**2. High-energy radiation and polymers**

Edible polymer film is a thin layer of edible material formed on a food as a coating or placed (pre-formed) on or between food components; in this case, edible films can improve the quality of multicomponent foods. The sanitary condition of the edible packaging would need to be maintained during storage, transportation and marketing. The end result would be source reduction and or improved recyclability of the remaining elements of the packaging system.

One major advantage of using edible films and coatings is that several active ingredients (antimicrobials, antibrownings, texture enhancers and nutraceuticals) can be incorporated into the polymer matrix and consumed with the food, thus enhancing safety or even nutritional

Many potential uses of edible films can be described such as inhibit migration of moisture, oxygen, carbon dioxide, aromas; carry food ingredients (e.g., antioxidants, antimicrobials, flavor) and or improve mechanical integrity or handling characteristics of the food [2]. Practical uses of edible films include wrapping various products; individual protection of dried fruits, meat and fish; control of internal moisture transfer in pizzas, pies, which are based on the film's properties (e.g., sensory, mechanical, gas and solute barrier). Also, ingredients can be delivered to processors on water-soluble and edible packaging films in premeasured amounts of ingredients to food processors and foodservice operations. Future applications of the concept have been envisioned in consumer-sized pouches of dried products ready for reconstitution

The functionality and performance of edible polymer mainly depend on their barrier, me‐ chanical and color properties, which in turn depend on film composition and its formation process. Polysaccharide (cellulose, starch, dextrin, vegetable and other gums) and protein (gelatin, gluten, casein) based films can have suitable mechanical and sensory properties, while wax (beeswax, carnauba wax) and lipid or lipid derivative films have enhanced water vapor barrier properties. It was shown that the incorporation of oil between plasticized starch layers could reduce oxygen and water vapor permeability of the films [3]. The film-forming technol‐ ogy, solvent characteristics, plasticizing agents, temperature effects, solvent evaporation rate, coating operation and usage conditions of the film (relative humidity, temperature) can also

Important properties to be evaluated in an edible coating are its microbiological stability, adhesion, cohesion, wettability, solubility, transparency, mechanical properties, sensory and permeability to water vapor and gases. In order to choose the best edible film to be employed in each case, it is necessary to take in account the characteristics of the food intended to be protected. For coating a fresh fruit, for instance, it will be desired to induce low water vapor permeability to preserve texture and moderated O2/CO2 permeability to permit respiration. On the contrary, when it is intended to protect dried fruits and nuts, low water vapor perme‐

ability is required to maintain crispiness and low O2 permeability to avoid oxidation.

Proteins as components of edible films (gelatin, zein, casein) has usually better performance than polysaccharides (chitosan, starch, pectin); lipids have excellent water vapor barrier but do not form stand-alone films, and can be used as coatings, such as waxes on fruit surfaces. Besides the barrier efficiency, edible films and coatings have to be also sensory acceptable.

substantially modify the ultimate properties of the film [4].

and sensory attributes [1].

162 Radiation Effects in Materials

on water.

Gamma rays and electron beam are two commonly used ionizing radiation sources in industrial process. Gamma rays, 1.17 and 1.33 MeV, are emitted continuously from radioactive source such as cobalt-60, whereas electrons are generated from an accelerator to produce a stream of electrons called electron beam. The energy of electrons depends on the type of machines and can vary from 200 keV to 10 MeV. High-energy radiation using conventional gamma or e-beam sources is an adequate tool for the modification of polymer materials through degradation, grafting and cross-linking [10].

The absorbed dose of radiation is expressed in units of gray (Gy). The Gy was adopted by the International Commission of Radiation Units and measurements (ICRU) in 1975 as the special name for the standard international (SI) unit of absorbed dose, being 1Gy = 1 Joule/kg of matter. When processing food products very seldom the employed dose exceeded 20 kGy, and most of the cases remained below 10 kGy [11]. The adsorbed radiation dose is a consequence of the radiation exposure time and the distance between the product and the radiation source in the irradiation camera.

The long molecular chains of polymers can be broken by the absorption of a quantum of energy above the energy of the covalent bond of the main carbon chain, which typically is in the range of 5–10 eV. The energy of beta and gamma photons of 1 to 10 MeV surpasses by many orders of magnitude this minimum value, representing a high probability of affecting all kinds of polymers, naturals and synthetics alike [12]. Usually, there is a competition between crosslinking and chain scission reaction. If chain scission reaction predominates the material degrades.

Radiation, as a non-selective, highly efficient tool of ionization, may form excited sites, ions and free radicals in almost all kinds of materials. Radiation treatment of polymer mixtures, even if they are (partially) incompatible, gives a chance for bridge-forming bonds [13].

The key effect of radiation is the production of reactive oxygen species (ROS), which affects biomolecules (e.g., lipid, protein, polysaccharides). One of the important radiation-induced free-radical species is the hydroxyl radical, which indiscriminately attacks neighboring molecules often at near diffusion-controlled rates. Hydroxyl radicals are generated by ionizing radiation either directly by oxidation of water, or indirectly by the formation of secondary partially ROS. These may be subsequently converted to hydroxyl radicals by further reduction ("activation"). Secondary, radiation injury is therefore influenced by the surrounding antiox‐ idant status and the amount and availability of activating mechanisms. The biological response to radiation may be modulated by alterations in factors affecting these secondary mechanisms [14].
