Today I made Pineapple smoothie with milk. It was tasty. But I kept it in fridge cause I wanted the drink to be even cooler. I forgot that pineapple will curdle the milk and turned the smoothie bitter. However, what I don't understand is that I always thought curdling happens with heat. And I used chilled milk with ice cubes and even frozen pineapple and I immediately put in the refrigerator. Is there any way to make pineapple smoothie with milk/curd and not let it get bitter?