pH-responsive color indicator film based on sodium carboxymethylcellulose and natural acid buffering matrix for real-time beef sub-freshness monitoring

Abstract

The good water solubility and film-forming properties of sodium carboxymethylcellulose (CMC–Na) render it an excellent matrix for a pH-responsive color indicator film, but its basic character inhibits its direct application. Natural acids can be used to solve this problem, but the buffering system formed in these processes may decrease the sensitivity and improve the heterogeneous compound distribution of the film. Previous studies have tried to address this problem by adjusting the species and amounts of the acids. However, the underlying mechanism has not been comprehensively clarified. Herein, a CMC–Na and CaCl₂ mixture was used as the matrix for a pH-responsive color indicator film, alizarin (AZ) was used as the pH indicator, cinnamaldehyde (CMA) was used as an ultraviolet (UV) shielding agent, and natural acids were used as pH-controlling agents. Three protic acids with distinct numbers of acidic protons and pK_a, acetic acid (monoacid), tartaric acid (diacid) and citric acid (triacid), were compared to elucidate the effect of these properties on the matrix pH, dye dispersion, and film response. The results showed that tartaric acid can be a good candidate in solving the problem on a basic CMC–Na matrix owing to its good performance in adjusting the film color and the sensitive color response of the resulting film at pH 6.0–7.0. The hydrogen bonds, hydrophobic interactions and chelation reactions formed among the compounds improve the mechanical property and oxygen barrier performance of the film, which are also beneficial to the homogeneous distribution of AZ and CMA, leading to a uniform color response and good UV-shielding performance. This film displays excellent color response quality in beef sub-freshness detections, suggesting its high sensitivity in “point-of-care” detections of real meat samples via naked eyes.