Which of the following is a common problem caused by improper storage temperatures of milk?

Improper storage temperatures of milk can lead to a variety of quality issues, one of which is the development of a sour smell. This unpleasant odor is typically caused by the growth of spoilage bacteria that thrive in warmer temperatures. When milk is stored outside the recommended temperature range, these bacteria can multiply rapidly and begin to produce lactic acid, which increases acidity and results in the sour smell associated with spoiled milk.

The other options do not accurately reflect issues that arise from improper storage temperatures. Increased viscosity may occur due to other factors, such as the presence of certain additives or the natural process of cream separation, but it is not a direct result of temperature misuse. Improved nutritional value contradicts the effects of improper storage, as spoilage generally leads to a decrease in quality and safety. Finally, crystallization of lactose is more related to drying processes and freezing rather than improper temperature during storage, as lactose typically remains dissolved in liquid milk under proper conditions. Thus, the sour smell is a clear indicator of spoilage linked to improper storage temperatures.