Question

The decimal reduction time 'D' is defined as the time necessary for reduction in the microbial population by

• A. 70%
• B. 80%
• C. 90%
• D. 100%

Hint:

Thermal Processing Values:
D-VALUE = Time to reduce microbial population by 90% (1-log reduction).
Z-VALUE = Temperature increase required to reduce D-value by 90% (10-fold).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Thermal bacteriology and microbial death kinetics: the Decimal Reduction Time ($D$-value) is the time required at a specified constant temperature to reduce a microbial vegetative or spore population by one logarithmic cycle (by 90% or to one-tenth of its initial count).
Key Formula or Approach:
\[ \log_{10} \left( \frac{N_0}{N} \right) = \frac{t}{D} \quad \xrightarrow{N = 0.10 N_0 \text{ (90\% Destruction)}} \quad \log_{10}(10) = 1 = \frac{t}{D} \implies \mathbf{t = D} \]

Step 2: Detailed Explanation:
In thermal processing and food canning microbiology (Bigelow and Ball kinetics):
- Thermal destruction of microorganisms follows first-order reaction kinetics:
\[ \frac{dN}{dt} = -k N \implies \log_{10}\left(\frac{N_0}{N}\right) = \frac{t}{D} \]
- The Decimal Reduction Time (D-value) (C):
1. Defined as the exposure time (in minutes or seconds) at a given constant temperature required to destroy 90% of the viable microbial population (reducing surviving population from $N_0$ to $0.10 N_0$, or a 1-log reduction).
2. For example, if $N_0 = 100,000\text{ cfu/mL}$, after $1\;D$, remaining count is $10,000\text{ cfu/mL}$ ($90\%$ killed).
3. (A $12D$ process for Clostridium botulinum achieves a $10^{12}$-fold or $99.9999999999\%$ destruction).

Step 3: Final Answer:
Hence, decimal reduction time 'D' is the time for reduction in microbial population by 90%, matching option (C).