Theories Behind Food Drying: Heat and Mass Transfer Explained

Food drying is one of the oldest and most widely used methods of food preservation. At its core, the process works by removing moisture from food to levels where microorganisms can no longer grow and spoil the product. But what actually happens during drying at a scientific level? Two fundamental phenomena drive the entire process: heat transfer and mass transfer. Heat provides the energy needed to evaporate water, while mass transfer describes how that moisture physically moves from inside the food to the surrounding environment. Understanding these two mechanisms – and how they interact – is key to designing efficient drying systems and producing safe, high-quality dried food products.

What is drying and why does it matter? 🔗

Drying, in simple terms, is the removal of water or another solvent by evaporation from a solid or semi-solid material. In food processing, the solvent being removed is almost always water. When we reduce the moisture content of a food product, we lower its water activity – the measure of available water that microorganisms need to survive and multiply. This directly inhibits the growth of bacteria, yeasts, and moulds, extending the product’s shelf life significantly.

But drying is not just about making food last longer. It also reduces the weight and volume of food, making storage and transport easier and cheaper. Think of dried fruits, milk powder, instant coffee, or dehydrated vegetables – all products that owe their convenience and long shelf life to controlled drying processes. The effectiveness of these processes depends entirely on how well we manage heat and mass transfer.

Heat transfer in food drying 🔗

Before moisture can leave a food product, the water molecules inside it need enough energy to change phase – from liquid to vapour. This energy comes from heat transfer. As described by Chemical Engineering magazine, heat is transferred to the product to evaporate liquid, and the resulting vapour is then carried away into the surrounding gas. In food drying, heat can reach the product through three distinct mechanisms: conduction, convection, and radiation.

Conduction 🔗

Conduction is the transfer of thermal energy through direct physical contact. When a food item is placed on a heated surface – such as a metal tray or a drum dryer – heat flows from the hot surface into the food material. The energy passes from molecule to molecule through the food matrix, gradually raising the temperature from the contact point inward.

This method is particularly effective for thin layers of food. Drum drying, for example, uses conduction to dry liquid or semi-liquid foods like mashed potatoes or cereal pastes. A thin layer of the food is spread onto a heated rotating drum, and the heat conducted from the drum surface rapidly evaporates the moisture. However, conduction is inherently limited by the need for direct contact and the thermal conductivity of the food itself – foods with higher water content generally conduct heat better, but as they dry, conductivity decreases and heat penetration slows down.

Convection 🔗

Convection is the most commonly used heat transfer mechanism in food drying. It involves the transfer of thermal energy through a moving fluid – typically heated air. In a convective dryer, hot air is blown over and around the food product, delivering heat energy to the food surface. According to the Thermopedia encyclopedia, for drying to take place, moist material must obtain heat from its surroundings by convection, radiation, or conduction, and the moisture then evaporates and is received by a carrier gas.

What makes convection so effective is that the moving air serves a dual purpose: it delivers heat to evaporate moisture, and it simultaneously carries the resulting water vapour away from the food surface. Without adequate air movement, the area immediately around the food becomes saturated with humid air, which slows down further evaporation. This thin layer of nearly still air close to the food surface is called the boundary layer, and its thickness is directly influenced by air velocity – faster-moving air creates a thinner boundary layer, improving both heat delivery and moisture removal.

Hot air drying, fluidized bed drying, and spray drying are all examples of convection-based methods used extensively in the food industry.

Radiation 🔗

Radiation transfers heat through electromagnetic waves and does not require any physical contact or a medium like air or water. The most familiar example is solar drying, where food is exposed to sunlight and absorbs infrared radiation, which raises its temperature and drives moisture evaporation. As TeachEngineering notes, solar dryers actually utilise all three types of heat transfer – radiation from the sun, convection from circulating air, and conduction from heated trays touching the food.

In industrial settings, infrared heaters can be used to deliver radiant energy directly to the food surface. This approach is especially useful for quickly drying surfaces without overheating the interior of the product. Many modern dehydrators combine infrared radiation with convective airflow for optimal results.

Mass transfer in food drying 🔗

While heat transfer provides the energy for evaporation, mass transfer describes the actual movement of moisture – from inside the food, to its surface, and then into the surrounding environment. This is often the more complex of the two processes, because it involves moisture migration through the food’s internal structure, phase change at the surface, and vapour removal by the drying medium.

