Cream liqueur is a combination of spirits, heavy cream and other flavorings. Most, though not all, are heavily sweetened. Irish cream is a common example of a cream liqueur. It is made of Irish whiskey, heavy cream, sugar and flavorings. Other popular cream liqueurs are made with rum, tequila or vodka. The cream softens the taste and strength of the alcohol while allowing the flavor to come through.

Cream liqueurs curdle because of their high dairy content. Dairy products, such as milk or cream, curdle in the presence of acid. Acids have a very low pH, which lowers the overall pH of the mixture. As the pH drops below 5.5, the casein proteins in the dairy ingredients begin to coagulate. This breaks the natural emulsion of fat and water in the dairy ingredients, causing the fat and proteins to separate from the water, which results in curdling.

Safeguard your cocktails against curdling by choosing high-fat cream liqueurs, or varieties made with heavy cream rather than milk. Heavy cream resists curdling because there are relatively few protein strands and plenty of fat to insulate them from the acid.

Mix other ingredients, including those with acidic components, first. This will dilute the acid, minimizing the chances of curdling. Add the cream liqueur at the very end, and mix lightly.

Avoid mixing cream liqueurs with high-acid mixers such as citrus juices and soft drinks. Most soft drinks do not taste sour, but they do contain high levels of citric and phosphoric acids.

Common mixers for cream liqueurs include coffee, hot chocolate and other cream liqueurs. Cream liqueurs are prevalent in milkshake-style cocktails, in which the dairy enhances the smooth mouthfeel of the ice cream. Cream liqueurs are also delicious served alone over ice.