Monitoring Lemon Juice-Induced Coagulation of Cow’s Milk: The Impact of Heat Treatment and Calcium Addition on the Quality of Gels

The present research investigated the effect of moderate heat treatment (65 °C for 5 min) and calcium enrichment (10 mM CaCl2) on the quality of gels formed by lemon juice at 30 °C for 180 min. Raw milk, calcium-fortified raw milk, heated milk, and calcium-fortified heated milk were used. Rheological measurements showed that the addition of calcium to milk significantly improved the elastic modulus (G’), which passed from 21.2 Pa to 80.18 Pa. However, the combination of heat treatment and calcium produced weaker gels with G’ = 3.71 Pa. Turbiscan analysis revealed higher instability in calcium-fortified heated milk samples that have high Turbiscan Stability Index (TSI) values. Mid-infrared spectral regions (3000–2800 cm−1, 1700–1500 cm−1, and 1500–900 cm−1) and fluorescence spectroscopy indicated some structural changes in protein–water, protein–protein, and protein–lipid interactions depending on coagulation conditions. Principal component analysis (PCA) applied to the fluorescence and MIR datasets allowed the differentiation of gel samples depending on heat treatment and calcium addition. Scanning electron microscopy (SEB) indicated dense and uniform gels produced with calcium-enriched raw milk and porous structures with heated and calcium-enriched milk. These results reveal new information on how thermal treatment and calcium supplementation affect protein network structure formation and the gel microstructure during lemon juice-induced coagulation.