What Chemical Makes Milk Sour? Lactic Acid Explained

January 10, 2026 •

4 min read

The chemical process of milk turning sour is a natural phenomenon driven by microscopic organisms. The specific chemical that makes milk sour is lactic acid, which is produced when certain bacteria in the milk consume its natural sugar, lactose, in a process known as fermentation.

Quick Summary

Lactic acid, created through bacterial fermentation of lactose, is the chemical responsible for milk's sour taste. This process lowers the milk's pH, causing proteins to coagulate and leading to the distinct tang and texture of soured dairy products like yogurt and cheese. The article explores the bacterial action, the chemistry involved, and how it differs from milk spoilage.

Key Points

Lactic acid is the culprit: The chemical that makes milk sour is lactic acid, a byproduct of bacterial metabolism.
Bacteria are the catalysts: Lactic acid bacteria (LAB) ferment lactose (milk sugar) to produce lactic acid, leading to the sour taste.
Acidity causes curdling: The increased acidity from lactic acid production causes milk's casein proteins to coagulate, creating a thicker, curd-like texture.
Temperature controls the rate: Souring accelerates at warmer temperatures because it promotes faster bacterial growth.
Not all soured milk is spoiled: Intentionally fermented milk is made with specific, beneficial bacteria, while spoiled milk results from uncontrolled bacterial contamination.
Fermentation acts as a preservative: The low pH created by lactic acid inhibits the growth of many other spoilage and pathogenic bacteria.

The Lactic Acid Process: From Lactose to Tang

The souring of milk is not a random event but a precise chemical transformation initiated by bacteria. These microorganisms, primarily from the Lactobacillus and Lactococcus genera, are collectively known as lactic acid bacteria (LAB). The process begins when milk is left unrefrigerated, allowing these heat-resistant bacteria, which may survive pasteurization, to multiply rapidly.

These bacteria feed on lactose, the disaccharide sugar present in milk. Through anaerobic respiration, they metabolize the lactose into glucose and galactose, which are then further fermented into pyruvic acid and finally, lactic acid. The chemical reaction can be summarized as: $C{12}H{22}O_{11}$ (Lactose) + $H_2O$ → 4$C_3H_6O_3$ (Lactic Acid).

The Effect of Acidity on Milk’s Texture

The production of lactic acid fundamentally changes the chemical environment of the milk. The increasing concentration of acid causes the pH of the milk to drop significantly, moving from its natural, slightly acidic state (around pH 6.7) toward a much more acidic state. This drop in pH is what causes the characteristic sour taste.

The acidity also affects the milk's proteins, most notably casein. In fresh milk, casein exists as a colloidal suspension, tiny particles that stay evenly distributed and give milk its white, liquid appearance. As the pH drops, the acidic environment causes the casein proteins to lose their stability and clump together, a process known as coagulation or curdling. This is what creates the thickened texture seen in soured milk, yogurt, and cheese. The solid curds separate from the watery whey, completing the transformation.

Spoiled vs. Fermented: The Key Difference

It is crucial to distinguish between intentionally fermented milk and milk that has simply spoiled. While both are driven by bacterial action, the outcome depends on the type of bacteria present and the storage conditions.

Intentional Fermentation: This is a controlled process using specific, beneficial LAB starter cultures, as is the case for making yogurt, kefir, or cultured buttermilk. The resulting product is safe to consume and has a pleasant tangy flavor and thick texture.
Spoilage: This occurs when milk is contaminated by a variety of opportunistic bacteria and kept at an inappropriate temperature. This can result in foul smells and unpleasant, unpalatable textures, and potentially cause digestive issues due to harmful bacterial byproducts. The key takeaway is that not all soured milk is the same; controlled fermentation is a culinary art, while uncontrolled spoilage is a food safety issue.

How to Prevent Milk from Souring

Since bacterial growth is the cause of milk souring, the main methods for preventing spoilage focus on controlling these microorganisms. These techniques are standard practice in the dairy industry but are also useful for household management.

Refrigeration: Keeping milk at a constant temperature between 0 and 4 °C (32 and 40 °F) is the simplest and most effective method. This dramatically slows down the metabolic activity of bacteria.
Pasteurization: This process involves heating milk to a high temperature for a short period to kill harmful bacteria and reduce the overall bacterial load, extending its shelf life.
Ultra-High Temperature (UHT) Treatment: This involves heating milk to an even higher temperature than pasteurization, killing all bacteria and spores. This allows the milk to be stored at room temperature before opening.

Comparison of Souring vs. Preservation


Feature	Souring (Fermentation)	Preservation (Refrigeration)
Mechanism	Beneficial bacteria convert lactose into lactic acid.	Low temperature inhibits bacterial growth and metabolism.
Bacterial Role	Specific, desired lactic acid bacteria are active.	All bacterial activity, both good and bad, is suppressed.
Primary Goal	Create a new, distinct dairy product (e.g., yogurt).	Maintain the milk's original, fresh state.
Resulting Flavor	Tangy, tart, and acidic.	Retains its sweet, creamy flavor.
Changes to Protein	Casein proteins coagulate, forming a thicker, gel-like texture.	Casein remains in a stable, colloidal suspension.
pH Level	Decreases significantly, becoming more acidic.	Remains stable at a neutral or slightly acidic level.
Shelf Life	Extends shelf life by creating a less hospitable environment for pathogens.	Temporarily extends shelf life by slowing spoilage.

Conclusion

Lactic acid is the definitive chemical responsible for the souring of milk, a direct result of lactic acid bacteria metabolizing lactose. This natural process is harnessed for beneficial purposes in producing a wide range of fermented dairy products, from yogurt to cheese. However, when left uncontrolled, the same bacterial action leads to spoilage. Understanding this core chemical and biological process not only demystifies why milk turns sour but also highlights the importance of proper temperature control and sanitation for food preservation. Fermented milk products have been consumed for millennia, offering unique flavors and nutritional benefits, and this entire process is grounded in a simple but powerful chemical reaction.

What are some examples of lactic acid bacteria?

Frequently Asked Questions

Sour milk is intentionally fermented with specific lactic acid bacteria to create a palatable, tangy product. Spoiled milk is the result of uncontrolled bacterial growth, which can produce off-flavors, odors, and potentially harmful byproducts.

Yes, intentionally soured or cultured milk is excellent for cooking and baking, often used in recipes for pancakes, biscuits, and marinades. However, drinking spoiled milk is not recommended and can cause digestive upset.

Yes, pasteurized milk can still go sour. While the pasteurization process kills most bacteria, some heat-resistant strains can survive and multiply over time, especially if the milk is not kept consistently cold.

The lactic acid produced during fermentation lowers the pH of the milk, creating an acidic environment. This low pH inhibits the growth of other spoilage and pathogenic bacteria, thereby extending the product's shelf life.

Drinking a small, accidental sip of spoiled milk is unlikely to cause serious harm. However, consuming a larger amount can cause gastrointestinal distress, including nausea, vomiting, and diarrhea.

Freezing is an effective way to prevent milk from spoiling, but it can alter the texture of the milk, causing it to separate upon thawing. It is best used for cooking and baking rather than for drinking.

Examples of fermented milk products include yogurt, kefir, buttermilk, and various types of cheese. These products are made by intentionally adding specific lactic acid bacteria cultures to milk.