Chitosan-Graft-Gelatin Hydrogel Containing Bromothymol Blue as a Food Spoilage Indicator for Intelligent Food Packaging

A remarkable one-third of global food production is wasted, resulting in significant economic damage and escalating food insecurity. To address this sustainability challenge, freshness identification in food packaging using intelligent materials has gained traction. This work introduces a chitosan-graft-gelatin (CS-g-GEL) hydrogel, incorporating a pH-responsive dye, bromothymol blue (BTB), as a colorimetric freshness indicator to monitor the spoilage of chicken breasts. The successful formulation of the hydrogels is verified using FTIR, AFM, SEM, TGA, and ¹HNMR characterization techniques. At pH 5 and pH 7.4, the swelling capacity of the hydrogel is 790% and 470%, respectively. A rise in colony-forming units (CFU) from 5.5 on day 1–7.4 log CFU g⁻¹ on day 7 indicate microbial activity, and the production of carbon dioxide due to deteriorating food freshness decreases the pH in the food package. Additionally, CO₂ concentrations rise from 3.2 on day 1–22.1 (% v/v) on day 7, while O₂ levels decreases from 23.8 to near 0.5 (% v/v) in the same period. The developed hydrogel is demonstrated to serve as a smart and visual indicator of food freshness and offers a new dimension in the use of intelligent materials toward a global challenge.