A solution for detecting concentrations of CO.sub.2 is sealed within a pouch located on or in a food container and visually observable from outside the container. A microporous material places the indicator in gaseous communication with the food contents of the container, while maintaining isolation...http://www.google.com.au/patents/US4003709?utm_source=gb-gplus-sharePatent US4003709 - Visual spoilage indicator for food containers
