistry, 47, 1878 1886. Avena-Bustillos, R. J., & Krochta, J. M. (1993). Water vapor permeability of caseinate-based edible films as affected by ph, calcium crosslinking and lipid content. Journal of Food Science, 58, 904 907. Badia, J. D., Gil-Castell, O., & Ribes-Greus, A. (2017). Long-term properties and end-of-life of polymers from renewable resources. Polymer Degradation and Stability, 137, 35 57. Barone, J. R., & Schmidt, W. F. (2006). Nonfood applications of proteinaceous renewable materials. Journal of Chemical Education, 83, 1003. Barone, J. R., Dangaran, K., & Schmidt, W. F. (2006). Blends of cysteine-containing proteins. Journal of Agricultural & Food Chemistry, 54, 5393 5399. Barrer, R. M. (1941). Diffusion in and through solids. Cambridge: Cambridge University Press. Belitz, H. D., Grosch, W., & Schieberle, P. (2007). Lehrbuch der Lebensmittelchemie. Berlin: Springer. Belitz, H. D., Grosch, W., & Schieberle, P. (2008). Lehrbuch der Lebensmittelchemie. Berlin: Springer. Bouaouina, H., Desrumaux, A., Loisel, C., & Legrand, J. (2006). Functional properties of whey proteins as affected by dynamic high-pressure treatment. International Dairy Journal, 16, 275 284.

References

Bräuer, S., Meister, F., Gottlöber, R. P., & Nechwatal, A. (2007). Preparation and thermoplastic processing of modified plant proteins. Macromolecular Materials & Engineering, 292, 176 183. Brault, D., D’Aprano, G., & Lacroix, M. (1997). Formation of free-standing sterilized edible films from irradiated caseinates. Journal of Agricultural & Food Chemistry, 45, 2964 2969. Braun, D. (2003). Kunststofftechnik für Einsteiger. München: Hanser. Brennan, J. G. (Ed.), (2006). Food processing handbook. Weinheim: Wiley-VCH. Bugnicourt, E., Schmid, M., McNerney, O., & Wild, F. (2010). WHEYLAYER: The barrier coating of the future. Coating International, 43, 7 10. Bugnicourt, E., Schmid, M., Nerney, O.M., Wildner, J., Smykala, L., Lazzeri, A., & Cinelli, P. (2013). Processing and validation of whey-protein-coated films and laminates at semiindustrial scale as novel recyclable food packaging materials with excellent barrier properties. Advances in Materials Science & Engineering, 2013, Article 496207. Calvo, M. M., Leaver, J., & Banks, J. M. (1993). Influence of other whey proteins on the heatinduced aggregation of α-lactalbumin. International Dairy Journal, 3, 719 727. Cheftel, J., Cuq, J., & Lorient, D. (1985). Amino acids, peptides, and proteins. Food Chemistry, 2, 245 369. Cinelli, P., Schmid, M., Bugnicourt, E., Wildner, J., Bazzichi, A., Anguillesi, I., & Lazzeri, A. (2014). Whey protein layer applied on biodegradable packaging film to improve barrier properties while maintaining biodegradability. Polymer Degradation & Stability, 108, 151 157. Cinelli, P., Schmid, M., Bugnicourt, E., Coltelli, M. B., & Lazzeri, A. (2016). Recyclability of PET/ WPI/PE multilayer films by removal of whey protein isolate-based coatings with enzymatic detergents. Materials, 9, 473. Coltelli, M.-B., Wild, F., Bugnicourt, E., Cinelli, P., Lindner, M., Schmid, M., . . . Lazzeri, A. (2016). State of the art in the development and properties of protein-based films and coatings and their applicability to cellulose based products: An extensive review. Coatings, 6, 1 59. Condés, M. C., Añón, M. C., & Mauri, A. N. (2015). Amaranth protein films prepared with highpressure treated proteins. Journal of Food Engineering, 166, 38 44. Creuzenet, C., Touati, A., Dufour, E., Chobert, J. M., Haertle, T., & Choiset, Y. (1992). Acylation and alkylation of bovine. beta-lactoglobulin in organic solvents. Journal of Agricultural & Food Chemistry, 40, 184 190. De Wit, J.N. (2001a). Lecturer’s handbook on whey and whey products. Brüssel: European Whey Products Association. De Wit, J.N. (2001b). Lehrbuch der Molke und Molkenerzeugnisse. Brüssel: European Whey Products Association. DeJong, G., & Koppelman, S. (2002). Transglutaminase catalyzed reactions: Impact on food applications. Journal of Food Science, 67, 2798 2806. Debeaufort, F., Quezada-Gallo, J. A., & Voilley, A. (1998). Edible films and coatings: Tomorrow’s packagings: A review. Critical Reviews in Food Science & Nutrition, 38, 299 313. Dewit, J. N., & Klarenbeek, G. (1984). Effects of various heat-treatments on structure and solubility of whey proteins. Journal of Dairy Science, 67, 2701 2710. Domininghaus, H., Eyerer, P., Elsner, P., & Hirth, T. (2008). Kunststoffe: Eigenschaften und Anwendungen ; mit 240 Tabellen. New York: Springer-Verlag GmbH. Dumay, E. M., Kalichevsky, M. T., & Cheftel, J. C. (1994). High-pressure unfolding and aggregation of. beta-lactoglobulin and the baroprotective effects of sucrose. Journal of Agricultural & Food Chemistry, 42, 1861 1868.

