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Ultrasound effect on oxygen barrier properties of two different polypropylenes aimed for food packaging

Application of ultrasound in food industry is attracting much attention nowadays. High power ultrasound – within the frequency range 20-100 kHz and of energy intensity 10-1000 W cm-2 – generates intense pressure, shear and temperature gradients within food that can disrupt the structure of bacteria in the food. Ultrasonic waves can be readily propagated through certain optically opaque materials including many foods and packaging materials. For food products expected to exhibit extended shelf-life, a loss of packaging integrity may be a precursor to major food safety and quality issues. The location and small size of some of these packaging defects make them difficult to detect. However, some defects can be detected by molecular probes capable of measuring the permeation of gases and vapours through the defective material. Two different samples of OPP (biaxial oriented transparent polypropylene) films were treated for 2, 4 and 6 minutes with power ultrasound, high intensity and low frequency, 20 kHz probe with constant amplitude of 50%, 75% and 100% (S-4000, Misonix Sonicators, Newtown, Connecticut, SAD). The barrier characteristic of the oxygen permeance studied samples at 20°C-0% RH was performed using a Brugger (GDP-C model) permeability tester. The permeability (P), the diffusion (D) and the solubility (S) constants of the oxygen in two different samples of OPP films was determined before and after ultrasound treatments. Oxygen permeability decreases for first OPP film (from 1010 to 570 ml m-2d-1 bar-1) and increases for second OPP film sample (from 203 to 627 ml m-2d-1 bar-1) after ultrasound treatment. The present work has clearly illustrated the suitability of ultrasound-induced cavitation phenomena as a technique for degradation of polypropylene. Ultrasound could be potentially applied to premium quality, vacuum-packaged meat if an immersion system was used, for example during heat shrinking of the bag in a waterbath.

polypropylene; ultrasound; barrier properties; food packaging

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