Figure 5. Two-stage conveyor dryer. Source: Courtesy of Proctor and Schwartz Inc.

Table 4. Advantages and Limitations of Parallel Flow, Counter-Current Flow, Center-Exhaust, and Cross-Flow Drying

Type of air flow



Parallel or cocurrent type:


Counter-current type:

Center-exhaust type:

Cross-flow type:

Airflow 11

Rapid initial drying. Little shrinkage of food. Low bulk density. Less heat damage to food. No risk of spoilage

More economical use of energy. Low final moisture content as hot air passes over dry food

Combined benefits of parallel and counter-current dryers but less than cross-flow dryers

Flexible control of drying conditions by separately controlled heating zones, giving uniform drying and high drying rates

Low moisture content difficult to achieve as cool, moist air passes over dry food

Food shrinkage and possible heat damage. Risk of spoilage from warm moist air meeting wet food

More complex and expensive then single-direction air flow

More complex and expensive to buy, operate, and maintain of dried food is ground to a free-flowing powder that has good rehydration properties. The rapid drying and low product temperatures result in high-quality product. However, a large surface area is required for high production rates, so capital costs are therefore high.

Fluidized-Bed Dryers. Metal trays with mesh or perforated bases contain a bed of particulate foods up to 15 cm deep. Hot air is blown through the bed (Fig. 6), causing the food to become suspended and vigorously agitated (fluid-ized). The air thus acts as both the drying and the fluidiz-

Figure 6. Fluidized-bed drying. Source: Courtesy of Petrie and McNaught Ltd.

ing medium and the maximum surface area of food is made available for drying. A sample calculation of the air speed needed for fluidization is described in sample problem 2. Dryers may be batch or continuous in operation; the latter are often fitted with a vibrating base to help move the product. Continuous cascade systems, in which food is discharged under gravity from one tray to the next, employ up to six dryers for high production rates.

Fluidized-bed dryers are compact and have good control over drying conditions, relatively high thermal efficiencies, and high drying rates. In batch operation, products are mixed by fluidization, this leads to uniform drying. In continuous dryers, there is a greater range of moisture content in the dried product; bin dryers are therefore used for finishing. Fluidized-bed dryers are limited to small particulate foods that are capable of being fluidized without excessive mechanical damage (eg, peas, diced or sliced vegetables, grains, powders, or extruded foods). These considerations also apply to fluidized-bed freeze-dryers and freezers.

A development of the fluidized-bed dryer, named the Torbed dryer, has potential applications for drying particulate foods. A fluidized bed of particles is made to rotate around a torus-shaped chamber, by hot air blown directly from a burner (Fig. 7). The dryer has very high rates of heat and mass transfer and substantially reduced drying times. It is likely that some products (eg, vegetable pieces) would require a period of equilibration to allow moisture redistribution before final drying. The dryer operates semi-continuously under microprocessor control and is suitable for agglomeration and puff drying in addition to roasting, cooking, and coating applications.

Kiln Dryers. These dryers are two-story buildings in which a drying room with a slatted floor is located above a furnace. Hot air and the products of combustion from the furnace pass through a bed of food up to 20 cm deep. These dryers have been used traditionally for drying apple rings or slices in the United States, and hops or malt in Europe. There is limited control over drying conditions, and drying

Figure 7. Torbed dryer: 1, rotating disc distributor to deliver raw material evenly into processing chamber; 2, rotating bed of particles; 3, fixed blades with hot gas passing through at high velocity; 4, burner assembly. Source: Courtesy of Tbrftech Ltd.

times are relatively long. High labor costs are incurred by the need to turn the product regularly, and by manual loading and unloading. However, the dryers have a large capacity and are easily constructed and maintained at low cost.

Pneumatic Dryers. In pneumatic dryers, powders or particulate foods are continuously dried in vertical or horizontal metal ducts. A cyclone separator is used to remove the dried product. The moist food (usually less than 40% moisture) is metered into the ducting and suspended in hot air. In vertical dryers the airflow is adjusted to classify the particles; lighter and smaller particles, which dry more rapidly, are carried to a cyclone more rapidly than are heavier and wetter particles that remain suspended to receive the additional drying required. For longer residence times the ducting is formed into a continuous loop (pneumatic ring dryers) and the product is recirculated until it is adequately dried. High-temperature short-time ring dryers are used to expand the starch-cell structure in potatoes or carrots to give a rigid, porous structure, which enhances subsequent conventional drying and rehydration rates. Calculation of air velocities needed for pneumatic drying is described in equation 16.

Pneumatic dryers have relatively low capital costs, high drying rates and thermal efficiencies, and close control over drying conditions. They are often used after spray drying to produce foods that have a lower moisture content than normal (eg, special milk or egg powders and potato granules). In some applications the simultaneous transportation and drying of the food may be a useful method of materials handling.


Figure 7. Torbed dryer: 1, rotating disc distributor to deliver raw material evenly into processing chamber; 2, rotating bed of particles; 3, fixed blades with hot gas passing through at high velocity; 4, burner assembly. Source: Courtesy of Tbrftech Ltd.