431

432

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

Dumay, E. M., Kalichevsky, M. T., & Cheftel, J. C. (1998). Characteristics of pressure-induced gels of β-lactoglobulin at various times after pressure release. LWT - Food Science & Technology, 31, 10 19. Eissa, A. S., Puhl, C., Kadla, J. F., & Khan, S. A. (2006). Enzymatic cross-linking of β-lactoglobulin: Conformational properties using FTIR spectroscopy. Biomacromolecules, 7, 1707 1713. Embuscado, M. E., & Huber, K. C. (2009). Edible films and coatings for food applications. New York: Springer. Endres, H.-J., & Siebert-Raths, A. (2011). Engineering biopolymers - Markets, manufacturing, properties and applications. Munich: Hanser-Verlag. Etzel, M. R. (2004). Manufacture and use of dairy protein fractions. The Journal of Nutrition, 134, 996S 1002S. Famelart, M.-H., Chapron, L., Piot, M., Brulé, G., & Durier, C. (1998). High pressure-induced gel formation of milk and whey concentrates. Journal of Food Engineering, 36, 149 164. Fetzer, R.W., & Ramachandran, K.S. (1979). Protein film process: US patent publication number: US4133901 A. Galietta, G., Di Gioia, L., Guilbert, S., & Cuq, B. (1998). Mechanical and thermomechanical properties of films based on whey proteins as affected by plasticizer and crosslinking agents. Journal of Dairy Science, 81, 3123 3130. Gennadios, A. (Ed.), (2002). Protein-based films and coatings. Boca Raton: CRC Press. Gennadios, A., Weller, C. L., & Testin, R. F. (1993). Modification of physical and barrier properties of edible wheat gluten-based films. Cereal Chemistry, 70, 426 429. Gennadios, A., Hanna, M. A., & Kurth, L. B. (1997). Application of edible coatings on meats, poultry and seafoods: A review. Food Science & Technology-Lebensmittel-Wissenschaft & Technologie, 30, 337 350. Gennadios, A., Rhim, J.-W., Handa, A., Weller, C. L., & Hanna, M. A. (1998). Ultraviolet radiation affects physical and molecular properties of soy protein films. Journal of Food Science, 63, 225 228. Ghorpade, V. M., Li, H., Gennadios, A., & Hanna, M. A. (1995). Chemically modified soy protein films. Transactions of the ASAE, 38, 1805 1808. Giles, H. F., Wagner, J. R., & Mount, E. M. (2013). Extrusion: The definitive processing guide and handbook. Waltham, MA: Elsevier Science. Guilbert, S., Gontard, N., & Gorris, L. G. (1996). Prolongation of the shelf-life of perishable food products using biodegradable films and coatings. LWT-Food Science & Technology, 29, 10 17. Guilbert, S., Cuq, B., & Gontard, N. (1997). Recent innovations in edible and/or biodegradable packaging materials. Food Additives & Contaminants, 14, 741 751. Gülseren, I., Güzey, D., Bruce, B. D., & Weiss, J. (2007). Structural and functional changes in ultrasonicated bovine serum albumin solutions. Ultrasonics Sonochemistry, 14, 173 183. Gupta, P., & Nayak, K. K. (2015). Characteristics of protein-based biopolymer and its application. Polymer Engineering & Science, 55, 485 498. Hammann, F., & Schmid, M. (2014). Determination and quantification of molecular interactions in protein films: A review. Materials, 7, 7975 7996. Hassannia-Kolaee, M., Khodaiyan, F., Pourahmad, R., & Shahabi-Ghahfarrokhi, I. (2016). Development of ecofriendly bionanocomposite: Whey protein isolate/pullulan films with nano-SiO2. International Journal of Biological Macromolecules, 86, 139 144. Hernandez-Izquierdo, V. M., & Krochta, J. M. (2008). Thermoplastic processing of proteins for film formation - a review. Journal of Food Science, 73, R30 R39.

