Difference Between TDP and TDT

The key difference between TDP and TDT is that thermal death point (TDP) is the lowest temperature at which all microbes are killed in a 10-minute exposure while thermal death time (TDT) is the time it takes to kill a specific bacterium at a specific temperature.

Heat is one of the most common forms used in microbial control. Dry heat and moist heat are two forms of heat. Moist heat is more effective than dry heat. Heat alters cell membranes and denatures proteins of microorganisms, causing their thermal death. Thermal death point and thermal death time are two parameters related to heat sterilization. Thermal death point is the lowest temperature at which all microbes are killed in a 10-minute exposure. Thermal death time is the length of time needed to kill all microorganisms in a sample at a given temperature.

CONTENTS

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

What is TDP?

Thermal death point refers to the lowest temperature that kills a population of a target microorganism within 10 minutes. At this TDP, all microbes are killed in a 10-minute exposure. TDP is one aspect that measures thermal death of microorganisms.  TDP is an important measurement when purifying water through boiling. The TDP of E. coli is 80 ⁰C. It means, E. coli population in a solution can be killed within 10 minutes at 80 ⁰C.

The thermal death point varies among different microorganisms. Bacillus subtilis and Clostridium perfringens have high TDPs. Those two bacteria produce heat resistant spores. Therefore, in order to kill their spores within 10minutes, a high temperature is required.

What is TDT?

Thermal death time refers to the time taken to kill a population of the target microorganism at a specific temperature. Hence, it measures how long it will take to kill all microbes in a sample in a given temperature. It can be easily measured when the microorganisms are in a water-based solution.

Generally, the TDT of microorganisms varies because different microorganisms have different temperature tolerance levels. For example, Thermophilus aquaticus is an extremely thermophilic bacterium which is active across a wide range of temperature. It can tolerate extremely high temperatures. In contrast, E. coli is a heat-sensitive bacterium which can tolerate 4 to 45 ⁰C and grows best at 37 ⁰C. It cannot tolerate high temperatures. Therefore, the thermal death time of E. coli is less than Thermophilus aquaticus. Another example is that the thermal death time of B. subtilis is 20 minutes at 100 °C.

TDT can be measured using a graph or using a mathematical formula. Thermal death time calculation often uses a measurement called Z value. Therefore, Z value is the number of degree of temperature that has to be increased to achieve tenfold reduction in the D value. D value is concerned with the time taken to kill a specific number of microorganisms (around 90%) at a constant temperature.

TDT was developed for several applications at the beginning. They are food canning, production of cosmetics, producing salmonella-free feeds for animals (e.g. poultry) and pharmaceuticals, etc. Nowadays, TDT is used frequently in the food industry to ensure food safety.

What are the Similarities Between TDP and TDT?

• TDP and TDT are two aspects of measuring thermal death of microorganisms.
• These parameters are often used to describe sterilization procedures that use high heat.
• TDP and TDT vary among the microorganisms.

What is the Difference Between TDP and TDT?

TDP refers to the lowest temperature needed to kill a microbial population in a liquid culture within 10 minutes, while TDT refers to the time required to kill a microbial population in liquid culture at a given temperature. So, this is the key difference between TDP and TDT. In short, TDP is a measurement of temperature, while TDT is a measurement of time.

The below info-graphic lists the differences between TDP and TDT in tabular form.

Summary – TDP vs TDT

Heat tolerance varies among different species of microorganisms. When considering the thermal death of a microorganism, it is necessary to pay attention to the temperature of exposure and the time required at a given temperature in order to kill the population completely. TDP and TDT are two aspects. TDP is the lowest temperature required to kill a population of microorganism within 10 minutes. TDT, on the other hand,  is the time required to kill a population of the target microorganism in a sample at a given temperature. Thus, this is the key difference between TDP and TDT.

Reference:

1. “Thermal Death.” Encyclopedia.com, Encyclopedia.com, 28 Dec. 2020, Available here.
2. “Thermal Destruction of Microorganisms.” Thermal Destruction of Microorganisms | Food Science, Available here.

Image Courtesy:

1. “Autoclave for trash” By Matylda Sękpl.wiki: Cygaretkacommons: Cygaretka – Own work (CC BY-SA 3.0) via Commons Wikimedia