What Causes Milk to Curdle? Demystifying the Process

Have you ever poured milk in your coffee only to find that it curdled? Or, perhaps you have left a carton of milk in the fridge for too long and found that it had curdled. Either way, curdled milk is a common occurrence that can be frustrating and confusing. In this article, we will explore what causes milk to curdle and demystify the process behind this natural phenomenon.

The Chemistry of Milk

To understand what causes milk to curdle, it’s important to first understand the chemistry behind it. Milk is made up of water, fat, protein, lactose, vitamins, and minerals. The two main proteins in milk are casein and whey. Casein makes up about 80% of the total protein in milk.

The pH of milk is around 6.5, which is slightly acidic. Milk also contains calcium ions, which play a vital role in curdling. When milk is heated, its proteins denature and lose their shape. This causes them to clump together and form curds, which are solid mounds of milk protein.

The Role of Bacteria in Milk Curdling

Bacteria play a significant role in causing milk to curdle. Milk contains harmless bacteria called lactobacillus. This bacteria feeds on lactose sugar and produces lactic acid as a byproduct. Initially, this process can make the milk taste sour, but it is not yet curdled.

However, if other bacteria are able to grow in the milk, such as E. coli or Salmonella, they can produce enzymes that promote curdling. These enzymes break down the protein and produce lactic acid, which causes the milk to curdle rapidly. This type of curdling is not safe to consume and can cause illness if ingested.

The Role of Acids in Milk Curdling

Acids can also cause milk to curdle. When an acid is added to milk, such as lemon juice or vinegar, the pH of the milk drops. This causes the casein protein to denature and clump together, forming curds. This process is often used in making cheese, where an acid such as rennet is added to hot milk to cause curdling.

Factors that Influence Milk Curdling


Temperature plays a crucial role in milk curdling. As mentioned earlier, when milk is heated, its proteins become denatured and clump together to form curds. However, if milk is heated too much or too quickly, the proteins can become too tightly woven, making it difficult for them to form curds.

Age of the Milk

The age of the milk plays a significant role in curdling. Fresh milk contains fewer bacteria and other microorganisms, making it less likely to curdle. However, as milk begins to age, its bacteria count goes up, making it more likely to curdle. Thus, it is essential to consume milk within its expiry date.

Acid Content

The acid content of a liquid can also impact milk curdling. Highly acidic liquids, such as juices, can cause curdling to occur more quickly. This is because the pH of the liquid is already low, and the addition of milk further lowers it, causing curdling to occur.

How to Prevent Milk from Curdling

Store Milk Properly

Proper storage is vital to preventing milk from curdling. Milk should always be stored in a clean, well-sealed container in the refrigerator. Be sure to keep the temperature of the fridge below 40 degrees Fahrenheit. If the milk is not stored correctly, it will go bad and curdle more quickly.

Avoid Mixing Certain Substances

Some substances can cause milk to curdle more easily. For instance, mixing acidic drinks like orange juice with milk can cause curdling. Thus, it’s essential to avoid combining these substances.

Use Fresh Milk

Using fresh milk can help prevent curdling. Purchase milk that has a later expiry date or freeze it for later use for long-term storage. If you are using a recipe that requires curdling, use a fresher, colder milk.


In summary, curdled milk is a common occurrence that can happen to anyone. It can be caused by various factors such as bacteria growth, acid content or temperature. Nevertheless, with proper storage, handling, and care, you can avoid curdling and enjoy fresh, high-quality milk.


  • What is curdled milk?
  • Curdled milk is when the proteins in milk clump together and form solid mounds called curds, which are often the result of acidification or bacterial growth in the milk.

  • What causes milk to curdle?
  • The most common causes of milk curdling are bacterial growth or the addition of acids, such as vinegar or lemon juice. Additionally, high temperatures and age of the milk can also contribute to curdling.

  • Is curdled milk safe to consume?
  • It depends on the cause of the curdling. If it is due to bacterial growth and contamination, then it is not safe to consume. However, if it is due to acidification, such as in cheese-making, then it is safe to eat.

  • How can I prevent milk from curdling?
  • The best way to prevent milk from curdling is to properly store it in the fridge, avoid mixing it with acidic substances, and use fresh milk whenever possible.


  1. American Chemical Society- Reactions. (2018, December 13). Why Does Milk Curdle? Retrived from: https://www.youtube.com/watch?v=CO28ONCCNqk.
  2. Grigorakis, S., & Massouras, T. (2019). Microbiological spoilage of dairy products. In Microbial Spoilage of Foods (pp. 739-762). Elsevier.
  3. Lindsay, R. C. (1980). Chemistry of milk clarity and stability. Journal of dairy science, 63(5), 665-671.

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