**4. Radiation and proteins**

All proteins are polymers and their monomeric units are α-amino acids. Twenty chemically different amino acids are incorporated in proteins. Proteins in foods serve dual roles as nutrients and structural building blocks. The concept of protein functionality has historically been restricted to non-nutritive functions—such as creating emulsions, foams and gels—but this places sole emphasis on food quality considerations and potentially overlooks modifica‐ tions that may also alter nutritional quality or allergenicity. Foegeding proposed recently a new model that addresses the function of proteins in foods based on the length scale(s) responsible for the function. Properties such as flavor binding, color, allergenicity and digestibility are explained based on the structure of individual molecules, placing this functionality at the nano/molecular scale. At the next higher scale, applications in foods involving gelation, emulsification and foam formation are based on how proteins form secondary structures that are seen at the nano- and microlength scales, collectively called the mesoscale. The macroscale structure represents the arrangements of molecules and mesoscale structures in a food. Macroscale properties determine the overall product appearance, stability and texture. For applications in food products, protein functionality should start with the identification of functional needs a0 scales. Those needs are then evaluated relative to how processing and other ingredients could alter desired molecular scale properties, or proper formation of mesoscale structures. This allows for a comprehensive approach to achieving the desired function of proteins in foods [34].

Proteins have long been empirically used to make biodegradable, renewable and edible packaging materials. Numerous cereal and vegetable proteins, such as corn zein, wheat gluten and soy proteins, and animal proteins (such as milk proteins, collagen, gelatin, keratin and myofibrillar proteins) are commonly used to form agricultural packaging materials.

Lacroix and Vu, among others, had described the methods to produce protein-based films and coatings, the properties of them and their applications in food systems. They focus on selected proteins originated from animal and plant sources consisting of caseins, whey proteins, collagen and gelatin, plasma proteins, myofibrillar proteins, egg white proteins, soy protein, wheat gluten and zein [35–37].

Several other reports were published about the beneficial use of gamma radiation on the improvement of properties of different biopolymeric materials. Ionizing radiation was employed for the synthesis of polysaccharide derivatives to be used as oral delivery system for a colon-specific drug carrier [31]; other works mention the increase of mechanical and barrier properties of polysaccharide materials and the induction of grafting of methylcellulose-

Some authors reported the development of edible films from potato peel. High-pressure, gamma rays and ultrasound were applied to potato peel solutions to break down biopolymer particles in the solution small enough to allow for biopolymer film formation. Film properties, including moisture barrier and tensile properties, color and microstructures, were investigated from the films formed with different concentrations of plasticizer (glycerol) and emulsifier (soy lecithin). The authors concluded that the concentrations of both plasticizer (glycerol) and emulsifier (soy lecithin) were important variables in producing biopolymer films from potato

All proteins are polymers and their monomeric units are α-amino acids. Twenty chemically different amino acids are incorporated in proteins. Proteins in foods serve dual roles as nutrients and structural building blocks. The concept of protein functionality has historically been restricted to non-nutritive functions—such as creating emulsions, foams and gels—but this places sole emphasis on food quality considerations and potentially overlooks modifica‐ tions that may also alter nutritional quality or allergenicity. Foegeding proposed recently a new model that addresses the function of proteins in foods based on the length scale(s) responsible for the function. Properties such as flavor binding, color, allergenicity and digestibility are explained based on the structure of individual molecules, placing this functionality at the nano/molecular scale. At the next higher scale, applications in foods involving gelation, emulsification and foam formation are based on how proteins form secondary structures that are seen at the nano- and microlength scales, collectively called the mesoscale. The macroscale structure represents the arrangements of molecules and mesoscale structures in a food. Macroscale properties determine the overall product appearance, stability and texture. For applications in food products, protein functionality should start with the identification of functional needs a0 scales. Those needs are then evaluated relative to how processing and other ingredients could alter desired molecular scale properties, or proper formation of mesoscale structures. This allows for a comprehensive approach to achieving the

Proteins have long been empirically used to make biodegradable, renewable and edible packaging materials. Numerous cereal and vegetable proteins, such as corn zein, wheat gluten and soy proteins, and animal proteins (such as milk proteins, collagen, gelatin, keratin and

Lacroix and Vu, among others, had described the methods to produce protein-based films and coatings, the properties of them and their applications in food systems. They focus on selected

myofibrillar proteins) are commonly used to form agricultural packaging materials.

based films [32].

166 Radiation Effects in Materials

peel [33].

**4. Radiation and proteins**

desired function of proteins in foods [34].

