On the Bacterial Non-permeability of Plastic Films

Ronsivalli et al. (Food Tech., 20, 1074, 1966) have devised a method to determine the bacterial permeability of plastic films, in which, pouches of various plastic films are filled with suitable bacterial growth medium, sealed hermetically, sterilized by means of γ ray, and immersed in a highly contaminated bath of spoiling fishes slurry at low temperature for a long period. The pouches are then removed from the contaminated bath and incubated. After incubation is complate, contents of the pouches are inspected for any visual evidences of microbial growth-e.g., turbidity, gas formation, etc. If a medium in a pouch remain sterile, the pouch materials is judged to be impermeable to the bacteria contained in the bath. The authors of the present paper adopted Ronsivalli's method to the plastic films shown in Table I. Seven or eight pouches (7.8×7.8 cm) of each film filled with the growth medium of Difco-Bact-Trypton and a few dry pease which had been previously sterilized by heating at 120°C. for 15 min., and heat-sealed by an impulse sealer, were subjected to the irradiation by γ ray of 60Co at 4.5 Mrad for the purpose of cold sterilization, followed by immersion in a contaminated medium with Cl. Pasteurianum of 107/cc concentration. After removal from the contaminated bath, the pouches are incubated at 35°C. for 7 days, and then inspected as to the gas formation and turbidity. The anaerobic Cl. Pasteurianum has been proved by the authors to grow rapidly in the aerobic Difco-Bact-Trypton medium. In the preliminary test, one of eight pouches showed the bacterial contamination, which, probably due to the failure of heat-sealing. Therefore the operaters became more careful for the heat-sealing of pouches. In the first experiment carried out, with greater care of heat sealing. All of the test pouches were observed to remain sterile. The same result was obtained from the second experiment. Thus, it is safe to say that the plastic films are impermeable to the bacteria. The films as pouch material were tested with respect to UV transmittance, (a) before irradiation by γ ray (eg. the original film), (b) after irradiation, keeping in air, (c) after irradiation, keeping in contaminated growth medium. And, as the result, any noticeable differences between (b) and (c) were not detected (Fig. 2-5).