A Novel Method to Determine Thermal Death Kinetics of Microorganisms in Low-moisture Foods: Thermal-Death-Time Sandwich

Introduction: Traditionally, the thermal death kinetics of microorganisms in foods are determined using tubes, disks, pouches, or similar vessels in a water or oil bath. However, this method suffers from prolonged come-up time (CUT) for low moisture foods. A long CUT results in microbial destruction during the CUT which confounds the results. A heating device with fast-responding controls can reduce the CUT, thus increasing the precision of the thermal destruction parameters.

Purpose: This study was undertaken to develop a precision controlled heating system that reduces CUT and, subsequently, compare it to traditional thermal-death-time (TDT) disks immersed in an oil bath.

Methods: A TDT sandwich consists of a sample packaged in an aluminum-lined pouch that is sandwiched between resistive heating pads adhered to aluminum plates. The TDT sandwich was insulated with ceramic fiber boards and controlled with an Arduino microcontroller. Pouches filled with 10.0 g of water were heated to 90°C for 15 min to test the permeability of the pouches. The CUT (time taken to reach 90±0.5°C) of the TDT sandwich was compared against traditional oil bath method using 0.5 g samples of ground black pepper. Samples for the oil bath were packaged in aluminum TDT disks. All samples were tested in at least triplicate.

Results: The moisture loss in water samples after heating was 0.00027±0.00016 g, indicating low moisture permeability of the pouch material at 90°C. The ground black pepper CUT to attain 90°C was 26.3±1.7 s using the TDT sandwich system, whereas the CUT in the oil bath was between 190.2±26.6 s and 283.2±35.1 s, depending on the location.

Significance: The TDT sandwich method had shorter and more consistent CUTs compared to the traditional oil bath method. The low moisture permeability and dry nature of the pouches also avoided leakage or contamination of samples during thermal treatment.