Key Factors Influencing Drying Rates in Food Dehydration

Drying is one of the most widely used methods for preserving food, but its efficiency depends on how well we control the conditions during the process. Whether you’re working in a commercial food processing facility or experimenting with a home dehydrator, the speed at which moisture leaves the food – known as the drying rate – is governed by a set of interconnected factors. Understanding these factors helps you produce high-quality dried products, save energy, and reduce processing time.

What is drying rate and why does it matter? 🔗

Drying rate refers to the amount of moisture removed from a food product per unit of time per unit of drying surface area. It is typically expressed in kilograms of water removed per hour per square metre (kg/h/m²). According to ScienceDirect’s overview on food dehydration, understanding drying kinetics is essential for optimal design and operation of drying systems. The drying rate directly determines how long the process takes, how much energy is consumed, and what quality the final product will have.

A faster drying rate generally means less opportunity for spoilage, enzymatic browning, or microbial growth during the process. However, drying too quickly can lead to defects such as case hardening, where the outer surface dries and hardens while moisture remains trapped inside. The goal is always to balance speed with product quality.

Temperature of the drying medium 🔗

Temperature is arguably the most influential factor in drying. When the temperature of the drying air increases, water molecules within the food gain more kinetic energy. This accelerates the transformation of liquid water into vapour at the food surface. As the FAO’s guide on drying principles explains, the drying rate generally increases with rising air temperature and decreases with increasing air humidity.

Higher temperatures also increase the capacity of the surrounding air to hold moisture, which strengthens the vapour pressure gradient between the wet food surface and the drier air stream. This gradient is the fundamental driving force behind moisture removal.

Temperature trade-offs in food dehydration 🔗

While higher temperatures speed up drying, they must be chosen carefully. Excessively high heat – especially in the early stages – can cause several problems. According to Gelgoog’s technical overview on fruit and vegetable drying, rapid heating of high-moisture raw materials can rupture cell walls as juice expands suddenly, damage sugars and organic compounds through decomposition, and create a hard crust on the food surface that blocks further moisture escape.

For most fruits and vegetables, a drying temperature in the range of 50-70°C is considered standard. Heat-sensitive products like herbs or certain leafy greens may require even lower temperatures to retain colour, flavour, and nutritional value. Commercial processors often use a staged approach: beginning at a higher temperature during the initial constant-rate period when the surface stays cool due to evaporative cooling, and then reducing the temperature once the falling-rate period begins to protect the product.

Relative humidity of the drying air 🔗

The moisture content of the drying air – measured as relative humidity (RH) – has a direct impact on how efficiently the air can absorb water from the food. Dry air with low relative humidity creates a steeper moisture gradient between the food surface and the surrounding environment, which accelerates evaporation. Conversely, humid air already carrying significant moisture has little remaining capacity to pick up water from the product.

A study published in Heliyon (PMC) on onion drying demonstrated this clearly. When the relative humidity of the drying air was reduced through dehumidification, both drying time and the degradation of phenolic compounds decreased significantly. The researchers found optimal onion slice drying at 49.6°C with a relative humidity of just 0.65%, achieving approximately 96% retention of beneficial phenolic compounds.

The concept of equilibrium moisture content 🔗

Every food product reaches an equilibrium moisture content (EMC) determined by the temperature and humidity of the surrounding air. This is the lowest moisture level the product can reach under those conditions. As the FAO notes, it is impossible to dry a product below the EMC of the drying air. For example, paddy rice exposed to air at 25°C and 90% relative humidity can only reach about 16.7% moisture content. To dry further, you must either raise the temperature or reduce the air’s humidity.

This is precisely why drying operations in tropical regions with naturally humid climates often struggle with sun-drying methods. Using dehumidified air – through desiccants like zeolite or mechanical dehumidifiers – can improve results dramatically, even at lower temperatures.

Air velocity and circulation 🔗

Air velocity refers to the speed at which drying air moves across the food surface. Faster-moving air serves two critical purposes: it delivers heat to the food more efficiently, and it sweeps away the layer of moisture-saturated air that forms around the product surface. Without adequate air movement, a thin boundary layer of humid air develops around the food, slowing further evaporation.

According to a study published in Procedia Engineering, both increasing air temperature and air velocity led to higher evaporation rates. The research found that the gradient of evaporation rate increased with velocity, rising from 5 grams per m/s to 9 grams per m/s, suggesting that at higher speeds, each incremental increase in velocity has a proportionally greater effect on moisture removal.

When does air velocity stop helping? 🔗

The impact of air velocity is most pronounced during the constant-rate period of drying, when the food surface is still wet and moisture removal is governed by external conditions. During this phase, faster airflow continuously replaces humid air with dry air, maintaining a strong moisture gradient.

However, once the product enters the falling-rate period – when the surface has dried out and internal moisture must migrate to the surface before it can evaporate – increasing air velocity yields diminishing returns. At this stage, the rate-limiting step becomes internal moisture diffusion, which depends on the food’s structure and composition rather than external airflow. The FAO notes that at very low air flows, increasing velocity causes faster drying, but at greater velocities, the effect becomes minimal.

