Microbiology of Dried Foods: From Harvest to Storage

Dried foods – from sun-dried tomatoes and raisins to powdered spices and jerky – are staples in kitchens worldwide. Drying is one of the oldest methods of food preservation, and it works primarily by removing moisture that microorganisms need to grow. But here’s the catch: drying does not make food sterile. Microbes can still be present on dried foods, and under the right conditions, they can become active again. Understanding the microbiology of dried foods, from the moment a fruit or vegetable is harvested to the day it sits on a storage shelf, is essential for food safety.

How fresh produce picks up microorganisms before drying 🔗

Contamination of food with microorganisms doesn’t start in the drying room – it begins in the field. Fresh fruits and vegetables are naturally exposed to a wide range of microbes during their growth cycle. Soil is a rich reservoir of bacteria, fungi, and spores. Irrigation water, if sourced from untreated or contaminated supplies, can introduce pathogens like Salmonella, E. coli, and various mould species onto the surface of crops. Even the air carries fungal spores and bacteria that settle on produce as it grows.

Harvesting adds another layer of risk. Equipment, human hands, transport containers, and even the ground surface can transfer microorganisms to freshly picked produce. Improper handling – like placing produce directly on soil or using unwashed tools – significantly increases the initial microbial load on the raw material. This is why adopting Good Agricultural Practices (GAPs) during cultivation and harvest is the very first line of defence in keeping dried foods safe.

The role of water activity in controlling microbial growth 🔗

To understand how drying preserves food, you need to understand the concept of water activity (a_w). Water activity is not the same as moisture content. It measures how much of the water present in food is “free” and available for microbial use, chemical reactions, and enzymatic processes. Pure water has an a_w of 1.0, while most fresh foods sit at around 0.95-0.99.

According to the U.S. FDA, most bacteria, yeasts, and moulds need a_w above 0.95 to thrive. By reducing the a_w below certain thresholds, you can effectively shut down microbial growth. Here’s how different groups of microorganisms respond to decreasing water activity:

Bacteria: Most pathogenic bacteria, including Salmonella and E. coli, require a_w above 0.91 for growth. Staphylococcus aureus is more tolerant and can grow at a_w as low as 0.83-0.86 under aerobic conditions, as noted by the FAO. This makes it one of the organisms of highest concern in intermediate-moisture foods.

Yeasts: These generally need a_w above 0.88 to grow. However, some osmophilic (sugar-loving) yeasts can survive at a_w values as low as 0.60-0.65, which explains why improperly stored dried fruits sometimes develop a fermented odour.

Moulds: Moulds are the most resilient group. Many can grow at a_w levels around 0.70-0.80, and certain xerophilic (dry-loving) species can manage even lower. This is why mould growth on dried figs, nuts, or grains remains a persistent challenge.

The general rule in food science is that dried foods with a_w below 0.60 are considered microbiologically stable. If they remain dry during storage, their shelf life is not limited by microbial spoilage.

What happens to microorganisms during the drying process 🔗

When food is dried, the removal of moisture creates an environment hostile to most microbes. Many vegetative bacterial cells are destroyed or inactivated during the drying process, especially when higher temperatures are involved. However, drying is not equivalent to sterilisation. Several types of microorganisms can survive the process:

Bacterial spores: Species like Bacillus cereus and Clostridium spp. form heat-resistant spores that easily survive drying. These spores remain dormant at low a_w and do not produce toxins, but they can become active again if the food absorbs moisture during storage or rehydration.

Salmonella: This pathogen is particularly concerning in dried foods. Although it cannot multiply at low water activity, it can survive for extended periods in dry environments. Dried spices, nuts, powdered milk, and cereal products have all been linked to Salmonella outbreaks over the years.

Mould spores: Many fungal spores are highly resistant to desiccation. They can remain viable in dried foods and germinate when moisture conditions become favourable, even slightly. Some moulds, particularly Aspergillus species, can produce dangerous mycotoxins such as aflatoxins in dried products like figs, groundnuts, and cereals – posing serious public health risks.

Drying method matters 🔗

The type of drying method used has a significant impact on microbial survival. Sun drying, while cost-effective and widely used, exposes food to environmental contaminants such as dust, insects, bird droppings, and airborne microorganisms. Temperature fluctuations during sun drying can also create pockets of higher moisture within the product, providing safe havens for microbial survival.

Hot air drying and oven drying offer more consistent temperature control and reduce the risk of environmental contamination. Freeze drying preserves food quality well but operates at low temperatures, meaning it may not significantly reduce pathogen levels – many dried foods with microbiological safety issues are produced by freeze drying or similar low-temperature techniques.

Newer technologies like infrared drying, microwave drying, and superheated steam drying show promise in combining effective moisture removal with improved microbial inactivation. These methods can achieve higher surface temperatures that help destroy vegetative cells and even some spore-forming organisms.

Sources of contamination during processing 🔗

Even after the raw material has been properly harvested and washed, contamination can still occur during processing. Every step – washing, cutting, blanching, drying, and packaging – presents an opportunity for microorganisms to enter the food chain.

The post-processing environment is, in fact, a major source of microbial contamination. According to Food Safety Magazine, the manufacturing environment around dry food processing lines is a common origin of microbial issues. Equipment surfaces, conveyor belts, storage bins, and packaging materials can harbour bacteria, yeasts, and moulds, especially in areas where moisture accumulates due to condensation, leaky pipes, or improper cleaning.

Key sources of contamination during processing include:

Human handlers – workers’ hands, clothing, and hair can introduce Staphylococcus, Salmonella, and other organisms. Strict personal hygiene and use of protective gear are essential.

Equipment and surfaces – cutting boards, drying trays, slicers, and packaging machines that are not properly cleaned between batches become breeding grounds for biofilms and microbial colonies.