Soy protein isolate (SPI), a protein with good biocompatibility, biodegradability and process‐ ability, has a significant potential in the food industry, agriculture, bioscience and biotechnol‐ ogy. Up to now, several technologies have been applied to prepare SPI-based materials with equivalent or superior physical and mechanical properties with petroleum-based materials [38].

Among protein-based films, those made of gelatin are specially important and can be produced from a variety of origins, with or without the addition of other components as plasticizers, surfactants or mixture with others substances [39–42].

Fish gelatin is a potential alternative to current mammalian (beef and pork) gelatin. However, its physical and thermal properties limit its use in many applications. The treatment of microbial transglutaminase as a cross-linking agent could be a practical way to increase the use of fish gelatin films in various applications [43].

Edible protein film characteristics can be enhanced with chemical and enzymatic methods, combining with hydrophobic material or some polymers or using a physical method, and the resulting film properties will depend on modification methods and conditions [44].

The enzyme and chemical modifications are efficient in lowering water vapor permeability. Composite edible protein films in combination with lipids can result in better functionality than the films produced with only proteins, especially with respect to their barrier properties. Of the lipids, waxes produce the best water vapor barrier properties, but produce fragile and/ or brittle films.

The preparation of gelatin and glycerol mixture with microbial transglutaminase as the crosslinking agent was described [45]. A composite casein-gelatin was prepared using also trans‐ glutaminase [46]. In some cases, however, the chemical/enzymatical cross-linking was not satisfactory [47].

Application of radiation has been extended to the modification of proteins. Depending on the adsorbed radiation dose or radiation exposure time, various effects can be achieved resulting in the polymerization (cross-linking) or depolymerization of protein molecules. Most food proteins, however, undergo irradiation-induced cross-linking and subsequent improvement on the film properties [48].

Radiation-induced lipid oxidation can be readily detected by our body's olfactory cells rancidity, off-flavor and, sometimes on the positive aspect, aroma; similarly, it is easy to notice discoloration that results from the oxidation of pigments. In contrast, protein oxidation occurs undetected by sensory organs; instrumental analysis is therefore required. Yet, proteins are very susceptible to reactive oxygen species (ROS) and impart both desirable and undesirable consequences when oxidatively modified. For example, oxidant-initiated disulfide bond formation among gluten molecules is responsible for the desirable rheology and sponginess of bread. And functional myosin (or actomyosin) aggregates produced by the reaction of cysteine, lysine and tyrosine residues with low concentrations of free radicals promote protein gel networks in processed muscle foods, hence the products' firmness and mouthfeel.

Chemically, protein oxidation that can result from radiation treatment in the presence of O2 involves the initial modification of amino acid side chain groups by ROS. The radiation sensitivity of the amino acids is the highest for cysteine and decrease following the sequence: cysteine, methionine, tyrosine, tryptophan, phenylalanine, valine, leucine, histidine, glutamyl, proline, threonine, arginine and lysine. The initial modification of amino acid side chain groups by ROS leads to the conversion to carbonyl and other derivatives. Electron-deficient carbonyl groups are highly reactive with amines and thiol moieties to produce cross-links between polypeptides or segments within the same protein molecule. Disulfide and dityrosine are other covalent linkages in oxidatively stressed proteins. Protein radicals, which usually have a long half-life, are precursors of polymers as well [49].

The field of health science has pioneered protein oxidation research. Similarly, research in food science over the past two decades has established ubiquitous occurrences of protein oxidation in both fresh and processed foods, especially meat products. Most of the studies point to the negative aspect because uncontrolled oxidation leads to deleterious consequences: tissue hardening (as in frozen fish), loss of water-binding potential and off-flavor due to thiol oxidation. For that reason, developments about novel antioxidants, such as phenolic deriva‐ tives, peptides/protein hydrolysates, phospholipids and polysaccharides, and their role in food quality preservation are going on [50].

Many peptides and protein hydrolysates, being preferred targets of ROS and excellent functional compounds (water-binding, foaming, rheology and so on), are qualified as "multifunctional" natural food additives. Aside from the negative impacts of oxidation, mildly oxidized lipids give us a complex, highly desirable aroma of foods as that coming from frying. Today, the beneficial effects of limited protein oxidation are also no longer ignored. That must be taking in account whenever any protein food is submitted to oxidative processes such as irradiation.

Gamma-irradiation affects proteins by causing conformational changes, oxidation of amino acids, rupture of covalent bonds and formation of protein free radicals that can be beneficial for specific further applications. Chemical changes in the proteins that are caused by gamma irradiation are fragmentation, cross-linking, aggregation and oxidation by oxygen radicals that are generated in the radiolysis of water. For example, the hydroxyl and super oxide anion radicals that are generated by radiation of film-forming solution could modify the molecular properties of the proteins, which results in the alteration of protein films by covalent crosslinkages formed in protein solution after irradiation.

