What off-flavor is most likely found in milk that has not been properly cooked?

Sour off-flavors in milk are commonly associated with the presence of lactic acid bacteria, which can thrive if milk is not properly pasteurized or cooked. When pasteurization is inadequate, these bacteria can multiply, leading to fermentation of lactose into lactic acid. This shifts the milk's pH and results in a sour taste.

In contrast, other off-flavors have different causes. Fruity flavors are often linked to spoilage microorganisms that produce esters, while oxidized flavors arise from exposure to light or air, leading to the rancidity of fat. Rancid flavors are usually the result of lipolysis, where fat breaks down into free fatty acids, often associated with poor storage conditions rather than cooking. Therefore, sour is the most accurate description for an off-flavor resulting specifically from improper cooking or pasteurization.