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.
In addition to @FuzzyChef's answer, pineapple contains acids, mostly in the form of citric acid, but also malic acid and a few others in lesser quantities. The citric acid content ranges from between 0.4% to 1.2%, depending on the report (e.g. this one, PDF, probably paywalled). Because of these acids, the juice of pineapples has a pH of around 3-4, making it substantially more acid than milk with a pH of 6.7-6.9.
Acidity can curdle milk, as you will have noted if you have ever added lemon juice or vinegar to milk to make a substitute for buttermilk.
So, even if you do manage to kill off the bromelain by cooking you will still have to contend with the acidity, which will still curdle your milk.
Not sure if this will work for you, but personally I tend to sidestep the issue by using a milk substitute - typically soy milk - if I'm going to be mixing it with fresh pineapple.
If you do go for the cooking route, I wouldn't worry about the acidity so long as you're mixing the milk and cooked pineapple cold - but if you mix them while the pineapple's still warm, it's likely to curdle.