A scavenger in chemistry is a chemical substance added to a mixture in order to remove or inactivate impurities or unwanted reaction products.

 

Oxygen scavengers are small sachets or self adhesive labels that are placed inside modified atmosphere packs to help extend product life ( notably cooked meats ) and help improve product appearance. They work by absorbing any oxygen left in the pack by oxidation of the iron powder contained in the sachet / label.

 

source :  http://en.wikipedia.org/wiki/Scavenger_%28chemistry%29