Theory of Drying: Heat and Mass Transfer in Food Preservation

Drying is one of the oldest and most effective methods of food preservation. At its core, the process works by removing water from food, which limits the growth of spoilage-causing microorganisms like bacteria, yeast, and moulds. But what actually drives this moisture removal? The answer lies in two interconnected physical processes: heat transfer and mass transfer. Understanding how these two processes work – both independently and together – is essential for designing efficient drying systems that produce safe, high-quality food products.

What happens when food dries? 🔗

When food is placed in a drying environment, two things happen simultaneously. First, thermal energy (heat) is delivered to the food product. Second, moisture within the food moves toward the surface, evaporates, and is carried away by the surrounding air. These two processes – heat transfer and mass transfer – are tightly coupled. Research published in the journal Energies describes drying as a complex process involving simultaneous heat, mass, and momentum transport phenomena with continuous phase changes. If either heat supply or moisture movement becomes a bottleneck, the entire drying process slows down, which can compromise food quality and safety.

Heat transfer in food drying 🔗

Heat transfer is the process of moving thermal energy from a heat source to the food product. This energy is what gives water molecules enough kinetic energy to evaporate. In drying operations, heat reaches the food through three primary mechanisms: conduction, convection, and radiation. Most commercial and household drying systems use one or more of these mechanisms in combination.

Conduction 🔗

Conduction is the transfer of heat through direct physical contact. When food sits on a heated surface – such as a metal tray in a drum dryer or a heated belt in a continuous dryer – thermal energy passes from the hot surface into the food. According to food engineering course material from GCW Gandhinagar, conduction-based heat transfer within a product is governed by the food’s thermal conductivity and is described mathematically by Fourier’s Law. This law states that the rate of heat flow through a material is proportional to the temperature gradient and the material’s thermal conductivity.

Conduction is particularly effective for drying thin layers of food because the heat can quickly penetrate the material. However, for thicker or denser foods, conduction alone is slow because it relies on energy passing molecule-by-molecule through the food structure.

Convection 🔗

Convection transfers heat through the movement of a fluid – usually hot air in the case of food drying. This is the most widely used mechanism in commercial drying systems such as tray dryers, fluidised bed dryers, and cabinet dryers. When hot air flows over food, it delivers thermal energy to the surface and simultaneously carries away the water vapour that forms.

There are two types of convection relevant to drying. Natural convection occurs when warmer, lighter air rises and is replaced by cooler, denser air – driven purely by temperature differences. Forced convection uses fans or blowers to push heated air across the food, which is significantly more efficient. As noted in a study published in the International Journal of Food Engineering, increasing the inlet air temperature from 40°C to 80°C during convective drying of carrot samples reduced drying time by 50% and energy consumption by 30%.

Air velocity plays a critical role too. When air moves slowly, a thick boundary layer of humid air forms around the food. This layer acts as a barrier, reducing both heat transfer to the food and moisture removal from its surface. Higher air velocity thins this boundary layer, increases the vapour pressure gradient, and speeds up drying.

Radiation 🔗

Radiation transfers heat through electromagnetic waves. Unlike conduction and convection, it does not require a physical medium – heat travels directly from the source to the food. Sun drying, one of the oldest preservation methods, relies entirely on solar radiation. In modern applications, infrared heaters are used to deliver radiant energy directly to food surfaces. This approach is especially useful for surface-level drying without overheating the food’s interior. Many modern dehydrators now combine infrared radiation with convective airflow for better results.

Mass transfer in food drying 🔗

While heat transfer supplies the energy for evaporation, mass transfer describes the actual movement of moisture. It occurs in two stages: first, water moves from the interior of the food to its surface (internal mass transfer), and second, surface moisture evaporates into the surrounding air (external mass transfer). Both stages must work efficiently for drying to succeed.

Internal moisture movement 🔗

Inside a food product, moisture moves from regions of higher concentration (the moist interior) toward regions of lower concentration (the drying surface). This movement is driven by concentration gradients and occurs through several mechanisms:

Liquid diffusion is the most commonly observed mechanism. Water molecules migrate through the food matrix from high-concentration areas to low-concentration areas, following Fick’s Law of Diffusion. A review published in the MOJ Food Processing & Technology confirms that most mathematical models for drying are built on Fick’s law and Fourier’s law as their foundational equations. This process can be slow, especially in thick or dense food items, and often becomes the rate-limiting step in the overall drying process.

Capillary flow is another important mechanism, particularly in porous foods like fruits and vegetables. Water moves through the tiny pores and intercellular channels of the food, driven by capillary forces – the same forces that cause water to travel up a narrow tube. According to a detailed review in the International Journal of Food Properties, capillary flow tends to dominate moisture movement in the early stages of drying, while vapour diffusion takes over in the later stages as the food becomes drier.

Vapour diffusion occurs within the air-filled pores of the food as it dries. As surface moisture evaporates and the outer layers become drier, moisture deeper inside the food converts to vapour and diffuses outward through the pore spaces.

Pressure-driven flow can also push moisture outward. When food is heated, internal pressure builds up (particularly in high-temperature methods like microwave drying), forcing liquid toward the surface.

External moisture removal 🔗

Once moisture reaches the food surface, it evaporates into the surrounding air. The rate of this external mass transfer depends on the vapour pressure gradient between the food surface and the drying air. Warm, dry, fast-moving air creates a steep gradient that promotes rapid evaporation. If the surrounding air is already humid or stagnant, the gradient weakens and drying slows considerably.

Drying rate periods: constant and falling 🔗

The interaction between heat and mass transfer produces a characteristic drying curve that can be divided into distinct periods.

