Question

The sequence of metabolic events in raw milk held at room temperature for long period is
(A) Streptococci converts lactose to lactic acid
(B) Mold grow on the surface and reduce acidity by oxidizing lactic acid to \(\text{CO}_2\) and \(\text{H}_2\text{O}\)
(C) Lactobacilli further converts remaining lactose to lactic acid
(D) Proteolytic sporeformers degrade casein fraction and lipolytic bacteria utilize fat fraction

• A. (A), (B), (C), (D).
• B. (A), (D), (C), (B).
• C. (A), (C), (B), (D).
• D. (C), (B), (D), (A).

Hint:

The logical pathway for raw milk fermentation is:
Acid production by Streptococci \(\rightarrow\) Further acid production by Lactobacilli \(\rightarrow\) Acid consumption by Molds \(\rightarrow\) Protein/fat decay by Sporeformers.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
When raw milk is held at room temperature without preservation, it undergoes a predictable ecological succession of microorganisms.
This sequence of metabolic events, known as "microbial succession," is driven by changes in pH, oxygen levels, and nutrient availability.

Step 2: Detailed Explanation:
The natural decomposition of raw milk proceeds through four distinct, overlapping stages:
1. Streptococci Development (A): Initially, raw milk has a near-neutral pH (\(\approx 6.6\)).
Fast-growing, homofermentative streptococci, such as Lactococcus lactis (formerly Streptococcus lactis), dominate the microflora.
They ferment the milk sugar lactose into lactic acid, lowering the pH to approximately \(4.5\), which eventually stops their own growth.
2. Lactobacilli Proliferation (C): As the environment becomes highly acidic, the more acid-tolerant lactobacilli (such as Lactobacillus casei and Lactobacillus bulgaricus) thrive.
They continue fermenting the remaining lactose, dropping the pH further to around \(4.0\text{--}3.5\).
3. Mold and Yeast Growth (B): The high acidity halts further bacterial fermentation.
Acid-tolerant yeasts and molds (such as Geotrichum candidum) then establish on the milk surface.
They consume the accumulated lactic acid, oxidizing it to carbon dioxide (\(\text{CO}_2\)) and water (\(\text{H}_2\text{O}\)), which gradually raises the pH toward neutrality.
4. Proteolysis and Lipolysis (D): Once the acidity is neutralized and the pH rises, proteolytic spore-forming bacilli (such as Bacillus and Clostridium species) and lipolytic bacteria multiply.
They degrade milk proteins (casein) and fats, causing curd digestion, rot, and bitter putrefaction.

Step 3: Final Answer:
The correct sequential order of metabolic events is (A) \(\rightarrow\) (C) \(\rightarrow\) (B) \(\rightarrow\) (D).