Surface area and food geometry 🔗

The surface area of the food exposed to drying air plays a major role in determining how quickly moisture can escape. When food is sliced thinner or cut into smaller pieces, the total surface area available for evaporation increases substantially. At the same time, the distance that internal moisture must travel to reach the surface decreases.

This is why pre-processing steps like slicing, dicing, and shredding are standard practice in food dehydration. As Gelgoog’s technical guide explains, cutting raw materials increases the specific surface area and proportionally increases the evaporation rate – the finer and thinner the cut, the shorter the drying time.

Uniform sizing matters 🔗

It’s not just about cutting food smaller – uniformity in size and shape is equally important. If pieces are of different thicknesses, thinner portions will dry faster and potentially over-dry or burn, while thicker portions remain moist. This unevenness leads to inconsistent product quality and possible microbial safety concerns in the under-dried sections. Sorting food by size before loading it into the dryer is a simple but effective way to ensure even dehydration.

Nature and composition of the food material 🔗

The internal properties of the food itself significantly affect how it dries. These include initial moisture content, cellular structure, porosity, chemical composition, and the way water is bound within the food matrix.

Foods with high initial moisture content – like cucumbers or watermelons at over 90% water – naturally take longer to dry than denser items like nuts. The ScienceDirect overview on drying rate categorises the factors affecting drying into two groups: external conditions (temperature, humidity, velocity, surface area) and internal conditions (shape, size, structure, porosity, and moisture-binding characteristics).

How food structure influences moisture movement 🔗

The internal structure of the food determines how easily water can migrate from the interior to the surface. Porous foods like bread or some vegetables allow moisture to travel relatively freely through their open structure. Dense, compact foods – or those with waxy skins like grapes and blueberries – resist moisture movement, which slows the drying process considerably.

Foods with high sugar content present additional challenges. As drying progresses, sugars concentrate and can form a sticky surface layer that impedes further moisture escape. Similarly, high-fat foods may develop a hydrophobic surface layer that repels water movement. This is why pre-treatments like blanching, peeling, or even light alkali soaking are used before drying – they alter cell membrane permeability and reduce the water-holding capacity of cells.

Pre-treatment and loading density 🔗

Several pre-drying treatments can dramatically improve drying efficiency. Peeling removes the protective epidermis of fruits and vegetables, exposing the moist interior directly to the drying air. Blanching in hot water or steam denatures enzymes, modifies cell structure, and improves moisture mobility. Sulphuring or sulphite dipping helps preserve colour and further changes cell membrane permeability, making water removal easier.

How tray loading affects drying performance 🔗

The amount of food loaded onto each drying tray – often called loading density – directly affects drying rate. Overloading trays creates a thick layer of product that restricts air circulation between and around individual pieces. This limits both heat transfer to the food and the removal of evaporated moisture from the product surface. A thinner, more spread-out arrangement allows air to flow freely around every piece, treating each as an individually exposed element with maximum surface area contact. Proper spacing on drying trays is one of the simplest and most effective ways to improve drying uniformity and reduce processing time.

How these factors work together 🔗

None of these factors operate in isolation. Temperature, humidity, air velocity, surface area, and food properties all interact with each other in complex ways. Raising the temperature, for instance, simultaneously lowers the relative humidity of the air (since warmer air can hold more moisture), which creates a dual benefit for drying. But if air velocity is too low, the moisture removed from the food accumulates in a stagnant boundary layer and negates some of the temperature advantage.

Similarly, maximising surface area through thin slicing benefits drying only if there is adequate airflow to carry moisture away from all exposed surfaces. Without it, pieces packed closely together effectively reduce their available surface area. Commercial food processors use psychrometric charts and mathematical models to balance these factors and predict drying times accurately. For smaller-scale operations, understanding these relationships helps you make better decisions about temperature settings, food preparation, and equipment arrangement.

The most energy-efficient approach often involves adjusting conditions in stages – starting with moderate-to-high temperatures and strong airflow when surface moisture is abundant, then reducing intensity as the product transitions into the falling-rate period when internal diffusion becomes the limiting factor.

Practical takeaways 🔗

To optimise drying rate in food dehydration, keep these key principles in mind. Use the highest safe temperature for the product being dried, as this maximises heat transfer without compromising quality. Ensure the drying air is as dry as possible – using dehumidified air can be especially beneficial in humid climates or during rainy seasons. Maintain strong and consistent airflow across all drying surfaces. Cut food into uniform, thin slices to maximise surface area. Apply appropriate pre-treatments like blanching or peeling to improve moisture mobility. And finally, avoid overloading drying trays to maintain proper air circulation around every piece.

What do you think? Given that temperature, humidity, and air velocity all interact during drying, which factor do you think would be the most challenging to control in a small-scale drying operation? How might local climate conditions in your area influence the drying strategies you’d need to adopt?

References

1. https://www.sciencedirect.com/topics/food-science/food-dehydration
2. https://www.fao.org/4/t1838e/t1838e0u.htm
3. https://www.gelgoogdryer.com/news/technical-blog/fruits-drying-rate-affecting-factors.html
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC7339060/
5. https://www.sciencedirect.com/science/article/pii/S1877705817311980
6. https://www.sciencedirect.com/topics/food-science/drying-kinetics
7. https://www.sciencedirect.com/topics/engineering/drying-rate