A spoilage indicator card based on distance-based color change of paper impregnated with acid-base indicator for freshness monitoring of shrimp

Food freshness sensors can help informing consumers regarding the quality of the food. An acid-base indicator card was developed for freshness monitoring of shrimp. This indicator card was prepared by using a filter paper impregnated with purple sweet potato solution at pH 4.5 to detect the color change from the pink to the green color of anthocyanins upon in contact with the volatile amines released from shrimps during storage. Before immersing the filter paper in purple sweet potato solution, the paper was preprinted with a word “ B A D ” on a detection area. The alphabet was printed by wax magenta color on filter paper number 4 and heated for 5 min to melt the wax. Then, the filter paper was immersed in purple sweet potato solution for 4 h before left until dried to hide the alphabet on a card. Subsequently, the indicator card was totally laminated except only one side for use as the inlet of the volatile amines. This indicator card was then placed in the storage box containing shrimp samples. As the storage time of shrimp increased, the color of the indicator card gradually changed from pink to purple for 2 mm in one day, 5 mm in two days, and then turned to blue green color for 3 mm in three days, 5 mm in four days, causing the word “B A D” to display on the card. This newly designed text-displaying indicator card based on distance-based color change of paper impregnated with acid-base indicator shows potential to be used for freshness monitoring of shrimp.