Making the Milk

• Sour milk is milk that is starting to go bad. Buttermilk is a good substitute for sour milk, but it is a different product that is purposefully cultured to create a sour flavor. Buttermilk may also consist of the milk left over from the butter-churning process. Both sour milk and buttermilk impart a tangy flavor to recipes. You can simulate this flavor by adding 1 tablespoon of lemon juice for every 1 cup of milk. Let the mixture sit for five to 10 minutes before using in a recipe.

Why It Works

• The acid of the lemon juice makes the milk coagulate and become thicker. In traditional soured milk, bacteria does this work. The lemon juice option is safer as it doesn't impart the potentially harmful toxins of naturally soured milk.

Additional Substitutes

• Lemon juice can be added to any type of milk -- skim, low-fat or whole -- to create the soured milk effect. The ratio remains at 1 tablespoon of juice to 1 cup of milk. If you don't have any lemon juice, you could also sour 1 cup of milk with a tablespoon of vinegar or 2 tablespoons of cream of tartar. Mixing 1 part milk and 2 parts plain yogurt is another way to create the tang of sour milk.

Sour Milk Can Be Unhealthy

• Even if you have some sour milk on hand, you may opt for lemon-induced sour milk instead. Milk that tastes just slightly sour and isn't quite pleasant to drink may be fine for cooking and baking, but it may also contain high amounts of dangerous bacteria. Your safest bet is to dispose of milk that's starting to sour and acidify your own with lemon juice.