Internal moisture movement 🔗

Inside the food product, water exists in different forms. Free water fills cell spaces and intercellular gaps and is relatively easy to remove. Bound water, on the other hand, is chemically or physically associated with proteins, carbohydrates, and other food components, making it much harder to extract.

As drying begins and surface moisture evaporates, a moisture concentration gradient develops between the wetter interior and the drier surface. This gradient is the driving force that causes internal moisture to migrate outward. According to ScienceDirect’s overview of drying kinetics, moisture within food can migrate through several mechanisms: liquid diffusion, vapour diffusion, and capillary flow, among others.

Here is a closer look at these mechanisms:

Liquid diffusion occurs when water molecules move through the food matrix from regions of high moisture concentration to regions of low concentration, following Fick’s law of diffusion. The rate depends on the food’s structure, temperature, and the steepness of the concentration gradient.

Capillary flow is significant in porous foods like fruits and vegetables. Moisture is drawn through tiny capillary channels within the food structure by surface tension forces – similar to how water moves upward through a paper towel.

Vapour diffusion happens when water evaporates within the food’s internal pores and the resulting vapour migrates outward. The International Journal of Food Properties reports that many researchers have identified vapour diffusion as a key mechanism during drying of porous materials, with the vapour pressure gradient serving as the primary driving force.

Pressure-driven flow can occur during high-temperature drying, where heating generates internal pressure that physically pushes moisture toward the surface.

Surface evaporation 🔗

Once moisture reaches the food’s surface, it must undergo a phase change from liquid to vapour. This is where the vapour pressure gradient between the food surface and the surrounding air becomes critical. The food surface, being wet and warm, has a higher vapour pressure than the drier surrounding air. This difference in vapour pressure acts as the driving force that pulls moisture away from the surface and into the air.

Several factors influence the rate of surface evaporation. Air temperature – higher temperatures increase the energy available for the liquid-to-vapour phase change. Air humidity – lower humidity in the surrounding air creates a larger vapour pressure difference, accelerating evaporation. Air velocity – faster airflow sweeps away the humid boundary layer near the food surface, maintaining a strong vapour pressure gradient.

How heat and mass transfer work together 🔗

In practice, heat transfer and mass transfer are not separate, independent events – they happen simultaneously and are deeply coupled. As an MDPI Energies journal paper explains, drying is a complex process involving simultaneous heat, mass, and momentum transport with continuous phase changes. Heat flows inward from the surface to the centre, while moisture moves in the opposite direction – outward from the centre to the surface.

This coupling is what makes drying a non-trivial engineering challenge. If heat is delivered too aggressively, the surface dries out rapidly and forms a hardened crust – a phenomenon known as case hardening. This crust acts as a barrier, trapping moisture inside and slowing further drying. On the other hand, if heat delivery is too slow, the process becomes inefficient and costly in terms of time and energy.

The drying curve: constant rate and falling rate periods 🔗

The interaction between heat and mass transfer is best understood through the drying curve – a graph that plots moisture content against drying rate over time. Drying typically proceeds through two distinct phases.

Constant rate period 🔗

In the early phase of drying, the food surface is saturated with moisture. Water evaporates freely from the surface at a steady rate. During this period, the drying rate is primarily dependent on the rate of heat transfer to the material. As long as internal moisture can reach the surface fast enough to replace what is being evaporated, the rate stays constant. The food surface remains at or near the wet-bulb temperature – the lowest temperature that air can reach through evaporative cooling.

During this phase, increasing air temperature, reducing air humidity, or boosting air velocity all directly increase the drying rate, because these factors enhance heat delivery and vapour removal at the surface.

Falling rate period 🔗

Eventually, the food surface begins to dry out and can no longer maintain a saturated condition. This transition point is called the critical moisture content. Beyond this point, the drying rate drops because internal moisture can no longer migrate to the surface fast enough. The process shifts from being heat-transfer limited to mass-transfer limited.

