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

Milk is a highly perishable food product, and its quality is greatly influenced by storage and handling conditions. Consequently, there is a critical need for facile and reliable approaches, such as pH-responsive colorimetric indicators, to monitor milk freshness throughout the supply chain. In this context, the current study developed a novel and environmentally friendly colorimetric indicator by dip-coating cellulose paper (CP) with a polyvinyl alcohol (PVA)/alizarin solution for real-time monitoring of milk freshness. 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 were 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 and excellent UV-blocking properties, with transmittance below 22.9%, 9%, and 3% in the UV-A, UV-B, and UV-C regions, respectively. It also showed a pronounced antioxidant activity of 68.3 ± 3.1%. Furthermore, the potential of CP/PVA/alizarin indicator in monitoring milk freshness was demonstrated through its 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).