What causes raw milk to sour?

January 2, 2026 •

4 min read

Over 90% of milk in some regions is sold raw, and this unpasteurized product undergoes a natural change when left out. So, what causes raw milk to sour, creating a tangy, clabbered state? The answer lies in the microbiological activity present from the moment the milk is collected.

Quick Summary

Raw milk sours naturally through lactic acid fermentation, where bacteria convert lactose into lactic acid. This process thickens the milk and creates its tangy flavor, in contrast to pasteurized milk which rots due to different microbes.

Key Points

Lactic Acid Fermentation: Raw milk sours naturally due to beneficial lactic acid bacteria consuming lactose and producing lactic acid.
Casein Curdling: The increased acidity from lactic acid causes the milk's casein proteins to clump together, forming curds or 'clabber'.
Temperature Dependence: Souring is heavily influenced by temperature; warmer conditions accelerate the process, while colder temperatures slow it down.
Different from Spoilage: Unlike pasteurized milk which rots, raw milk typically undergoes a clean souring process unless it is contaminated with unwanted bacteria.
Hygiene is Key: The quality and cleanliness of the milking process directly impact the initial bacterial load and, therefore, the speed and type of souring.
Culinary Uses: Naturally soured raw milk, or clabber, can be used for baking or cheesemaking, unlike spoiled pasteurized milk.

The Natural Fermentation Process

Unlike pasteurized milk, which has its microbial load significantly reduced by heat, raw milk contains a diverse population of beneficial microorganisms from the cow and the milking environment. The primary culprits behind its pleasant souring are lactic acid bacteria (LAB), which are harmless and even desirable in many fermented foods like yogurt and cheese.

Lactic Acid Bacteria at Work

The process is called lactic acid fermentation. Lactic acid bacteria feed on lactose, the natural sugar in milk. As they metabolize the lactose, they produce lactic acid as a byproduct. This acid gradually lowers the milk's pH level over time. This change in acidity is what triggers the physical transformation of the milk.

The Curdling of Casein Proteins

Milk's proteins are predominantly casein and whey. Casein proteins exist in spherical structures called micelles, which normally repel each other and remain suspended in the milk, giving it its liquid consistency. However, as the lactic acid increases and the pH drops, the negative charge on the micelles is neutralized. With the repulsive forces gone, the proteins begin to clump together, or coagulate, forming the solid curds and thickened texture known as 'clabber'.

Factors That Influence Raw Milk Souring

Several key factors determine the speed and quality of the souring process in raw milk.

Temperature: This is arguably the most significant factor. Lactic acid bacteria thrive in warmer temperatures. Storing raw milk at room temperature dramatically accelerates fermentation, causing it to clabber within a day or two. Keeping it consistently cold in a refrigerator (below 4°C) slows the process considerably, allowing it to remain 'fresh' for a week or more.
Initial Bacterial Load: The number and type of bacteria present at the time of milking play a crucial role. Clean and hygienic milking practices result in a lower initial bacterial count, which extends the shelf life. Poor hygiene can introduce undesirable bacteria that lead to spoilage instead of clean souring.
Time: The natural progression of time allows the bacteria to multiply and produce enough lactic acid to alter the milk's chemical composition. The longer the raw milk sits, the more acidic it becomes, and the more it will thicken.

Raw Milk vs. Pasteurized Milk Spoiling: A Comparison


Feature	Raw Milk	Pasteurized Milk
Microbial Content	Contains a diverse and active population of naturally occurring bacteria, including beneficial LAB.	Has been heat-treated to kill most bacteria, both good and bad, leaving a very low microbial load.
Souring Process	Naturally undergoes lactic acid fermentation, becoming tangy and thick (clabber).	Does not ferment cleanly. Spoilage is caused by fewer, hardier bacteria that survive pasteurization, leading to a foul odor and putrefaction.
Safety	High risk of carrying harmful pathogens like E. coli or Listeria if not from a controlled, tested source.	Safe for consumption due to the elimination of pathogens during the heating process.
End Product	Can be intentionally cultured into products like sour cream or cottage cheese.	Becomes inedible and unsafe; must be discarded once spoiled.

Potential for Undesirable Spoilage in Raw Milk

While natural souring is the typical path, poor conditions can lead to other forms of raw milk spoilage. Psychrotrophic bacteria, which can grow even in refrigerated temperatures, may produce off-flavors and textures by breaking down fats and proteins. Additionally, if raw milk is contaminated with pathogens like E. coli or Salmonella through poor farm hygiene, it can become dangerous to consume and still appear spoiled in taste and smell. The Centers for Disease Control and Prevention warns against the risks of consuming raw milk due to potential illness-causing germs.

Conclusion: Souring as a Natural Evolution

In summary, what causes raw milk to sour is a natural and predictable biological process driven by lactic acid bacteria. This fermentation converts lactose into lactic acid, which causes casein proteins to coagulate. This leads to the clabbering of the milk, a distinctly different outcome from the putrefaction that occurs when pasteurized milk spoils. The speed and nature of this change are controlled by temperature, time, and the initial quality of the milk. For those who consume raw milk, this souring is not an indication of spoilage but a natural evolution that can be used for culinary purposes, but the health risks associated with pathogens in raw milk are still a major concern.

Key Factors Affecting the Souring Rate

Initial Bacterial Count: The higher the initial load of LAB, the faster the fermentation will begin.
Temperature: Warm temperatures significantly accelerate the growth and metabolic activity of the bacteria.
Container and Surface Hygiene: Contamination from unclean surfaces can introduce new, potentially unwanted, bacteria to the milk.
Air Exposure: Leaving milk open to the air can introduce airborne microorganisms that affect the process.
Storage Duration: The longer the milk is stored, the more time the bacteria have to produce lactic acid and initiate curdling.

Frequently Asked Questions

Raw milk advocates suggest naturally soured raw milk, or clabber, is safe and even beneficial. However, since raw milk can contain harmful pathogens, health organizations like the CDC advise against its consumption in any state due to risk of foodborne illness.

Sour raw milk is a result of natural fermentation by beneficial bacteria, turning tangy and thick. Spoiled pasteurized milk is caused by residual microbes that survive processing, leading to putrefaction and an unpleasant odor and taste.

To accelerate the natural souring process, leave the milk covered at a warmer temperature, such as a countertop instead of the refrigerator. This encourages the lactic acid bacteria to multiply and ferment the lactose more quickly.

Naturally soured raw milk typically has a pleasant, tangy, and slightly cheesy aroma. If raw milk smells foul, rotten, or has a slimy texture, it may be contaminated with undesirable microbes and should not be consumed.

Yes, naturally soured raw milk is an excellent substitute for buttermilk in recipes. The acidity reacts with baking soda to act as a leavening agent for fluffier baked goods.

The acid in the lemon juice lowers the milk's pH, causing the casein proteins to coagulate and clump together. This is a chemical reaction, similar to what happens in natural souring, but it is not a sign of spoilage.

The time it takes for raw milk to sour depends on temperature and initial bacterial load. At room temperature, it can take 12 to 24 hours, while in the refrigerator, it can take a week or longer to develop a noticeable sour taste.