Impact of Cold‐Chain Interruption on the Stability and Quality of Pasteurized Milk: Physical, Enzymatic, and Microbial Insights

Pasteurized milk was popular for its nutrition and taste, but it needs strict temperature control to keep its quality. This study investigated the impact of cold chain interruption on the stability of pasteurized milk stored at 25-35°C for 1-12 h. Parameters measured included particle size, zeta potential, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) profiles, microbial activity, peroxide levels, microstructural changes, and protease and lipase activities. The results showed that a 3 h cold chain interruption at 35°C sharply increased protease and lipase activities in pasteurized milk to 0.85 U/mL and 5.99 U/mL, with growth rates of 5.81% and 5.12%. Bacillus species grew rapidly, reaching 39.31% relative abundance and becoming the dominant microbiota. Oxidative markers, including lipid hydroperoxides and thiobarbituric acid (TBA) reactive substances (TBARS), rose significantly to 8.58 g/100 g and 3.22 g/100 g. These results indicated a critical point within the 3 h cold chain interruption at 35°C, leading to pasteurized milk deterioration. During a 6 h interruption at 25°C, protease and lipase activities rose to 0.87 U/mL and 5.93 U/mL, while Bacillus species reached 13.18% relative abundance. Lipid hydro peroxides and TBARS increased to 8.01 and 3.26 g/100 g, indicating that a 6 h cold chain interruption at 25°C represented a critical time point for the deterioration of pasteurized milk stability. This study assessed pasteurized milk quality deterioration at 25°C and 35°C under cold chain interruption, providing a scientific basis for quality control during disruptions.