Which of the following is a sign of spoilage in dairy products?

A sign of spoilage in dairy products is an unusual odor. Spoilage often results from the growth of bacteria, molds, or yeast, which can produce off-putting smells that are distinctly different from the natural scent of fresh dairy items. When dairy products begin to spoil, the breakdown of proteins and fats can lead to the formation of various compounds that generate unpleasant aromas, indicating that the product is no longer safe for consumption. An unusual odor typically signifies that microbial activity has occurred, which raises concerns about food safety.

The other options either do not indicate spoilage or may be characteristic of certain dairy products under specific conditions. Opaque appearance can occur naturally in certain types of dairy or due to dissolving or mixing many ingredients, while normal acidity is typical in fermented dairy products and does not indicate spoilage. Consistent texture suggests that the product remains stable and has not undergone changes typically associated with spoilage. Thus, an unusual odor is a clear and direct indicator of spoilage in dairy products.