Which acid is present in milk? Unveiling the truth behind its sourness

December 11, 2025 •

3 min read

Fresh milk typically has a pH between 6.5 and 6.7, classifying it as slightly acidic. However, a specific bacterial process determines which acid is present in milk as it ages and becomes sour. This article explores the science behind milk's acidity and the vital role of lactic acid.

Quick Summary

Milk's primary acidity comes from lactic acid, a byproduct of bacteria fermenting lactose. This process is fundamental to the taste and texture of many dairy products.

Key Points

Lactic acid is the primary acid in sour milk: The tangy taste of sour milk and other fermented dairy products is due to lactic acid, a byproduct of lactose fermentation.
Fresh milk contains very little lactic acid: Fresh milk's slightly acidic pH is mainly due to its mineral and protein content, not lactic acid.
Bacteria ferment lactose into lactic acid: Lactic acid bacteria (LAB) convert the sugar lactose into lactic acid as milk ages, causing it to become more acidic.
Acidity affects milk's texture: As lactic acid concentration increases, the milk protein casein coagulates, causing milk to curdle and thicken, as seen in yogurt and cheese.
Acidity acts as a preservative: The lower pH created by lactic acid inhibits the growth of harmful bacteria, extending the shelf life of fermented dairy products.
Different bacteria produce different flavour profiles: Depending on the type of bacteria, fermentation can produce just lactic acid (homofermentative) or other compounds like CO2 and acetic acid (heterofermentative), leading to different flavours and textures.

The chemistry of milk's natural acidity

Fresh milk is a complex emulsion containing water, proteins, fats, and lactose. Its slightly acidic pH (around 6.5–6.7) is influenced by salts like phosphates and proteins such as casein. This initial acidity is low and stable under proper storage. The significant change in acidity, or souring, is primarily due to microbial action, not the innate composition of fresh milk.

The crucial role of lactic acid bacteria

Lactic acid ($C_3H_6O_3$) is the main acid found in milk as it sours. It is largely absent in fresh milk but is produced through fermentation by Lactic Acid Bacteria (LAB). LAB, naturally present in milk, convert lactose into lactic acid, especially in warmer conditions:

$Lactose + Lactic Acid Bacteria \rightarrow Lactic Acid$

This fermentation is key in the dairy industry, intentionally used with starter cultures like Lactobacillus bulgaricus and Streptococcus thermophilus to make yogurt and cheese with specific textures and flavors.

Homolactic vs. heterolactic fermentation

LAB are categorized by their fermentation products:

Homofermentative bacteria: Produce mainly lactic acid, resulting in a clean sourness. Examples include Lactobacillus acidophilus and Lactococcus lactis.
Heterofermentative bacteria: Produce lactic acid plus other compounds like CO2, acetic acid, and ethanol. This creates more complex flavors and textures, like the holes in some cheeses.

Acidity's impact on milk's structure: Casein coagulation

Increased lactic acid lowers milk's pH, causing the protein casein to coagulate below pH 5.2. Casein micelles, normally suspended due to negative charges, lose their charge near their isoelectric point (pH ~4.6) as acidity rises. This loss of repulsion leads to clumping and curd formation, essential for cheesemaking and yogurt thickening.

The preservative effect of developed acidity

Lactic acid acts as a natural preservative by lowering pH, creating an environment unsuitable for many spoilage and pathogenic bacteria that prefer a more neutral pH. This is why fermented dairy products have a longer shelf life than fresh milk. This preservation method is vital for dairy product safety and longevity.

Comparison of different milk acid profiles

The table below compares the acid profiles of fresh and fermented milk products:


Feature	Fresh Milk	Sour Milk	Yogurt
Primary Acid	Phosphates, minor acids (citric)	Lactic Acid	Lactic Acid
Primary Cause of Acidity	Natural minerals and proteins	Bacterial fermentation of lactose	Intentional bacterial fermentation
Typical pH Range	6.5–6.7	< 6.5, dropping as it sours	4.0–4.6, varies by type
Key Chemical Change	Stable, slight acidity	Lactose breakdown into lactic acid	Lactose breakdown into lactic acid
Texture	Thin, fluid liquid	Thickening, clumping, curdled	Thick, smooth coagulum (gel)
Taste Profile	Mild, sweet	Tart, tangy	Tangy, sour (can vary)
Bacterial Profile	Low bacterial count	Uncontrolled bacterial growth	Controlled starter cultures

Conclusion: Lactic acid is the definitive answer

To summarize, fresh milk's slight acidity is due to minerals and proteins, but the characteristic sourness of aged or fermented milk is caused by lactic acid. This acid is produced when lactic acid bacteria ferment lactose, milk's main sugar. This process is crucial for creating fermented dairy products like yogurt and cheese and acts as a long-standing preservation method. Understanding this transformation is key to appreciating milk chemistry.

For further reading on the role of lactic acid bacteria in dairy, consult the National Institutes of Health (NIH) website: Dairy Lactic Acid Bacteria and Their Potential Function in Human Health.

Frequently Asked Questions

No, fresh milk contains only negligible amounts of lactic acid. Its natural, slightly acidic pH is due to phosphates and other minerals. Lactic acid is produced by bacteria after the milk has been exposed to the air for some time, especially at warm temperatures.

The pH of fresh cow's milk typically ranges from 6.5 to 6.7, which is slightly acidic. The pH drops as the milk sours and lactic acid is produced.

No, lactic acid and lactose are different. Lactose is a type of sugar naturally present in milk. Lactic acid is the acid produced by bacteria when they ferment the lactose.

Lactic acid bacteria (LAB) are responsible for producing lactic acid in milk. Common examples include Lactobacillus and Streptococcus species, which are also used as starter cultures in fermented dairy products like yogurt and cheese.

Milk curdles because the increasing lactic acid concentration lowers the milk's pH. This causes the casein proteins to lose their negative charge and coagulate, forming solid curds.

Yes, lactic acid is present in many fermented foods. It is responsible for the sour flavour in items such as sauerkraut, sourdough bread, and pickles.

While the acidity from lactic acid is not inherently harmful, the uncontrolled bacterial growth in naturally soured milk can introduce other, potentially pathogenic, bacteria. It is generally not recommended to drink sour milk unless it has been deliberately fermented under controlled, sanitary conditions, such as in yogurt or kefir production.