Understanding the Drying Rate in Dehydration Processes

When fruits and vegetables are dried – whether in a commercial dehydrator or a traditional solar dryer – water does not leave the product at the same speed from start to finish. The rate at which moisture is removed changes throughout the process, and understanding this change is what the drying rate is all about. For anyone involved in post-harvest management or food processing, knowing how drying rate works is the difference between a well-preserved product and a spoiled or quality-compromised one.

What is drying rate? 🔗

Drying rate is defined as the mass of water removed from a product per unit time per unit area of the drying surface, typically expressed in kilograms per hour per square metre (kg/h/m²). In simpler terms, it tells you how fast moisture is leaving the food at any given moment during drying. It is not a fixed value – it shifts constantly as the product loses water and its internal structure changes.

Drying rate is measured and plotted against the remaining moisture content of the product to produce what is known as a drying rate curve. This curve is one of the most important tools in food dehydration, as it visually maps out how the process evolves from start to finish. Drying rate data are usually represented in the form of drying curves, where the drying rate is plotted against the remaining water content, expressed as kg of water per kg of dry matter.

Phases of the drying process 🔗

The drying process does not happen all at once. It moves through distinct phases, each with its own behaviour, controlling mechanisms, and implications for product quality. Based on drying rate, the process is divided into three periods: the constant rate drying period, the first falling rate period, and the second falling rate period.

Warming-up or initial period 🔗

When a fresh fruit or vegetable is first placed in a dryer, there is a brief initial phase where the surface temperature of the product rises to the wet-bulb temperature of the drying air. During this induction or rising rate period, different water flow mechanisms are coupled and the drying rate increases as water is removed. This phase is usually short and is often not taken into account for drying time calculation. Once the surface stabilises, the process moves into the constant rate period.

Constant rate period 🔗

The constant rate period is the phase during which the drying rate stays steady and does not change with time. During constant rate drying, the surface of the food behaves as a free water surface. The rate of evaporation of water from the surface equals the rate of moisture migration from the interior of the product to its surface. As long as this balance holds, the drying rate remains flat on the curve.

During this phase, the drying period is governed fully by external heat and mass transfer, as the surface of the product is always covered with a film of free water. The product temperature is usually at the wet-bulb temperature of the drying air. This is also the most energy-efficient phase of the entire operation, since most of the supplied heat goes directly into evaporating water rather than raising the product temperature.

Three air conditions are essential for effective drying during this phase: a moderately high dry-bulb temperature, a low relative humidity, and a high air velocity. Processors who get these conditions right can move through the constant rate period efficiently without risking surface damage.

The constant rate period continues until the product reaches its critical moisture content (CMC). The critical moisture content is the moisture content of the product at which the constant rate drying period ceases and the falling rate period begins. It depends on the type of product and its thickness. It is worth noting that for some produce with low initial moisture content, the constant rate period may be very brief or entirely absent, and drying proceeds directly into the falling rate period.

The falling rate period 🔗

Once the product drops below its critical moisture content, the drying rate begins to decline. When the supply of water to the surface drops below the rate of evaporation, the moisture content of the surface begins to decrease rapidly. From that moment, internal transport – and not evaporation – becomes the rate-limiting factor, and the falling rate period begins.

Unlike the constant rate period, where external air conditions drive drying, the falling rate period is governed by how efficiently moisture can travel through the internal structure of the food to reach the surface. The falling rate phase occurs when the drying rate sharply decreases as the rate of water migration from inside to the surface of the sample is lower than the evaporation rate from the surface to the air.

Most agricultural products are dried in the falling rate period. This phase is the longest and the most critical from a quality standpoint, because it is here that heat damage to the food is most likely to occur.

First falling rate period 🔗

The first falling rate period is unsaturated surface drying. As drying proceeds, the wet surface area decreases, which causes the drying rate to also decrease. The reduction of the wet surface area to zero marks the end of the first falling rate period. During this sub-phase, capillary forces still assist in moving moisture toward the surface, and the drying rate decreases roughly in a linear relationship with moisture content.

Second falling rate period 🔗

In the second falling rate period, subsurface evaporation takes place and drying continues until the equilibrium moisture content is reached. At this stage, moisture movement is controlled almost entirely by molecular diffusion through the dry interior structure of the food. The drying rate decreases continuously with time and decreasing moisture content, and the predominant mechanism of mass transfer is internal mass transfer.

This is also when case hardening becomes a serious risk. If the external drying conditions are too aggressive during this phase, the outer surface of the food dries rapidly and forms a hard, impermeable shell that traps residual moisture inside. The result is a product that appears dry externally but contains unsafe moisture levels internally, which can lead to microbial growth and spoilage during storage.

Factors that affect drying rate 🔗

Multiple variables interact to determine how fast moisture leaves a product at any stage of drying. The factors affecting drying rate can be divided into external conditions – drying-air temperature, humidity, velocity, and turbulence, as well as material surface area and thickness – and internal conditions, including the nature of the material and the mechanisms by which moisture moves within the solid.

