Mycotoxins remain one of the most significant food safety challenges for the dried fruit and tree nut industries worldwide. These naturally occurring toxins, produced by certain molds, can develop under the right environmental and storage conditions and pose serious health risks to humans and animals. With evolving global regulations, shifting climate conditions, and the emergence of new contaminants of concern, the industry faces ongoing pressure to strengthen preventive strategies and ensure compliance.
What Are Mycotoxins?
Mycotoxins are secondary metabolites produced by molds. They are not essential for mold growth but can have harmful effects on plants, animals, and humans. More than 300 different mycotoxins have been identified, though only a subset are well understood. In food production, the most concerning are produced by three genera of molds: Aspergillus, Fusarium, and Penicillium.
While many mycotoxins remain unstudied, some are known carcinogens or mutagens. Because of these risks, regulatory agencies around the world set limits for certain mycotoxins, though these limits vary by country and commodity.
Conditions Favoring Mycotoxin Development
Mold growth, and consequently mycotoxin production, is favored by warm and moist conditions. Key contributing factors include:
- Improper drying or storage (e.g., rain during harvest, high humidity during storage)
- Insect damage (notably from navel orangeworm in almonds and pistachios)
- Plant stress (such as drought in crops like peanuts or corn)
Improper storage is one of the most preventable risks. For example, if nuts or dried fruits are stockpiled with inadequate airflow or moisture control, mold can flourish even after a crop appears to be safely dried. Rainfall at the wrong stage of harvest can also trap moisture inside the product, creating conditions for rapid mold growth. Once contamination occurs, it is extremely difficult to reverse.
Insect damage is another critical factor. Pests such as navel orangeworm not only harm the crop directly but also create openings that allow mold to penetrate the product. Damaged nuts and fruits are much more susceptible to aflatoxin contamination, making integrated pest management a key preventive measure.
Finally, plant stress—such as drought conditions—can weaken natural defenses and make crops more vulnerable. In regions dependent on rainfall rather than irrigation, drought stress has historically led to spikes in mycotoxin levels, particularly in peanuts and corn. While irrigation provides some protection in California’s orchards, shifts in climate may increase stress-related risks in the future.
Understanding how these factors interact is essential for designing preventive measures that address both field conditions and storage environments.
Major Mycotoxins in Dried Fruits & Tree Nuts
Aflatoxin
- Produced by: Aspergillus flavus and Aspergillus parasiticus
- Commodities affected: Corn, peanuts, cottonseed, tree nuts, dried fruits (especially figs and dates)
- Health effects: Liver damage, cirrhosis, liver cancer, immunosuppression
- Regulatory note: Very strict limits exist worldwide. For example, aflatoxin M1 in milk is limited to 0.5 ppb in the U.S. and 0.05 ppb in Europe.
For a deeper dive into aflatoxin risks specific to almonds and pistachios, including export challenges and best practices, see our article: Aflatoxin Guide for Almonds and Pistachios
Controls for aflatoxin include orchard sanitation (removal of “mummy nuts”), pest control for navel orangeworm, use of atoxigenic strains of Aspergillus for biocontrol, and post-harvest sorting and storage practices. Heat treatments such as roasting are not reliable controls, though blanching and oil extraction may reduce levels under certain conditions.
Ochratoxin A (OTA)
- Produced by: Penicillium verrucosum (cooler climates), Aspergillus carbonarius and Aspergillus ochraceus (warmer climates)
- Commodities affected: Raisins, coffee, wine, pistachios, cereal grains
- Health effects: Potential kidney damage (nephritis); suspected carcinogen
- Regulatory note: The U.S. has no current limits, but Europe sets strict thresholds (e.g., 5 ppb in pistachios).
OTA is often present at low but consistent levels, particularly in raisins, with occasional spikes in pistachios.
