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Comparison of Thermal Resistance Parameter Measurement of Salmonella in Skim Milk between Isothermal and Non-Isothermal Heat Inactivation Methods

Food Science and Nutrition Project
Ren Yang
Graduate Student Seminar Days
Spring 2014

Isothermal heat inactivation is a traditional method to measure thermal death time parameters, DT and z. Because of the number of samples required this method is time and labor intensive. This study explored the measurement of DT and z values using a non-isothermal method which promises to significantly reduce the number of samples required for their calculation. To develop a non-isothermal method in measuring DT and z values. The non-isothermal heat treatment used a differential scanning colorimeter (DSC) with a constant heating rate of 5°C/minute, and the isothermal treatment a water bath using capillary tubes. In the DSC, Salmonella Senftenberg in skim milk was heated from 25ºC to target temperatures. In the capillary tubes, it was heated at a constant 65.9, 67, and 68ºC 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. Under isothermal heat treatment, the DT values at 65.9, 67, and 68°C were 0.49, 0.2 and 0.14 min minimum minute respectively, and the derived z value was 3.9ºC. Under non-isothermal treatment, a temperature delay was observed, requiring a correction to obtain the actual temperature. Based on the energy flow profile (q=Cp*M*(target temperature increase-lapse)), a temperature correction was determined for each experiment. When corrected to the skim milk temperatures of 66, 67, and 68ºC, the respective DT- values were 0.35, 0.195, and 0.114 min minimum minute. 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. It greatly reduces the time and effort involved in measurement when compared to the traditional capillary tube method.

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