---
title: "Decimal Reduction Time (DRT) | Microbiology"
description: "Decimal reduction time (DRT) is the time needed to cut a microbial population by 90% at a set temperature, used to judge sterilization and food safety."
canonical: "https://fiveable.me/microbio/key-terms/decimal-reduction-time-drt"
type: "key-term"
subject: "Microbiology"
unit: "Unit 13"
---

# Decimal Reduction Time (DRT) | Microbiology

## Definition

Decimal reduction time (DRT), or D-value, is the time needed at a specific temperature to reduce a microbial population by 90%. In Microbiology, it helps measure how well heat or sterilization methods kill microbes.

## What It Is

Decimal reduction time (DRT) is the amount of time needed at a specific temperature to reduce a microbial population by one log, which means a 90% drop. If you start with 1,000 cells, one DRT leaves about 100. If you start with 100,000, one DRT leaves about 10,000. Microbiology classes often call this the D-value.

DRT is not a fixed property of a microbe in every situation. It depends on the organism, the temperature, and sometimes the environment around the cells. A spore-forming bacterium usually has a higher DRT than a less resistant vegetative cell, which means it survives longer under the same heat treatment. That is why some microbes are much harder to control than others.

The temperature part matters a lot. As temperature goes up, the DRT usually goes down, because heat damages proteins, membranes, and DNA faster. That is why a sterilization process can be described in terms of time and temperature together, not just one number by itself. The same organism may need a much shorter exposure at a higher temperature to get the same 90% reduction.

DRT is a practical way to describe killing curves. Microbial death in heat is often logarithmic, not a neat straight drop to zero. Each equal time interval removes a similar fraction of the remaining cells, so the population falls in steps of 90%, then 90% of what is left, and so on. That pattern is why DRT is useful for comparing methods and microbes.

In food preservation and medical sterilization, DRT helps you judge whether a treatment is strong enough. It can be used when looking at pasteurization, autoclaving, or other thermal control methods to see how long a target microbe can survive under a given condition. If the DRT is low, the microbe is easier to kill. If it is high, the process has to be longer or hotter to reach the same reduction.

## Why It Matters

Decimal reduction time shows up whenever Microbiology turns from naming microbes to controlling them. It gives you a way to compare how resistant different organisms are to heat, especially in the topic of controlling microbial growth. Without a number like DRT, saying a method is "hot enough" is too vague to use in labs, food processing, or sterilization planning.

It also connects the biology of the microbe to the effect of the treatment. A bacillus that forms endospores can survive much longer than a nonspore-forming cell, so DRT helps explain why some species are harder to eliminate. That makes it useful for understanding why one sterilization protocol works for one organism but not another.

In lab settings, DRT gives you a way to read survival curves and make sense of thermal death data. In food and medical contexts, it helps explain why time and temperature are paired together in safety standards. Once you know DRT, you can move from a simple idea like "heat kills microbes" to a more precise idea about how fast that killing happens.

## Connections

### Sterilization

Sterilization is the broader process DRT helps measure. A DRT value tells you how long a specific microbe survives at one temperature, which is useful when deciding whether a sterilization method will fully eliminate microbial life or just reduce it. In practice, DRT helps compare different heat treatments before you choose one for a lab, medical device, or food product.

### Thermal Death Time (TDT)

Thermal Death Time is the total time needed to kill all cells in a population at a given temperature, while DRT focuses on a 90% reduction. They are related, but they answer different questions. TDT is about complete killing, and DRT is about the speed of log reduction, which makes it a building block for understanding death curves.

### Z-value

Z-value describes how much the temperature must change to change the DRT by a factor of 10. That means DRT and Z-value work together when you compare heat resistance across temperatures. If you know one DRT at one temperature and the Z-value, you can estimate how the treatment will change when the temperature shifts.

### [Bacillus subtilis](/microbio/key-terms/bacillus-subtilis)

Bacillus subtilis is a useful example because Bacillus species are often heat resistant compared with many other bacteria. When you look at DRT data, spore-formers like Bacillus help show why some microbes survive heat treatment longer. That makes them good organisms for discussing why resistance varies so much across microbial groups.

## On the AP Exam

A quiz question might give you a survival curve or a heating scenario and ask what a DRT means. You should identify it as the time for a 90% reduction at a specific temperature, then use that idea to compare resistance across microbes or treatment conditions. If a graph shows a log drop in survivors, you may be asked to read how many DRT intervals passed.

In a lab report or problem set, you might explain why one sample needed a longer heat treatment than another. The best answer connects the DRT to the organism, the temperature, and the shape of the killing curve instead of just saying "it took longer."

## decimal reduction time (DRT) vs Thermal Death Time (TDT)

DRT and TDT both deal with heat killing microbes, but they are not the same. DRT measures the time for a 90% reduction, while TDT measures the time needed to kill everything in a population at one temperature. If a question asks about log reduction or surviving fraction, think DRT. If it asks when no viable cells remain, think TDT.

## Key Takeaways

- Decimal reduction time (DRT) is the time needed at a set temperature to reduce a microbial population by 90%.
- A lower DRT means a microbe is killed faster under that condition, while a higher DRT means it is more heat resistant.
- DRT changes with temperature, so a hotter treatment usually shortens the time needed for the same reduction.
- The term is useful because microbial death often happens in log steps, not all at once.
- You will see DRT when comparing sterilization methods, heat resistance, and survival curves in Microbiology.

## FAQs

### What is decimal reduction time (DRT) in Microbiology?

Decimal reduction time, or DRT, is the time required at a specific temperature to reduce a microbial population by 90%. It is also called the D-value. Microbiology uses it to describe how quickly heat or another thermal treatment lowers the number of surviving microbes.

### Is decimal reduction time the same as Thermal Death Time?

No. DRT measures a one log, or 90%, reduction in the population, while Thermal Death Time measures the time needed to kill all cells at a given temperature. They are related because both describe microbial death under heat, but they answer different questions.

### Why does temperature change DRT?

Higher temperatures damage microbial cells faster, so the DRT usually gets shorter as temperature rises. That means the same microbe may need a much longer time at a lower temperature than at a higher one. This is why time and temperature are usually discussed together in sterilization and food safety.

### How do you use DRT in a microbiology lab question?

You use it to interpret survival data or compare resistance between microbes. If a problem gives you a heat treatment and a starting population, DRT helps you figure out how many log reductions happened or which organism is harder to kill. It is a common way to read thermal death curves.

## Related Study Guides

- [13.1 Controlling Microbial Growth](/microbio/unit-13/1-controlling-microbial-growth/study-guide/HjGpZtFLLj9zmkNv)

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