Tainting via high extrusion temperatures
Yet another situation is the migration of off-flavors and odors from the packaging into the product, altering the taste or smell of the product; this is called “tainting.” Tainting is often times the result of an excessive extrusion temperature for a particular polymer, which degrades the polymer of the film creating low molecular weight, oxidized polymer components in the packaging materials. For instance, excessive melt temperatures in polyethylene can give a burned wax odor that can be absorbed into a product altering its taste, or appearing in the package head space so you get a whiff of odor when the package is opened.
Another common example is the production of acetaldehyde in PET bottle preforms by too high an extrusion temperature for the bottle performs, and the acetaldehyde will be absorbed by the drinks and alter their flavor. Styrene-based packaging also must be watched carefully for maximum extrusion temperature to prevent styrene from being generated during the extrusion step and tainting food products.
Variations in polymer-degradation mechanisms should be understood to anticipate the potential for and type of degradation products and the melt-temperature ranges to be avoided during film production. Thermal and oxidative degradation in polymers has been studied for a long time, and the information is generally available in the literature and, of course, directly from resin suppliers who can make recommendations as to extrusion conditions and melt temperatures to be avoided.
There are also instances where the polymer itself can taint a product, and the case I know best is the absorption of thiol- (sulfur) based stabilizers, present in some resins, to particularly delicate or highly controlled taste profiles such as premium chocolate. Of course, most of the time polymer additives do not pose a problem for taste or odor profiles for most products.