Curdling in milk has to do with a specific protein it contains called casein. Casein molecules normally float around in milk with a negative electrical charge, which evenly disperses them. Certain processes neutralise the charge, causing the casein molecules to clump, forming curds. Juices do not curdle on their own but can accelerate the curdling of milk in recipes that include both juice and milk. Adding acidic juices to milk, such as lime or orange juice, helps to neutralise the casein molecules, thus curdling milk faster. You can prevent curdling in milk and juice recipes using a variety of methods.
Use the freshest milk when combining it with juice. Fresher milk will last longer among the acidity of juices and you will have more time to use it before it curdles.
Store milk and juice in a cold refrigerator. Heat allows bacteria to grow which sours the milk, creating the acids that accelerate the neutralisation of casein molecules. Serve beverages containing milk and juice with ice to keep them colder longer.
Use milk with higher fat content. Skimmed milk curdles much faster than whole milk. Fat serves as a defence mechanism against the curdling process, and can add flavour to recipes with juice and milk such as smoothies.
Serve beverages containing both milk and juice immediately after preparing them. This keeps the ingredients fresh and minimises the opportunity for curdling.