A high acid (sour) flavor in milk is caused by ___________________________.

The growth of bacteria in the milk is correct because certain bacteria can produce acidic byproducts as they metabolize lactose, the sugar found in milk. When these bacteria proliferate, they lower the pH of the milk, leading to sourness. This sour flavor often indicates spoilage, as the presence of unwanted bacteria signifies that the milk may not be safe to consume and has likely lost its fresh taste and quality.

While improper storage temperatures can accelerate bacterial growth, they don't specifically account for the presence of a high acid flavor on their own. Similarly, excessive processing can lead to changes in texture and flavor, but it is not the primary cause of high acidity. Lastly, milk coming from old cows may have different qualities, but this factor does not typically cause a high acid flavor. The sourness in milk is primarily a result of bacterial activity, which is why the option related to bacteria is the most appropriate answer.