The vinegar and milk react to form curds (a solid) and whey (a liquid). These curds cannot be returned to vinegar and milk.
Instructions. Pour the tablespoon of vinegar into a 1 cup measurer. Fill the rest of the 1 cup measurer with milk. Gently stir and let the mixture sit for about 5 minutes. This will allow the milk to slightly curdle, giving it a buttermilk-like texture and taste.
Milk curdles when mixed with vinegar because the acid from the vinegar causes the protein molecules in milk to come together to form visible solids. The solids float in the clear whey, forming curdled milk.
Sour milk is not the same as buttermilk.
Buttermilk is either purposefully cultured to get a sour taste or is the byproduct of butter making. If raw milk sours, it's perfectly fine to drink and that's the way most of the world drinks milk. But if pasteurized milk sours, it's just on its way to going bad.
Soured milk that is produced by bacterial fermentation is more specifically called fermented milk or cultured milk.
What is buttermilk? Buttermilk is a slightly sour milk. The sourness of buttermilk comes from acids in the milk, most notably, lactic acids. Because the proteins in buttermilk are curdled, buttermilk is slightly thicker than regular milk, but not quite as thick as cream.
Acidic ingredients like lemon juice, vinegar, tomatoes, or even wine can cause your milk to curdle. Prevent this reaction by reducing your acids first before adding dairy. Remember this tip when making creamy adobo, tomato soup, or a citrusy cream-based pasta.
The milk needs to be at near boiling temperatures when you add the acid. The combination of heat and acid will cause the milk proteins to unravel (denature) and tangle up with each other (coagulate) which will result in the curd you are looking for.
Lemon Juice or Vinegar
As with many chemical reactions, temperature controls the rate at which the reaction occurs. When adding lemon juice or vinegar to hot milk, it will curdle almost immediately, but adding it to cold milk will not produce a reaction for quite some time.
This is because adding an acid, such as vinegar, to the milk changes the pH of the milk and makes the casein molecules unfold and reorganize into a long chain, curdling the milk. The white chunks are curds. You should have been able to use a spoon to separate the curds from most of the liquid.
The vinegar will cause the milk to quickly separate into curds and whey. The acid in the vinegar causes the protein strands in the milk to coagulate. The curds are casein, which is used in the production of glue and some plastics.
Observations and results
This is because adding an acid (such as vinegar) to the milk changes the latter's pH (acidity) and makes the casein molecules unfold and reorganize into a long chain, thereby curdling the milk.
First, you get milk nice and hot. Then you add some vinegar or lemon juice, which curdles the milk. That is, it divides the milk into curds (lumpy white things) and whey (a thin, chalk-colored liquid). The curds make the cheese.
Casein can be precipitated from milk by the addition of acid [3]. Types of acids that can be used as a precipitate include lemon juice, citric acid, tartaric acid, and acetic acid. Acetic acid is the most common precipitate used for the separation of milk proteins [4,5].
Add an acid: For every 1 cup of milk, stir in 1 tablespoon lemon juice or vinegar. Let the mixture stand for 10 minutes. You can scale the recipe up or down depending on how much you need.
In a jar or a small bowl, combine milk with vinegar or strained lemon juice. Stir and let it sit until the mixture thickens or curdles, about 15 minutes, or a few minutes more.
With sauces and soups that contain milk, boiling or simmering can cause the milk to curdle. While curdled milk is safe to eat, it is not particularly appetizing.
The Bottom Line
Milk can often be consumed after its expiration date, generally for up to three days for opened milk and up to seven days for unopened milk. Before consuming, it's best to look for signs of spoilage. You can tell if milk is bad by checking the smell, color, consistency and taste.
Dust, dirt and manure can cause an unclean flavor of milk. Cows and their surroundings must be kept clean. Milking equipment that has not been properly cleaned and sanitized may be a factor. Washing a cow's udder with water and failing to dry them is one of the primary causes of unclean flavors.