Temperature 🔗

The higher the temperature, the faster the drying rate; conversely, the lower the temperature, the slower the drying rate. Higher temperatures increase the energy available for evaporation and widen the vapour pressure gradient between the product surface and the surrounding air, both of which accelerate moisture removal.

However, excessive heat during the early stages of drying creates problems. When fruits and vegetables with very high moisture content meet dry hot air suddenly, the juice in the tissue expands rapidly, which can rupture the cell wall. Additionally, sugars and other organic substances in the raw material can decompose or become scorched at high temperatures, detracting from the appearance and flavour of the finished product. Research on apple rings found that using drying air temperatures ranging from 50 to 70°C produced better-quality dried apples compared to drying at 80°C. This underscores why temperature must be matched carefully to the product type and stage of drying.

Relative humidity 🔗

When the temperature is raised and the relative humidity is lowered, the greater the partial pressure difference between the raw material and the outside water vapour, the easier the evaporation of water. Lower relative humidity means the drying air can absorb more moisture, maintaining a strong concentration gradient that pulls water out of the food. Research on lychee drying showed that drying time decreased from 52 hours to 11 hours by decreasing relative humidity from 90% to 40% at a constant temperature of 40°C. This highlights just how dramatically humidity control can affect drying efficiency.

Air velocity 🔗

The greater the flow rate of the drying air, the faster water evaporates from the surface of the fruits and vegetables. Increasing air velocity benefits the drying process by quickly transferring heat from the air to the raw material to maintain its evaporation temperature. This effect is most pronounced during the constant rate period, when surface evaporation is the controlling mechanism. During the falling rate period, the benefit of increased air velocity diminishes because internal diffusion – not surface conditions – is the limiting factor.

Initial moisture content and product characteristics 🔗

The starting moisture content of the product directly influences how long both drying phases last. Foods with higher initial moisture content generally have a more extended constant rate period before reaching the critical moisture content. Beyond moisture, the physical structure of the food matters significantly. Dense produce like whole potatoes or beets has longer falling rate periods than thin-sliced or porous foods, simply because moisture has more distance to travel internally. Studies on onion slices confirm that drying kinetics follow Fick’s diffusion model, where internal moisture diffusivity is a key variable affected by both temperature and the structural properties of the product.

Size and surface area of the produce 🔗

Smaller pieces and thinner slices dry faster because they expose more surface area relative to their volume, shortening both the constant rate and falling rate periods. If the amount of raw materials loaded on the drying tray is large, the thickness will be greater, which is not conducive to air circulation, affects the evaporation of water, and slows the drying rate. This is why uniformity of cut size is a standard practice in commercial dehydration – it ensures all pieces move through the drying phases at similar rates, preventing some pieces from over-drying while others remain under-processed.

Why drying rate matters in practice 🔗

Understanding the drying rate and its two main phases is not just an academic exercise – it has direct consequences for the quality, safety, and economics of food preservation. During the constant rate period, processors can safely apply higher temperatures and air velocities to remove bulk moisture quickly. Once the product enters the falling rate period, conditions must be moderated to prevent case hardening, nutrient loss, and colour deterioration.

Modern commercial drying facilities increasingly use automated systems that monitor product weight and surface temperature continuously, allowing them to detect the transition from the constant rate to the falling rate period and automatically adjust drying conditions. This precision helps maximise drying speed while safeguarding final product quality – a balance that manual methods struggle to achieve consistently.

From a food safety perspective, when the moisture content of the food falls below the critical moisture content, the rate of drying slowly decreases until it approaches zero at the equilibrium moisture content. Reaching the correct equilibrium moisture content – not just a visually dry surface – is what determines whether a product will be stable during storage. Stopping the process prematurely, especially during the second falling rate period, is one of the most common causes of spoilage in dried fruits and vegetables.

What do you think? If two batches of the same fruit are dried under identical conditions but one is sliced thicker than the other, how do you think this difference in thickness would affect the transition point between the constant rate and falling rate periods? And considering that most agricultural products spend the majority of their drying time in the falling rate period, what practical adjustments could processors make to reduce heat damage during this critical phase?

References

1. https://www.sciencedirect.com/topics/engineering/drying-rate
2. https://www.slideshare.net/slideshow/rate-of-drying-for-food-products/238759080
3. https://ift.onlinelibrary.wiley.com/doi/10.1111/1541-4337.12845
4. https://slidetodoc.com/food-dehydration-constant-rate-period-and-falling-rate/
5. https://www.sciencedirect.com/topics/engineering/falling-rate-period
6. https://www.gelgoogdryer.com/news/technical-blog/fruits-drying-rate-affecting-factors.html
7. https://www.researchgate.net/publication/257028862_Experimental_Determination_of_the_Factors_Affecting_the_Drying_Rates_of_Apple_rings_under_Varying_Controlled_Drying_Air_Conditions
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC11000004/
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC7339060/