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CLOSURES PLAY a significant role in the overall shelf life of the plastic container system. The relative role that closures play is often not well understood. Currently most closures are measured separately from containers and are often not considered when determining the shelf life of a product. Product shelf life, when referring to container permeability, is usually evaluated by looking at carbon dioxide egress or oxygen ingress. This article will look at the effect of oxygen ingress on container shelf life through the container and the closure. The closures studied here are 28mm closures. The importance of closure barrier will increase with closure size. As wide mouth plastic containers become more popular, closures will play an increasing role in container shelf life. There are four sources of oxygen in a plastic container. Permeation through the container and closure is usually the most significant source of oxygen, but is not the only source. The container absorbs oxygen that will be released into the product. The product itself contains oxygen and the headspace, (the air space left after the product is filled) contains residual oxygen. Food products and beverages have different levels of tolerance for oxygen. To determine shelf life, it is important to establish how much oxygen is too much and then evaluate the total system oxygen barrier provided by the container and closure. The size of the container has an effect on the relative amount of oxygen per unit volume. The smaller the container the greater the exposure per unit volume. Small bottles often have the same size closure as larger bottles. The surface area of the closure plays a larger role in a small container than it would with a larger container. Adding barrier to a closure will have a stronger effect in a small bottle than in a larger bottle. Bottle barrier also affects the relative role of the closure. With a barrier bottle, the relative role of the closure increases as the bottle barrier improves. A non-barrier closure would account for 17% of the container permeation in a 600 mL bottle; it becomes 51% of the permeation in a five-fold improvement and 77% in a 10-fold improvement. The cap is not getting worse, but it's relative effect on overall container barrier is getting larger. Closure barrier also has an effect. Barrier caps can improve shelf life: the better the barrier of the bottle, the more important the barrier of the cap becomes. Traditional measurement systems have looked at the barrier of the cap separate from the container. Twinpak has been evaluating an oxygen indicator system developed by the Guelph Food Technology Centre for use in measuring total system oxygen. The oxygen indicator will detect the absorbed oxygen, the permeate and the headspace oxygen. Twinpak's initial work with the indicator has confirmed the importance of the closure in overall system permeability. The development of the blue color in the neck area of the container precedes development in other areas. The development of the colour in the bottle neck is a measure of the dissolved oxygen in the neck area and the oxygen from permeation through the closure. The oxygen indicator measures all sources of oxygen that will effect the product. In conclusion, closures play an important role in container barrier. The closure has a significant influence on container shelf life in small containers and in high barrier containers. The relative role of the closure increases with the size of the closure and the barrier of the bottle. Barrier closures can improve shelf life.