Answer: The milk proteins were denatured due to the acidity of the orange juice.
Step-by-step explanation:
Any acidic solution, such as the orange juice in this example, would make milk curdle. The milk curdled because of the acidity of the orange juice made the milk proteins denatured.
That's because orange juice is an acidic solution while milk acts as a buffer, meaning it shields the acidity of the orange juice, making the milk curdle.