Achieving Safe and Clean Pecans with the Help of Peracetic Acid Washing

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Pecans are a favorite ingredient in pies, snacks, and salads, but like many foods, they can carry hidden dangers. One of the biggest risks is Salmonella, a group of bacteria that causes severe food poisoning. To tackle this problem, scientists have been searching for better ways to clean pecans during processing.

A groundbreaking study published in Food Control in 2025 offers a promising solution: washing pecans with a sanitizer called peracetic acid (PAA). This method not only kills Salmonella but also stops it from spreading between nuts during processing. Let’s explore how this discovery works and why it matters for everyone who loves pecans.

The study was led by researchers from the USDA and Fort Valley State University. They compared PAA to chlorine, the sanitizer most pecan processors use today. The results showed that PAA is just as effective as chlorine but comes with fewer downsides.

For example, PAA doesn’t corrode equipment or release harmful fumes, making it safer for workers and cheaper for businesses.

The Problem of Salmonella in Pecans

Pecans grow on trees and often fall to the ground during harvest, where they can come into contact with dirt, animals, or contaminated water. This makes them vulnerable to Salmonella, a type of bacteria that survives in soil and water.

Salmonella infections cause symptoms like diarrhea, fever, and stomach cramps, and severe cases can lead to hospitalization.

The bacteria can survive on pecan shells for over 78 weeks (more than a year and a half), according to past studies.

Even worse, Salmonella can move from the shell to the edible part of the nut during processing. Once contaminated, pecans can cause serious illnesses. For instance, a 2004 outbreak linked to almonds sickened 29 people across 12 U.S. states.

To reduce these risks, many pecan processors use hot water or steam treatments to kill bacteria. These methods heat nuts to temperatures that destroy pathogens. However, they require expensive equipment and energy, which smaller facilities can’t always afford.

Instead, most rely on chlorine-based sanitizers in their washing tanks. Chlorine, often in the form of sodium hypochlorite, works by breaking down bacterial cell walls. While chlorine works, it has major drawbacks.

Over time, chlorine damages machinery, releases toxic fumes, and becomes less effective as the water gets dirtier. This led scientists to look for alternatives—and peracetic acid (PAA) emerged as a top candidate.

Understanding Key Terms: What is Peracetic Acid (PAA)?

Peracetic acid (PAA) is a chemical sanitizer made by mixing acetic acid (vinegar) and hydrogen peroxide. It’s widely used in food processing because it kills bacteria, viruses, and fungi without leaving harmful residues.

When PAA breaks down, it turns into water, oxygen, and acetic acid—all of which are safe for humans and the environment. Unlike chlorine, PAA doesn’t produce toxic gases, making it safer for workers.

How the Study Worked: Testing PAA Against Salmonella

The researchers began by creating a mix of five Salmonella strains. These strains were chosen because they’ve been linked to past outbreaks in nuts and other foods. For example, one strain, Salmonella Enteritidis, caused a 2004 almond outbreak, while Salmonella Montevideo was tied to contaminated pistachios in 2009.

Each strain was modified to resist an antibiotic called rifampicin, a drug used to treat bacterial infections. By making the bacteria antibiotic-resistant, the scientists could easily track them during experiments without interference from other microbes.

Next, they contaminated in-shell pecans with these bacteria. Some nuts were given a high dose of Salmonella (about 7 million cells per gram), while others received a low dose (around 45,000 cells per gram).

These levels mimic real-world scenarios where nuts might be lightly or heavily contaminated. After drying, the pecans were stored in a refrigerator to mimic real-world conditions.

The real test came when the pecans were washed. The team soaked them in water treated with three different solutions: plain water (as a control), PAA at 30 parts per million (ppm), PAA at 90 ppm, and chlorine at 1,000 ppm (the industry standard).

Parts per million (ppm) is a measurement used to describe very low concentrations of chemicals. For example, 90 ppm means 90 grams of PAA dissolved in 1 million grams of water. The goal was to see how well each solution killed Salmonella over time.

Samples were taken after 2 minutes, 15 minutes, 60 minutes, and 4 hours. To test cross-contamination (the transfer of bacteria from contaminated to clean items), uncontaminated pecans were soaked in the same water as the contaminated ones.

Key Findings: PAA’s Remarkable Effectiveness

The results were clear. PAA at 90 ppm reduced Salmonella by 4.2 log CFU/g—a 99.99% reduction—on heavily contaminated pecans after 60 minutes. To understand log CFU/g, it’s important to break down the term:

• Log refers to a logarithmic scale. A 1-log reduction means killing 90% of bacteria, a 2-log reduction eliminates 99%, and so on.
• CFU/g stands for colony-forming units per gram, a way to measure live bacteria in a sample.

