Question

In the HTST (High-Temperature Short-Time) pasteurization of milk, the minimum holding time of 15 seconds at $72^\circ\text{C}$ is mathematically derived to ensure ______.

• A. a 12-log reduction of Clostridium botulinum
• B. a 5-log reduction of Coxiella burnetii
• C. complete inactivation of Alkaline Phosphatase only
• D. total destruction of thermophilic spores

Hint:

Coxiella burnetii is the most heat-resistant vegetative pathogen in raw milk. Milk pasteurization kinetics are designed to achieve a $5$-log reduction ($5\text{-D}$ kill) of this specific bacterium.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the primary microbiological target and log-reduction standard used to mathematically derive the minimum thermal process parameters ($72^\circ\text{C}$ for $15$ seconds) for HTST pasteurization of milk.

Step 2: Key Formula or Approach:
Thermal death kinetics of microorganisms follow a first-order reaction:
\[ \log \left( \frac{N_0}{N_t} \right) = \frac{t}{D} \] where:
$N_0$ = initial bacterial population,
$N_t$ = surviving population after time $t$,
$t$ = heating holding time ($15$ seconds),
$D$ = decimal reduction time at $72^\circ\text{C}$ for the target pathogen.
To achieve a $5$-log reduction:
\[ \log(10^5) = 5 \implies t = 5 \times D \]

Step 3: Detailed Explanation:

• Target Pathogen: Coxiella burnetii (the causative agent of Q-fever) is identified as the most heat-resistant, non-spore-forming pathogen commonly found in raw milk.

• Pasteurization Standard: To ensure public safety, thermal milk pasteurization must reduce the population of this target pathogen by at least $5$ orders of magnitude ($99.999\%$ destruction or a $5\text{-D}$ reduction).

• Kinetic Derivation:

• At $72^\circ\text{C}$, the $D$-value of Coxiella burnetii in milk is approximately $3$ seconds.

• Therefore, to achieve a safe $5$-log reduction, the minimum thermal holding time must be:
\[ t = 5 \times D = 5 \times 3\text{ s} = 15\text{ seconds} \]
• Alkaline Phosphatase (ALP): ALP is an endogenous milk enzyme with a heat inactivation profile slightly more stable than Coxiella burnetii. While its inactivation confirms successful pasteurization, it is used as a chemical indicator, not as the primary target for microbial safety calculations.

• 12-log reduction of C. botulinum: This is the target for commercial sterilization of low-acid canned foods (the "botulinum cook"), not pasteurization.

Step 4: Final Answer:
The minimum pasteurization parameters of $72^\circ\text{C}$ for $15$ seconds are derived to guarantee a $5$-log reduction of Coxiella burnetii, corresponding to option (B).