It's a genuinely unwary visitor who drinks the milk in a bachelor's fridge without first taking a cautious sniff to see if it has gone "off." Several families of bacteria are just as enthusiastic about milk's nutritional value as humans are, and most reveal their presence by excreting lactic acid. It turns the milk sour and unpalatable, but that doesn't mean it's unusable. The baking soda in many recipes requires soured milk to trigger its leavening power, and if you don't have the genuine article on hand, you'll need to produce it artificially.
How to Make Sour Milk
You'll need a few basic items, including a measuring spoon, a fork and lemon juice, vinegar or cream of tartar, to get started. Follow these steps for how to sour milk for a recipe. To sour milk for pancakes, or other sweeter recipes, be sure sour milk without vinegar. Measure the quantity of milk called for in your recipe.
Heat the milk in your microwave until it's just slightly warm, or leave it on your counter for an hour or two ahead of time so it comes to room temperature. This step is optional, but the milk will sour more quickly if it's warmer.
Stir in one tablespoon of lemon juice or vinegar, or half the amount, half a tablespoon, of cream of tartar, for every cup of soured milk your recipe requires. If you're using the cream of tartar, a dry acid, whisk it for several seconds with a fork to ensure that it's thoroughly dissolved. So, with a one to one ratio or one to half ratio when using cream of tartar, a recipe that calls for two cups of milk will require two tablespoons liquid acid or one tablespoon dry acid.
Rest the milk on your counter for three to five minutes, until it begins to separate or "clabber." This is normal, and indicates that the milk's casein proteins have begun to curdle and the milk is ready to use. Combine the now-soured milk with your remaining wet ingredients or stir it into your dry ingredients, as directed in the recipe.
Recipes Containing Additional Baking Soda
If your recipe contains more than half a teaspoon of baking soda, you might need to increase the quantity of acidity in the milk. If you've made the recipe previously and detected the distinctively bitter, soapy taste that baking soda often has, that is a giveaway that you need more acidity in the milk.
Add another half of a teaspoon of liquid acid, or a quarter teaspoon of dry acid, each time you make a batch. The baking soda taste should eventually vanish. Remember to use the same amount of liquid or dry acid in future. If you find the finished baked goods aren't as browned as they should be, you have used too much and should cut back slightly. If preparing a baked good for an event or party, try a small test batch first.
Alternative Sources of Acid
Plain white vinegar, apple cider vinegar and mild-flavored white wine or rice vinegar are all appropriate, alternative options. If compatible with the flavor of your baked goods, more adventurous alternative choices of acid, such as raspberry-flavored vinegar, can make an interesting variation while adding complexity to the dish's flavor profile.
An Alternatives to Buttermilk
You can also use milk soured this way as a substitute for buttermilk in recipes. It will provide an equivalent degree of leavening, though the end result will lack buttermilk's distinctive flavor. If you only use buttermilk infrequently, consider keeping dry buttermilk powder in your cupboard as an alternative. A quarter cup of buttermilk powder incorporated into most recipes, along with the soured milk, provides a very satisfactory substitute that fulfills both leavening needs and an accurate taste.