Emerging Concern: Alternaria Toxins
Alternaria molds, common in soil and plant diseases, can produce over 70 metabolites. Four are of key concern: AOH, AME, TEA, and TEN. Toxicology is still under review, but the European Union is actively monitoring these compounds. Recent data revealed TEA exceedances in pistachios from California, prompting closer scrutiny.
Sampling and Testing Challenges
Sampling is one of the most challenging—and often frustrating—parts of mycotoxin testing. For food safety professionals already juggling tight timelines and regulatory pressure, it can feel like an uphill battle. Studies show that 83% of testing variability comes from sampling, compared to just 1% from laboratory analysis. Because contamination is often uneven—sometimes just a single “hot” nut can skew results—large, representative samples are essential to getting reliable results and protecting your product.
- U.S. FDA protocols: 10 lb for surveillance, 50 lb for compliance
- European Union protocols: Up to 100 incremental samples, with 10–30 kg total depending on the commodity
This means that sampling is not simply about collecting a bag of nuts or dried fruit. It requires a structured process to ensure every portion of a shipment is represented. For example, a container of pistachios may appear uniform, but a few highly contaminated kernels could cause an entire lot to exceed allowable limits. Without sufficient sampling, such “hot spots” may be missed, leading to inaccurate results.
Preparation of samples also plays a role. Grinding and homogenization must be thorough enough to ensure toxins are evenly distributed in the test portion. For sticky dried fruits such as figs and raisins, Safe Food Alliance laboratories often use slurry methods to ensure consistency. For nuts, fine dry grinding is more effective. Each commodity requires a tailored approach, and these technical details can make the difference between reliable and misleading results.
International Regulations and Market Realities
Different markets impose different limits:
- United States: Generally 20 ppb total aflatoxin (with tighter rules for dairy and pistachios)
- Europe: Typically 2–10 ppb for aflatoxins depending on commodity, plus OTA limits
- Japan: Less than 10 ppb for aflatoxin, though sampling challenges previously led to high rejection rates
While these numbers may appear small, they carry significant weight. The difference between a U.S. limit of 20 ppb and a European limit of 10 ppb can determine whether a shipment is accepted or rejected. For companies that export globally, this means compliance must be aimed at the strictest standards, not just domestic regulations.
Rejections are costly. Shipments flagged at import may be returned, reconditioned, or destroyed. The process involves additional sampling, testing, and regulatory oversight, often leading to weeks of delays. In some cases, products may be placed under “detention without physical examination,” requiring extensive documentation and clearance before they can re-enter commerce. These costs extend beyond money—they also affect brand reputation and customer trust.
For exporters, understanding the nuances of each market is essential. Europe not only enforces lower limits but also requires more extensive sampling protocols, increasing the likelihood of detecting even trace contamination. Japan has recently revised its protocols to align more closely with European approaches, reducing inconsistencies but maintaining strict expectations. The lesson is clear: companies must treat international compliance as a core part of food safety, not an afterthought.
Looking Ahead
As climate conditions change and global markets demand stricter standards, mycotoxins will remain a significant challenge. Emerging toxins like Alternaria are likely to receive more attention from regulators, especially in Europe. For growers and processors, the best defense remains:
- Rigorous field and orchard practices (pest control, sanitation, irrigation management)
- Effective post-harvest handling and storage
- Robust sampling and testing protocols
Safe Food Alliance remains committed to supporting the industry through training, consulting, and accredited laboratory testing. By staying proactive, the dried fruit and tree nut industries can better manage mycotoxin risks and meet the high expectations of global markets.
Ensuring safe, high-quality products requires constant vigilance. By combining sound agricultural practices, careful post-harvest handling, and rigorous laboratory testing, the dried fruit and tree nut industries can reduce risk and maintain consumer trust. Safe Food Alliance works alongside the industry to address the challenges of mycotoxin management and meet the demands of global regulations. Together, we can strengthen food safety and protect the reputation of California’s specialty crops in markets worldwide.
