What bacteria is in nuts and how to ensure safety

November 25, 2025 •

4 min read

Despite having low moisture content, a key factor that inhibits bacterial growth, nuts are still susceptible to contamination by pathogens. Outbreaks of illness, primarily caused by Salmonella, have been linked to nuts like almonds, pistachios, and peanuts, underscoring the importance of understanding what bacteria is in nuts.

Quick Summary

Nuts can be contaminated with pathogenic bacteria like Salmonella, E. coli O157:H7, and Listeria monocytogenes during harvest or processing. Due to low water activity, bacteria can survive for extended periods, though proper handling and pasteurization significantly mitigate risk.

Key Points

Prevalence: Nuts can be contaminated with pathogenic bacteria like Salmonella, E. coli, and Listeria, even with low moisture content.
Survival: These bacteria can survive for long periods in low-moisture environments, with their persistence sometimes enhanced by the nuts' high fat content.
Contamination Sources: Contamination can happen at various stages, including harvest from contact with soil and wildlife feces, and in processing facilities from equipment or poor hygiene.
Pasteurization: Commercial nuts, especially almonds, are often pasteurized to reduce microbial load through methods like heat, steam, or infrared processing.
Home Safety: Consumers should store nuts properly (refrigerated for longer shelf life) and wash hands and surfaces to prevent cross-contamination.
Raw vs. Roasted: Roasting significantly reduces bacterial risk, while raw nuts offer a potentially higher level of heat-sensitive nutrients but require more careful handling.

Sources of Bacterial Contamination in Nuts

Bacteria do not naturally grow within the kernel of nuts, but they can be introduced at multiple points along the supply chain. The journey from orchard to consumer presents several opportunities for microbial contamination.

On-farm Contamination

Soil and Feces: Many nuts are harvested directly from the orchard floor after shaking. Contact with soil, which may contain animal feces from wildlife like birds, deer, and rodents, is a primary risk factor for bacterial transfer, especially for groundnuts like peanuts.
Water Sources: Contaminated irrigation water or runoff from livestock operations can spread pathogens like E. coli and Salmonella to nuts still on the tree or lying on the ground.
Equipment: Harvesting machinery can become contaminated and transfer pathogens to the nuts.

Processing and Handling Risks

Shelling and Washing: Contaminated water used for processing, such as hulling, shelling, or blanching, can cause cross-contamination. If not properly dried, nuts can also become a breeding ground for bacteria.
Improper Storage: Storing nuts in high-humidity conditions can increase the risk of mold growth and mycotoxin production, though proper drying and storage are key to controlling moisture.
Cross-Contamination: Poor hygiene practices by workers or improperly sanitized equipment within a processing facility can introduce bacteria to the final product.

Common Pathogenic Bacteria Found on Nuts

The low moisture and high fat content of nuts inhibit bacterial growth but allow certain hardy pathogens to survive for extended periods. Some of the most common include:

Salmonella: Considered the main target organism for safety protocols concerning dry foods, Salmonella can persist for long periods and is highly resistant to heat when dehydrated. Outbreaks have been linked to a variety of nuts, including almonds, pistachios, pecans, macadamia nuts, and peanuts.
Escherichia coli O157:H7: This strain of E. coli has been associated with outbreaks linked to walnuts and hazelnuts. It can survive on surfaces for many months, especially when protected by the nut's shell.
Listeria monocytogenes: This pathogen has been isolated from nuts and nut products, leading to recalls. It is particularly dangerous for at-risk populations, including pregnant women, young children, and immunocompromised individuals.
Other Bacteria: Additional bacteria occasionally detected on nuts include Bacillus cereus, Clostridium spp., and Staphylococcus aureus.

Commercial Decontamination and Pasteurization

To mitigate the risk of bacterial contamination, the nut industry employs several methods, particularly for raw almonds, which are subject to mandatory pasteurization in some regions.

Thermal Pasteurization: This includes dry roasting, oil roasting, or using steam to heat the nuts to temperatures sufficient to kill pathogens. Careful control is needed to avoid compromising the nut's quality.
Non-thermal Treatments: Newer methods like infrared heating, radiofrequency treatment, and cold plasma are used to reduce microbial load with minimal impact on product quality.
Chemical Washes: While less common for the final product, chemicals like sodium hypochlorite are sometimes used in processing water to reduce microbial counts.