Rotary Dryers. A slightly inclined rotating metal cylinder is fitted internally with flights to cause the food to cascade through a stream of hot air as it moves through the dryer. Airflow may be parallel or counter-current (Table 4). The agitation of the food and the large area of food exposed to the air produce high drying rates and a uniformly dried product. The method is especially suitable for foods that tend to mat or stick together in belt or tray dryers. However, the damage caused by impact and abrasion in the dryer restrict this method to relatively few foods (eg, sugar crystals and cocoa beans).

Spray Dryers. A fine dispersion of preconcentrated foods is first atomized to form droplets (10-200 fim in diameter) and sprayed into a current of heated air at 150-300°C in a large drying chamber. The feed rate is controlled to produce an outlet air temperature of 90-100°C, which corresponds to a wet-bulb temperature (and product temperature) of 40-50°C. Complete and uniform atomization is necessary for successful drying, and one of the following types of atomizer is used.

1. Centrifugal Atomizer. Liquid is fed to the center of a rotating bowl (with a peripheral velocity of 90-200 m/s). Droplets, 50-60 jum in diameter, are flung from the edge of the bowl to form a uniform spray (Fig. 8a).

2. Pressure Nozzle Atomizer. Liquid is forced at a high pressure (700-2000 kPa) through a small aperture. Droplet sizes are 180-250 jum. Grooves on the inside of the nozzle cause the spray to form into a cone shape and therefore to use the full volume of the drying chamber.

3. Two-Fluid. Nozzle Atomizer. Compressed air creates turbulence, which atomizes the liquid (Fig. 8b). The operating pressure is lower than the pressure nozzle, but a wider range of droplet sizes is produced.

Both types of nozzle atomizer are susceptible to blockage by particulate foods, and abrasive foods gradually widen the apertures and increase the average droplet size. Studies of droplet drying, including methods for calculating changes in size, density, and trajectory of the droplets are reported in Refs. 10, 16, and 17.

Rapid drying takes place (1-10 s) because of the very large surface area of the droplets. The temperature of the product remains at the wet-bulb temperature of the drying air, and there is minimum heat damage to the food. Airflow may be co- or counter-current (Table 4). The dry powder is collected at the base of the dryer and removed by a screw conveyor or a pneumatic system with a cyclone separator. There are a large number of designs of atomizer, drying chamber, air heating, and powder collecting systems (10,18). The variations in design arise from the different requirements of the very large variety of food materials that are spray dried—eg, milk, egg, coffee, cocoa, tea, potato, ground chicken, ice cream mix, butter, cream, yogurt and cheese powder, coffee whitener, fruit juices, meat and yeast extracts, encapsulated flavors (19), and wheat and corn starch products. Spray dryers may also be fitted with

Figure 8. Atomizers: (a) centrifugal atomizer; (b) two-fluid nozzle atomizer (15). Source: Courtesy of Elsevier Applied Science.

Figure 8. Atomizers: (a) centrifugal atomizer; (b) two-fluid nozzle atomizer (15). Source: Courtesy of Elsevier Applied Science.

fluidized bed facilities to finish powders taken from the drying chamber.

Spray dryers vary in size from small pilot-scale models for low-volume high-value products (eg, enzymes and flavors) to large commercial models capable of producing 80,000 kg of dried milk per day (20) (Fig. 9). The main advantages are rapid drying, large-scale continuous production, low labor costs, and simple operation and maintenance. The major limitations are high capital costs and the requirement for a relatively high feed moisture content to ensure that the food can be pumped to the atomizer. This results in higher energy costs (to remove the moisture) and higher volatile losses. Conveyor band dryers and fluidized bed dryers are beginning to replace spray dryers, as they are more compact and energy efficient (21).

The bulk density of powders depends on the size of the dried particles and on whether they are hollow or solid.

Figure 9. Spray dryer. Source: Courtesy of De Melkindustrie Veghel.

This is determined by the nature of the food and the drying conditions (eg, the uniformity of droplet size, temperature, solids content, and degree of aeration of the feed liquid). Instant powders are produced by either agglomeration or non-agglomeration methods. Agglomeration is achieved by remoistening particles in low-pressure steam in an ag-glomerator, and then redrying. Fluidized-bed, jet, disc, cone, or belt agglomerators are described in Ref. 22. Alternatively, straight-through agglomeration is achieved directly during spray drying. A relatively moist powder is agglomerated and dried in an attached fluidized bed dryer. Nonagglomeration methods employ a binding agent, (eg, lecithin), to bind particles. This method was previously used for foods with a relatively high fat content, (eg, whole milk powder) but agglomeration procedures have now largely replaced this method (23).

