Milk after a period of time can be unfit for consumption in its current form. It is then converted to other dairy products or if left idle for long period of time becomes spoiled.
Souring and curdling can be desired outcomes and are, in fact, necessary to the making of certain kinds of foods, cheese and yoghurt among them.
When acidity of milk increases due to microbial activity (generally desirable for certain products manufacturing otherwise undesirable) to an extent where the natural sweet taste turns to acidic but the milk still remains in liquid. This milk is termed as sour milk (usually when acidity reaches beyond 0.19%).
Sour milk has many uses you may find helpful, and you’ll no longer feel the need to rush to consume it all before it expires.
The first thing I think of when I find soured milk is making cheese. After all, the main ingredient in cheese is milk that’s gone bad. You can make cottage cheese by cooking sour milk in a double boiler over simmering water until it begins to whey (when the watery part of milk separates from the curd, or cheese). Strain it through a towel to remove excess milk, and add in some cream and salt and pepper to taste.
Though sour milk can be delicious in baked goods, desserts (think custards and cheesecakes), eggs and casseroles, there are many other uses for it that don’t involve food:
- Beauty. Sour milk is a common ingredient in facial masks. It’s great for the skin — making it smoother and giving it a more even, consistent color. Place sour milk all over the face, let sit, then rinse off with milk (strange, I know), followed by a final rinsing of water.
- Pets. Use sour milk for pet food — make dog or cat biscuits, or feed it to hens once it’s curdled (you can sit it outside in the sun to speed up this process).
- Home and garden. Pour it around plants to keep deer away, and put it at the base of rosebushes to help them grow. You can even pour sour milk over silver and let sit overnight to make it shine.
When the acidity is further increases to an extent where protein denaturation takes place and the milk turns into a gel like structure (again this can be done intentionally for product manufacturing like yoghurt and cheese). This gel like structure is called curd and milk is said be curdled.
Spoiled and rotten, though, are terms used to describe the undesirable outcomes.
When the said microbial changes occur in uncontrolled conditions, the milk is considered to be not fit for consumption. In that case it is said to be spoiled or rotten. Most of the spoilage (like liolysis: breaking of milk fat , proteolysis: breakdown of protein, sweet curdling, ropiness etc.) are caused by microbial contamination of milk.
Spoiled milk can’t be used elsewhere. It is best to throw out the milk.