The Science Behind Drying Fish: Traditional and Modern Methods

Fish is one of the most perishable foods on the planet, with a moisture content of around 70-80% in its fresh state. That high water content makes it an ideal breeding ground for bacteria and spoilage organisms. Drying – one of the oldest preservation techniques known to humankind – addresses this problem by removing water from the fish, thereby creating conditions where microorganisms simply cannot thrive. Whether practiced on a sunlit beach in Southeast Asia or inside a high-tech solar dome in Cambodia, fish drying remains a cornerstone of post-harvest processing, particularly in tropical developing countries where cold storage is expensive or unavailable.

Why drying works: the role of water activity 🔗

To understand fish drying, you first need to understand water activity (a_w). Water activity is a measure – on a scale of 0 to 1 – of how much water in a food product is available for microbial growth and chemical reactions. Fresh fish typically has a water activity of about 0.98-0.99. Most spoilage bacteria require a water activity above 0.91 to grow, while moulds and yeasts can survive at slightly lower levels, down to about 0.70-0.80. The goal of drying is to bring the fish’s water activity below 0.60, a level that is hostile to virtually all spoilage organisms.

Water activity can be described as the ratio of the water vapour pressure in the fish flesh to the vapour pressure of pure water at the same temperature and pressure. When you reduce this ratio through drying, you effectively starve bacteria and enzymes of the moisture they need to function. The result is a product that remains stable at most ambient temperatures and can last for months or even years when stored properly.

An added benefit: as water evaporates, the proteins, minerals, and vitamins in the fish become more concentrated per gram of the final product. Dried fish is, gram for gram, an exceptionally nutrient-dense food – something that matters enormously in regions where protein deficiency is a public health concern.

The two stages of the drying process 🔗

Fish drying does not happen uniformly. Food scientists divide the process into two distinct phases, each with different characteristics, challenges, and control requirements.

The constant rate period 🔗

During the first phase of drying, known as the constant rate period, moisture evaporates from the surface of the fish at a steady and predictable rate. As long as the fish’s interior contains enough free water, that moisture migrates easily to the surface to replace what has evaporated. The product temperature remains relatively constant during this phase because the evaporative cooling effect offsets the heat being applied.

Three external factors primarily control the speed of this stage: air temperature, relative humidity, and air velocity. Higher temperatures and faster-moving, drier air will remove surface moisture more quickly. This phase typically lasts for the first several hours of drying, depending on environmental conditions and the size and thickness of the fish.

The falling rate period 🔗

Once the easily accessible surface moisture has been removed, the process transitions into the falling rate period. In this phase, the fish’s internal temperature rises, and moisture must travel from deep within the flesh outward to the surface. This internal migration becomes progressively harder as the outer layers dry and form a semi-rigid barrier.

The drying rate now decreases gradually. The moisture gradient between the fish’s interior and its surface shrinks over time, and the process continues to slow until an equilibrium point is reached – the point at which the water activity of the fish matches the humidity of the surrounding air.

This stage demands patience. Applying excessive heat to speed things up can cause a defect called case hardening – the outer surface dries too quickly and forms a tough, impermeable shell that traps residual moisture inside. Case-hardened fish may appear dry on the outside but harbour enough internal moisture to support microbial growth, leading to spoilage from the core outward. Controlling temperature and airflow during the falling rate period is therefore critical to producing a safe, uniformly dried product.

Sun drying: the most common method 🔗

Sun drying is by far the most widely practised fish drying method globally, especially in tropical and subtropical countries across Asia, Africa, and Latin America. It is also the simplest and cheapest – requiring nothing more than sunlight, moving air, and a surface to lay the fish on. The FAO notes that sun drying of fish, with or without the addition of salt, is practised in many tropical countries as a low-cost form of preservation.

How sun drying works 🔗

The process begins with cleaning and preparing the fish – gutting, splitting, or filleting depending on the species and local practice. Often, the fish is salted before drying to further reduce water activity and inhibit bacteria during the initial hours when the flesh is still moist. The prepared fish is then spread on mats, racks, rocks, rooftops, or raised platforms and left in direct sunlight. The combination of solar heat and wind drives moisture evaporation. Under ideal conditions – clear skies, low humidity, and good airflow – fish can be adequately dried within 2 to 7 days, depending on the species and fish size.

