Prediction Of Shelf-life Of Beverages Stored In PET Bottles With Passive And Active Walls
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media - II (T2-7b)
Keywords: shel-life, mass transport in porous material, passive barrier, active barrier
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 from the computed change of gas concentration in the container as a function of time. Although the approach is quite general, specific attention is given to the case of liquid filled PET bottles (with both passive and active gas scavenging walls), with either oxygen or carbon dioxide as the permeating gas. The work demonstrates how basic chemical engineering principles can be applied to the problem in hand: the net flux of the permeating component across the container boundaries is inserted in the overall material balances, which take into account mass transfer and chemical reaction within the boundary material.
Two situations are considered: when the walls simply provide a passive resistance to the flux (as is the case for standard PET or PET aided by some other low permeability material) and when an active gas scavenger is incorporated within the boundary material. For the first case (passive barrier) a theoretical analysis has been carried out to establish whether effectively steady conditions would prevail within the bottle wall. Having demonstrated that this is the case for the vast majority of practical cases, permeability data relative to oxygen and carbon dioxide have been collected from many source in the scientific literature and have been further verified with specific oxygen transmission rate (OTR) experiments carried out in the our laboratory. A general dependency of permeability on the material structural condition has been found for PET layers. The validity of the Nielsen relationship for the reduction of permeability for PET blends has also been verified. All this information has been utilised to provide a numerical routine for the prediction of oxygen (or carbon dioxide) concentration in a container as a function of time; these predictions have been verified by comparison with data on gas concentration in water filled bottles (and carbonated-water filled bottles, for the CO2 measurements) maintained under controlled conditions for periods of up to 6 months.
Analysis for the case of an active scavenger wall differs from the above approach due to the reaction of the scavenging material with the oxygen, which results in a gas flux changing continuously with time until all of the scavenger has been consumed. This situation has been modelled by Cussler and co-workers who formulated the unsteady state material balance equations for oxygen and the scavenger component in the barrier wall; the gas flux follows from the calculated concentration profiles. Clearly, such numerical predictions require knowledge of the kinetic constant for the oxygen-scavenger reaction. For the specific oxygen scavenger investigated in this work, the kinetic constant was first estimated from data obtained using test bottles prepared with varying scavenger concentration, then utilised to carry out predictions for other bottles of differing shape and volume. Overall the results were very good, with predictions always close to measured gas concentration, thereby supporting the general procedure proposed in this work
Presented Tuesday 18, 15:40 to 16:00, in session Transport Phenomena in Porous/Granular Media - II (T2-7b).
