How Microorganisms Survive in Dried Foods and What It Means for Food Safety

Drying is one of the oldest food preservation methods known to humanity. By removing moisture, we make food hostile to microbial growth and extend its shelf life significantly. But here’s the critical point most people overlook – drying does not sterilize food. Many microorganisms, including some dangerous pathogens, can survive the drying process and remain dormant in dried foods for weeks, months, or even years. Understanding how and why microorganisms persist in dried foods is essential for anyone involved in food production, storage, or safety.

What makes dried foods resistant to microbial growth? 🔗

The primary reason dried foods resist microbial spoilage is reduced water activity (a_w). Water activity is not the same as moisture content. It measures the availability of water for biological processes – the “free” water that microorganisms can actually use. According to the UC Master Food Preserver Program, water activity is expressed on a scale from 0 (completely dry) to 1.0 (pure water). Dried foods generally have a_w values ranging from 0.03 to 0.70, depending on the product and drying method used.

When a_w drops below certain thresholds, microorganisms can no longer grow or reproduce. However – and this is the key takeaway – many of them do not die. They simply enter a dormant or metabolically inactive state, waiting for conditions to become favourable again. This is why low-a_w foods should never be considered sterile, even though they may appear perfectly safe on the surface.

Water activity thresholds for different microorganisms 🔗

Not all microorganisms respond to low water activity the same way. There is a clear hierarchy of tolerance that directly affects which organisms pose the greatest concern in dried foods.

Bacteria 🔗

Bacteria are the most sensitive to reduced water activity. Most pathogenic bacteria need a_w above 0.91 to grow. For example, Clostridium botulinum requires a minimum a_w of approximately 0.93, and Salmonella species need a_w above 0.93 for active growth. Staphylococcus aureus is a notable exception – it can grow at a_w values as low as 0.85 under aerobic conditions, making it the best-adapted pathogenic bacterium to low-moisture environments. Despite their inability to grow, many bacteria remain viable in dried foods for extended periods.

Yeasts 🔗

Yeasts generally require a_w above 0.88 for growth. However, osmophilic yeasts – species adapted to high-sugar environments – can tolerate a_w values as low as 0.65. This explains why improperly stored dried fruits sometimes develop fermentation odours even when they seem dry enough to be safe.

Molds 🔗

Molds are the most resilient of the three groups. Many species can grow at a_w levels as low as 0.70, and some xerophilic (dry-loving) molds can survive at a_w of 0.60 or even lower. This is why mold growth can appear on foods that seem too dry to spoil, such as nuts, grains, dried spices, and old bread. Aspergillus species, commonly found on stored grains and dried fruits, are a notable example of molds that thrive where bacteria and most yeasts cannot.

Microbial survival under different temperature conditions 🔗

Temperature plays a major role in determining how long microorganisms survive in dried foods. The interaction between temperature and low water activity creates distinct survival patterns.

Survival at freezing temperatures 🔗

Freezing immobilises water molecules, making them unavailable for microbial metabolism. Most bacteria cannot grow at temperatures below freezing. However, freezing does not kill them – they enter a state of suspended animation and can resume growth once the food thaws. Research has shown that pathogens like Salmonella dehydrated on surfaces can remain at constant numbers for up to 24 months when stored at 4°C. Counterintuitively, cold storage can actually enhance microbial survival in dried foods compared to storage at room temperature, because the cold slows the rate of microbial die-off.

Survival at moderate (room) temperatures 🔗

At room temperature (20-25°C), the dynamics of microbial survival become more complex. Microorganisms face dual stress from both moderate temperature and low water activity. Some gradually die off over time, while others adapt and persist. Studies on Salmonella have demonstrated that the pathogen can be recovered from dried cranberries and raisins stored at 25°C for up to 21 days, from dried strawberries for 42 days, and from date paste for up to 84 days. The food matrix and its composition – particularly its fat, sugar, and protein content – strongly influence how long organisms survive at these temperatures.

