Development and characterization of a novel pH-responsive colorimetric indicator based on cellulose paper and polyvinyl alcohol loaded with alizarin for monitoring milk freshness

Abstract

pH-responsive colorimetric indicators provide a simple, reliable, and real-time approach for assessing food freshness, thereby minimizing food wastage and reducing the risk of foodborne illness. In this study, a novel pH-responsive colorimetric indicator was fabricated by dip-coating cellulose paper (CP) with polyvinyl alcohol (PVA)/alizarin solution. The developed CP/PVA/alizarin indicator exhibited a compact morphology without any phase separation, indicating excellent compatibility among its constituents. FTIR analysis disclosed intermolecular hydrogen bonding interactions between alizarin and the CP/PVA. The water solubility and swelling index of the indicator are observed to be 9.5±0.6 % and 77.2±3.1 %, respectively. In comparison with neat CP, the CP/PVA/alizarin indicator demonstrated an increase in crystallinity from 69 to 73%, an improvement in tensile strength from 5.2±1.5 to 37.2±1.1 MPa, and an enhancement in elongation at break from 2.6±0.5 to 11.9±1.1%. In addition, it exhibited distinct pH-sensitive color responses, excellent UV-blocking properties with transmittance below 22.9, 9, and 3% in the UV-A, UV-B, and UV-C regions, respectively, along with a pronounced antioxidant activity of 68.3±3.1%. Notably, the potential of CP/PVA/alizarin indicator in monitoring milk freshness was demonstrated through its distinct and perceivable color changes: pale brown for fresh milk (pH 6.5), orange-brown for semi-fresh milk (pH 5.5), and yellow-orange for spoiled milk (pH 4.5).