Understanding the Drying Phenomena in Food Preservation

Drying is one of the oldest and most reliable methods of preserving food – particularly fruits and vegetables. The core idea is straightforward: remove enough water from the food so that bacteria, yeast, and molds simply cannot grow. But behind this simple concept lies a set of physical phenomena involving heat transfer, mass transfer, moisture gradients, and potential defects like case hardening. Understanding these drying phenomena is essential for anyone working in food processing or studying food engineering.

Why drying works: the role of water activity 🔗

Fresh fruits and vegetables typically contain 80% to 95% moisture, making them highly perishable. Microorganisms – bacteria, yeasts, and molds – need available water to survive and multiply. The key term here is water activity (aw), which measures how much water in a food product is actually available for microbial use. It is not the same as total moisture content. Water activity is expressed on a scale from 0 to 1, where pure water has an aw of 1.0.

According to the UC Master Food Preserver Program, most bacteria need a water activity above 0.91 to grow, yeasts need above 0.88, and molds can survive down to about 0.65. When drying reduces the aw of a food product to below these thresholds, microbial growth is effectively halted. Dried fruits and jerky, for example, typically have water activities below 0.75, which is well below the threshold needed for most spoilage organisms.

As noted by Food Safety Magazine, water activity has been incorporated into FDA and USDA regulations, Good Manufacturing Practices (GMP), and HACCP requirements – reflecting just how critical this parameter is in commercial food preservation.

The two pillars of drying: heat transfer and mass transfer 🔗

At the core of every drying process are two simultaneous phenomena: heat transfer and mass transfer. These work together to remove moisture from food materials.

Heat transfer 🔗

Heat transfer is the movement of thermal energy from the drying medium (usually hot air) to the food product. This energy raises the temperature of water molecules inside the food, giving them enough kinetic energy to evaporate. There are three main mechanisms through which heat reaches the food:

Conduction occurs when heat passes through direct contact – for instance, from a heated tray to the food sitting on it. Convection is the most common mechanism in food drying, where hot air flows over and around the food, transferring heat to its surface. This is the principle behind most commercial hot-air dryers and home dehydrators. Radiation involves the transfer of energy through electromagnetic waves, such as infrared radiation, and does not require a medium. According to a review published in the journal Foods, drying involves the application of energy to vaporize and mobilize moisture within porous food products, with heat and mass transfer occurring simultaneously throughout the process.

Mass transfer 🔗

While heat transfer supplies the energy for evaporation, mass transfer describes how moisture actually moves from inside the food to its surface, where it can then evaporate into the surrounding air. This movement is driven by a moisture gradient – the difference in moisture content between the wet interior and the drier surface of the food.

Inside the food, moisture migrates through several mechanisms. Diffusion is the primary one, where water molecules move from areas of higher concentration (the centre) toward areas of lower concentration (the surface). Capillary flow also plays a role in porous food structures, where water is drawn through tiny channels toward the surface. As research published in the MOJ Food Processing & Technology journal explains, the heat and mass transfer mechanisms during drying are complex and depend on the internal properties of the food such as porosity and permeability, as well as external factors like air temperature and velocity.

Stages of the drying process 🔗

Drying does not happen at a uniform rate from start to finish. The process is typically divided into distinct stages, each with different characteristics.

The constant rate period 🔗

In the early phase of drying, the food surface is saturated with moisture. Heat from the drying air reaches the surface and rapidly evaporates water. Because there is plenty of moisture available at the surface, the rate of drying remains relatively constant during this stage. The surface temperature of the food stays close to the wet-bulb temperature of the drying air, because evaporative cooling balances the heat input.

During this phase, the moisture gradient between the interior and the surface is steep, and internal moisture can replenish what is lost from the surface relatively quickly. The rate of drying here is largely controlled by external conditions – air temperature, air velocity, and humidity of the drying environment.

The falling rate period 🔗

As drying continues, the surface begins to dry out and a dry layer forms on the outside of the food. This marks the transition to the falling rate period. Now, the internal moisture has to travel a longer distance through the increasingly dry outer layer before it can evaporate. The rate of drying drops because internal mass transfer becomes the limiting factor, not external heat supply.

According to a study published in the Journal of Food Engineering, during this stage, the food material transitions from a soft, rubbery state to a hard, glassy state as moisture is lost. This phase transition significantly affects the material’s mechanical properties and is closely linked to shrinkage, texture development, and case hardening.

The falling rate period is usually the longest stage of drying and requires careful management to ensure the product dries uniformly without quality loss.

The moisture gradient and its importance 🔗

The moisture gradient is essentially the driving force behind mass transfer during drying. It refers to the difference in moisture concentration between the centre (high moisture) and the surface (low moisture) of the food. This gradient is what causes water to migrate outward.

At the start of drying, this gradient is steep, which means moisture moves relatively quickly. As drying progresses and the surface dries out, the gradient flattens, slowing down moisture movement. Maintaining an appropriate moisture gradient throughout the process is critical. If the gradient becomes too extreme – for example, due to very high drying temperatures – the surface dries far too quickly while the interior remains wet. This leads to a serious quality defect known as case hardening.

Case hardening: a common drying defect 🔗

Case hardening occurs when the outer surface of the food dries and hardens too quickly, forming a tough shell that traps moisture inside. The food may look and feel dry on the outside, but the interior retains significant moisture. This trapped moisture creates conditions favourable for mould growth during storage, leading to spoilage.

As the Penn State Extension explains, when drying temperature is too high, the outer surface hardens and prevents moisture from escaping from the centre of the food slice. Similarly, the University of Minnesota Extension warns that raising oven temperatures to speed up drying will cook the food on the outside before it dries on the inside, resulting in a product that appears dry but will develop mould in storage.

