Sour milk is not necessarily bad unless it's ultra-pasteurized. There could still be ways of putting it to good use.
If you have some milk that’s gone sour in the fridge before you were able to drink it, don’t dump it down the drain. There may be some way of salvaging it.
Keep in mind, however, that the ultra-pasteurized milk sold in the supermarket and raw milk are very different things. The former is essentially a dead product, with no live bacteria; when it goes bad, it rots and should be thrown out, unless you can catch it early on. The latter turns into “clabber,” which is thickened sour milk that’s still perfectly useful.Historically, clabber was used as a leavening agent. It worked together with baking soda to produce fluffy quick breads and cakes, but once baking powder was invented, it was no longer needed (via The Prairie Homestead).
I can only get organic pasteurized milk, which means I’m unable to enjoy the versatile clabber of raw milk; but I still use my bags of sour milk (yes, I live in Canada, where milk is always sold in bags!) when it’s just slightly off. Once it separates and starts to stink, though, there’s nothing more to do with it.
Here are some ways to put sour milk to good use whenever possible:
BakingSour milk is a good replacement for buttermilk, yogurt, or sour cream. Some recipes even call for “soured milk,” which requires you to add a spoonful of vinegar to milk. Make pancakes, waffles, biscuits, a fruit-bottomed upside-down cake, or my aunt Jane's best-ever scones.
CookingAdd small amounts of sour milk to dishes that have a creamy, cheesy consistency, such as casseroles, seafood stews, or potato bakes. Just be careful to ensure that the sour flavor doesn’t overpower.
Tenderize meat such as chicken or fish by soaking in sour milk prior to cooking. You can also mix up a flavorful marinade – the same way as you’d use buttermilk for chicken.
Soak grains such as wheat berries, barley, and farro in sour milk.
CheesemakingHere is a recipe for Old-Fashioned Cottage Cheese from The Self-Sufficient Home Acre blog. All you need is four ingredients and some cheesecloth for wonderfully rich cottage cheese.
Skin CareIt’s called “lactic acid facial.” Rubbing sour milk (or sour cream or yogurt) onto your skin will make it smoother, firmer, and lighter. Some fair-skinned people say that rubbing whey onto their skin assists with tanning if you get out into the sunshine soon afterward. (I’ll have to try it and report back since tanning is nearly impossible for my redheaded complexion.)
Add a cup of sour milk to the bathtub for extra-smooth skin. You may want some essential oils, too, if the odor is strong.
GardeningDilute sour milk with water and pour onto garden beds to increase calcium content. It’s supposed to be particularly good for tomato plants.
Use the ClabberIf you have raw milk that clabbers, you can strain and use the curds for ricotta-like cheese, in place of sour cream, or in salad dressing, depending on the texture.
Pet FoodMix sour milk into feed for hens, pigs, dogs, and cats, or add to a batch of homemade baked treats.
Craft ProjectCasein plastic is a fun science experiment-type craft that kids will enjoy. Here are directions for recreating this unusual plastic at home.
In the FutureYou can preemptively avoid milk spoilage by making yogurt if you have more milk in the fridge than you can use. It’s very easy, has superior texture and flavor to store-bought yogurt, and is much cheaper. You don't need a yogurt maker, despite what the recipe says. Pour into glass Mason jars, swaddle in a towel, and leave in the oven overnight with the light on. Refrigerate to firm up.
Milk also freezes well, especially if it comes in Canadian-style plastic bags. Toss in the freezer and defrost overnight in the fridge as needed.