To avoid curdling in acidic liquids combined with milk or cream please read the followings
A stock needs to be thickened before adding milk or thicken the milk before adding it to stock. Roux or cornstarch can be used as a thickened agent .
Never add cold milk to hot stock. If milk or cream needs to be added to a hot soup, heat the milk first before adding or temper the milk by adding small amount of soup to liquid until same temperature is reached.
Add milk or cream only at the end of making a sauce or soups and never boil after is added.
Tip. Lemon juice for example will curd milk very fast, add lemon juice at the end of the sauce or cream soups when all ready thicken. Same is valid for tomato sauce or juice.
If any curdling occur add more thickening agent.