The need to improve the shelf life and quality of foods, combined with the urgency to reduce waste generated by synthetic non-biodegradable packaging, have resulted in an increasing interest in biodegradable or edible materials (Chen, 1995). The challenge in the development of such materials lies in achieving controlled lifetime together with compatibility with foods and the environment. Natural polymers or polymers derived from natural monomers such as food proteins offer the greatest opportunities, since their biodegradability and environmental compatibility are assured (Krochta & de Mulder-Johnston, 1997).

Milk proteins such as caseins have been formulated into coatings and films for improving food quality (Baker et al. 1994). Unfortunately, the highly hydrophilic nature of these proteins limits the possibility of producing films with the required properties. Gamma irradiation, which induces the formation of crosslinks in the protein structure, is known to improve the mechanical properties and water resistance of such materials (Brault et al. 1997; Mezgheni et al. 1998a; Ressouany et al. 1998), and can also sterilize materials for possible biomedical applications (Kaetsu, 1995).

Gamma irradiation is slowly becoming accepted in the food industry as a means of safely improving the shelf life of various fruits and vegetables and eliminating bacterial contamination in meats (Pszczola, 1997). All these features (sterility, increased mechanical strength, controlled life time) make gamma irradiation a promising technique for the production of commercial biodegradable films.

In the present study, we have investigated the microbial resistance of calcium caseinate films irradiated at 4 or 64 kGy. A minimum dose of 4 kGy was required to produce complete sterility in the films without inducing many crosslinks in the protein. A dose of 64 kGy was used to induce maximum crosslinking density (Ressouany et al. 1998). The rate of modification of the films was evaluated using three methods. Firstly, microbiological counts were performed on films incubated with a strain of Pseudomonas aeruginosa. This strain was chosen because it produces a wide range of proteinases and is often implicated in the process of food spoilage. These results were compared with those obtained for soluble N. Since conversion to CO2 is one of the standard test procedures to assess biodegradability (El-Din Sharabi & Bartha, 1993), we also analysed the CO2 produced by the biodegradation of the crosslinked films using a modified version of a standard test procedure (American Society for Testing Materials, 1992). This measures the aerobic degradation of plastic materials inoculated with municipal sewage sludge. We have replaced the sludge by a concentrated bacterial strain (Ps. aeruginosa) and compared these results with the two previous methods. This last method was used to measure the resistance of the crosslinked films to bacterial degradation.