What causes case hardening? 🔗

Several factors contribute to this defect. Excessively high drying temperatures are the most common cause, as they accelerate surface evaporation far beyond the rate at which internal moisture can migrate outward. Low humidity in the drying environment combined with fast airflow can also strip surface moisture too quickly. Cutting food pieces too thick or in uneven sizes makes it harder for interior moisture to reach the surface before the outer layer hardens. Foods with intact skins, such as blueberries or grapes, are also more prone to case hardening because the skin acts as an additional barrier to moisture escape.

How to prevent case hardening 🔗

Preventing case hardening requires controlling the balance between heat supply and moisture removal. Here are the key strategies:

Use appropriate drying temperatures. Lower temperatures over longer periods produce better results than high temperatures for shorter durations. The University of Missouri Extension recommends watching temperatures closely, particularly at the beginning and end of drying, since early high temperatures are the main cause of shell formation.

Cut food uniformly and thinly. Consistent size ensures even drying across all pieces. Thinner slices allow moisture to reach the surface more easily, reducing the risk of the exterior hardening before the interior dries.

Ensure adequate air circulation. Good airflow around the food helps carry away evaporated moisture and prevents localised humidity buildup around the food surface. Rotating dehydrator trays periodically helps maintain even airflow.

Pretreat foods appropriately. For fruits with tough skins, techniques like blanching, freezing, or puncturing the skin before drying allow moisture to escape more freely. Blanching weakens the cell wall structure, which helps water evaporate more quickly during dehydration.

Control humidity levels. High humidity in the drying environment slows down surface evaporation and can sometimes contribute to uneven drying. A well-ventilated drying setup helps maintain optimal conditions.

Factors that influence the drying rate 🔗

Several variables determine how quickly and effectively a food product dries. Understanding these factors allows processors to optimise their operations.

Air temperature 🔗

Higher air temperatures increase the rate of heat transfer to the food, providing more energy for evaporation. However, as discussed above, excessively high temperatures can cause case hardening and also degrade heat-sensitive nutrients like vitamins A and C.

Air velocity 🔗

Increasing the speed of air flowing over the food surface improves both heat transfer and the removal of moisture-laden air from around the food. This accelerates drying, particularly during the constant rate period. However, very high air velocities at the start of drying can contribute to case hardening.

Relative humidity 🔗

The humidity of the drying air determines its capacity to absorb moisture. Drier air (lower relative humidity) creates a stronger driving force for evaporation. If the drying air is already moist, the rate of moisture removal decreases.

Food characteristics 🔗

The composition, thickness, and structure of the food itself have a major impact. Foods with higher initial moisture content take longer to dry. Dense or thick pieces slow down internal moisture diffusion. The journal Processes notes that drying significantly reduces the water activity and moisture content of foods, but the process must be done thoroughly – low-aw foods should not be considered sterile, as they can still be contaminated under unhygienic conditions.

Chemical and physical changes during drying 🔗

Drying does more than just remove water. It triggers several changes in the food that affect its final quality.

Shrinkage is one of the most visible changes. As water leaves the food cells, the cells collapse and lose their structure. This is why dried fruits and vegetables are significantly smaller than their fresh counterparts. The extent of shrinkage depends on the drying method and conditions used.

Maillard browning is a chemical reaction between sugars and amino acids that occurs during drying with warm air. It causes brown discolouration and can alter the flavour of the dried product. This reaction is most rapid when the moisture content of the food passes through the 15-20% range, so drying systems are often designed to move through this range as quickly as possible.

Nutrient loss also occurs during drying. Heat-sensitive vitamins, particularly vitamin C, can be destroyed by high temperatures and prolonged exposure to air. Pretreating foods with acidic solutions (such as ascorbic acid dips) helps preserve colour and reduce vitamin degradation.

Loss of volatile compounds affects the aroma and flavour of dried products. Higher drying temperatures generally lead to greater losses of the volatile substances that contribute to a food’s characteristic taste and smell.

Ensuring uniform drying 🔗

The goal of any well-designed drying process is to achieve uniform moisture removal throughout the product. This means managing the interplay of heat transfer, mass transfer, and food properties to avoid both under-drying (which leaves the product susceptible to microbial spoilage) and over-drying (which wastes energy and degrades quality).

Key practices for uniform drying include maintaining consistent food piece sizes, using calibrated temperature and airflow settings, rotating trays or repositioning food during the process, and allowing for a conditioning period after drying. Conditioning involves loosely packing dried food in a container for several days and shaking it periodically. This allows moisture to redistribute evenly among the pieces. If condensation appears inside the container, the food needs further drying.

What do you think? How might a better understanding of moisture gradients and heat transfer change the way dried food products are designed in the future? And if you have experience with drying food – whether at home or in a commercial setting – have you ever encountered the problem of case hardening?

References

1. https://ucanr.edu/program/uc-master-food-preserver-program/article/water-activity-and-its-role-food-preservation
2. https://www.food-safety.com/articles/4420-water-activitye28099s-role-in-food-safety-and-quality
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC7554907/
4. https://medcraveonline.com/MOJFPT/heat-and-mass-transfer-modeling-for-fruit-drying-a-review.html
5. https://www.sciencedirect.com/science/article/abs/pii/S0260877415002435
6. https://extension.psu.edu/lets-preserve-drying-fruits-and-vegetables-dehydration
7. https://extension.umn.edu/preserving-and-preparing/drying-food
8. https://extension.missouri.edu/publications/gh1562
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC8017434/