Considerations on Drying

In past discussions of drying foods, I’ve considered some different types of dryers and modeling approaches. This article contains some of the other factors that might affect the choice of a dryer and its operation.

First, here is a brief review of the fundamentals. Drying involves four transport processes:
• External heat transfer from source to surface of food.

• Internal heat transfer from surface to moisture.

• Internal mass transfer of moisture to surface of food.

• External mass transfer of moisture from surface of food to a sink, such as the atmosphere.

The detailed mechanisms of each of these processes can be complex and depend on the nature of the food and of the environmental conditions. During the course of drying, different mechanisms and processes may dominate. For instance, as long as there is freely mobile water present, evaporation of moisture occurs from the surface of the food, and external processes dominate. During this period, the food temperature is the wet bulb temperature for the pressure and temperature of the environment. Once moisture content is reduced so that the surface is dry, internal mechanisms become important, and the surface temperature can rise. It is more difficult to affect internal processes than it is to affect external processes.

For example, external heat transfer can be improved by increasing the temperature of the heat source and, often, by increasing the velocity of hot gas contacting the food. However, internal heat transfer is predominantly by conduction through a porous solid layer and can only be affected by raising the surface temperature. There is usually an upper limit to the permissible surface temperature, determined by color changes or scorching.

External mass transfer is rarely a limiting process but can be improved by higher velocity, reduced relative humidity, and reduced total pressure. Reduced pressure is primarily used to reduce the temperature of drying for heat sensitive materials but has the added benefit of also improving internal and external mass transfer. However, low total pressure also reduces the thermal conductivity of the porous dry layer and thus reduces internal heat transfer rates. Drying is complex.

Objectives in Drying
Normally, the objectives in a given situation are to dry as much material as possible in the shortest possible time, using energy efficiently and the least expensive equipment subject to constraints on the properties of the finished product, such as shape, color, and flavor. Because the rates of internal heat and mass transfer are proportional to characteristic dimension squared, (thickness, radius of drop, and so on), it is tempting to make size as small as practical. For spray-drying, a liquid feed is atomized into hot air, and droplet size is controlled by atomizer design and pressure drop through the atomizer. Small droplets dry quickly but also may drift to the walls of the dryer and may be difficult to collect in cyclones or bag filters. Such small droplets may also be unsuitable for their intended use. For example, consumers object to the very fine particles of spray-dried soluble coffee, which leads to the use of additional processes to agglomerate fine powder into larger particles and to provide a surface that simulates crystals. For industrial use, the fine powder is acceptable.

In cases such as dried vegetables and meats for soup mixes, well-defined shapes are desirable (for example, cubes and slices). Conventional drying can cause shrinkage and case hardening if not performed carefully. Shrinkage occurs when the piece structure collapses as moisture is removed and the structure is soft because of the plasticizing effect of lingering moisture or the reduction in viscosity of the remaining concentrate caused by high temperature. Case hardening can be caused by the migration of soluble solids to the surface with liquid water, where the solids are deposited as moisture evaporates at the surface. Both effects are undesirable because they inhibit rehydration of the pieces.

Some Special Dryers
Freeze drying avoids these effects by immobilizing water as ice, which is then sublimed typically under vacuum. Radiant or conduction heat transfer becomes important in vacuum drying because convection is less effective. Convective heat transfer can be limited by the risk of blowing product around as it becomes lighter due to loss of moisture.

In a fluid bed dryer, weight changing can be used to selectively remove dried product from a fluidized bed of mixed particles, which may dry at different rates due to different sizes. Dried pieces are often fragile, so air velocities in a fluidized bed dryer may need to be reduced to minimize particle degradation. An interesting compromise was developed at the U.S. Dept. of Agriculture (USDA) Western Regional Research Center some years ago, called the belt trough dryer. A continuous loop of open weave chain link belt was used to form a bed through which hot air was blown. The belt moved to agitate the bed gently. It could be operated in batch or continuous mode.

The same USDA center also developed foam mat drying, in which liquid foods are foamed and deposited on perforated trays through which warm air is blown. The very thin films of the foam bubbles dry quickly at mild conditions and the foam retains its structure when dried. The dried foam is scraped from the trays and ground to produce a fine powder.

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Material Handling
Getting materials into and out of a dryer can be as important as what occurs in the dryer. For example, if slices are being dried, as in the case of fruit or vegetable snacks, it may be necessary to have a single layer on a belt because overlapping pieces do not dry well and may stick together. In this case, a special slicer can lay down pieces precisely. Once dried, the pieces can be tumbled into bags for packaging. Belt dryers with a bed of product must be loaded evenly to ensure uniform drying.

Fine powders are usually collected with cyclones and filters, but if they are too fine, they may escape capture, reducing yield, clogging filters, and causing an air mission issue. Fine powders of many materials can also pose an explosion hazard, so ducts must be grounded to prevent static charge accumulation, and fans and motors must be properly specified for dusty conditions.

Baking, toasting, and roasting can be interpreted as drying processes in which other phenomena besides moisture removal are important. For example, color, texture, and flavor development are the objective in baking. Color develops exponentially faster as moisture decreases, so careful control of temperature is important toward the end of the process. Sometimes radio frequency is used as the source of energy at the end of baking low moisture products such as cookies because it delivers energy selectively to the moisture without overheating the dried material.

Sometimes moisture loss is undesirable in a thermal process as when cooking meat patties. In such cases, establishing conditions for poor drying can give good results. Steam can be injected in conveyor ovens to increase relative humidity and retard moisture loss while cooking the meat using radiant energy. Radiant energy is created by a hot surface, such as electrical resistance heaters or special burners. The rate of heat transfer is proportional to the difference of the absolute temperatures each raised to the fourth power. (Absolute temperature is degrees Celsius plus 273 or degrees Fahrenheit plus 456.) Radiant energy travels in a straight line from the source, so equipment relying on this type of energy must be designed to eliminate shadowing of the material.

Sanitary Design
Like most food equipment, food dryers—at least the food contact surfaces—should be constructed of stainless steel. Many are equipped with clean in place (CIP) systems, which can become complex. Typically there are spray devices arranged to clean all the interior walls and others to clean belts. There must be drains and return lines, and velocities must be sufficient to achieve turbulence for effective cleaning. Cleaning agents can be corrosive, so all components should be resistant.

Dryers can have food burnt onto surfaces and belts, and small pieces always find their way into obscure corners between belts, for instance. Quality of fabrication and construction matter. Other considerations are convenience of access for inspection, maintenance, and cleaning. Welded seams should be continuous, corners should be rounded, and sheet metal should be heavy enough to resist denting. Quality construction may carry a price premium but is worth the cost over the life of the equipment.

Drying is a venerable process with many useful variations. The principles of drying can be applied to other processes where moisture removal is a secondary effect or even undesirable. Modern process control systems with control of zones and the ability to adjust conditions during a run enable sophisticated optimization of drying processes. Understanding the principles and the impact of size, in particular, enables a designer or developer to maximize the economic benefits of drying. For example, controlling droplet size in spray-drying can maximize yield. Likewise, controlling final moisture content can optimize both quality and yield.

by J. Peter Clark,
Contributing Editor,
Consultant to the Process Industries, Oak Park, Ill.
[email protected]