Understanding the Mechanisms of Dehydration in Food Preservation

Food dehydration is one of the most widely practised preservation techniques in the world, and at its core lies a set of physical processes that determine how effectively moisture is removed from a food product. Whether you are drying mango slices under the sun or operating an industrial tunnel dryer, the same fundamental mechanisms – heat transfer and mass transfer – govern the outcome. Understanding how these two phenomena interact, and how the food itself changes during drying, is key to producing safe, shelf-stable products without sacrificing quality.

What dehydration actually does to food 🔗

Dehydration preserves food by lowering its water activity, which in turn inhibits the growth of bacteria, yeasts and moulds. When water is removed, the product also becomes lighter and more compact, reducing transportation and storage costs. But the process is far from simple. As the IntechOpen food dehydration review notes, drying is a simultaneous heat-and-mass-transfer operation that involves moving water particles out of the food matrix while supplying enough thermal energy to convert that liquid water into vapour. The efficiency and safety of the final product depend on how well these two transfers are balanced.

Heat transfer: getting energy into the food 🔗

Before any moisture can leave, the food must receive enough thermal energy to raise the temperature of its water above the point of evaporation. This energy reaches the food through three main modes.

Conduction 🔗

Conduction occurs when heat flows through direct physical contact. In a drum dryer, for instance, a thin layer of food paste is spread onto a heated metal cylinder, and thermal energy travels from the hot surface into the food. The rate of conductive heat transfer depends on the temperature difference between the heat source and the food, the thickness of the food layer, and the thermal conductivity of the material. Dense, wet foods generally conduct heat more readily than porous, dry ones.

Convection 🔗

Convection is the most common mode of heat transfer in food drying. Hot air is blown over or through the food, transferring thermal energy as it moves. Cabinet dryers, tunnel dryers, fluidized-bed dryers and simple solar dryers all rely predominantly on convective heating. The rate depends on the temperature and velocity of the air, as well as the humidity of the drying medium. Forced convection – where fans actively circulate air – is significantly more efficient than natural convection driven only by temperature differences.

Radiation 🔗

Infrared radiation transfers energy through electromagnetic waves and can heat the food surface without requiring a physical medium. Infrared dryers can achieve rapid surface drying, but they require specialised equipment and careful control to avoid overheating. In many modern commercial systems, radiation is combined with convection to speed up the early stages of drying.

Mass transfer: moving moisture out 🔗

While heat transfer supplies the energy, mass transfer describes the actual movement of water from inside the food to the surrounding environment. This movement is driven by a moisture gradient – water naturally migrates from regions of higher concentration (the food’s interior) to regions of lower concentration (the surface and then the air). According to the University of British Columbia’s food science curriculum, as water migrates to the surface it also carries dissolved soluble substances like sugars and salts, which can accumulate on the outer layers and affect rehydration properties later.

How moisture moves inside the food 🔗

Water does not simply evaporate from the centre of a food piece and vanish. It must first travel through the internal structure to reach the surface. This internal migration happens through several pathways:

Liquid diffusion is the most common internal transport mechanism in dense, non-porous foods. Water molecules move through the food matrix along a concentration gradient – from wetter regions to drier ones. This process is relatively slow and is often the rate-limiting step when drying thick or dense products like potato slices or meat.

Capillary flow plays an important role in porous foods such as fruits and vegetables. Water is pulled through the tiny channels and pores within the food’s cellular structure by surface-tension forces, much like water rising through a paper towel. The more porous the food, the more effectively capillary action can move moisture outward.

Vapour diffusion becomes significant once the interior temperature is high enough to generate steam within the food. The vapour then moves through air-filled pores toward the lower-pressure environment at the surface. In high-temperature drying methods, internal pressure gradients can actively push moisture outward, accelerating drying.

Evaporation at the surface 🔗

Once moisture reaches the food’s outer surface, it evaporates into the surrounding air. The rate of this surface evaporation is governed by the temperature of the food surface, the humidity of the drying air, and the velocity of air flowing over the food. Low-humidity, fast-moving air creates a steeper moisture gradient between the surface and the environment, promoting faster evaporation.

The drying curve: constant rate and falling rate periods 🔗

The interplay of heat and mass transfer creates a characteristic drying curve that all food products follow, though the specifics vary by product. As explained by the IRRI Rice Knowledge Bank, the drying process can be divided into distinct periods, each with different characteristics and implications for quality.

The constant rate period 🔗

In the early phase of drying, the food surface remains saturated with moisture. Heat energy arriving at the surface is used almost entirely to evaporate water, and the rate of moisture removal stays steady. During this stage, the food surface temperature stays relatively low because evaporating water carries heat away – a natural cooling effect that protects the product from thermal damage. The constant rate period is the most energy-efficient phase of the drying process. External factors such as air temperature, air velocity and humidity dominate the drying rate during this phase, while the food’s internal structure has relatively little influence.

The critical moisture content 🔗

As drying progresses, the surface eventually can no longer remain fully wet. The point at which internal moisture supply can no longer keep up with surface evaporation is called the critical moisture content. According to ScienceDirect’s engineering reference, in real food systems – unlike inert materials – the transition from constant to falling rate is gradual rather than abrupt, though the concept of a critical moisture content is still used in process modelling.

