# Kill Step Log Reduction Time Calculator

> Source: https://netray.co/tools/kill-step-log-reduction-time-calculator
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Find the hold time needed for a target log reduction from D-value, z-value and process temperature, with a sweep showing time against temperature.

## Category

quality

## Industry

Food and Beverage

## Audience

- Food safety manager
- Process engineer
- Quality engineer

## Tool

**Kind:** calculator

**Inputs:**

#### D-value at reference temperature

**Key:** d_ref

**Type:** number

**Default:** 1.2

**Min:** 0.001

**Step:** 0.1

**Unit:** min

**Help:** Minutes for a 1-log reduction at the reference temperature.

#### Reference temperature

**Key:** t_ref

**Type:** number

**Default:** 65

**Min:** 40

**Max:** 130

**Step:** 0.5

**Unit:** C

#### z-value

**Key:** z

**Type:** number

**Default:** 6

**Min:** 1

**Step:** 0.5

**Unit:** C

**Help:** Temperature rise that cuts D by a factor of ten.

#### Process temperature

**Key:** temp

**Type:** number

**Default:** 72

**Min:** 40

**Max:** 130

**Step:** 0.5

**Unit:** C

#### Target log reduction

**Key:** logs

**Type:** slider

**Default:** 5

**Min:** 1

**Max:** 12

**Step:** 1

**Unit:** log

**Outputs:**

#### D-value at process temperature

**Key:** d_t

**Expr:** d_ref * pow(10, (t_ref - temp) / z)

**Format:** decimal

**Description:** Minutes per log at the process temperature.

#### Hold time required

**Key:** time_min

**Expr:** logs * d_t

**Format:** minutes

**Primary:** true

#### Hold time in seconds

**Key:** time_sec

**Expr:** time_min * 60

**Format:** number

**Primary:** true

#### Fraction surviving

**Key:** survivors

**Expr:** pow(10, 0 - logs) * 100

**Format:** percent

**Chart:**

#### Hold time (min) vs process temperature

**Type:** line

**X:** temp

**From:** 60

**To:** 80

**Steps:** 21

**Series:**

- time_min

**Disclaimer:** D and z values are organism, matrix and water activity specific. Use values from validated literature or your own challenge study, and add a safety margin. This is not a substitute for a validation study.

## Content

**Intro:** Most thermal kill steps are specified as a log reduction of a target pathogen. Given a D-value and z-value, this tool gives the hold time at any process temperature.

**Sections:**

#### How to use it

**Body:** Enter the D-value you have at a reference temperature, the z-value, the temperature you will actually run and the number of logs you need. The tool adjusts D to your process temperature and multiplies by the target logs. Use the sweep chart to see how quickly the required time collapses as temperature rises, which is useful when deciding between a lower-temperature long hold and a hotter short hold.

**Bullets:**

- D-value is time for one log, 90 percent kill
- z-value is degrees per tenfold change in D
- Use the coldest point of the product for temperature

#### Method and formula

**Body:** D at temperature T equals D at reference multiplied by 10 raised to (reference minus T) divided by z. Required time equals logs times D at T. Fraction surviving equals 10 to the minus logs. This is the classical log-linear model; it ignores shoulders and tailing that show up in dry or high-fat products, so validation data wins over the model.

#### Worked example and mistakes

**Body:** With D of 1.2 minutes at 65 C and z of 6 C, running at 72 C gives D of about 0.08 minutes, so a 5-log reduction takes about 0.4 minutes, roughly 24 seconds. Mistakes to avoid: taking D values from a buffer when your product is a high-fat paste, mixing Celsius and Fahrenheit, and forgetting that come-up time at the product cold spot is not hold time.

**Cta:** If kill step records and deviations live in several systems, Netray can build a read-only view over your ERP and quality data on your own servers.

**Faq:**

#### What log reduction do I need?

**Answer:** It depends on the hazard and regulation or customer requirement. Many validated lethality steps target 5 log for Salmonella in low-moisture foods and higher for ready-to-eat meat. Your HACCP plan and validation study set the number.

#### Where do D and z values come from?

**Answer:** Peer-reviewed literature, regulator guidance and your own challenge studies. Values vary by strain, matrix, pH and water activity, so use ones matched to your product and apply a safety factor.

#### Why does a few degrees change the time so much?

**Answer:** The relationship is exponential. With a z-value of 6 C, each 6 C rise cuts the time to one tenth. That is why accurate temperature measurement is the key control in a kill step.

#### Does this work for cold plasma or high pressure processing?

**Answer:** No. The D and z model describes thermal kill. Non-thermal processes use different dose-response models and validations.

## Search terms

log reduction calculator, D value z value calculator, pasteurization hold time calculator, 5 log reduction time, thermal death time calculator, kill step validation food, lethality log reduction formula, salmonella log reduction time

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