The aim of this paper is to provide a framework, whereby gas permeation rates through plastic packaging walls, and hence, food shelf life may be estimated. Although the approach is quite general, specifi c attention is given to the case of liquid-fi lled polyethylene terephthalate (PET) bottles with oxygen as the permeating gas.Two situations are considered: when the walls simply provide a passive resistance to the fl ux (as is the case for standard PET or PET blended with some other low permeability material) and when an active gas scavenger is incorporated within the boundary material.For the passive wall, permeability data relative to oxygen have been collected from literature sources and also measured using specifi c oxygen transmission rate experiments. For the active walls, scavenger kinetic constants were estimated from data obtained using test bottles prepared with varying scavenger concentrations. Numerical predictions in both cases have been verifi ed by comparison with data on gas concentration in water-fi lled bottles maintained under controlled conditions for periods of up to 6 months.

Steady-and unsteady-state gas permeation rates through packaging walls containing active (scavenger) materials are determined as functions of the system's physical parameters and the scavenger load. With the simplifying assumptions of constant scavenger concentration and first-order reaction kinetics, steady-state analysis shows that there is a minimum quantity of scavenger that must be added to the packaging wall if any reduction of gas permeation is to be achieved. Unsteady-state studies have established the dependence of the time needed to reach stationary behaviour on the system's physical parameters.