Question

Repasteurization and prolonged holding of milk at pasterurization temperature leads to predominance of ________.

• A. Mesophils
• B. Thermophils
• C. Thermodurics
• D. Psychrotrophs

Hint:

To remember the difference:
- Thermodurics = Survive pasteurization but do not grow at high temperatures.
- Thermophils = Survive AND grow at pasteurization temperatures, so they multiply during prolonged hot holding.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Pasteurization is a heat treatment designed to kill pathogenic microorganisms and reduce the overall microbial load in milk.
However, different groups of bacteria respond differently to elevated temperatures based on their optimal growth ranges and thermal resistance profiles.
Detailed Explanation:
Let us evaluate how different bacterial groups respond to prolonged exposure or repeated exposure to pasteurization temperatures (typically $63^\circ\text{C}$ for 30 minutes in vat pasteurization or $72^\circ\text{C}$ for 15 seconds in HTST):
1. Mesophils: These organisms have an optimum growth temperature of $20^\circ\text{C}$ to $45^\circ\text{C}$.
They are easily killed by pasteurization temperatures and cannot survive prolonged holding.
2. Psychrotrophs: These organisms can grow at low temperatures ($7^\circ\text{C}$ or below).
They are highly heat-sensitive and are destroyed during pasteurization.
3. Thermodurics: These organisms can survive pasteurization temperatures, but they do not grow at these high temperatures.
Their optimum growth temperature is typically in the mesophilic range ($30^\circ\text{C}$ to $37^\circ\text{C}$).
4. Thermophils: These organisms have an optimum growth temperature of $55^\circ\text{C}$ to $65^\circ\text{C}$ and can grow at temperatures as high as $75^\circ\text{C}$.
When milk is subjected to repasteurization or held at pasteurization temperatures for extended periods, the high temperature acts as a selective enrichment environment.
Under these conditions, vegetative mesophils and psychrotrophs are destroyed, while thermophilic bacteria (such as Streptococcus thermophilus or Anoxybacillus flavithermus) thrive, multiply rapidly, and become the predominant microflora.

Step 2: Final Answer:
Repasteurization and prolonged holding at pasteurization temperature leads to the predominance of thermophils.