There’s a simple explanation for why you may have experienced curdling when mixing your ingredients. Both the orange concentrate and the milk and ice cream need to be at the same temperature when mixed together to prevent any possible curdling.  Since I make use of ice cream in this Orange Julius recipe, I made sure to add frozen orange concentrate straight to the blender with the other cold ingredients. If you add room-temperature orange juice to ice cream and chilled milk, then you risk ending up with a curdled drink instead!