a) CAS: sodium caseinate.
b) LMP: low methoxyl pectin.
c) WPC: whey protein concentrate.
d) WPI: whey protein isolate.
e) WPNF: whey protein nanofibrils.
f) PLA: poly(lactic acid).
Whey proteins are composed of different globular proteins such as β-lactoglobulin, α-lactalbumin, bovine serum albumin, immunoglobulins, and the polypeptides proteose-peptone. Particularly, β-lactoglobulin is the major whey protein responsible for gelation and aggregation behavior in whey [31].
Films and coatings based on whey are transparent materials, and they have better mechanical and barrier properties when compared with those manufactured with polysaccharides. Protein network in whey can be modified by means of thermal treatments, and the resulting modified whey can be used to manufacture materials with improved tensile and barrier properties [29, 31].
Finally, films and coatings based on casein and whey have limited elasticity and water sensitivity, limiting their applications. The films elasticity can be improved using plasticizers such as glycerol or sorbitol into the film-forming solution [18, 29]. Table 2.1 shows some studies using WPC and WPI with addition of plasticizers for antimicrobial and antioxidant food packaging.
2.2.2 Cellulose and Derivatives
Cellulose is considered the most abundant biopolymer in the world, being a component of the plant tissues and vegetal cell wall. This biopolymer can be obtained from several plants such as cotton, sugarcane bagasse, eucalyptus, and wood, among others, which it become an environmental and economically feasible by-product [32]. In addition, derivatives of cellulose have also been used for food packaging applications, especially the cellulose ethers as carboxymethyl cellulose (CMC), ethylcellulose (EC), hydroxyethyl cellulose (HEC), hydroxypropyl methylcellulose (HPMC), and methylcellulose (MC) [33].
Cellulose is conformed of β-D-glucose monomers, which are 6-membered rings called pyranoses. The β-D-glucose units are bonded by acetal linkages (oxygen atoms) produced from dehydration reactions between the C-1 of the pyranose and C-4 of a neighbor pyranose [34]. The equatorial position of β-D-glucose hydroxyl functional groups causes a considerable linearization of cellulose chains that facilitates the formation of long chains (fibers) of the biopolymer [34]. The hydrogen bonds between hydroxyl groups (–OH) from different cellulose chains promote the formation of microcrystalline regions, while other micro-regions are characterized by a disordered structure (amorphous) due to low amount of hydrogen bonds [35].
Strong hydrogen bonds of the cellulose crystalline regions caused it to become mechanical and thermally resistant and insoluble in several solvents including water [36]. On the other hand, hydroxyl groups of cellulose amorphous regions are more distant between them than hydroxyl groups of crystalline regions, which allows physical interactions between cellulose and other molecules. In the case of water, cellulose is able to absorb a large number of molecules without dissolving them. This characterizes a high swelling ability of this biopolymer [33].
Cellulose ethers (MC, EC, CMC, HEC, HPMC) are biopolymers produced from substitution reactions of cellulose hydroxyl groups or alkylation and differ as to the substituting group and number of hydroxyl groups substituted (substituting degree) [33]. Generally, the cellulose etherification reactions are performed in alkaline medium using halides and alkyl sulfates as etherifying agents. The biggest advantage of the cellulose ethers in comparison with cellulose is their higher water solubility [36]. These materials are used to modify the rheology of solutions by changing of viscosity, increasing of water swelling ability, stabilizing of suspensions, gelling, and emulsifying and to form films and coatings that are more flexible than cellulose [35, 37].
Applications of films and coatings based on cellulose and its derivatives are presented in Table 2.2. It is observed that cellulose and its derivatives have the ability to form composites with a variety of other biopolymers for both active and smart packaging applications.
2.2.3 Chitin and Chitosan
Chitin is considered the second most abundant biopolymer in the world. Usually, this biopolymer is isolated from the exoskeletons of crustaceans such as crabs, lobsters, shrimps, squid, among others [51–53]. The molecular formula of chitin consists of the 2-acetamido-2-deoxy-β-D-glucose through a β (1,4) linkage, considered a polysaccharide (cellulose) with hydroxyl groups at position C-2 replaced by an acetamido group [51].
Chitosan is a derivative obtained by the deacetylation of chitin (deacetylation >50%) by means of alkaline treatment [44]. Chitosan is a cationic polysaccharide with linear structure integrated by two monomers, D-glucosamine (2-amino-2-deoxy-β-glucopyranose)