Theories Behind Drying and Dehydration Processes in Food Preservation

Drying and dehydration are among the oldest and most reliable methods of preserving food, particularly fruits and vegetables. According to the Food and Agriculture Organization (FAO), removing moisture from food directly inhibits the growth of bacteria, yeasts, and molds – the primary agents of spoilage. But the process is far more than simply applying heat. At its core, effective drying depends on two simultaneous physical phenomena: heat transfer and mass transfer. Understanding how these two processes work – and how they interact – is fundamental to producing safe, high-quality dried food products.

Why drying works: the role of water activity 🔗

Before exploring the theories, it helps to understand why moisture removal preserves food. As documented in food engineering literature, fresh fruits and vegetables typically contain 80-95% moisture, making them highly perishable. Microorganisms need available water to survive and multiply. The key measure is water activity (aw) – not total moisture content, but how much water is actually available for microbial use. Successful drying reduces water activity to levels where microbial growth is no longer possible, extending shelf life significantly without the need for refrigeration or chemical preservatives.

According to ScienceDirect’s overview of food dehydration, the process involves the simultaneous transfer of heat into the food and moisture out of it – these two phenomena must be managed together to achieve efficient, uniform drying. This is where the two main theories come in.

Theory 1: heat transfer in drying 🔗

Heat transfer is the movement of thermal energy from a heat source into the food product. This energy raises the temperature of water molecules within the food, giving them sufficient kinetic energy to evaporate. Without adequate heat transfer, moisture removal is sluggish and the food may spoil before it dries. There are three distinct mechanisms through which heat reaches food during drying.

Conduction 🔗

Conduction is the transfer of heat through direct physical contact between materials. In industrial drying systems, conduction transfers heat from heated trays, drum surfaces, or conveyor belts directly to the food placed on them. This mechanism is particularly effective for thin, flat food products – such as fruit leathers or thinly sliced vegetables – where the food maintains consistent contact with the heated surface. Heat moves progressively from the hot surface into the interior of the food. The limitation of conduction is that it is relatively slow and confined to the contact area; it cannot reach internal portions of thick food pieces on its own.

Convection 🔗

Convection is the most widely used heat transfer mechanism in food drying. It involves the movement of a heated fluid – almost always hot air – over and around the food surface. Research published in the Agricultural Engineering International journal confirms that convection is the dominant mode of heat transfer in most commercial food dryers, including hot-air cabinet dryers, tunnel dryers, and fluidised bed dryers. There are two types: natural convection, driven by temperature-induced air density differences, and forced convection, where fans actively circulate heated air. Forced convection is far more efficient for food drying because it consistently delivers hot, dry air to the food surface and carries away moisture-laden air. The rate of convective heat transfer depends on air temperature, velocity, and the surface area of the food exposed to the airstream.

Radiation 🔗

Radiation is the transfer of thermal energy through electromagnetic waves, requiring no physical medium or direct contact. In solar drying applications, infrared radiation from the sun directly heats the food surface and the air within the drying chamber simultaneously. In industrial settings, infrared emitters are used to deliver targeted heat directly to food surfaces – this is especially useful for surface drying and for foods with delicate internal structures that must not be overheated. Radiation heats the outermost layers of the food very efficiently, which can accelerate initial surface evaporation. However, since it does not penetrate deeply into most food products, it is typically used alongside convection rather than as a standalone mechanism. A review published in Foods (MDPI) notes that effective modern drying methods incorporate all three heat transfer modes – convection, conduction, and radiation – to maximise energy efficiency and product quality.

Theory 2: mass transfer in drying 🔗

While heat transfer supplies the energy for evaporation, mass transfer describes how moisture actually moves from the interior of the food to its surface, where it can evaporate into the surrounding air. As reviewed in the journal International Journal of Food Properties, drying of biological materials like fruits and vegetables is predominantly an internal mass-transfer-controlled process – meaning the rate at which moisture migrates from inside the food is often the limiting factor in overall drying speed.

The moisture gradient: the driving force 🔗

Mass transfer in drying is driven by a moisture gradient – the difference in moisture concentration between the wet interior and the drier surface of the food. Water molecules naturally move from areas of higher concentration (deep inside the food) toward areas of lower concentration (the surface), following the principles of diffusion. Wikipedia’s drying entry explains that in the falling-rate period of drying, this internal moisture migration is mainly by molecular diffusion, where water flux is proportional to the moisture content gradient. As the surface moisture evaporates, the gradient is maintained, and internal moisture continues to migrate outward – as long as the surface does not seal prematurely.

How moisture moves inside food 🔗

Inside a food product, moisture migrates through several mechanisms simultaneously. Liquid diffusion is the primary mechanism, where water molecules move through the food’s cellular matrix from high to low concentration zones. Capillary flow also plays a significant role in porous foods – water is drawn through tiny channels and pores toward the surface, much like water rising through a sponge. In fresh fruits and vegetables, which have a naturally porous cellular structure, capillary flow can be quite active in the early stages of drying. The FAO’s technical guide on fruit and vegetable processing highlights that cutting food into smaller pieces or thinner slices substantially improves mass transfer by reducing the distance internal moisture must travel to reach the surface – a key practical application of mass transfer theory.

