The time required to kill 90% of microorganisms in a sample at a specified temperature is called: (Choose the single correct sterilization parameter.)

Introduction:
Thermal lethality parameters describe how microorganisms die under heat. This question targets the definition of the D value, widely used in sterilization and food processing validation.

Given Data / Assumptions:

• We are assessing the metric for a one-log (90%) reduction at a constant temperature.
• Other parameters (F value, thermal death time/point) have different meanings.

Concept / Approach:
The D value (decimal reduction time) is the time at a given temperature to reduce the viable population by 1 log (i.e., to 10% of its initial value). It reflects the slope of the survivor curve under isothermal conditions.

Step-by-Step Solution:
Define 90% kill at fixed temperature. Map to terminology: one-log reduction → D value. Differentiate from F value (integrated lethal time, usually at 121 °C) and from TDT/TDP. Select “D value.”

Verification / Alternative check:
Survivor curve mathematics shows N_t = N_0 * 10^(−t/D); when t = D, N_t = 0.1 * N_0 (90% kill).

Why Other Options Are Wrong:

• Thermal death time (TDT): Time to kill a defined population completely at a set temperature.
• Thermal death point (TDP): Lowest temperature that kills a test population in 10 minutes.
• F value: Equivalent exposure time at reference temperature (e.g., 121 °C) delivering required lethality.

Common Pitfalls:
Confusing “90% kill” (D value) with “complete kill” (TDT); they are distinct.

Final Answer:
D value is the time for 90% reduction at a specified temperature.