Safe Handling and Storage at Home

For consumers, especially those who prefer raw nuts, safe handling is critical to minimizing risk.

Source Responsibly: Choose products from reputable retailers with good food safety records. For unpasteurized raw nuts, consider the risks involved.
Proper Storage: Store nuts in airtight containers in a cool, dry place. For extended storage, refrigeration or freezing is recommended, as it significantly extends shelf life and inhibits fungal growth.
Roast at Home: Roasting raw nuts at home, particularly if they were sourced with higher risk, is a simple way to reduce bacterial load. Roasting them on a baking sheet at 350°F for about 10-15 minutes is often recommended.
Wash Hands and Surfaces: Always wash your hands and any surfaces that come into contact with nuts to prevent cross-contamination, especially if handling raw products.
Avoid Soaking at Room Temperature: Soaking nuts in water, a practice used by some to soften them, creates a moist environment where bacteria can grow. If soaking, do so in the refrigerator and use treated nuts.

Raw vs. Roasted Nuts: Microbial Safety and Nutrition


Feature	Raw Nuts	Roasted Nuts
Microbial Safety	Higher risk of bacterial and mold contamination, especially if unpasteurized. Requires careful handling and storage.	Significantly lower risk due to heat treatment during roasting. Pasteurization is mandated for some nuts sold raw.
Nutrient Retention	Generally retain more heat-sensitive nutrients like some antioxidants and vitamin E. Phytic acid is present, which can inhibit mineral absorption but can be reduced by soaking.	Some heat-sensitive nutrients may be degraded.
Fat Content	Healthy unsaturated fats are intact. Lower total fat and calories by weight due to higher moisture content.	Higher total fat and calories by weight as moisture is lost. Healthy fats can be damaged if exposed to high heat for too long, leading to rancidity.
Acrylamide Formation	Not present.	Can form in some nuts (especially almonds and pistachios) during the roasting process as a result of the Maillard reaction.
Flavor and Texture	Milder, more subtle flavor; softer texture.	Deeper, more intense flavor and aroma; crunchier texture.
Digestibility	Can be less digestible for some due to phytic acid, though soaking can help.	More digestible due to heat breaking down some compounds.

Conclusion

While the low moisture content of nuts makes them a relatively low-risk food, contamination by pathogenic bacteria such as Salmonella, E. coli O157:H7, and Listeria monocytogenes can and does occur. Sources of contamination range from the orchard floor to processing facilities and improper storage conditions. For this reason, the food industry has implemented rigorous safety measures, including mandatory pasteurization for certain nuts. For consumers, minimizing risk involves understanding the bacteria present in nuts and practicing proper food hygiene, including responsible sourcing, correct storage, and considering home roasting for raw varieties. By being mindful of these factors, you can safely continue to enjoy nuts as a healthy part of your diet. For more detailed food safety guidelines, resources like those from UC Davis are helpful.

Frequently Asked Questions

Yes, although it is rare, you can get food poisoning from raw nuts if they have been contaminated with pathogenic bacteria like Salmonella or E. coli. Many raw nuts, particularly almonds, are pasteurized to mitigate this risk.

Yes, soaking nuts, especially at room temperature, can significantly increase the risk of bacterial growth. The addition of moisture creates a favorable environment for pathogens to multiply.

Yes, consuming commercially roasted nuts is generally safer from a microbial standpoint than consuming raw nuts. The heat treatment used in roasting is an effective method for killing harmful bacteria.

Salmonella can survive on nuts for very long periods, potentially over a year, especially at refrigerated or frozen temperatures where its population declines very slowly. At ambient temperatures, the population declines more significantly but can still persist for months.

To prevent contamination, store nuts in an airtight container in a cool, dry place. For longer shelf life, refrigeration or freezing is recommended, as this also inhibits mold growth.

Not necessarily. Studies have shown no significant statistical difference in Salmonella prevalence between organic and conventional nuts. Contamination routes, such as contact with soil or wildlife, exist regardless of farming method.

Yes, reputable nut processing companies implement robust food safety protocols, including testing and pasteurization, to ensure their products are safe for consumption.