Airborne contamination – open-air drying setups and poorly ventilated processing rooms allow dust and airborne spores to settle on food during drying.

Cross-contamination – mixing raw and partially dried or fully dried products, or using the same equipment for different batches without sanitising, can spread pathogens across the production line.

Why dried foods are not “automatically safe” 🔗

There is a common misconception that dried foods are inherently safe from foodborne illness. Research clearly shows this is not the case. A review published in the journal Comprehensive Reviews in Food Science and Food Safety concluded that dried foods are not inherently safe from a microbiological standpoint and require additional hurdles to ensure safety.

Real-world outbreaks reinforce this point. In 1998, dry cereal in the United States was linked to a Salmonella outbreak affecting 209 people across 11 states, leading to a recall of around three million pounds of product. More recently, dried spices and powdered supplements have been repeatedly recalled due to Salmonella contamination. An FDA review of spice recalls from 1970 to 2003 found that nearly all involved Salmonella, with paprika being the most frequently recalled spice.

The reason Salmonella is so problematic in dried foods is its ability to survive – not grow, but survive – at very low a_w values for months or even years. When a consumer rehydrates or consumes the product, even a small number of surviving cells can cause illness, especially in vulnerable populations like children, the elderly, or immunocompromised individuals.

Storage conditions and their impact on microbial stability 🔗

Proper storage is just as important as proper drying. Even well-dried foods can spoil if storage conditions allow moisture to creep back in. The main factors that influence microbial stability during storage are:

Humidity and moisture reabsorption 🔗

Dried foods are hygroscopic – they tend to absorb moisture from the surrounding air. If stored in a humid environment without adequate packaging, their a_w can rise above the critical thresholds that support microbial growth. This is particularly dangerous because the change may not be visible to the naked eye. A dried product can look and feel dry while its internal water activity has increased enough to support mould germination.

Temperature 🔗

High storage temperatures accelerate chemical degradation reactions and can also promote microbial activity if moisture levels are borderline. Ideally, dried foods should be stored at cool temperatures – below 21°C (70°F) – to slow down all forms of deterioration. Even for products with safely low a_w, elevated temperatures can cause rancidity, colour changes, and nutrient loss.

Packaging 🔗

The choice of packaging material is a critical barrier against moisture reabsorption and environmental contamination. Moisture-proof packaging – such as metallised films, laminated pouches, or glass jars with airtight seals – protects dried foods from humidity and airborne contaminants. Vacuum packaging or the use of oxygen absorbers can further extend shelf life by creating an environment hostile to aerobic moulds and reducing oxidative reactions.

Pest and insect control 🔗

Stored dried foods are attractive to insects and rodents, which can physically damage packaging and introduce microbial contamination. Fumigation, hermetic (airtight) storage, and regular inspection of storage areas are all important practices for maintaining product integrity.

Prevention strategies across the supply chain 🔗

Ensuring the microbiological safety of dried foods is not a single-step process. It requires a multi-hurdle approach that addresses contamination risks at every stage.

Pre-harvest and harvest 🔗

Using clean irrigation water, applying Good Agricultural Practices (GAPs), training farm workers in hygiene, and avoiding contact between produce and contaminated soil or equipment – these are the foundational steps in reducing the initial microbial load.

Pre-treatment before drying 🔗

Washing produce thoroughly with clean, treated water removes surface dirt and many microorganisms. Blanching – briefly exposing vegetables to boiling water or steam – can significantly reduce vegetative microbial cells. Chemical treatments with food-grade sanitisers like citric acid, sodium metabisulfite, or diluted chlorine solutions also help lower surface contamination.

Controlled drying 🔗

Ensuring uniform drying with consistent temperature and airflow prevents the formation of moisture pockets where microbes can survive. Monitoring the a_w of the final product – not just its moisture content – is essential to confirm microbiological stability.

Post-drying handling and packaging 🔗

Adopting Good Manufacturing Practices (GMPs) in the processing facility, including regular sanitation of equipment, environmental monitoring for pathogens, and use of appropriate packaging materials, reduces the risk of recontamination after drying.

Storage and distribution 🔗

Maintaining cool, dry storage conditions, using moisture-barrier packaging, and implementing a HACCP (Hazard Analysis and Critical Control Points) system throughout the supply chain ensures that the safety achieved during drying is preserved all the way to the consumer’s plate.

The importance of monitoring and testing 🔗

Regular microbiological testing of both the product and the processing environment is essential. Key tests include aerobic plate counts (to assess overall microbial load), yeast and mould counts (particularly relevant for dried foods, since these organisms tolerate low a_w), and targeted testing for specific pathogens like Salmonella and Staphylococcus aureus.

Environmental monitoring – swabbing equipment surfaces, walls, floors, and air ducts in the processing area – helps identify contamination hotspots before they lead to product recalls or outbreaks. As food safety experts emphasise, controlling microbial growth in the factory environment is critical because many dried foods are not treated with any kill step after packaging.

What do you think? Given that dried foods are often perceived as inherently safe, how can food manufacturers and consumers be better educated about the microbial risks that persist even after drying? And in regions where sun drying is the only accessible method, what low-cost practices could be promoted to improve safety?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC8017434/
2. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
3. https://www.fao.org/4/y4358e/y4358e06.htm
4. https://www.sciencedirect.com/topics/food-science/dried-food
5. https://ift.onlinelibrary.wiley.com/doi/10.1111/1541-4337.12224
6. https://www.food-safety.com/articles/4640-microbiological-sampling-in-the-dry-foods-processing-environment
7. https://pubmed.ncbi.nlm.nih.gov/29672137/
8. https://pubmed.ncbi.nlm.nih.gov/16416926/