Milk that turns sour on its own in the refrigerator has traditionally been used in baking, but it is not considered a safe ingredient because the microorganisms that caused it to sour are uncontrolled. There is a way to get the same result safely: add a few drops of lemon juice or vinegar to fresh milk. Milk with a sharp odor, lumps, a changed color, or mold should unquestionably be discarded.
In older cooking traditions, soured milk is an ingredient: it is used in cakes, pancakes, and some other baked goods, where it adds acidity and tenderness. This practice was based on raw milk souring naturally due to lactic acid bacteria. Today’s pasteurized milk is different: its lactic acid bacteria have already been reduced, and what causes it to sour is spoilage microorganisms. In other words, the same-looking result does not come about in the same way. So soured milk in an old recipe is not the same as milk from today’s refrigerator. The same distinction applies to souring butter and cream.
This distinction has a practical consequence. If pasteurized milk is forgotten in the refrigerator and turns sour, you do not know which microorganism has multiplied; even if the smell is mild, its contents are uncontrolled. Baking does not make everything safe, because some spoilage products are heat-resistant. For this reason, the cautious choice is not to use milk that has soured on its own. The same caution applies to homemade cheese and curds made from soured milk.
The good news is that recipes do not actually call for soured milk, but for acidified milk. You can make it safely in a few seconds. Add 1 tablespoon (15 ml measure) of lemon juice or white vinegar to 1 measure of fresh milk, stir, and let it stand for a few minutes; the milk will curdle slightly and reach exactly the desired consistency. This mixture can replace soured milk in cakes, pancakes, and savory pastries measure for measure. Do not use warm milk; acid works more evenly and predictably in cold milk.
It is easier to decide what to use when you know what the acid does in a recipe. Acid reacts with baking soda to start leavening; it also loosens the gluten network and softens the crumb. For this reason, recipes using baking soda often require an acidic liquid. Yogurt, ayran, and whey do the same job; use whichever is available, adjusting the amount as needed. If you use yogurt, thin it with some milk because its thickness changes the liquid balance of the recipe. Check the measurement too: since yogurt is thicker than milk, increase the recipe’s liquid slightly.
The discard criteria are clear and nonnegotiable. Discard milk if it smells sharp or bitter, has lumps, has turned yellow or gray, or has mold on its surface. Discard a swollen or puffed-up carton without opening it. If in doubt, do not taste it; tasting it to check is not a safe method. Use opened milk within a few days and keep the cap tightly closed. Smelling the milk each time you open the carton is also a simple, useful habit.
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