Step 1: Concept Overview:
"Sandiness" in dairy products refers to a gritty texture, resembling fine sand in the mouth. This question identifies the cause of this defect specifically in milk powder.
Step 2: In-Depth Explanation:
Lactose, the primary sugar in milk, exists in a supersaturated state. When water is removed during the production of milk powder, ice cream, or condensed milk, lactose concentration significantly increases.
If processing, particularly cooling and drying rates, isn't controlled precisely, lactose can crystallize, forming large, hard alpha-lactose monohydrate crystals. These crystals dissolve slowly in the mouth and are large enough to be detected on the tongue, resulting in the "sandy" texture.
Proteins, fats, and calcium are not responsible for sandiness.
Step 3: Conclusion:
Lactose crystallization causes the sandiness defect in milk powders and other concentrated dairy products.