References

Hernandez-Izquierdo, V. M., & Krochta, J. M. (2009). Thermal transitions and heat-sealing of glycerol-plasticized whey protein films. Packaging Technology & Science, 22, 255 260. Hernandez-Izquierdo, V. M., Reid, D. S., McHugh, T. H., Berrios, J. D. J., & Krochta, J. M. (2008). Thermal transitions and extrusion of glycerol-plasticized whey protein mixtures. Journal of Food Science, 73, E169 E175. Ichinose, A., Bottenus, R. E., & Davie, E. W. (1990). Structure of transglutaminases. Journal of Biological Chemistry, 265, 13411 13414. Janjarasskul, T., Rauch, D. J., McCarthy, K. L., & Krochta, J. M. (2014). Barrier and tensile properties of whey protein candelilla wax film/sheet. LWT-Food Science & Technology, 56, 377 382. Joerger, R. D. (2007). Antimicrobial films for food applications: A quantitative analysis of their effectiveness. Packaging Technology & Science, 20, 231 273. Jovanovi´c, S., Miroljub, B., & Ognjen, M. (2005). Whey proteins-properties and possibility of application. Mljekarstvo, 55, 215 233. Kaiser, W. (2007). Kunststoffchemie für ingenieure. München: Hanser Fachbuchverlag. Kanno, C., Mu, T.-H., Hagiwara, T., Ametani, M., & Azuma, N. (1998). Gel formation from industrial milk whey proteins under hydrostatic pressure: Effect of hydrostatic pressure and protein concentration. Journal of Agricultural & Food Chemistry, 46, 417 424. Khwaldia, K., Perez, C., Banon, S., Desobry, S., & Hardy, J. (2004). Milk proteins for edible films and coatings. Critical Reviews in Food Science & Nutrition, 44, 239 251. Klein, G. A. (2010). Industrial color physics. New York: Springer. Koshy, R. R., Mary, S. K., Thomas, S., & Pothan, L. A. (2015). Environment friendly green composites based on soy protein isolate - A review. Food Hydrocolloids, 50, 174 192. Lazarevic, D., Aoustin, E., Buclet, N., & Brandt, N. (2010). Plastic waste management in the context of a European recycling society: Comparing results and uncertainties in a life cycle perspective. Resources, Conservation & Recycling, 55, 246 259. Lee, S.-Y., & Krochta, J. (2002). Accelerated shelf life testing of whey-protein-coated peanuts analyzed by static headspace gas chromatography. Journal of Agricultural & Food Chemistry, 50, 2022 2028. Lent, L. E., Vanasupa, L. S., & Tong, P. S. (1998). Whey protein edible film structures determined by atomic force microscope. Journal of Food Science, 63, 824 827. Li, J., & Chen, H. (2000). Biodegradation of whey protein-based edible films. Journal of Polymers & the Environment, 8, 135 143. Lin, S.-Y. D., & Krochta, J. M. (2005). Whey protein coating efficiency on surfactant-modified hydrophobic surfaces. Journal of Agricultural & Food Chemistry, 53, 5018 5023. Mahmoud, R., & Savello, P. A. (1993). Solubility and hydrolyzability of films produced by transglutaminase catalytic cross-linking of whey protein. Journal of Dairy Science, 76, 29 35. Mate, J. I., & Krochta, J. M. (1996). Comparison of oxygen and water vapor permeabilities of whey protein isolate and beta-lactoglobulin edible films. Journal of Agricultural & Food Chemistry, 44, 3001 3004. McHugh, T. H., & Krochta, J. M. (1994). Sorbitol- vs glycerol-plasticized whey protein edible films: Integrated oxygen permeability and tensile property evaluation. Journal of Agricultural & Food Chemistry, 42, 841 845. McHugh, T. H., Avenabustillos, R., & Krochta, J. M. (1993). Hydrophilic edible films - modified procedure for water vapor permeability and explanantion of thickness effects. Journal of Food Science, 58, 899 903. Mihindukulasuriya, S. D. F., & Lim, L. T. (2014). Nanotechnology development in food packaging: A review. Trends in Food Science & Technology, 40, 149 167.

