Can Bacteria Grow in Butter? What You Need to Know

November 17, 2025 •

5 min read

While butter is a dairy product, its high fat content and low water activity make it an inhospitable environment for many microbes. However, this doesn't mean bacteria can't grow in butter under certain conditions, making proper handling crucial for safety.

Quick Summary

Butter's composition generally discourages bacterial growth due to high fat and low water content. Key factors like salt, pasteurization, and storage conditions determine its resistance to microbes. Proper storage methods are vital to prevent spoilage and maintain freshness.

Key Points

Fat and Water Content: Butter's high fat-to-water ratio and the distribution of water in small droplets generally inhibit bacterial growth, but it does not eliminate it entirely.
Salt is a Preservative: Salted butter is significantly more resistant to bacterial spoilage than unsalted butter because salt lowers the water activity, creating a less hospitable environment for microbes.
Pasteurization Kills Bacteria: Commercial butter is made from pasteurized cream, which dramatically reduces the initial microbial count and makes the final product safer for consumption.
Storage Temperature is Critical: High temperatures accelerate both bacterial growth and the oxidation of fats, which causes butter to go rancid. Keep butter refrigerated for long-term storage.
Unsalted and Raw Butter Risk: Unsalted, whipped, and raw (unpasteurized) butters are more perishable and should always be stored in the refrigerator due to a higher risk of bacterial contamination.
Airtight Containers are Essential: For any countertop storage of salted butter, use an opaque, airtight container like a butter bell or crock to protect it from light, air, and external odors.

Understanding Butter's Composition

Butter's unique makeup is what grants it relative resistance to microbial growth compared to other dairy products like milk or cream. At least 80% of butter is milk fat, with the remaining portion primarily being water and milk solids. The fat and water are not mixed uniformly but exist as a water-in-oil emulsion, where tiny droplets of water are encased within the fat. This physical structure is key to its longevity. Microorganisms require moisture to thrive, but in butter, this moisture is broken up into minute, isolated droplets. This makes it difficult for bacteria to spread and multiply throughout the product.

The Impact of Salt and Pasteurization

Two of the most significant factors influencing whether and how bacteria can grow in butter are the addition of salt and the pasteurization process.

Pasteurization: The process of pasteurizing the cream used to make butter involves heating it to high temperatures for a short period. This step effectively kills harmful bacteria and reduces the overall microbial load, making the final product much safer. The vast majority of commercially produced butter is pasteurized, providing an important line of defense against foodborne pathogens.
Salt: Salt acts as a preservative by reducing the water activity ($a_w$) of the food. Lowering the available water makes it a less suitable environment for bacterial growth. This is why salted butter has a longer shelf life and greater resistance to spoilage than its unsalted counterpart. Studies have shown that bacterial counts decrease significantly in salted butter during storage, while they may increase in unsalted butter under the same conditions.

Factors That Encourage Bacterial Growth

While butter is resilient, certain conditions can undermine its natural defenses and allow bacteria to proliferate.

Unsalted and Raw Butter: Without the preservative effects of salt, unsalted butter is more vulnerable to spoilage. Similarly, raw (unpasteurized) butter retains beneficial enzymes but also carries a higher risk of containing harmful bacteria, requiring refrigeration and more cautious handling. Pathogens like Salmonella and Staphylococcus aureus have been found in raw and improperly handled butter.
Temperature Abuse: Leaving butter out at high temperatures for extended periods can accelerate bacterial growth. Warm temperatures not only encourage the growth of any pre-existing psychrotrophic bacteria but also increase the rate of fat oxidation, leading to rancidity. The USDA recommends refrigerating butter if the room temperature is above 70°F.
Contamination: Post-processing contamination can introduce microbes into butter. This can happen through unclean utensils, airborne spores, or contact with other contaminated foods. Once introduced, bacteria can multiply, especially in the larger water droplets of poorly worked butter.
Whipped Butter: Whipped butter contains more air and is often sold unsalted, making it more perishable than regular butter. The increased air and moisture content create a more favorable environment for bacterial growth.

Signs of Butter Spoilage

Beyond bacterial growth, butter can also go rancid, a chemical spoilage process. It's crucial to distinguish between the two types of spoilage, as the signs differ.

Rancidity: Caused by the oxidation of fats. Signs include an off-smell (often described as 'sweaty feet' or 'old cheese'), a sour or bitter taste, and changes in color (dark yellow or brown).
Bacterial/Mold Spoilage: Indicated by visible mold, unusual spots, or very sour, cheesy flavors from bacterial action. Some microbes, like certain pseudomonads, can produce fruity or putrid flavors.