Limitations of sun drying 🔗

Despite its widespread use, sun drying has several well-documented problems:

Weather dependency is the most obvious. Cloud cover, rain, or high humidity can stall the drying process entirely. In many tropical regions, peak fishing seasons unfortunately overlap with monsoon periods, creating a situation where large catches coincide with the worst possible drying conditions. The FAO highlights that this mismatch between catch volumes and weather is a major cause of post-harvest fish losses in small-scale fisheries.

Inconsistent quality is another concern. Without controlled conditions, some pieces dry too fast (risking case hardening), while others dry too slowly (risking spoilage). The final product can vary widely in moisture content, texture, and safety – even within a single batch.

Contamination is a persistent issue in open-air drying. Fish laid out in the open are exposed to dust, sand, insect infestation, animal droppings, and microbial contamination from the environment. Research from Cambodia found that Salmonella was detected in traditionally sun-dried fish but not in fish dried using enclosed solar dryers, underlining the hygiene advantage of covered drying systems.

Long drying times further compound these problems. Because sun drying can take five to seven days, the fish remains in a vulnerable, partially dried state for an extended period – time during which bacterial and enzymatic spoilage can advance if conditions are not favourable.

Innovations in fish drying technology 🔗

Recognising the limitations of open-air sun drying, researchers and development organisations have introduced a range of improved drying technologies. These innovations aim to increase heat absorption, improve air circulation, protect fish from contamination, and reduce dependency on perfect weather.

Solar dryers 🔗

Solar dryers represent a significant step up from traditional sun drying. These are enclosed structures – often tent-shaped, cabinet-style, or dome-shaped – covered with transparent or translucent materials (glass, polycarbonate, or UV-stabilised polyethylene) that trap solar radiation and create a greenhouse effect. The enclosed design protects the fish from insects, dust, and rain while generating higher internal temperatures and lower humidity than open-air drying.

The results are substantial. A UNIDO programme in Cambodia found that solar dryer domes could dry fish in about 8-12 hours compared to three to seven days for traditional methods. The domes also produced more consistent, higher-quality products with better colour and flavour retention.

In the Philippines, researchers at Ateneo de Manila University developed an affordable multilevel solar dryer equipped with solar-powered sensors that monitor temperature and humidity inside the drying chamber and automatically regulate conditions. Tests showed the dried fish met the Philippine national standard for quality.

Several types of solar dryers are used in fish processing: solar cabinet dryers (small box-type units ideal for household or small-scale use), solar tunnel dryers (longer, tunnel-shaped structures suited for larger volumes), and solar dome dryers (larger semi-spherical or dome-shaped structures). The FAO has documented that these dryers achieve higher drying temperatures, reduced humidity, increased drying rates, and significantly improved product quality compared to open-air methods.

Black surface technology 🔗

A simple but effective innovation involves placing fish on black-painted metal screens or incorporating black-coloured surfaces inside the drying chamber. Dark surfaces absorb more solar radiation than lighter ones and re-radiate that energy as heat. This can raise the temperature around the fish by 10-15°C above ambient air temperature, significantly accelerating moisture evaporation. This technique is particularly useful during periods of reduced sunlight or in regions where solar intensity is moderate, as it helps maintain consistent drying conditions throughout the day.

Hybrid drying systems 🔗

To address the limitation of nighttime or cloudy-day drying, engineers have developed hybrid solar-biomass dryers. These systems use solar energy during the day and switch to a biomass-fuelled heat source (burning wood, rice husks, or coconut shells) at night or during overcast periods. A study from Indonesia described a hybrid dryer with a glass-covered drying chamber and a crossflow biomass heat exchanger that allowed continuous 24-hour drying, dramatically reducing total processing time.

Even more advanced systems combine solar thermal collectors with photovoltaic panels and infrared lamps. One such PV-powered solar-infrared hybrid dryer designed for anchovy drying in India achieved a drying efficiency of over 41% and produced samples with superior colour and texture compared to conventionally dried fish.

Improved air circulation designs 🔗

Effective air circulation is often the overlooked factor in fish drying. Stagnant air quickly becomes saturated with moisture, slowing evaporation and creating favourable conditions for mould growth. Modern dryer designs address this through chimney effects (where heated air rises and exits at the top, drawing in fresh dry air from below), forced convection using small electric or solar-powered fans, and strategic vent placement to create cross-ventilation patterns. Even simple improvements like using raised racks instead of drying fish on the ground can make a meaningful difference, since air movement at ground level is typically very slow.