433

434

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

Molinaro, S., Cruz-Romero, M., Sensidoni, A., Morris, M., Lagazio, C., & Kerry, J. P. (2015). Combination of high-pressure treatment, mild heating and holding time effects as a means of improving the barrier properties of gelatin-based packaging films using response surface modeling. Innovative Food Science & Emerging Technologies, 30, 15 23. Morr, C., & Ha, E. (1993). Whey protein concentrates and isolates: Processing and functional properties. Critical Reviews in Food Science & Nutrition, 33, 431 476. Müller, K., Bugnicourt, E., Latorre, M., Jorda, M., Echegoyen Sanz, Y., Lagaron, J., . . . Schmid, M. (2017). Review on the processing and properties of polymer nanocomposites and nanocoatings and their applications in the packaging, automotive and solar energy fields. Nanomaterials, 7, Article74. Nicolai, T., Britten, M., & Schmitt, C. (2011). β-Lactoglobulin and WPI aggregates: Formation, structure and applications. Food Hydrocolloids, 25, 1945 1962. Nicorescu, I., Loisel, C., Vial, C., Riaublanc, A., Djelveh, G., Cuvelier, G., et al. (2008). Combined effect of dynamic heat treatment and ionic strength on the properties of whey protein foams Part II. Food Research International, 41, 980 988. Nielsen, P. M., Petersen, D., & Dambmann, C. (2001). Improved method for determining food protein degree of hydrolysis. Journal of Food Science, 66, 642 646. Olsen, K., Ipsen, R., Otte, J., & Skibsted, L. H. (1999). Effect of high pressure on aggregation and thermal gelation of β-lactoglobulin. Milchwissenschaft, 54, 543 546. Onwulata, C., & Huth, P. (2008). Whey processing, functionality and health benefits. Ames, IA: John Wiley & Sons. Oussalah, M., Caillet, S., Salmiéri, S., Saucier, L., & Lacroix, M. (2004). Antimicrobial and antioxidant effects of milk protein-based film containing essential oils for the preservation of whole beef muscle. Journal of Agricultural & Food Chemistry, 52, 5598 5605. Oymaci, P., & Altinkaya, S. A. (2016). Improvement of barrier and mechanical properties of whey protein isolate based food packaging films by incorporation of zein nanoparticles as a novel bionanocomposite. Food Hydrocolloids, 54, 1 9. Parris, N., Purcell, J. M., & Ptashkin, S. M. (1991). Thermal denaturation of whey proteins in skim milk. Journal of Agricultural & Food Chemistry, 39, 2167 2170. Pérez-Gago, M., Serra, M., & Del Rio, M. (2006). Color change of fresh-cut apples coated with whey protein concentrate-based edible coatings. Postharvest Biology & Technology, 39, 84 92. Pérez-Gago, M. B., & Krochta, J. M. (1999). Water Vapor Permeability of Whey Protein Emulsion Films as Affected by pH. Journal of Food Science, 64, 695 698. Pérez-Gago, M. B., & Krochta, J. M. (2001a). Denaturation time and temperature effects on solubility, tensile properties, and oxygen permeability of whey protein edible films. Journal of Food Science, 66, 705 710. Pérez-Gago, M. B., & Krochta, J. M. (2001b). Lipid particle size effect on water vapor permeability and mechanical properties of whey protein/beeswax emulsion films. Journal of Agricultural & Food Chemistry, 49, 996 1002. Pérez-Gago, M. B., & Krochta, J. M. (2002). Formation and properties of whey protein films and coatings. In A. Gennadios (Ed.), Protein-based films and coatings (pp. 159 180). Boca Raton: CRC Press. Petersen, K., Nielsen, P. V., Bertelsen, G., Lawther, M., Olsen, M. B., Nilsson, N. H., & Mortensen, G. (1999). Potential of biobased materials for food packaging. Trends in Food Science & Technology, 10, 52 68. Phillips, L. G., German, J. B., O'Neill, T. E., Foegeding, E. A., Harwalkar, V. R., Kilara, A., et al. (1990). standardized procedure for measuring foaming properties of three proteins, a collaborative study. Journal of Food Science, 55, 1441 1444.