During the falling rate period, the bottleneck is no longer at the surface but inside the food. Moisture must travel longer distances through increasingly dry material, and the mechanisms of internal transport – diffusion, capillary flow – become slower. The food temperature begins to rise above the wet-bulb temperature, approaching the air temperature. This is where careful control becomes critical, as excessive temperatures can damage heat-sensitive nutrients, alter flavour, or cause undesirable textural changes.

For most food products – especially fruits and vegetables – the falling rate period dominates the overall drying process, meaning that internal mass transfer is the primary factor determining total drying time.

Factors that influence drying efficiency 🔗

Understanding the theory behind heat and mass transfer allows food engineers to optimise drying processes by manipulating several key variables.

Air temperature: Higher temperatures provide more energy for evaporation and increase the air’s capacity to hold moisture. However, exceeding safe limits can degrade the food’s nutritional and sensory quality.

Relative humidity: Lower humidity air creates a larger driving force for moisture removal. The drier the air, the faster evaporation occurs at the food surface.

Air velocity: Faster airflow reduces the boundary layer thickness, improving both heat delivery and vapour removal. However, as noted by ScienceInsights, once the food enters the later stages of drying, increasing airflow has diminishing returns because the bottleneck shifts to internal moisture movement.

Food piece size and shape: Smaller, thinner pieces dry faster because moisture has a shorter distance to travel from the interior to the surface. Slicing food before dehydration is one of the simplest ways to speed up the process.

Food composition and structure: Porosity, fibre orientation, and cellular structure all affect how easily moisture can move internally. For instance, moisture moves more rapidly along fibres in vegetables than across them.

Practical applications in the food industry 🔗

The principles of heat and mass transfer are applied across a wide range of drying technologies used in food processing today:

Hot air drying is the most widely used method. It relies primarily on convective heat transfer and is suitable for fruits, vegetables, grains, herbs, and spices. The equipment ranges from simple cabinet dryers to large-scale tunnel and belt dryers.

Spray drying atomises liquid food (such as milk or coffee extract) into fine droplets and exposes them to a stream of hot air. The enormous surface area of the droplets allows extremely rapid drying – often within just a few seconds – making it ideal for heat-sensitive products.

Freeze drying (lyophilisation) takes a different approach. The food is first frozen, then moisture is removed by sublimation under vacuum – ice converts directly to vapour without passing through the liquid phase. This preserves the food’s structure, colour, and nutritional content better than most other methods, but it is significantly more expensive.

Vacuum drying reduces the pressure in the drying chamber, which lowers the boiling point of water and allows evaporation at lower temperatures. This makes it particularly useful for heat-sensitive foods and ingredients.

Solar drying uses sunlight as the primary energy source and remains widely practised in developing regions for drying grains, fruits, fish, and vegetables. Modern solar dryers are engineered to maximise radiation capture, promote air circulation, and protect the food from contamination.

Why this theory matters for food safety 🔗

From a food microbiology perspective, the theory of drying is not just an engineering curiosity – it has direct implications for food safety. If drying is insufficient or uneven, pockets of high moisture can remain within the food, creating microenvironments where bacteria and moulds can survive and proliferate. Understanding how heat penetrates the food and how moisture migrates out helps processors design systems that ensure uniform drying throughout the product.

Proper control of drying parameters – temperature, humidity, airflow, and time – based on a solid understanding of heat and mass transfer ensures that the final product reaches a moisture level low enough to be microbiologically stable. This is especially critical for products that will be stored at ambient temperatures without refrigeration.

What do you think? How might the balance between heat transfer and mass transfer vary when drying a whole fruit versus a thinly sliced vegetable? And what trade-offs do food manufacturers face when choosing between faster, hotter drying and slower, gentler methods that better preserve nutrition?

References

1. https://en.wikipedia.org/wiki/Drying
2. https://www.chemengonline.com/solids-drying-basics-and-applications/?printmode=1
3. https://www.thermopedia.com/content/711/
4. https://www.teachengineering.org/activities/view/uoh-2785-heat-transfer-solar-dryers-activity
5. https://www.sciencedirect.com/topics/food-science/drying-kinetics
6. https://www.tandfonline.com/doi/full/10.1080/10942910601161672
7. https://www.mdpi.com/1996-1073/15/24/9347
8. https://scienceinsights.org/how-does-drying-work-physics-stages-and-more/
9. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/vacuum-drying