Introduction to Food Engineering

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Thermal Resistance Characteristics of Microorganisms

Learning Outcomes

At the end of this module, you will be able to:

  1. describe a microbial survivor curve and determine D-value from thermal inactivation data
  2. determine z-value from given data
  3. describe how to use rate constant, k, and activation energy, Ea, to describe thermal resistance characteristics

Thermal processing of foods is about pasteurizing and sterilizing foods to increase their shelf life. To design thermal processes, we need an understanding of how the application of heat inactivates spoilage or pathogenic bacteria. In this module, we will explore various parameters used in describing thermal effects on bacteria, such as D-value, z-value, and activation energy. In the following video, you will learn an essential concept, called Decimal Reduction Time or D-value.

In the next video, you will view a solved example using a spreadsheet to obtain D-value for the given data.

The influence of temperature on D-value is expressed by another parameter called z-value. In the following video, you will learn how to describe z-value and a solved example about calculating z-value from given data using a spreadsheet.

We can represent thermal resistance characteristics such as D-value and z-value with terms commonly used in chemical kinetics such as the rate constant, k, and activation energy, Ea. In the next video, you will learn how to convert D value into rate constant, k.

In the next video, we will consider how z-value and activation energies are related.


In this module, you learned thermal resistance characteristics of microorganisms such as D-value and z-value. You also learned how you might obtain these values from given data on thermal inactivation of bacteria. We also explored the relationship between these thermal parameters and coefficients used in describing chemical kinetics, such as rate constant, k, and activation energy, Ea.