Real-Time Monitoring and Rapid Determination of Pork Freshness Using a Colorimetric Sensor Array Based on Porphyrin and pH Indicator Ligand Combination

This study presents the development of a colorimetric sensor array (CSA) that combines pH indicators with porphyrin solutions to create a sensitive and stable sensor for monitoring pork freshness. Four pH indicators─bromophenol blue, bromocresol green, bromocresol violet, and chlorophenol red─were chosen for their ability to detect volatile gases produced during pork spoilage. Color changes were quantified by extracting grayscale values, and various porphyrins, including TPPNi, TPPMnCl, and TPPCu, were screened for their interactions with the pH indicators. Principal component analysis was used to determine the optimal concentration of the porphyrin solution. The CSA was then constructed by combining the four pH indicators with the selected porphyrins, resulting in an average stability improvement of 21.94%, 23.59%, 28.84%, and 23.31%, respectively. The occurrence of coordination is verified by Fourier transform infrared (FTIR), and theoretical calculations are applied to explain the possible reasons for the increased stability. The CSA was tested on pork samples stored for different periods, and a partial least squares regression model was built using grayscale data to predict freshness. Furthermore, storage experiments were conducted to evaluate the CSA's stability over 3, 7, and 15 days, and the retention of color in CSA was 99.02%, 99.01%, and 98.84%, respectively. The models achieved 98.89% average accuracy (n = 30 samples) in predicting the freshness grades of pork samples from different batches and in assessing the stability of the CSA, demonstrating its potential as a reliable tool for rapid and accurate pork freshness discrimination.