3M onitor ark is a diffusion-based indicator label. The action is activated d fatty acid ester diffusing along a wick(Ahvenainen and Hurme, 1997. Wh o tapes are brought together, the e tape migrate into a light reflective r activates a he other tape, at a temperature-dependent and progressive change in the light transmission of the reflective porous mattiu induces a color change. melting temperature of the colored fatty acid determines the range of temperatures food stored at(de Kruijf et al., 2002 )