Question

The most common contaminant in dahi is

• A. Coliforms
• B. Yeasts
• C. Spores
• D. Anaerobes

Hint:

Fermented Milk Spoilage: In low-pH products (Dahi, Yoghurt, Cultured Butter), the MOST COMMON spoilage contaminants are ACID-TOLERANT YEASTS (Kluyveromyces, Candida).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Microbiological spoilage of fermented dairy products: low pH ($< 4.5$) suppresses standard bacteria while permitting acid-tolerant lactose-fermenting yeasts (Kluyveromyces marxianus, Candida) to cause gas, yeasty taints, and gassy frothing in dahi.
Key Formula or Approach:
\[ \text{Dahi Acidic Milieu (pH } 4.0 - 4.5\text{)} \implies \text{Suppresses Bacteria} \longrightarrow \mathbf{Yeasts \text{ (Acid-Tolerant Spoilage Dominates)}} \]

Step 2: Detailed Explanation:
In the microbiology and shelf-life of traditional Indian fermented milk (Dahi):
- During dahi fermentation, lactic acid bacteria (LAB) lower the pH to $4.0 - 4.5$ and produce antimicrobial diacetyl and bacteriocins, which naturally inhibit most vegetative bacteria, coliforms, and anaerobes.
- However, Yeasts (B) (such as Kluyveromyces marxianus, Candida pseudotropicalis, Debaryomyces hansenii, and Saccharomyces) are highly acid-tolerant and proliferate freely in low-pH environments.
- They ferment residual lactose and galactose, producing carbon dioxide gas (gassy bubbling, curd disruption) and alcohol/esters (fruity, yeasty, off-flavors), making Yeasts the most common post-production microbial contaminant and spoilage agent in dahi.

Step 3: Final Answer:
Therefore, the most common contaminant in dahi is Yeasts, matching option (B).