The falling rate period 🔗

Once the critical moisture content is reached, the drying rate begins to decline. The surface starts to dry out in patches, and internal moisture transport – not surface evaporation – becomes the bottleneck. The food temperature also begins to rise because less evaporative cooling is taking place. This period is often subdivided into two stages. In the first falling rate period, some surface areas remain moist while others dry out, and the drying rate decreases roughly in proportion to the remaining moisture. In the second falling rate period, the entire surface is dry, and moisture removal depends entirely on how fast water can diffuse through the food’s internal structure to the receding evaporation front.

The insulation barrier: how the dried outer layer slows everything down 🔗

One of the most important phenomena in food dehydration is the formation of a dried outer layer as the surface loses its moisture. This layer acts as an insulation barrier with two significant effects.

First, it reduces the rate of heat transfer inward. Because dried food material is a poorer conductor of heat than wet food, thermal energy must now pass through this low-conductivity layer before it can reach the still-moist interior. The thicker the dried layer grows, the slower the heat penetrates.

Second, it impedes the outward movement of moisture. Water vapour from the interior must now diffuse through the porous dried matrix to escape, and this added resistance slows mass transfer. In effect, the evaporation front retreats deeper into the food as drying proceeds, and the path that moisture must travel keeps getting longer.

This double barrier effect is the primary reason why the falling rate period takes much longer than the constant rate period, even though less total moisture is being removed. According to Professor R. Paul Singh’s food engineering course at UC Davis, in highly moist foods the constant rate period typically precedes the falling rate period, but the falling rate phase accounts for the bulk of the overall drying time.

Case hardening: when the barrier becomes a problem 🔗

If drying conditions are too aggressive – particularly if the air temperature is too high – the outer layer can dry so rapidly that it forms a hard, almost impermeable shell. This phenomenon is known as case hardening. Research published in the Journal of Food Engineering shows that at high drying rates, the surface dries out much faster than the core, forming a case-hardened layer that causes early deviations in shrinkage patterns and traps moisture inside the product.

Case hardening is especially problematic in high-sugar foods like tropical fruits, where concentrated sugars at the surface can form a glassy, impermeable casing. The trapped interior moisture creates ideal conditions for microbial growth during storage, making the product unsafe despite appearing fully dried on the outside. Controlling temperature, using intermittent drying, and ensuring adequate air circulation are the primary strategies for preventing case hardening.

Factors that influence the overall dehydration process 🔗

Temperature 🔗

Higher temperatures increase the driving force for both heat and mass transfer, generally speeding up drying. However, as the IRRI points out for grain drying, raising the temperature below the critical moisture content does not necessarily speed up drying – it just heats the grain and can damage quality. This principle applies broadly across food types: high temperatures are useful for removing surface moisture quickly, but gentler conditions are needed once internal diffusion becomes the limiting step.

Air velocity and humidity 🔗

Faster-moving air removes the boundary layer of humid air that forms around the food surface, increasing the moisture gradient and promoting faster evaporation. Similarly, lower humidity in the drying air creates a steeper concentration difference, pulling moisture away more efficiently. Both factors are most impactful during the constant rate period when surface evaporation is the rate-limiting step.

Food structure and composition 🔗

A food’s porosity, cell structure, thickness and composition all influence how easily moisture can migrate internally. Porous foods like leafy greens dry much more quickly than dense foods like cheese or meat. Pre-treatments such as blanching, slicing into thinner pieces, or puncturing skins (as with berries) can create additional pathways for moisture escape and significantly reduce drying time.

Surface area 🔗

Increasing the exposed surface area – by slicing, dicing or shredding – provides more space for evaporation and reduces the distance that internal moisture must travel. This is one of the simplest and most effective ways to speed up the dehydration process.

Practical strategies to optimise drying 🔗

Understanding the mechanisms of dehydration directly informs how processors and home food preservers can optimise their results. Intermittent drying, where periods of heating alternate with resting (tempering) phases, allows internal moisture to redistribute toward the surface before the next drying cycle begins. This reduces the insulation effect of the outer dried layer and can improve both drying efficiency and product quality. Pre-treatment methods like blanching or chemical dipping alter the food’s cellular structure to facilitate moisture movement. Uniform slicing ensures that all pieces dry at approximately the same rate, reducing the risk of some pieces being overdried while others remain moist inside.

Why this matters for food stability 🔗

The ultimate goal of dehydration is to reach a moisture level – typically corresponding to a water activity below 0.6 – at which microorganisms cannot grow and enzymatic reactions are minimised. How efficiently and uniformly you reach that target depends entirely on how well heat and mass transfer are managed throughout the process. Poorly managed drying leads to uneven moisture distribution, case hardening, loss of nutrients, and products that spoil despite appearing dry. Well-managed drying, guided by an understanding of these mechanisms, produces food that is safe, nutritious and shelf-stable for extended periods.

What do you think? How might the growing interest in solar-powered and energy-efficient drying technologies change the way small-scale farmers preserve their produce? And in your experience, what has been the biggest challenge in getting uniformly dried food products?

References

1. https://www.intechopen.com/chapters/84932
2. https://wiki.ubc.ca/Course:FNH200/Lessons/Lesson_08/Page_08.2
3. http://www.knowledgebank.irri.org/step-by-step-production/postharvest/drying/drying-basics/drying-process/fundamentals-of-grain-drying
4. https://www.sciencedirect.com/topics/engineering/falling-rate-period
5. https://www.rpaulsingh.com/course/lectures/dehydration1.html
6. https://www.sciencedirect.com/science/article/abs/pii/S0260877415002435