detection the carriers of the present invention generally have thickness of less than about 1 mm a variety of barrier sheets which can be used for separating the carrier from fluid contact with the food product are also known in the art these include various hydrophobic materials low or medium density polyvinyl chlorides polyethylenes and other plastics with extractable hexane and monomer levels meeting food and drug administration standards for food wrappings the barrier sheets preferably have thicknesses in the range of about 1 u03bcm to about 13 u03bcm the exemplary indicator materials carriers barrier sheets and outer layers discussed above are presented for illustrative purposes only and are not intended to limit the invention the following non limiting examples further illustrate the efficacy of the invention in each of examples 1 2 and 3 aqueous u201c donor u201d compartments containing several concentrations of acidic or basic metabolite were separated by a barrier from aqueous u201c receptor u201d compartments containing the indicator material changes in indicator color were monitored in triplicate as a function of time and acid concentration this example was designed to show that acids which can be produced in microbial spoilage can trigger visual effects in a spoilage indicator as described herein permeation of lactic and acetic acids at different molar concentrations through separating films consisting of food packaging grade u201c cling wrap u201d polyvinyl chloride films was tested over a 48 hour period these films were of the type used and sold by supermarkets and had a thickness of 11 4 microns the bromocresol purple concentration was 0 001 the bromocresol purple is purple at ph 6 8 and yellow at ph 5 2 the results of these experiments are shown in the table 3.