a flexible zipper incorporating a layer of gas - impermeable thermoplastic material in one or both flanges . the zipper is installed in the mouth of a reclosable package or bag made of gas - impermeable material shaped to form a receptacle . the gas - impermeable web material of the receptacle and the layer of gas - impermeable material of the zipper form a substantially gastight enclosure for preserving the perishable contents of the package . flexible zippers having a layer of gas - impermeable thermoplastic material joined to a layer of gas - permeable thermoplastic material are preferably manufactured by a coextrusion process . the resulting gas barrier inside the receptacle has a zone of preferential tearing .