## What is DRT curve?

DRT curve or Decimal Reduction Time is the heating time in minutes at a constant temperature that will result in reducing the microorganisms by a factor of 10. It is also known as the D value.

**DRT curve** = Decimal Reduction Time (the time it takes to reduce 90% of the population at a given temperature or by a factor of 10)

This tutorial will examine the DRT curve (D value), a characteristic of microorganisms’ thermal destruction.

## How to obtain DRT curve?

So, if we take a homogeneous suspension of microorganisms and heat it at a constant temperature, we have the microorganism logarithmic order of death.

Now, we say commonly because certain variations are also observed. So, let’s plot the number of microorganisms per unit volume, N versus Time, t. When we heat microorganisms in this unit volume at a constant temperature, we will obtain an exponential plot, as shown in Figure 1 below.

This is an exponential decrease; we also call it a Survivor curve. Survivor curve, because the points on this curve represent survivors at any time when the suspension is being heated at a constant temperature.

### Two ways to obtain useful parameters for the DRT curve (D value)

We can convert the y-axis into a log scale to obtain some useful parameters. There are two ways of doing that. One, we can take that number of microorganisms and take a log of that. Two, we can plot the data on a semi-log paper. So, what we have here on the y-axis is the log scale. Then, we have 100,000, for example, for the number of microorganisms per unit volume.

If we plot that survival curve, we will notice that we get a straight line. As seen in the plot above, the time required for one log cycle reduction is the D value. The D value or DRT curve is obtained from the x-axis, where the unit is in time, t. So, the D value (DRT curve) is called the Decimal Reduction Time.

## Decimal Reduction Time (DRT)

**Decimal Reduction Time** is the heating time in minutes at a constant temperature that will reduce the population of microorganisms by a factor of 10. So, we see in the figure above that this D value represents the time for the number of microorganisms to decrease from 1,000 to 100 or by a factor of 10.

We can also say that the D value is the time for the survivor curve to traverse one log cycle. So, the D value remains the same for each of the log cycles on this curve. Thus, as we observed before, the reduction is by a factor of 10 from 1,000 to 100.

## Percentage Reduction (%R)

If we want to know the percent reduction in microorganisms during one log cycle, we can say 10,000 minus 1,000. The decrease during one log cycle is divided by the initial number of microorganisms.

**%R = 10,000 – 1,000/10,000 x 100 = 9000/10,000 x 100 = 90%**

We had 1,000 at the start of that log cycle, multiplied by 100, which gives us 900 over 1,000 in 100 or 90%. So, there is a 90% reduction in the number of survivors going through one log cycle. And that reduction of 90% for each log cycle will remain the same along the straight line.

## Observations on DRT curve:

- The DRT curve remains the same for each log cycle
- D value does not depend upon the initial number of microorganisms present in the suspension. We obtain the D value from one log cycle reduction in the microbial population.
- DRT curve is expressed in time units, most often as “minutes.”
- And again, it is a 90% reduction in the existing microbial population.

## Time it takes to decrease the microbial population from 100,000 to 1000

We can also observe from Figure 3, that if we were interested in finding the time it requires to decrease the microbial population from 100,000 to 1000, we need to heat that suspension at that constant temperature twice the decimal reduction time. So, the process will be two multiplied by D.

Keep in mind that the D value for microorganisms and nutrients present in food varies considerably. In fact, the D values for nutrients can be 2000 multiplied by the D value for microorganisms. For a large D value at that constant temperature, the 90% reduction will take much longer than another component in the food that may have a shorter DRT curve.

## Why can’t we use spectrophotometers for DRT curves?

DRT curve = Decimal Reduction Time (the time it takes to reduce 90% of the population at a given temperature or by a factor of 10)

We can’t use spectrophotometers for DRT curves because they would not distinguish between the live and dead cells. Dead cells block light rays from the bulb as heat doesn’t lyse cells.

## How do you calculate the decimal reduction time?

We calculate decimal reduction time by taking a population size, for example, 100,000 microorganisms at time t = 0. Temperature, T must remain constant. The decimal reduction time is the time it takes the population to decrease by a factor of 10, i.e., from 100,000 to 1,000 microorganisms.

## What would account for plateauing of the DRT curve?

Endospores would account for plateauing of the DRT curve because they are heat resistant.

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