Question

The length of the longest edge of lactose crystal to prevent sandiness defect in sweetened condensed milk should be not more than ______.

• A. 23 microns
• B. 12 microns
• C. 15 microns
• D. 29 microns

Hint:

To prevent sandiness in sweetened condensed milk, seed the concentrated product with fine lactose powder below 10 microns to encourage the formation of a large number of small crystals.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Sweetened condensed milk contains high concentrations of lactose, exceeding its solubility limit at room temperature.
To prevent the formation of large, gritty crystals that cause a "sandiness" defect, the crystallization process must be controlled.

Step 2: Detailed Explanation:
During the cooling of sweetened condensed milk, the product is seeded with lactose micro-crystals at a specific temperature (typically 30--32$^{\circ}$C).
This seeding promotes rapid, controlled crystallization, forming a large number of very small crystals rather than a few large ones.
The size of these lactose crystals determines the texture of the product:
1. Crystals smaller than 10--12 microns in length are not detectable by the human tongue, resulting in a smooth, velvety texture.
2. Crystals between 12 and 15 microns may occasionally be perceived as slightly gritty.
3. Crystals larger than 15 microns cause a distinct "sandy" mouthfeel, which is a sensory defect.
4. Crystals larger than 30 microns settled out of suspension, forming a sediment layer at the bottom of the container.
Therefore, to prevent the sandiness defect, the manufacturing process is controlled so that the maximum length of the longest edge of the lactose crystals does not exceed 12 microns.

Step 3: Final Answer:
Thus, the maximum crystal size to prevent sandiness is 12 microns.