Question

Sweet curdling defect in milk is caused by

• A. Staphylococcus aureus
• B. Pseudomonas fragi
• C. Bacillus cereus
• D. Aspergillus nigrifaciens

Hint:

Milk Spoilage Defects:
Sweet Curdling Bitty Cream = BACILLUS CEREUS (Extracellular rennet-like proteases + Lecithinase).
Acid Curdling = Lactic Acid Bacteria (Lactococcus lactis).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Enzymatic coagulation defects in pasteurized milk: psychrotrophic spore-forming Bacillus cereus secretes extracellular rennet-like chymotrypsin-like proteases that cleave casein at near-neutral pH without lactic acid production.
Key Formula or Approach:
\[ \text{Pasteurized Milk (pH } 6.6 - 6.8) \xrightarrow{\textit{Bacillus cereus} \text{ (Extracellular Metallo-Proteinases)}} \text{Casein Cleavage} \implies \mathbf{Sweet \text{ } Curdling / Coagulation (No Acid)} \]

Step 2: Detailed Explanation:
In fluid milk defects and thermoduric spore-forming bacteriology:
- Sweet Curdling (Sweet Coagulation) is a major spoilage defect of pasteurized refrigerated milk characterized by firm clot/curd formation at normal, near-neutral pH ($6.4 - 6.7$) without any lactic acid production.
- It is caused by the psychrotrophic spore-forming bacterium {Bacillus cereus (C) (and Bacillus subtilis, B. megaterium).
- Surviving pasteurization as heat-activated endospores, B. cereus germinates and secretes potent extracellular rennet-like proteolytic enzymes that specifically cleave the Phe105-Met106 bond of $\kappa$-casein, coagulating milk into a sweet curd (often accompanied by lecithinase-mediated 'bitty cream' defect).

Step 3: Final Answer:
Therefore, sweet curdling defect is caused by Bacillus cereus, matching option (C).