What type of acid is present in milk? A Detailed Exploration

November 26, 2025 •

2 min read

While many people assume milk contains lactic acid, fresh milk is actually only slightly acidic due to natural compounds like phosphates and citrates. The distinctive sour flavor and higher acidity we associate with spoiled or fermented dairy is a result of lactic acid, which is produced by bacteria over time.

Quick Summary

Lactic acid is the primary acid in fermented milk, created by bacteria consuming lactose. Fresh milk's very minimal natural acidity is actually due to components like phosphates and proteins, not lactic acid.

Key Points

Primary Acid is Lactic: Lactic acid is the main acid in sour or fermented milk, produced by bacteria breaking down lactose.
Fresh Milk's Natural Acidity: The very slight acidity of fresh milk comes from natural components like phosphates, citrates, and proteins, not lactic acid.
Acidity Develops Over Time: Lactic acid levels in milk increase as bacteria multiply and ferment lactose, causing the milk to sour.
Fermentation is Controlled Souring: Dairy products like yogurt and cheese are created by carefully controlling the lactic acid fermentation process.
pH is a Quality Indicator: Monitoring the pH of milk is a critical quality control measure, as a drop in pH signifies bacterial growth and increasing lactic acid.
Buffer System in Fresh Milk: The presence of phosphates and proteins acts as a natural buffer, keeping the pH of fresh milk stable.
Different Acids Present: Other minor acids, like citric and acetic acid, can also be present, particularly in fermented products.

The Science Behind Milk's Acidity

Fresh milk has a pH between 6.4 and 6.8, indicating low natural acidity. This initial acidity is due to components like phosphates, citrates, and proteins (casein), which also act as buffers. Over time, developed acidity occurs as microbes ferment lactose, causing milk to sour.

Lactic Acid: The Key to Fermentation

Lactic acid is the main acid in sour or fermented milk. Lactic Acid Bacteria (LAB), such as Lactobacillus, consume lactose and produce lactic acid, lowering pH and creating a tangy flavor. This process is crucial for products like yogurt and cheese, where specific LAB cultures (Streptococcus thermophilus, Lactobacillus bulgaricus) contribute to texture and flavor by producing lactic acid.

The Natural Acids in Fresh Milk

Minimal acidity in fresh milk is attributed to several natural components:

Phosphates: Mineral salts contributing to buffering and slight acidity.
Citrates: Citric acid salts naturally present.
Proteins: Acidic amino acids in proteins like casein.
Dissolved Carbon Dioxide: Forms weak carbonic acid from air exposure during milking.

These differ from the stronger acidity developed through lactic acid fermentation.

Fresh vs. Sour Milk: A Chemical Comparison


Characteristic	Fresh Milk	Sour/Fermented Milk
Primary Acid	Phosphates, Citrates, Proteins	Lactic Acid
pH	Slightly Acidic (6.4–6.8)	More Acidic (pH drops significantly)
Taste	Sweet or slightly bland	Tangy, sour
Bacterial Activity	Very low to non-existent	High, with bacteria actively converting lactose
Shelf Life	Limited	Extended (fermentation helps preserve the product)
Coagulation	Does not occur naturally	Occurs as casein proteins precipitate due to increasing acidity

Beyond Lactic Acid: Other Organic Acids

Other organic acids can appear in fermented dairy. Acetic acid, found in vinegar, can be produced by some bacteria. Lipolysis, the breakdown of milk fat, releases fatty acids contributing to flavors in ripened cheese. The type and amount of these acids depend on bacterial strains and fermentation conditions.

The Importance of Acidity in Dairy Production

Controlling lactic acid is vital in the dairy industry for safety and product quality. Acidic environments created by LAB inhibit harmful bacteria, extending shelf life. Acidity also impacts texture and flavor, from yogurt's smoothness to cheese's firmness. For more on dairy chemistry, visit the Journal of Dairy Science.

Conclusion: From Neutral to Tangy

Fresh milk is slightly acidic due to natural components. Lactic acid is present only in sour or fermented milk, produced by bacteria fermenting lactose. This process creates various dairy products and helps preserve milk's nutrients.

Frequently Asked Questions

No, fresh milk is only slightly acidic with a pH between 6.4 and 6.8, which is very close to neutral (pH 7). Its mild acidity is primarily due to natural compounds like proteins, phosphates, and citrates.

Milk sours when naturally present bacteria, primarily Lactobacillus, ferment the lactose (milk sugar). This fermentation process produces lactic acid, which increases the milk's acidity and gives it a sour taste.

Fermented dairy products like yogurt, cheese, and buttermilk contain significant levels of lactic acid due to the fermentation process. Fresh, unfermented milk contains virtually no lactic acid, but its acidity will increase if it begins to spoil.

No, lactic acid is not harmful. It is a natural organic acid produced in many fermented foods and even in our own muscles during exercise. The acid itself is safe, though the bacterial growth that produces it in spoiled milk can be a health concern.

Bacteria, specifically Lactic Acid Bacteria (LAB), consume the lactose in milk and break it down into lactic acid. In dairy production, these bacteria are known as 'starter cultures' and are intentionally added to milk to control the fermentation process.

Milk curdles when its pH drops due to increasing acidity, such as from developing lactic acid. This acidic environment causes the milk protein casein to coagulate and separate from the liquid whey, resulting in a curdled texture.

Commercial dairy producers control lactic acid levels by regulating temperature and using specific starter cultures of Lactic Acid Bacteria. This control allows them to create consistent fermented products and ensures the safety and quality of their goods.