Gold–silver core-shell nanorods based time-temperature indicator for quality monitoring of pasteurized milk in the cold chain

A time-temperature indicator (TTI) based on gold-silver core-shell nanorods (Au@Ag NRs) was developed to monitor the quality degradation of pasteurized milk in the cold chain. Changes of longitudinal plasmon peak wavelength, micro-morphology and colour of the Au@Ag NRs TTI at 4 °C, 15 °C, 25 °C, and 35 °C were investigated, and kinetic characteristics and temperature dependence of the TTI were studied. Pasteurized milk was also stored at the same temperatures, and changes of titratable acidity (TA), pH values and total bacterial count (TBC) were determined and fitted to characterize the kinetic properties of each quality attributes at different storage temperatures. Besides, the Arrhenius equation was used to calculate the activation energy ( E a ) values, among which the Au@Ag NRs TTI, TA, pH and TBC showed E a values of 70.36, 65.67, 127.87 and 52.33 kJ/mol, respectively. Results showed that the TTI and TA possessed more similar E a values, suggesting that the TTI could predict the TA of pasteurized milk in the cold chain more accurately. Furthermore, a dynamic temperature test was performed to verify the applicability and reliability of the TTI. Temperature accumulation effects were calculated and effective temperatures ( T eff ) of the TTI and the pasteurized milk were determined as 287.4 K and 287.3 K, respectively, which indicated high sensitivity and good applicability of the TTI for quality monitoring of the real milk system, showing great application potentials for cold chain intelligent labels.