References

Piringer, O. G. (1993). Verpackungen für Lebensmittel: Eignung, Wechselwirkungen, Sicherheit. Weinheim: Wiley-VCH. Plackett, D. (2011). Biopolymers: New materials for sustainable films and coatings. Chichester: John Wiley and Sons. Ramos, Ó. L., Fernandes, J. C., Silva, S. I., Pintado, M. E., & Malcata, F. X. (2012). Edible films and coatings from whey proteins: A review on formulation, and on mechanical and bioactive properties. Critical Reviews in Food Science & Nutrition, 52, 533 552. Rastogi, V., & Samyn, P. (2015). Bio-based coatings for paper applications. Coatings, 5, 887. Renkema, J. M., Lakemond, C. M., Jongh, H. H. d, Gruppen, H., & van Vliet, T. (2000). The effect of pH on heat denaturation and gel forming properties of soy proteins. Journal of Biotechnology, 79, 223 230. Rhim, J.-W., Gennadios, A., Fu, D., Weller, C. L., & Hanna, M. A. (1999). Properties of Ultraviolet Irradiated Protein Films. LWT - Food Science & Technology, 32, 129 133. Riezs, P., & Kondo, T. (1992). Free radical formation induced by ultrasound and its biological implications. Free Radical Biology & Medicine, 13, 247 270. Sabato, S. F., Ouattara, B., Yu, H., D’Aprano, G., Le Tien, C., Mateescu, M. A., et al. (2001). Mechanical and barrier properties of cross-linked soy and whey protein based films. Journal of Agricultural & Food Chemistry, 49, 1397 1403. Schmid, M. (2013). Properties of cast films made from different ratios of whey protein isolate, hydrolysed whey protein isolate and glycerol. Materials, 6, 3254 3269. Schmid, M., Bischur, G., Wild, F., & Noller, K. (2009). Whey coated plastic films to replace expensive polymers and increase recyclability. Paper presented at the 12th Tappi European Place Conference, 18-20 May 2009, Budapest, Hungary, Budapest. Schmid, M., Dallmann, K., Bugnicourt, E., Cordoni, D., Wild, F., Lazzeri, A., & Noller, K. (2012). Properties of whey protein coated films and laminates as novel recyclable food packaging materials with excellent barrier properties. International Journal of Polymer Science, 2012, Article 562381. Schmid, M., Hinz, L.-V., Wild, F., & Noller, K. (2013a). Effects of hydrolysed whey proteins on the techno-functional characteristics of whey protein-based films. Materials, 6, 927. Schmid, M., Noller, K., Wild, F., & Bugnicourt, E. (2013b). Whey protein coated films: WO/ 2013/014493. Schmid, M., Müller, K., Sängerlaub, S., Stäbler, A., Starck, V., Ecker, F., & Noller, K. (2014). Mechanical and barrier properties of thermoplastic whey protein isolate/ethylene vinyl acetate blends. Journal of Applied Polymer Science, 131, Article41172. Schmid, M., Sängerlaub, S., Wege, L., & Stäbler, A. (2014). Properties of transglutaminase crosslinked whey protein isolate coatings and cast films. Packaging Technology & Science, 27, 799 817. Schmid, M., Held, J., Hammann, F., Schlemmer, D., & Noller, K. (2015a). Effect of UV-radiation on the packaging-related properties of whey protein isolate based films and coatings. Packaging Technology & Science, 28, 883 899. Schmid, M., Reichert, K., Hammann, F., & Stäbler, A. (2015b). Storage time-dependent alteration of molecular interaction property relationships of whey protein isolate-based films and coatings. Journal of Materials Science, 50, 4396 4404. Schmid, M., Zillinger, W., Müller, K., & Sängerlaub, S. (2015c). Permeation of water vapour, nitrogen, oxygen and carbon dioxide through whey protein isolate based films and coatings 2 Permselectivity and activation energy. Food Packaging & Shelf Life, 6, 21 29.

435

436

CHAPTER 11:

W h e y P r o t e i n - B a s e d Pa c k a g i n g F i l m s a n d C o a t i n g s

