Question

Heat stable proteinases of psychrotrophic organisms can lead to which one of the following defects in UHT milk?

• A. Souring
• B. Feathering
• C. Age thickening
• D. Age gelation

Hint:

UHT Milk Spoilage Defects: Heat-stable Psychrotrophic Proteases (Pseudomonas) $\rightarrow$ Cause AGE GELATION (Enzymatic clotting during ambient storage). Heat-stable Lipases $\rightarrow$ Rancidity.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Enzymatic shelf-life limitation in UHT milk: heat-stable metalloproteinases (AprX) secreted by raw milk psychrotrophic Pseudomonas survive commercial UHT sterilization ($140^\circ ext{C}$ for 4 s) and slowly hydrolyze micellar $\kappa$-casein during room temperature storage, inducing irreversible enzymatic Age Gelation.
Key Formula or Approach:
\[ \text{Residual Heat-Stable Protease (AprX)} + \kappa\text{-Casein} \xrightarrow{\text{Ambient Storage (3--6 Months)}} \text{Casein Network Cross-linking} \implies \mathbf{Age \text{ } Gelation \text{ (Custard-like Gel)}} \]

Step 2: Detailed Explanation:
In UHT processing and long-life milk storage defects:
- Psychrotrophic bacteria (predominantly {Pseudomonas fluorescens and Pseudomonas fragi) grow in refrigerated raw milk and secrete extracellular metalloproteinases (e.g., AprX protease).
- These bacterial proteases possess extreme thermal stability ($D_{140^\circ ext{C}} = 1 - 2\text{ minutes}$); they survive commercial Ultra-High Temperature (UHT) sterilization ($138^\circ - 142^\circ ext{C}$ for 2--4 seconds) with $> 10 - 20\%$ residual enzymatic activity.
- During subsequent room temperature storage of sterile UHT milk carton packages:
- The residual protease slowly hydrolyzes $\kappa$-casein and $\beta$-casein over 2 to 6 months, releasing hydrophobic peptide domains that form a continuous rigid three-dimensional protein matrix, causing the catastrophic defect of Age Gelation (D) (turning liquid milk into a solid, custard-like irreversible gel).

Step 3: Final Answer:
Hence, heat stable proteinases lead to Age gelation in UHT milk, matching option (D).