Cream is made up of proteins, or casein, and fat suspended in liquid. When acid is introduced, the proteins coagulate, or curdle. Heavy cream is less likely to curdle than light cream, half and half or milk, because it has more fat and less casein. The fat coats the casein molecules that are present, safeguarding them from the acid.

Alcohol is naturally acidic. Acid levels are measured on the pH scale. 7.0 is considered neutral. Distilled water has a pH of 7.0. Anything lower than 7.0 is acidic, while anything with a pH between 7.1 and 14.0 is alkaline. Most wines, for example, have a pH between 2.8 and 3.8. Beers are slightly less acidic, with a pH range of 4.0 - 5.0. This acidity is what can curdle cream when the two are mixed.

If you make cocktails that curdle with a stable cream liqueur, such as Irish cream or cream tequila, the alcohol may not be to blame. The most common cause of curdling in beverages made with cream liqueurs are other, highly acidic ingredients such as citrus juice. Adding very low pH juice, such as orange or lemon juice, will curdle even a high-fat cream.

To prevent cream from curdling when mixed with alcohol, use a high fat percentage cream. Avoid low-fat alternatives such as milk or half and half. Be sure your cream is very fresh. Old cream is more likely to curdle. Avoid adding additional ingredients to your cocktail or sauce that bring extra acidity, such as citrus juice. If you are making a cooked sauce, whisk a spoonful of cornstarch into the sauce before you add the cream. The starch will help coat the casein, preventing curdling.