Very hot milk : don't heat the milk more than the usual 160 F (70°C) otherwise the milk will surely curdle when adding any syrup.
Citric acids will surely curdle the milk so avoid them (any citric items and very acidic homemade syrups).
Citrus MONIN flavours with high citric acids content will curdle the milk so make sure to:
- Pour the syrup first with the espresso, stir for 10 to 15 seconds to decrease the acidic content then add the steamed milk
- Or decrease the citric content by just adding some simple syrups like Vanilla, or Pure Cane Sugar to the milk, and this works well
Dirty machines with unwashed steam wand can lead to that, because residues of citric acids will spoil the milk so clean your machine regularly.
Spoiled or expired milk : always check the expiry date. In the end this is the best way to control the milk!
Expired or damaged syrups will surely curdle the milk because of the increase in the acid content, so it's a way to check and control your syrups.
A good idea for not curdling the milk is by steaming the syrup with the milk, it will decrease the citric acids and will help surely. I tried it many times.
Try always to use steamed milk, better than cold milk, higher temperature prevents the proteins in the milk to coagulate and the milk to curdle.
