What bacteria causes discoloration in milk?

January 2, 2026 •

4 min read

Milk, a highly nutritious beverage, provides a favorable environment for numerous microorganisms, and microbial growth can lead to spoilage, including noticeable color changes. While most people are familiar with milk that has simply gone sour, discoloration can be an alarming sign of specific bacterial contamination, revealing a hidden microbiological world.

Quick Summary

Discoloration in milk is caused by specific chromogenic bacteria producing vibrant pigments. Common culprits include Pseudomonas species causing blue, yellow, or brown hues, Flavobacterium resulting in yellow milk, and Serratia marcescens, which is responsible for pinkish discoloration.

Key Points

Psychrotrophic Bacteria: Cold-tolerant bacteria, especially Pseudomonas, are major causes of spoilage and discoloration in refrigerated milk products.
Pseudomonas Color Spectrum: Different Pseudomonas species can cause blue, yellow, brown, or gray discoloration due to their specific pigment production.
Pink from Prodigiosin: Pink or reddish milk is often caused by Serratia marcescens, which produces the red pigment prodigiosin.
Hygienic Practices are Key: Preventing milk discoloration and spoilage requires strict hygiene during milking, processing, and storage to minimize bacterial contamination.
Refrigeration is Crucial: Storing milk at a consistent temperature of 4°C or below is critical for inhibiting bacterial growth and extending shelf life.
Environmental Sources: Contamination often comes from the farm environment, including manure, soil, and water, as well as improperly sanitized equipment.
Heat-Resistant Survivors: Some spore-forming bacteria, like certain Bacillus species, can survive pasteurization and cause spoilage if milk is stored improperly.

Common Bacteria That Cause Milk Discoloration

Several types of bacteria are known to cause milk discoloration, often producing distinct colors due to their pigmentation or metabolic byproducts. The spoilage is often influenced by factors like temperature, hygienic practices during milking and processing, and the duration of storage.

Pseudomonas Species: The Masters of Many Colors

Bacteria of the Pseudomonas genus are a major cause of milk spoilage, particularly in refrigerated products, because many species are psychrotrophic, meaning they can thrive at low temperatures. Different species within this genus can be responsible for various color changes, primarily due to the production of different pigments.

Blue Milk: Pseudomonas syncyanea is a classic example, causing a blue coloration. Studies have also linked species like Pseudomonas fluorescens to bluish discoloration.
Yellow Milk: A similar species, Pseudomonas synxantha, can impart a distinct yellow hue to the creamy layer of milk.
Brown or Grey Milk: Other Pseudomonas species, such as Pseudomonas putrefaciens and Pseudomonas azotoformans, have been documented to cause brown or even greyish discoloration.

Flavobacterium Species: The Yellow Culprits

Like Pseudomonas, some species of Flavobacterium can also cause milk to turn yellow. These Gram-negative, rod-shaped bacteria are often found in water and soil and produce yellow or orange pigments, which explains the characteristic discoloration in contaminated milk and other dairy products.

Serratia Marcescens: The Agent of Pink Milk

Pink or reddish discoloration is typically caused by Serratia marcescens, a bacterium that can produce a distinctive reddish-orange pigment called prodigiosin. While this is a rare occurrence in commercially processed milk, it is a recognized cause of pink-colored expressed breast milk, particularly in hospital settings where strict hygiene is not maintained for breast pumps. The pigment is often more visible on surfaces that have come into contact with the contaminated milk, such as breast pads or feeding equipment.

Chromobacterium Lividum: The Violet Threat

In one documented case, the psychrophilic bacterium Chromobacterium lividum was identified as the culprit behind a violet discoloration in refrigerated, pasteurized retail milk and cream. This rod-shaped, Gram-negative bacterium produces a deep violet pigment and grows well at low temperatures, making it a viable contaminant in cold storage environments.

Sources and Prevention of Bacterial Contamination

Bacterial contamination leading to milk discoloration originates from multiple points, from the dairy farm to the consumer's refrigerator. Preventing spoilage requires good hygienic practices at every stage of the milk's journey.

How Contamination Occurs

From the cow: Bacteria from the udder, manure, or soil can contaminate milk during milking. Mastitis, an udder infection, can also introduce harmful bacteria.
From equipment: Contaminated milking machines, pipes, and storage tanks are a significant source of psychrotrophic bacteria like Pseudomonas, which can form biofilms.
Post-processing: Poor hygiene during bottling or handling, and improper refrigeration, can introduce new contaminants or allow surviving thermoduric bacteria to grow.

