Development of a Thermoelectric-Based Thermal Death Time (TDT) Sandwich Device

A thermoelectric-based Thermal Death Time (TDT) sandwich device was developed with an active cooling feature to improve the precision and repeatability of microbial inactivation experiments. The system incorporates a Takagi–Sugeno (TS) fuzzy logic-based adaptive PID controller that can adjust PID gains in real time, ensuring stable temperature regulation and responsive during the heating, holding, and cooling phases. The fuzzy PID performed well compared to a conventional PID at 56, 58, and 60 °C, with 93.98%, 92.73%, and 81.21% of time hold within ±1 °C of the setpoint temperatures and lower RMSE values (0.60-0.74 °C vs. up to 1.60 °C). The device achieved rapid come-up times (44–50 s) and relatively fast cooling times of 22–26 seconds. The thermal performance was also evaluated across five diverse food matrices (blueberry juice, whole-fat milk, evaporated milk, chicken broth, and mashed potatoes), demonstrated consistent come-up times (43-45 seconds) and better thermal uniformity, with RMSE values ranging from 0.66 to 0.83°C, except chicken broth (1.08°C). Whole-fat milk and evaporated milk were kept within a ±1 °C tolerance level of around 88% of the holding time at 60°C. In contrast, mashed potatoes and blueberry juice were maintained at 72 and 78%, respectively. The isothermal inactivation of Escherichia coli K12 in blueberry juice was conducted utilizing the developed device. Isothermal treatments at 54 °C and 56 °C for 10 minutes resulted in decreases of 6.45 and 5.29 log CFU/mL, with D-values of 8.77 and 4.35 min, respectively. These findings demonstrate the device's ability to provide reliable thermal control and microbial lethality evaluation, which supports its use in food safety and thermal process validation.