Question

The defects encountered in UHT milk during storage is

• A. loss of vitamins
• B. mold button formation
• C. gelation/coagulation
• D. leakiness

Hint:

Age gelation in UHT milk is caused by residual bacterial proteases that degrade $\kappa$-casein, which destabilizes the casein micelles and leads to coagulation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Ultra-high-temperature (UHT) treatment ($135^{\circ}\text{C}$ to $150^{\circ}\text{C}$ for 2 to 5 seconds) produces commercially sterile milk.
However, physical and chemical changes can still occur during extended storage at room temperature.

Step 2: Detailed Explanation:
The primary physical defect that limits the shelf life of UHT milk during storage is age gelation (gelation/coagulation).
This defect is characterized by a marked increase in viscosity, followed by the formation of a self-supporting, three-dimensional protein gel network.
This phenomenon is primarily driven by:
Residual heat-stable proteases produced by psychrotrophic bacteria (such as Pseudomonas spp.) that survive the UHT process and slowly hydrolyze caseins.
Physico-chemical changes, including the slow dissociation of $\beta$-lactoglobulin-$\kappa$-casein complexes from the surface of casein micelles.
While other changes occur:
Vitamin loss is a nutritional degradation rather than a structural product defect.
Mold button formation is a defect associated with sweetened condensed milk, caused by species of Aspergillus or Penicillium.
Leakiness is a packaging defect rather than an inherent milk quality issue.

Step 3: Final Answer:
Therefore, gelation/coagulation is the primary physical defect encountered in UHT milk during storage.