Purpose: To develop a non-isothermal DSC method in measuring DT and z-values.
Methods: The non-isothermal heat treatment used a differential scanning colorimeter (DSC) with a constant heating rate of 5°C/min, and the isothermal treatment a water bath using capillary tubes. In the DSC, Salmonella Senftenberg in skim milk was heated from 25 to target temperatures and in capillary tubes, it was heated at a constant 65.9, 67, and 68°C for various lengths of time. Under non-isothermal treatment, DT and z values were calculated by fitting the data to the following equation: log(Nt/N)=(z/(K*DTref*ln10))*(10^((T-Tref)/Z)). Under isothermal conditions, the standard log linear approach was used to calculate these values. All experiments were conducted with three or more replicates.
Results: Under isothermal heat treatment, the DT values at 65.9, 67, and 68°C were 0.37, 0.2 and 0.14 min, respectively, and the derived z-value was 4.9°C. Under non-isothermal treatment, it was found that heat transfer uniformly affected results requiring a correction to the target temperature. This was based on the energy flow profile (q=Cp*M*(target temperature increase-lapse)) and a temperature correction of 2.82°C was determined. When corrected to the skim milk temperatures of 66, 67, and 68°C, the respective DT- values were 0.43, 0.23, and 0.13 min, and the z-value is 3.8°C.
Significance: Incorporating the difference between the sample and programmed temperature, this non-isothermal approach aligns perfectly with the capillary tube method in measuring DT and z-values, and reduces revolutionarily the time and effort involved in measurement.