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? 
Pineapple contains bromelain, which is a powerful enzyme that breaks down most animal proteins.  As such fresh pineapple will always curdle milk.  The bitter flavor is a side-effect of this curdling. The only way to prevent this is to break down the bromelain by cooking the pineapple.  So you can make a smoothie either by cooking the pineapple yourself, or by using canned pineapple. 