The role of salting before drying 🔗

In practice, drying is rarely used alone. Most traditional fish preservation combines drying with salting – and for good reason. Adding salt to fish before drying serves multiple purposes. Salt draws moisture out of the fish tissue through osmosis, giving the drying process a head start. It also lowers the water activity independently of moisture removal, providing an additional barrier against microbial growth. According to a comprehensive review published in the journal Foods, salt and spices are commonly added to dried fish to enhance flavour and further decrease water activity, which aids the overall preservation process.

Common salting methods before drying include dry salting (rubbing coarse salt directly onto the fish) and brining (soaking fish in a saltwater solution for a set period). After salting, the fish is rinsed and then transferred to the drying setup. The prior salt treatment means the fish reaches a safe, low water activity faster during drying, reducing the window of vulnerability to spoilage.

Factors affecting drying efficiency 🔗

Several variables determine how quickly and effectively fish can be dried:

Temperature: Higher temperatures accelerate moisture evaporation, but excessively high heat (above 60-70°C for most species) can damage proteins, degrade nutrients, and cause undesirable colour and flavour changes. Finding the right temperature balance is key.

Relative humidity: Dry air can absorb more moisture from the fish surface. In humid environments, drying is inherently slower and may be inadequate without mechanical dehumidification or enclosed drying systems.

Air velocity: Moving air carries away the moisture-laden boundary layer that forms around the fish surface, replacing it with drier air and maintaining the evaporation gradient. Even modest increases in airflow can markedly improve drying rates.

Fish thickness and preparation: Thinner slices dry faster than whole fish or thick fillets because moisture has a shorter distance to travel from the interior to the surface. Splitting, filleting, or scoring fish before drying increases the exposed surface area and reduces drying time. Research on Nile tilapia drying found that slice thickness (4 mm vs 12 mm) significantly affected total drying duration.

Pre-treatment: Salting, blanching, or applying other pre-treatments can change the tissue structure and moisture-binding properties of the fish, influencing how easily water migrates during the falling rate period.

Quality and safety considerations 🔗

Dried fish, when properly produced, is a safe and shelf-stable food. However, poor drying practices can introduce several quality and safety hazards. Lipid oxidation is a common problem, particularly in fatty fish species. As water is removed, the concentrated unsaturated fats become more exposed to oxygen, leading to rancidity and off-flavours during storage. Browning reactions – both enzymatic and non-enzymatic (Maillard reactions) – can darken the product and alter its taste. And as already mentioned, inadequate drying that leaves residual moisture above safe levels invites microbial spoilage, potentially including the growth of pathogenic organisms.

Proper packaging after drying is equally important. Dried fish should be stored in moisture-proof packaging in a cool, dry environment to prevent reabsorption of atmospheric moisture. In tropical climates with high ambient humidity, this can be a significant challenge for small-scale processors.

Economic and social significance 🔗

Fish drying is far more than a technical process – it has deep economic and cultural roots in coastal and inland fishing communities around the world. Dried fish products can be transported over long distances without refrigeration, opening up access to inland and urban markets that would otherwise be unreachable for perishable fresh fish. The weight reduction from drying also lowers transportation costs per unit of nutrition, making dried fish economically viable for trade and export.

For small-scale fishing communities, fish drying provides employment and income – particularly for women, who are heavily involved in post-harvest processing in many parts of Asia and Africa. Low-cost solar dryers and improved drying racks represent accessible technology that can meaningfully boost product quality and market value without requiring large capital investment.

What do you think? Given the clear advantages of solar dryers over traditional sun drying, what do you believe are the biggest barriers to their widespread adoption in small-scale fishing communities? How might governments and development organisations help bridge that gap?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC9562176/
2. https://www.sciencedirect.com/topics/food-science/dried-fish
3. https://www.britannica.com/topic/constant-rate-period
4. https://www.britannica.com/topic/falling-rate-period
5. https://www.fao.org/flw-in-fish-value-chains/value-chain/processing-storage/artisanal-fish-drying/en/
6. https://en.bdfish.org/2011/01/fish-drying-dehydration/
7. https://www.mdpi.com/2571-8797/6/3/48
8. https://www.unido.org/stories/solar-fish-drying-tech-has-potential-transform-cambodias-fisheries
9. https://www.ateneo.edu/news/2024/05/27/sustainable-solar-dryer-prototype-developed-philippine-fisherfolk
10. https://www.fao.org/3/t0685e/T0685E06.htm
11. https://www.sciencedirect.com/science/article/pii/S2214157X18300972
12. https://www.nature.com/articles/s41598-025-94194-9