So, a 4.2 log CFU/g reduction means that out of every 10,000 bacteria, only 1–2 cells survived. Even at 30 ppm, PAA achieved a 3.5 log CFU/g reduction (99.95%) on high-dose pecans.

For comparison, chlorine at 1,000 ppm reduced bacteria by 4.0 log CFU/g, but its effectiveness dropped over time as the water got dirtier.

Equally important, PAA prevented cross-contamination. In plain water, Salmonella spread quickly to uncontaminated pecans, reaching levels of 4.2 log CFU/g (about 16,000 cells per gram) after 4 hours.

But in PAA-treated water, no bacteria transferred to clean pecans at any point. This is critical because cross-contamination during washing is a major cause of foodborne outbreaks. Another advantage of PAA is its stability.

While chlorine levels dropped by 22% over 4 hours (from 1,034 ppm to 799 ppm), PAA stayed strong for the first hour before declining by 26% (from 90.7 ppm to 66.7 ppm).

Stability matters because sanitizers lose effectiveness as they react with dirt, pecan debris, and other organic matter in water.

Why PAA Is Better Than Chlorine

Chlorine has been the go-to sanitizer for decades, but PAA offers several upgrades. First, PAA works at much lower concentrations. For example, 90 ppm of PAA achieved the same results as 1,000 ppm of chlorine.

This means processors can use less chemical, saving money and reducing waste. Second, PAA breaks down into harmless byproducts—acetic acid (vinegar) and oxygen—unlike chlorine, which releases toxic fumes and corrodes stainless steel equipment.

Corrosion is a process where chemicals like chlorine eat away at metal, causing rust and equipment damage. Over time, this forces facilities to replace tanks, pipes, and valves—a costly problem. PAA avoids this issue entirely.

Cost is another factor. Chlorine requires frequent monitoring and replacement as it degrades, adding labor and material costs. PAA, on the other hand, stays stable longer, reducing the need for constant adjustments. For small processors, switching to PAA could mean fewer equipment repairs and safer working conditions.

Challenges and Limitations

While the study’s results are promising, there are still hurdles to overcome. One issue is organic matter. During washing, dirt, pecan debris, and other particles make the water “dirty,” measured as chemical oxygen demand (COD). COD is a test that estimates how much organic material is in water.

High COD means the water has lots of substances that can react with sanitizers, reducing their effectiveness. In this study, COD levels reached 261 ppm in PAA-treated water, which is lower than what many commercial facilities deal with.

If COD rises above 1,000 ppm, PAA’s effectiveness might decline, though it still performs better than chlorine in such conditions.

Another challenge is time. Some processors soak pecans for up to 48 hours during conditioning (a step where nuts are softened before shelling). The study only tested treatments for 4 hours, so longer soaking times need to be explored.

Additionally, PAA works best in slightly acidic water (pH 3), while chlorine requires alkaline water (pH 11). pH is a scale from 0 to 14 that measures how acidic or basic a solution is. Adjusting pH levels adds complexity, though it’s a manageable step for most facilities.

What This Means for the Pecan Industry

For small and medium processors, adopting PAA could be a game-changer. It’s cheaper, safer, and easier to use than chlorine, with no need for expensive equipment upgrades. The researchers recommend starting with 30–90 ppm PAA in washing tanks and monitoring sanitizer levels hourly to maintain effectiveness.

Large facilities that already use thermal treatments (like hot water or steam) could combine them with PAA for even better results. The National Pecan Shellers Association (NPSA) recommends a 5-log reduction (99.999%) in pathogens, and PAA’s 4.2-log reduction gets close. Adding a short hot water rinse could bridge the gap without major costs.

Looking ahead, scientists plan to study natural compounds like Resveratrol-7, a plant-based antioxidant that might boost PAA’s power. They’re also testing PAA in real-world conditions with higher organic matter and longer soaking times.

Conclusion

This study shows that switching to PAA could make pecans safer while saving money and protecting workers. For consumers, it means fewer worries about foodborne illness. For producers, it’s a chance to streamline operations and avoid costly recalls.

As lead researcher Dr. Cameron Bardsley notes, “PAA isn’t just a replacement for chlorine—it’s an upgrade. It’s better for people, equipment, and the bottom line.” While more research is needed, the future of pecan processing looks brighter—and safer—thanks to this simple yet powerful innovation.