Using gamma irradiation to induce cross-linking was found to be an effective method for the improvement of both barrier and mechanical properties of the edible films and coatings based on proteins. There are plenty of examples in the literature that corroborate the efficiency of radiation application on natural films, like those made of zein, the protein found in maize [51]. Different types of gelatin-based films with enhanced properties can be produced by means of application of ionizing radiation [52, 53].

The effect of electron beam accelerator doses on properties of plasticized fish gelatin film was studied. The electron spin resonance spectra indicated free radical formation during irradia‐ tion, which might induce intermolecular cross-linking. Tensile strength for gelatin film significantly increased after irradiation (improved by 30% for 60 kGy). The vapor permeability was weakly affected by irradiation. Surface tension and its polar component increased significantly in accordance with the increase of wettability. So, the authors suggest that irradiation may change the orientation of polar groups of gelatin at the film surface and crosslink the hydrophobic amino acids. They did not observed modification of the crystallinity of the film. So, they conclude that structure changes only occurs in the amorphous phase of the gelatin matrix. It is also observed that irradiation enhances the thermal stability of the gelatin film, by increasing the glass transition temperature and the degradation temperature [54].

cysteine, lysine and tyrosine residues with low concentrations of free radicals promote protein gel networks in processed muscle foods, hence the products' firmness and mouthfeel.

Chemically, protein oxidation that can result from radiation treatment in the presence of O2 involves the initial modification of amino acid side chain groups by ROS. The radiation sensitivity of the amino acids is the highest for cysteine and decrease following the sequence: cysteine, methionine, tyrosine, tryptophan, phenylalanine, valine, leucine, histidine, glutamyl, proline, threonine, arginine and lysine. The initial modification of amino acid side chain groups by ROS leads to the conversion to carbonyl and other derivatives. Electron-deficient carbonyl groups are highly reactive with amines and thiol moieties to produce cross-links between polypeptides or segments within the same protein molecule. Disulfide and dityrosine are other covalent linkages in oxidatively stressed proteins. Protein radicals, which usually have a long

The field of health science has pioneered protein oxidation research. Similarly, research in food science over the past two decades has established ubiquitous occurrences of protein oxidation in both fresh and processed foods, especially meat products. Most of the studies point to the negative aspect because uncontrolled oxidation leads to deleterious consequences: tissue hardening (as in frozen fish), loss of water-binding potential and off-flavor due to thiol oxidation. For that reason, developments about novel antioxidants, such as phenolic deriva‐ tives, peptides/protein hydrolysates, phospholipids and polysaccharides, and their role in food

Many peptides and protein hydrolysates, being preferred targets of ROS and excellent functional compounds (water-binding, foaming, rheology and so on), are qualified as "multifunctional" natural food additives. Aside from the negative impacts of oxidation, mildly oxidized lipids give us a complex, highly desirable aroma of foods as that coming from frying. Today, the beneficial effects of limited protein oxidation are also no longer ignored. That must be taking in account whenever any protein food is submitted to oxidative processes such as

Gamma-irradiation affects proteins by causing conformational changes, oxidation of amino acids, rupture of covalent bonds and formation of protein free radicals that can be beneficial for specific further applications. Chemical changes in the proteins that are caused by gamma irradiation are fragmentation, cross-linking, aggregation and oxidation by oxygen radicals that are generated in the radiolysis of water. For example, the hydroxyl and super oxide anion radicals that are generated by radiation of film-forming solution could modify the molecular properties of the proteins, which results in the alteration of protein films by covalent cross-

Using gamma irradiation to induce cross-linking was found to be an effective method for the improvement of both barrier and mechanical properties of the edible films and coatings based on proteins. There are plenty of examples in the literature that corroborate the efficiency of radiation application on natural films, like those made of zein, the protein found in maize [51]. Different types of gelatin-based films with enhanced properties can be produced by means of

half-life, are precursors of polymers as well [49].

quality preservation are going on [50].

linkages formed in protein solution after irradiation.

application of ionizing radiation [52, 53].

irradiation.

168 Radiation Effects in Materials

Cross-linked copolymers of gelatin and poly(vinyl alcohol) (PVA) with excellent water absorption and water retention abilities were successfully synthesized using Co-60 gamma radiation [55]. Also, gamma irradiation was applied on milk protein to improve characteristics of milk protein films [56–59].
