A paper impregnated with purple sweet potato anthocyanin as a natural acid-base indicator was developed for the freshness monitoring of shrimp. The indicator paper was laminated, except only one side as the inlet of the vapor to slowly diffuse through. With this laminated paper, the color change of the indicator paper gradually occurred from the inlet position through the whole paper in a “distance-based” color change manner, which was related to the length of shrimp storage time. In order to add a gimmick to the laminated indicator card and to make the card more user friendly, we also propose to use the “text-displaying” approach to allow the appearance of a word “BAD”, to allow the appearance of a word “BAD”, which was first hidden and became unhidden upon exposure to the released volatile nitrogen compounds from shrimp at a certain storage time.

Thanks are due to the  National Research Council of Thailand (NRCT) and Mahidol University under grant number N41A640159. The support from Center of Excellence for Innovation in Chemistry (PERCH-CIC) and the NRCT through the International Research Network Grant (IRN/502/2563) is gratefully acknowledged. We also appreciate the help from Miss Kamonrat Liabsiri for FTIR analysis and in-kind support from Dtect Tech Co., Ltd. for the permission to use some necessary equipment. This work is related to the SDGs 4 (Quality Education).