Comparison of Butter Types and Spoilage Risk


Feature	Salted Butter	Unsalted Butter	Raw Butter	Whipped Butter
Salt Content	Contains added salt, acting as a preservative.	No added salt, offers a sweeter flavor.	No pasteurization, may or may not be salted.	Incorporates air, often unsalted.
Pasteurization	Typically made from pasteurized cream.	Typically made from pasteurized cream.	Made from unpasteurized cream.	Typically pasteurized, but whipped after.
Water Activity	Lower $a_w$ due to salt, hindering bacterial growth.	Higher $a_w$, making it more hospitable to microbes.	Variable $a_w$, but higher risk due to microbes from cream.	Increased surface area and moisture access.
Shelf Life at Room Temp	Up to several days to two weeks with proper care.	Only a few hours; should be refrigerated.	Always refrigerate due to higher risk.	Keep refrigerated; highly perishable.
Primary Spoilage Concern	Rancidity (oxidation of fats).	Both rancidity and bacterial growth.	Pathogenic and spoilage bacteria.	Rancidity and faster microbial growth.

Proper Storage for Preventing Bacterial Growth

To minimize the risk of bacterial growth and keep butter fresh and flavorful, follow these guidelines:

Refrigeration for Long-Term Storage: For storing butter longer than a few days, refrigeration is the safest and most effective method. Storing butter in its original wrapper or an airtight container in the coldest part of the fridge (not the door) prevents it from absorbing odors and slows spoilage.
Countertop Storage (Salted Butter Only): If you prefer soft, spreadable butter, keep a small, salted portion in an opaque, airtight butter dish or a butter bell. Change the water in a butter bell every few days and ensure the room temperature stays below 70-75°F. Only leave out what you will use within a day or two.
Freezing: For long-term preservation, butter can be frozen for up to one year. Wrap it tightly in its original packaging plus an extra layer of foil or a freezer bag to prevent freezer burn and odor absorption.
Sanitary Practices: Always use a clean knife or utensil when cutting butter to avoid cross-contamination.

Conclusion

While the composition of butter makes it less prone to bacterial growth than other dairy products, the possibility of microbial contamination and spoilage is real and depends heavily on the type of butter and how it is stored. Factors like pasteurization, salt content, and temperature play critical roles in determining butter's shelf life and safety. Salted, pasteurized butter offers the most resilience and is generally safe for short-term countertop storage in a cool environment, but unsalted, whipped, or raw varieties require constant refrigeration. The best practice is to store butter properly, especially in the fridge for long-term use, and use a clean, airtight container to prevent both microbial growth and rancidity. When in doubt, trust your senses: any foul smell, sour taste, or discoloration is a clear sign that the butter should be discarded. For further reading on food preservation methods, refer to the USDA guidelines.

Frequently Asked Questions

Yes, but only for certain types of butter and for a limited time. Pasteurized, salted butter can be left out at a cool room temperature (below 70°F) for a few days to a couple of weeks if kept in an airtight, opaque container. Unsalted, whipped, or raw butter should always be refrigerated.

Yes, salt acts as a natural preservative by reducing the water activity in butter, which inhibits the growth of most bacteria. This is why salted butter has a longer shelf life and is less susceptible to bacterial spoilage than unsalted butter.

Rancidity is a chemical change caused by the oxidation of fats, resulting in a distinctly sour, bitter, or 'off' smell and taste. Bacterial spoilage is caused by microbial activity and can lead to mold growth, discoloration, and cheesy or putrid flavors.

Yes, unsalted butter is more susceptible to bacterial growth because it lacks the preserving effect of salt. It is therefore recommended to always store unsalted butter in the refrigerator and use it within a shorter timeframe.

Signs of spoiled butter include a rancid or foul smell, a sour or bitter taste, discoloration (yellowing or browning), and visible mold. If you notice any of these, it's best to discard the butter.

Properly wrapped and frozen butter can be stored for up to one year without significant loss of quality. Ensure it's in an airtight container or freezer bag to prevent freezer burn and absorbing odors.

No, it's not ideal. The temperature in the refrigerator door fluctuates the most due to frequent opening and closing. For long-term storage, keep butter in the coldest part of the fridge, such as on a back shelf.

Homemade butter, especially if made from raw or unpasteurized cream, has a higher risk of bacterial contamination. It should always be refrigerated and handled with strict hygiene to ensure safety.