Survival at elevated temperatures 🔗

Higher temperatures generally accelerate microbial death. However, here is a crucial detail for food processors: low water activity increases the heat resistance of microorganisms. This means that bacteria in dried foods can withstand significantly higher temperatures than the same bacteria in moist foods. According to research published in Frontiers in Microbiology, Salmonella in non-fat dried milk remained detectable even after 10 hours at 76.6°C. At 4% and 7% moisture, even 2 hours at 85°C was insufficient to eliminate the pathogen. Raising the moisture to 25%, however, required only 30 minutes for complete destruction. This increased thermal resistance is not unique to Salmonella – it has been observed in E. coli, Listeria monocytogenes, Bacillus species, and even Clostridium botulinum.

Factors beyond water activity that affect microbial survival 🔗

While a_w is the most critical factor, several other variables work alongside it to influence whether microorganisms survive, die, or potentially resume growth in dried foods.

pH (acidity) 🔗

Acidity works together with water activity to control microbial growth. When pH is low (acidic conditions), microorganisms become more sensitive to other stresses, including low a_w. As noted in research from Pressbooks (Microbiology: Canadian Edition), most bacteria thrive at near-neutral pH (6.0-8.0), and reducing pH below 4.6 can effectively inhibit bacterial growth even at relatively higher water activity levels. A food product with a_w of 0.92 and pH of 4.6, for instance, can be shelf-stable even though neither factor alone would fully prevent microbial growth. This combined effect – known as hurdle technology – is the basis of many traditional preservation methods like pickling and fermentation.

Oxygen availability 🔗

The presence or absence of oxygen determines which types of microorganisms can survive. Aerobic organisms like Pseudomonas and many mold species need oxygen and tend to cause surface spoilage on exposed dried foods. Anaerobic pathogens such as C. botulinum are a concern in vacuum-packed or tightly sealed products. Research published in Frontiers in Microbiology demonstrated that Salmonella in vacuum-packed halva (a_w 0.18) showed higher survival than in air-sealed packaging, indicating that reduced oxygen can actually help certain pathogens persist longer in dried foods.

Food composition and matrix effects 🔗

The chemical makeup of the food itself plays a significant role. Foods high in fat – such as chocolate, peanut butter, and nut products – tend to protect microorganisms during both drying and storage. The fat creates a physical barrier around bacterial cells, shielding them from environmental stress. Peanut butter, with a_w of 0.45-0.20, has been linked to multiple Salmonella outbreaks. In chocolate (a_w 0.40-0.50), Salmonella has been shown to survive for over 15 months at room temperature. The type of solute also matters – increasing sucrose content in dried foods has been shown to enhance Salmonella survival by up to 79-fold, while sodium chloride (salt) tends to decrease survival.

Why dormant microorganisms are a food safety concern 🔗

The fact that microorganisms cannot grow in properly dried foods does not mean they are gone. Several factors make dormant organisms a real food safety risk.

Rehydration reactivates dormant organisms 🔗

When dried foods are reconstituted with water, dormant microorganisms can quickly resume metabolic activity and multiply. This is particularly dangerous for products like powdered infant formula, dried soup mixes, and instant meals. If reconstituted food is then held at room temperature, bacteria that were dormant can reach infectious doses within hours. Proper cooking or processing after rehydration is essential to ensure safety.

Low infectious doses in dried foods 🔗

Some pathogens require surprisingly few cells to cause illness when ingested through low-moisture foods. For Salmonella in dried products, the infectious dose can be as low as 10 to 1,000 cells – far lower than the 100,000+ cells typically needed in moist foods. This is partly because the high-fat matrix of many dried foods (like chocolate or peanut butter) protects bacterial cells from stomach acid, allowing them to reach and colonise the intestine more effectively.