Constant rate period 🔗

During the early phase of drying, the food surface remains wet because internal moisture migrates to the surface fast enough to replace what evaporates. The drying rate stays roughly constant during this stage. The surface temperature of the food stays near the wet-bulb temperature of the drying air because the continuous evaporation produces a cooling effect. In this period, drying rate is controlled mainly by external conditions – air temperature, humidity, and velocity.

Falling rate period 🔗

As drying continues, a point called the critical moisture content is reached. Beyond this point, internal moisture can no longer move to the surface quickly enough to keep it wet. The surface begins to dry out, the evaporation front recedes into the food’s interior, and the drying rate drops. During the falling rate period, internal mass transfer – governed by the food’s structure, composition, and thickness – becomes the rate-limiting factor. The food surface temperature also begins to rise toward the dry-bulb temperature of the air, which is when most heat-related quality damage can occur.

As the GCW Gandhinagar food processing notes explain, hygroscopic foods (those that absorb moisture readily) may exhibit two or more falling rate periods. In the first, liquid diffusion still operates; in the second, moisture removal occurs primarily through desorption. The falling rate period is typically the longest part of the drying process and requires careful control of temperature to avoid damaging the food.

Factors that influence heat and mass transfer during drying 🔗

Several practical factors determine how quickly and uniformly food dries. Adjusting these factors allows food processors to optimise drying performance.

Temperature 🔗

Higher drying temperatures increase both the rate of heat transfer and the driving force for evaporation. However, excessive heat can cause case hardening – a condition where the food’s surface dries and hardens too quickly, trapping moisture inside. It can also degrade heat-sensitive nutrients, cause colour changes, and alter flavour.

Air velocity 🔗

Increasing air speed reduces the boundary layer thickness around the food, improving both heat supply and moisture removal at the surface. However, as the computational study on convective drying found, increasing air velocity reduces drying time but comes at the cost of higher energy consumption. Balancing speed and energy use is a key design consideration.

Relative humidity 🔗

Drier air (lower relative humidity) creates a larger vapour pressure gradient between the food surface and the surrounding air, which accelerates evaporation. Interestingly, the same study found that varying relative humidity between 10% and 30% had almost no measurable effect on drying performance within that range, suggesting that temperature plays a far more dominant role.

Food characteristics 🔗

The composition, structure, and size of food pieces heavily influence internal mass transfer. Porous foods dry faster because moisture can move through capillary channels more easily. Cutting food into smaller, uniform pieces shortens the distance moisture must travel, speeding up drying. As a comprehensive review in Energies notes, most food materials are hygroscopic capillary-porous media, meaning they retain water through a combination of capillary forces and adsorption – and this structure directly governs how moisture leaves the product during drying.

Why understanding these processes matters for food safety 🔗

From a food microbiology perspective, the entire point of drying is to reduce water activity below the level where microorganisms can grow. Most bacteria require a water activity above 0.90, while moulds can survive at levels as low as 0.70. Efficient heat and mass transfer ensure that moisture is removed uniformly and quickly enough to drop water activity below these thresholds throughout the product – not just on the surface.

Poor drying practices can leave moist pockets inside food where microbial growth continues unchecked. Understanding the theory of drying – how heat reaches every part of the food and how moisture migrates out – gives food processors the tools to design processes that are both safe and energy-efficient.

Common drying methods and the heat and mass transfer mechanisms they use 🔗

Hot air drying (cabinet or tray drying) primarily uses forced convection to deliver heat and remove moisture. It is versatile and cost-effective but can be slow for dense products.

Freeze drying (lyophilisation) first freezes the food, then uses low pressure to sublimate ice directly into vapour. Heat transfer is mainly by conduction from heated shelves. This method produces the highest quality dried products but is expensive.

Sun drying relies on solar radiation and natural convection. It is the most economical approach but depends entirely on weather conditions and is difficult to control.

Microwave drying generates heat internally by exciting water molecules with electromagnetic energy, creating rapid, volumetric heating. This speeds up internal mass transfer but requires careful control to prevent uneven drying.

Drum drying uses conduction from heated metal cylinders to dry thin films of food paste or slurry. It is fast and efficient for products like instant cereals and flaked foods.

Optimising the drying process 🔗

Effective drying is about finding the right balance between energy input and moisture removal speed – without compromising the food’s nutritional value, appearance, or taste. Some practical strategies include maintaining uniform piece sizes for consistent drying, using appropriate air temperatures that match the product’s heat sensitivity, ensuring adequate airflow across all food surfaces, and monitoring humidity to prevent reabsorption of moisture.

Advances in computational modelling, including physics-informed machine learning approaches, are now helping food engineers simulate and predict drying behaviour more accurately, opening the door to smarter dryer design and process optimisation.

What do you think? How might a better understanding of heat and mass transfer change the way drying processes are designed for local or traditional foods in your region? Could modern monitoring tools help small-scale food processors dry their products more safely and efficiently?

References

1. https://www.mdpi.com/1996-1073/15/24/9347
2. https://gcwgandhinagar.com/econtent/document/1588156061Unit%20V%20Drying-theory,%20drying%20curves,%20and%20heat%20transfer.pdf
3. https://www.degruyterbrill.com/document/doi/10.1515/ijfe-2025-0137/html
4. https://medcraveonline.com/MOJFPT/heat-and-mass-transfer-modeling-for-fruit-drying-a-review.html
5. https://www.researchgate.net/publication/43267718_Moisture_Transfer_in_Solid_Food_Materials_A_Review_of_Mechanisms_Models_and_Measurements