Prevention Strategies

Proper Refrigeration: Store milk at a constant temperature of 4°C (39°F) or lower. Avoid storing milk in the refrigerator door, where temperatures fluctuate with every opening.
Hygienic Practices: Ensure all milking equipment and processing machinery are thoroughly cleaned and sanitized. For home use, always use clean utensils and close the container tightly after use.
Observe Expiration Dates: While proper storage can extend shelf life, milk should be consumed before its expiration date to minimize the risk of bacterial proliferation.

Comparison Table: Common Discoloration-Causing Bacteria


Bacterium	Typical Discoloration	Contributing Factor	Primary Source of Contamination
Pseudomonas syncyanea	Blue	Pigment production	Water, milking equipment, refrigeration
Pseudomonas synxantha	Yellow (creamy layer)	Pigment production	Refrigeration, farm environment
Pseudomonas putrefaciens	Brown/Grey	Pigment production	Refrigeration, farm environment
Flavobacterium spp.	Yellow	Pigment production	Water, soil, refrigeration
Serratia marcescens	Pink/Reddish	Prodigiosin pigment	Environmental surfaces, improper handling (breast milk)
Chromobacterium lividum	Violet	Pigment production	Psychrophilic (cold-loving) nature, post-pasteurization contamination

Conclusion

Milk discoloration is a clear indicator of bacterial spoilage, with different species of chromogenic bacteria responsible for specific color changes. While Pseudomonas species can cause a variety of hues, Serratia marcescens is the primary culprit for pink milk, and Flavobacterium contributes to yellow coloration. Preventing this spoilage relies heavily on maintaining proper hygiene and cold-chain integrity throughout the entire production process, from the farm to the refrigerator. By understanding what bacteria causes discoloration in milk, producers and consumers can take better-informed steps to ensure the safety and quality of their dairy products. For additional information on milk processing, reputable sources like the Journal of Dairy Science offer further insights.

Keypoints

Psychrotrophic Bacteria: Cold-tolerant bacteria, especially Pseudomonas, are major causes of spoilage and discoloration in refrigerated milk products.
Pseudomonas Color Spectrum: Different Pseudomonas species can cause blue, yellow, brown, or gray discoloration due to their specific pigment production.
Pink from Prodigiosin: Pink or reddish milk is often caused by Serratia marcescens, which produces the red pigment prodigiosin.
Hygienic Practices are Key: Preventing milk discoloration and spoilage requires strict hygiene during milking, processing, and storage to minimize bacterial contamination.
Refrigeration is Crucial: Storing milk at a consistent temperature of 4°C or below is critical for inhibiting bacterial growth and extending shelf life.
Environmental Sources: Contamination often comes from the farm environment, including manure, soil, and water, as well as improperly sanitized equipment.

Frequently Asked Questions

Yes, milk discoloration caused by bacteria is a clear sign of spoilage and potential microbial contamination. While some bacteria may only affect the milk's appearance and taste, others, like certain strains of Serratia marcescens, can be opportunistic pathogens, particularly a risk to vulnerable populations like infants.

Blue milk is most commonly caused by contamination with specific pigment-producing Pseudomonas species, such as Pseudomonas syncyanea. These psychrotrophic bacteria can grow well even under refrigeration and produce a distinct blue pigment.

Milk may turn yellow due to contamination by bacteria such as Pseudomonas synxantha or Flavobacterium species, which produce yellow pigments. These microorganisms can thrive in refrigerated conditions, often affecting the cream layer first.

Pink or reddish milk, including in expressed breast milk, is a sign of contamination by Serratia marcescens. This bacterium produces a red pigment called prodigiosin. It is typically associated with hygiene lapses, especially with cleaning breast pump equipment.

To prevent milk spoilage, store it at 4°C or colder in the main part of the refrigerator, not the door, to maintain a consistent temperature. Always use clean utensils, close the container tightly, and avoid drinking directly from the carton to prevent introducing oral bacteria.

While boiling milk can kill many active bacteria, it does not destroy all bacterial spores or heat-stable enzymes produced by psychrotrophic bacteria like Pseudomonas. These enzymes can continue to cause quality defects even after the bacteria are eliminated. Boiling may also curdle milk that has already soured due to acid production.

Raw milk is more susceptible to contamination from a wider variety of bacteria found in the environment. Pasteurization significantly reduces the microbial load, but some thermoduric (heat-resistant) and spore-forming bacteria can survive, making pasteurized milk still vulnerable to spoilage, including discoloration, if improperly handled or stored.