Difference Between D Value and Z Value

The key difference between D value and Z value is that D value is the time required to kill 90% of microorganisms at a specific temperature while  Z value measures the number of degrees the temperature should be increased to achieve a tenfold reduction in D value.

Thermal death time refers to the time taken for a specific bacterium to kill at a specific temperature. This calculation uses a measurement called Z value. The Z value is the number of degrees of temperature that has to be increased to achieve a tenfold reduction in the D value. D value concerns the time taken to kill a specific number of microorganisms (around 90%) at a constant temperature. In simple words, Z value is a measure of the change of the D value in varying temperatures. Therefore, Z value explains the resistance of an organism to varying temperatures. These measurements are important in different fields, especially during food canning, manufacturing cosmetics and pharmaceuticals and preparation of animal feeds.

CONTENTS

1. Overview and Key Difference
2. What is D Value 
3. What is Z Value
4. Similarities Between D Value and Z Value
5. Side by Side Comparison – D Value vs Z Value in Tabular Form
6. Summary

What is D Value?

Decimal reduction time or D value tells how long it will take to kill 90% of a specific bacterial population at a specific temperature. Therefore, the D-value of a bacterium measures the time required for a tenfold reduction (reduce the population by one decimal) in the bacterial population in a given medium, at a given temperature. After achieving a tenfold reduction or one decimal reduction, the remaining bacterial population would be only 10% of the original population. D value basically depends on the temperature, the type of the microorganism and the medium composition.

Concerning usefullness, this measurement is very important in understanding the effectiveness of thermal inactivation of bacteria under different conditions, especially in food cooking and preservation.  If we know the D value at a known temperature and the Z value, we can easily calculate the D value at an unknown temperature of that particular bacterium.

What is Z Value?

Z value is a measurement useful for the thermal death time calculation of a specific bacterium. It refers to the temperature change needed for the D-value to change by a factor of ten. Thus, it is a measure of the change of the D-value with varying temperatures. In simple words, it is the number of degrees the temperature has to be increased to achieve a tenfold reduction in the D-value.

As an example, if the Z value of a population is 10 ⁰C, we can simply reduce the population from a log reduction of the D-value by increasing the sterilization temperature 10 ⁰C. It tells us how susceptible a specific bacterial population is to changes in temperature.

What are the Similarities Between D Value and Z Value?

• Both D value and Z value are important in assessing the effectiveness of the thermal inactivation processes in different applications.
• Z value is related to D value, and it is the number of degrees Celsius needed to change a D-value by one factor of ten.
• Both D value and Z value measure the heat resistance of microorganisms.

What is the Difference Between D Value and Z Value?

D value is the time required to destroy 90% of the target microorganism in a given medium at a given temperature while Z value is the temperature change required to change the D value by a factor of 10. So, this is the key difference between D value and Z value. Besides, the D value is measured in minutes while Z value is measured in Celsius. Thus, the unit of measurement is another difference between D value and Z value.

Summary – D Value vs Z Value

D value measures the time required to kill 90% of the population of a particular microorganism in a specific medium at a specific temperature. In contrast, Z value is the temperature change that is required to achieve a tenfold reduction in the D-value. Therefore, this is the key difference between D value and Z value. Measurements of D value and Z value are useful in different areas including food canning, manufacturing cosmetics and pharmaceuticals, etc.

Reference:

1.“Thermal Destruction of Microorganisms.” Thermal Destruction of Microorganisms | Food Science, Available here.
2. “F0 Value, D Value and Z Value Calculations.” Pharmaceutical Guidelines, Available here.

Image Courtesy:

1. “D-value” By Giangagliardi – Own work (Public Domain) via Commons Wikimedia
2. “Z-value” By Giangagliardi – Own work (Public Domain) via Commons Wikimedia