Question

Condensed milk is considered to be sandy, if the crystal size of lactose is more than -

• A. \(30\ \mu\)
• B. \(20\ \mu\)
• C. \(15\ \mu\)
• D. \(10\ \mu\)

Hint:

To ensure the formation of numerous, tiny, imperceptible crystals rather than a few large ones, condensed milk is "seeded" with very fine lactose crystals (particle size \({<} 1\ \mu\text{m}\)) at a specific temperature during cooling.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Sweetened condensed milk contains high concentrations of lactose.
During cooling, lactose tends to crystallize out of the supersaturated solution.
The size of these lactose crystals dictates the sensory mouthfeel of the final product.

Step 2: Detailed Explanation:
- Lactose crystal size \(< 10\ \mu\text{m}\): Completely imperceptible on the tongue, resulting in a smooth, creamy texture.
- Lactose crystal size \(11\text{ to }15\ \mu\text{m}\): May occasionally feel slightly powdery or mealy, but generally acceptable.
- Lactose crystal size \({>} 15\ \mu\text{m}\) (especially above \(15\text{--}20\ \mu\text{m}\)): The crystals are large enough to be felt as distinct, gritty particles by the sensory receptors of the tongue, a defect known as "sandiness."
Therefore, to prevent sandiness, crystallization is controlled (via seeding with fine lactose powder) to ensure crystal sizes remain below \(10\ \mu\text{m}\).

Step 3: Final Answer:
Condensed milk is considered sandy if the crystal size exceeds \(15\ \mu\), which corresponds to option (C).