The easiest way to solve this is to use as fresh as cream/milk as possible (age of the cream/milk is the biggest factor for this problem to occur) and let your coffee cool a little before adding cream or milk. To give you a brief explanation… All coffee is slightly acidic and made up of various organic acids. Lactic acid is one of these acids. When a coffee that is maybe a little higher in lactic acid is mixed with older milk (milk continues to build more and more lactic acid as it ages), then curdling can occur. The milk may not be spoiled enough to cause an off odor or flavor, but just enough acid and heat (in addition to its own) can cause curdling. Secondly, milk can also curdle when it is added to coffee that is too hot. So, in short, we would let your coffee cool a little bit more before adding your milk and also make sure you are using fresh milk.
