What indicator suggests that milk is no longer fresh?

Milk that is no longer fresh typically develops a sour smell and taste due to the proliferation of bacteria that can spoil the milk. These bacteria produce lactic acid as they metabolize lactose, leading to the characteristic sour flavor. Fresh milk should have a neutral smell and a mild flavor. When milk begins to sour, it indicates that it has undergone spoilage, making the smell and taste reliable indicators of freshness.

Other options do not provide clear signs of spoilage. For instance, milk being too cold does not denote freshness; rather, it’s important for maintaining freshness. The time since opening can vary based on storage conditions, but simply being opened for less than a week does not guarantee that the milk is fresh. Similarly, a high-fat content is an attribute of certain types of milk (like whole milk), not an indicator of freshness or spoilage. Thus, the odor and taste are the most definitive indicators of milk freshness.