Viable but non-culturable (VBNC) state 🔗

Some bacteria can enter a state where they remain alive but cannot be detected by standard laboratory culture methods. This VBNC state means that routine testing may show a product is “negative” for pathogens when, in reality, viable organisms are still present. When conditions improve – such as during rehydration – these cells can resuscitate and regain their ability to cause infection.

Common dried foods and their microbial risks 🔗

Understanding which dried foods carry the highest microbial risks helps both producers and consumers make informed decisions about storage and handling.

Powdered milk and infant formula – Spray drying does not guarantee complete pathogen elimination. Salmonella and Cronobacter (formerly Enterobacter sakazakii) can survive in these products for months to years. Multiple illness outbreaks worldwide have been traced to contaminated infant formula.

Nuts, peanut butter, and nut products – High fat content protects bacteria from heat and desiccation stress. These products have been implicated in numerous Salmonella outbreaks globally.

Dried fruits – While their lower pH provides some protection, pathogens can still survive for weeks to months depending on storage conditions and specific fruit composition.

Spices and herbs – Spices are frequently contaminated with Salmonella, Bacillus cereus, and mold species. Because spices are often added without further cooking, they represent a direct pathway for pathogens to enter ready-to-eat foods.

Chocolate and confectionery – Low a_w combined with high fat content allows long-term survival of Salmonella. Outbreaks involving contaminated chocolate have affected hundreds of people across multiple countries.

Practical implications for food safety 🔗

Given that microorganisms can survive in dried foods, several practical measures are critical for ensuring safety throughout the food chain.

Proper drying and process validation – Drying alone may not eliminate all pathogens, especially in high-fat or high-solid foods. Manufacturers should validate their processes specifically for the products they make, rather than relying on general time-temperature guidelines.

Storage conditions matter – Maintaining low humidity during storage is essential. If dried foods absorb moisture from the surrounding air, their a_w can rise above critical thresholds and support microbial growth. Proper packaging with moisture barrier materials is key.

Temperature control during storage – While higher storage temperatures accelerate microbial die-off, they can also degrade food quality. Cold storage preserves quality but may also preserve pathogens. Choosing the right storage temperature requires balancing safety and quality considerations.

Hygiene and post-processing contamination control – Many outbreaks involving dried foods originate from contamination after the drying step, not before it. Strict sanitation in processing environments, proper zoning between wet and dry areas, and robust environmental monitoring programmes are essential to prevent recontamination.

Safe rehydration practices – When reconstituting dried foods, using adequately hot water and consuming the product promptly reduces the risk of microbial growth. This is especially important for infant formula, which should be prepared with water at 70°C or above according to WHO guidelines.

The role of hurdle technology in dried food safety 🔗

Modern food science increasingly relies on hurdle technology – combining multiple preservation factors to create conditions where microbial survival and growth become extremely unlikely. Rather than depending on a single factor like low a_w, manufacturers combine reduced water activity with low pH, controlled atmosphere packaging, the addition of antimicrobial agents, and appropriate storage temperatures. Each factor serves as a “hurdle” that microorganisms must overcome, and the cumulative effect is far more powerful than any single barrier alone. This approach allows for milder processing conditions – preserving taste, nutrition, and texture – while still ensuring a safe product.

What do you think? Considering that pathogens like Salmonella can survive in dried foods for months and cause illness at very low cell counts, how should food labelling and consumer education be improved to communicate the risks associated with dried and low-moisture foods? And for food processors, is the current reliance on end-product testing sufficient, or should environmental monitoring take a more central role in ensuring the safety of dried food products?

References

1. https://ucanr.edu/program/uc-master-food-preserver-program/article/water-activity-and-its-role-food-preservation
2. https://link.springer.com/article/10.1007/s00217-021-03731-z
3. https://www.food-safety.com/articles/4420-water-activitye28099s-role-in-food-safety-and-quality
4. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2013.00331/full
5. https://pubmed.ncbi.nlm.nih.gov/24988015/
6. https://ecampusontario.pressbooks.pub/microbio/chapter/the-effects-of-ph-on-microbial-growth/