If you're a thrifty soul who has never met a failure you couldn't repurpose, spoiled milk is just a challenge in disguise. Milk that's not only past its sell-by date but its use-by date is always an iffy proposition, as is milk that tastes slightly off. Instead of tossing that carton, use its formerly suspect contents to lighten up your baked goods.
So Bad it's Good
Slightly soured milk is considered safe for use in baking because the oven's high temperatures kill harmful bacteria that cause food poisoning. Sour milk has a leavening effect on baked goods. The acidity in sour milk reacts when mixed with baking powder or baking soda by making gas bubbles. That reaction, in turn, results in food that's moister and lighter. Like buttermilk, sour milk imparts a tangy flavor that keeps baked goods for being overly rich or sweet.
There's definitely such a thing as too sour when it comes to sour milk. Make an assessment about whether your outdated milk is fine or foul. Usable sour milk has a tangy scent. This is the optimal stage for use as a baking ingredient -- when it's past its prime as a beverage, but not full of lumps and repulsive to taste or smell. If you're not sure where the outdated milk falls on that spectrum, play it safe and throw it out. You should not drink out-of-date milk.
Lighten the Mood
Use slightly spoiled milk to lighten and tenderize baked goods, especially when the recipe calls for buttermilk or sour milk. Using sour milk instead of regular milk in baking prevents cornbread and sweet muffins from turning dense or tough. Other baked goods improved by the tangy taste and lightening effect include coffee cake, cobbler batter, pound cake and baked puddings.
If the sour milk you were planning to salvage turns overly foul, you can still make that recipe you have your heart set on. Buttermilk is the most obvious alternative. If you don't want to make an extra trip to the store, try an equal amount of plain yogurt or sour cream in recipes for which the thicker texture will be a benefit. If something more akin to the thickness of buttermilk or sour milk is required, "spoil" regular milk by adding 1 tablespoon of lemon or vinegar, or 1 3/4 tablespoons cream of tartar for every 1 cup of plain milk.