Schmid, M., Herbst, C., Müller, K., Stäbler, A., Schlemmer, D., Coltelli, M.-B., et al. (2016). Effect of potato pulp filler on the mechanical properties and water vapor transmission rate of thermoplastic WPI/PBS blends. Polymer-Plastics Technology and Engineering, 55, 510 517. Shaw, N. B., Monahan, F. J., O’Riordan, E. D., & O’Sullivan, M. (2002). Physical properties of WPI films plasticized with glycerol, xylitol, or sorbitol. Journal of Food Science, 67, 164 167. Sothornvit, R., & Krochta, J. M. (2000a). Oxygen permeability and mechanical properties of films from hydrolyzed whey protein. Journal of Agricultural & Food Chemistry, 48, 3913 3916. Sothornvit, R., & Krochta, J. M. (2000b). Water vapor permeability and solubility of films from hydrolyzed whey protein. Journal of Food Science, 65, 700 703. Sothornvit, R., & Krochta, J. M. (2001). Plasticizer effect on mechanical properties of β-lactoglobulin films. Journal of Food Engineering, 50, 149 155. Suslick, K. S., Casadonte, D. J., Green, M. L. H., & Thompson, M. E. (1987). Effects of high intensity ultrasound on inorganic solids. Ultrasonics, 25, 56 59. Talens, P., & Krochta, J. M. (2005). Plasticizing effects of beeswax and carnauba wax on tensile and water vapor permeability properties of whey protein films. Journal of Food Science, 70, E239 E243. Tang, C.-H., & Jiang, Y. (2007). Modulation of mechanical and surface hydrophobic properties of food protein films by transglutaminase treatment. Food Research International, 40, 504 509. Taylor, S. M., & Fryer, P. J. (1994). The effect of temperature/shear history on the thermal gelation of whey protein concentrates. Food Hydrocolloids, 8, 45 61. Tedford, L.-A., & Schaschke, C. J. (2000). Induced structural change to β-lactoglobulin by combined pressure and temperature. Biochemical Engineering Journal, 5, 73 76. Thomas, C. R., & Geer, D. (2011). Effects of shear on proteins in solution. Biotechnology Letters, 33, 443 456. Tracton, A. A. (2005). Coatings technology handbook (3rd ed). Boca Raton: CRC Press. Truong, V. D., Clare, D. A., Catignani, G. L., & Swaisgood, H. E. (2004). Cross-linking and rheological changes of whey proteins treated with microbial transglutaminase. Journal of Agricultural & Food Chemistry, 52, 1170 1176. Ustunol, Z., & Mert, B. (2004). Water solubility, mechanical, barrier, and thermal properties of cross-linked whey protein isolate-based films. Journal of Food Science, 69, FEP129-FEP133. Utracki, L. A. (2003). Polymer blends handbook. Dordrecht: Kluwer Academic Publishers. Verbeek, C. J., & van den Berg, L. E. (2009). Recent developments in thermo-mechanical processing of proteinous bioplastics. Recent Patents on Materials Science, 2, 171 189. Verbeek, C. J. R., & van den Berg, L. E. (2010). Extrusion processing and properties of proteinbased thermoplastics. Macromolecular Materials & Engineering, 295, 10 21. Verheul, M., Roefs, S., & de Kruif, K. G. (1998). Kinetics of heat-induced aggregation of betalactoglobulin. Journal of Agricultural & Food Chemistry, 46, 896 903. Wihodo, M., & Moraru, C. I. (2013). Physical and chemical methods used to enhance the structure and mechanical properties of protein films: A review. Journal of Food Engineering, 114, 292 302. Winkler, H., Vorwerg, W., & Schmid, M. (2015). Synthesis of hydrophobic whey protein isolate by acylation with fatty acids. European Polymer Journal, 62, 10 18. Wolz, M., Mersch, E., & Kulozik, U. (2016). Thermal aggregation of whey proteins under shear stress. Food Hydrocolloids, 56, 396 404. Wong, D. W. S., Camirand, W. M., & Pavlath, A. E. (1996). Structures and functionalities of milk proteins. Critical Reviews in Food Science & Nutrition, 36, 807 844.

References

Wu, Y., Weller, C. L., Hamouz, F., Cuppett, S. L., & Schnepf, M. (2002). Development and application of multicomponent edible coatings and films: A review. Advances in Food & Nutrition Research, 44, 347 394. Xia, H. S., & Wang, Q. (2003). Preparation of conductive polyaniline/nanosilica particle composites through ultrasonic irradiation. Journal of Applied Polymer Science, 87, 1811 1817. Zink, J., Wyrobnik, T., Prinz, T., & Schmid, M. (2016). Physical, chemical and biochemical modifications of protein-based films and coatings: An extensive review. International Journal of Molecular Sciences, 17, Article1376. Zolfi, M., Khodaiyan, F., Mousavi, M., & Hashemi, M. (2014). The improvement of characteristics of biodegradable films made from kefiran whey protein by nanoparticle incorporation. Carbohydrate Polymers, 109, 118 125.

437