Impact of elevated and severe storage temperatures on the stability of UHT milk

The study evaluated the microbial, physicochemical and sensory stability of indirect UHT milk (138 °C, 4s) with varying fat content (0.5%, 2%, and 3.8%) stored at temperatures 20 °C, 30 °C and 50 °C simulating warehouse storage and transportation conditions. Milk samples stored at 20 °C for 12 months and at 30 °C for 6 months were examined monthly, and at 50 °C for 1 month (examined weekly). No changes in microbial load were detected. Under ambient storage UHT milk was stable. However, at temperatures 30 °C and 50 °C significant changes in color (ΔE) and thermal stability occurred. Free fatty acid (FFA) and hydroxymethylfurfural (HMF) levels significantly increased in 2% fat milk at elevated temperatures. Ionic calcium levels significantly decreased in 0.5% and 2% fat milk at 30 °C and 50 °C, respectively. Significant increase in non-protein nitrogen (NPN) content was observed in 3.8% fat milk at 30 °C. Lactulose concentrations increased significantly in 0.5% and 2% fat milk at 30 °C, and in all milk types at 50 °C. Moreover, correlation analysis revealed that lactulose strongly aligned with multiple markers of thermal degradation, such as color changes, FFA, HMF, and NPN, underscoring its potential as a key indicator of heat-induced physicochemical deterioration in UHT milk. • Microbiological stability of UHT milk was preserved across all evaluated storage conditions. • Elevated temperatures (30-50 °C) exerted significant effects on color, thermal stability, and critical chemical markers. • The fat content modulated the degree of physicochemical alterations during storage. • Lactulose demonstrated potential as a key indicator of heat-induced stability loss in UHT milk.