a closure that is able to prevent oxygen from entering the container upon which the closure is attached . preferably , the closure is comprised of one piece wholly molded of plastic , oxygen scavenging or anti - oxidant compounds , and catalysts homogenously mixed therein . more preferably , the closure is a composition of polypropylene or high - density polyethylene with an oxygen scavenger and catalyst intergrated within the polypropylene or high - density polyethylene .