Surface evaporation and vapor removal 🔗

Once moisture reaches the food surface, it evaporates and must be carried away by the drying air. According to FAO, air velocity is critical here: fast-moving air sweeps moisture-laden air away from the food surface, maintaining a low-humidity microenvironment that encourages continued evaporation. If the surrounding air becomes saturated with water vapor, evaporation slows dramatically – which is why ventilation and airflow design are central to efficient dryer engineering. Relative humidity of the drying air directly determines how much additional moisture the air can absorb; drier air creates a stronger driving force for mass transfer from the food surface into the airstream.

How heat transfer and mass transfer interact 🔗

In practice, heat transfer and mass transfer do not operate independently – they are tightly coupled. The rate of heat delivery to the food determines how much energy is available for evaporation, while the rate of internal moisture migration determines how quickly that evaporation can be sustained. ScienceDirect describes food dehydration as a process of simultaneous heat and mass transfer within the food and between the food and its environment.

This coupling leads to two characteristic drying phases observed in fruits and vegetables:

Constant rate period: In the early stage of drying, the food surface remains wet and moisture is freely available for evaporation. Heat transfer to the surface is quickly converted into evaporation, and the drying rate stays relatively stable. External conditions – air temperature, velocity, and humidity – primarily govern this phase.

Falling rate period: As the outer layers of the food dry out, internal moisture must travel farther and through an increasingly resistant dry layer before it can evaporate. As noted in drying literature, the drying rate drops because internal mass transfer becomes the limiting factor, not the external heat supply. This phase is dominant in hygroscopic products like fruits and vegetables, where the constant rate period may be negligible. Careful temperature management is critical here – excessive heat can cause case hardening, where the outer surface dries and contracts too quickly, sealing the food and trapping internal moisture.

Factors that influence the efficiency of heat and mass transfer 🔗

Several variables determine how quickly and effectively heat and mass transfer proceed during drying. Understanding these factors allows food processors to optimise operations and maintain product quality:

Air temperature raises the energy available for evaporation and reduces the relative humidity of the drying air. However, excessively high temperatures degrade heat-sensitive nutrients – particularly vitamin C – and can cause unwanted browning or textural changes. Air velocity accelerates both convective heat delivery and the removal of moisture-laden air from the food surface, reducing boundary layer resistance. Relative humidity of the drying medium determines the vapour pressure gradient between the food surface and the air; lower humidity creates a stronger driving force for mass transfer. Food surface area affects both heat and mass transfer – the FAO notes that slicing food into thinner pieces increases the surface exposed to the heating medium and shortens the internal path moisture must travel. Food porosity and cellular structure influence internal diffusivity – porous foods like apples and mushrooms allow moisture to migrate more freely than dense products like root vegetables.

Practical significance of these theories 🔗

The theories of heat and mass transfer are not purely academic – they directly shape decisions in both commercial food processing and smallholder food preservation. Research comparing traditional and novel drying techniques consistently shows that optimising the interplay of heat and mass transfer – through controlled temperature, airflow, and food preparation – is the primary lever for improving dried product quality, reducing energy consumption, and minimising nutrient loss. Engineers designing industrial dryers use these principles to determine equipment dimensions, airflow rates, and temperature profiles. In the field, extension workers apply the same concepts when recommending that farmers slice produce thinly, blanch before drying to improve cell permeability, and use well-ventilated drying structures to maintain low humidity around the product.

The relationship between these two transfer theories also underpins newer technologies such as infrared drying, microwave-assisted drying, and osmotic dehydration – each of which modifies the conventional heat and mass transfer pathways to achieve faster drying, better nutrient retention, or reduced energy use.

What do you think? Given that both heat transfer and mass transfer must work in balance for effective drying – if you had to prioritise one factor to improve in a low-resource, field-based drying setup (like a solar dryer for smallholder farmers), which would you focus on first: better heat delivery or better airflow for moisture removal? And do you think the principles of heat and mass transfer are sufficiently integrated into how drying equipment is designed and recommended for small-scale producers in developing countries?

References

1. https://www.fao.org/4/v5030e/v5030e0b.htm
2. https://en.wikipedia.org/wiki/Drying
3. https://www.sciencedirect.com/topics/food-science/food-dehydration
4. https://feeco.com/principles-of-heat-transfer-as-applied-to-rotary-dryers-rotary-kilns-and-rotary-coolers/
5. https://cigrjournal.org/index.php/Ejounral/article/download/964/958/958
6. https://www.teachengineering.org/activities/view/uoh-2785-heat-transfer-solar-dryers-activity
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC7554907/
8. https://www.tandfonline.com/doi/full/10.1080/10942910601161672