Trough Dryers (Belt-Trough Dryers). Small, uniform pieces of food, (eg, peas or diced vegetables) are dried in a mesh conveyor belt that hangs freely between rollers to form the shape of a trough. Hot air is blown through the bed of food, and the movement of the conveyor mixes and turns it to bring new surfaces continually into contact with the drying air. The mixing action moves food away from the drying air, and this allows time for moisture to move from the interior of the pieces to the dry surface. The mois ture is then rapidly evaporated when the food again contacts the hot air. The dryer operates in two stages, to 5060% moisture and then to 15-20% moisture. Foods are finished in bin dryers. These dryers have high drying rates (eg, 55 min for diced vegetables, compared with 5 h in a tunnel dryer), high energy efficiencies, good control, and minimal heat damage to the product. However, they are not suitable for sticky foods.

Tunnel Dryers. Thin layers of food are dried on trays, which are stacked on trucks programmed to move semi-continuously through an insulated tunnel. Different designs use one of the types of air flow described in Table 4. Food is finished in bin dryers. Typically, a 20-m tunnel contains 12-15 trucks with a total capacity of 5000 kg of food. This ability to dry large quantities of food in a relatively short time (5-16 h) made tunnel drying widely used, especially in the United States. However, the method has now been largely superseded by conveyor drying and fluidized-bed drying as a result of their higher energy efficiency, reduced labor costs, and better product quality.

Sun and Solar Drying. Sun drying (without drying equipment) is the most widely practiced agricultural processing operation in the world; more than 250,000,000 tons of fruits and grains are dried by solar energy per annum. In some countries foods are simply laid out on roofs or other flat surfaces and turned regularly until dry. More sophisticated methods (solar drying) collect solar energy and heat air, which in turn is used for drying. Solar dryers are classified into three categories (4):

1. Direct natural-circulation dryers (a combined collector and drying chamber).

2. Direct dryers with a separate collector.

3. Indirect forced-convection dryers (separate collector and drying chamber).

Both solar and sun drying are simple, inexpensive technologies, in terms of both capital input and operating costs. Energy inputs and skilled labor are not required. Sun drying is therefore the preferred option in developing countries that have a suitable dry season after harvesting food crops such as paddy or maize. The major disadvantages of sun drying are poor control over drying conditions; lower drying rates than in artificial dryers; dependence on sunlight, which causes cessation of drying operations at night or during rain; and contamination of the dried product by dust, etc. Each of these factors contributes to a more variable and generally lower quality product than that produced by artificial dryers.

Solar dryers aim to improve product quality by providing greater control over drying conditions, protection from rain or dust, and higher drying rates. However, they have a relatively small capacity for drying the large bulk of crops at harvest time when compared to sun drying. In addition, the higher capital investment may not result in a higher income from improved quality of the dried crop. More valuable crops such as herbs and spices offer better potential for solar drying owing to the smaller quantities involved and the increased income from improved quality. However, the dependence of solar dryers on sunlight and the better control over drying conditions achieved in artificial (fuel-fired) dryers again limit the potential for solar drying.

Heated-Surface Dryers. Dryers in which heat is supplied to the food by conduction have two main advantages over hot-air drying: (1) It is not necessary to heat large volumes of air before drying commences, and the thermal efficiency is therefore high; (2) Drying may be carried out in the absence of oxygen to protect components of foods that are easily oxidized.

Typically, heat consumption is 2000-3000 kJ/kg of water evaporated compared with 4000-10,000 kJ/kg of water evaporated for hot-air dryers. However, foods have low thermal conductivities that become lower as the food dries. There should therefore be a thin layer of food to conduct heat rapidly, without causing heat damage. Foods may shrink during drying and lift off the hot surface, therefore introducing an additional barrier to heat transfer. Careful control is necessary over the rheological properties of the feed slurry to minimize shrinkage and to determine the thickness of the feed layer.

Drum Dryers (Roller Dryers). Slowly rotating hollow steel drums are heated internally by pressurized steam to 120-170°C. A thin layer of food is spread uniformly over the outer surface by dipping, spraying, spreading, or auxiliary feed rollers. Before the drum has completed 1 rev (within 20 s/3 min), the dried food is scraped off by a doctor blade that contacts the drum surface uniformly along its length. Dryers may have a single drum (Fig. 10a) or double drums (Fig. 10b) or twin drums. The single drum is widely used as it has greater flexibility, a larger proportion of the drum area available for drying, easier access for maintenance, and no risk of damage caused by metal objects falling between the drums.

Drum dryers have high drying rates and high energy efficiencies. They are suitable for slurries in which the particles are too large for spray drying. However, the high capital cost of the machined drums and heat damage to sensitive foods from high drum temperatures have caused a move to spray drying for many bulk dried foods. Drum drying is used to produce potato flakes, precooked cereals, molasses, some dried soups, and fruit purees, and whey or distillers' solubles for animal feed formulations.

Developments in drum design to improve the sensory and nutritional qualities of dried food include the use of auxiliary rolls to remove and reapply food during drying, the use of high-velocity air to increase the drying rate, and the use of chilled air to cool the product. Drums may be enclosed in a vacuum chamber to dry food at lower temperatures, but the high capital cost of this system restricts its use to high-value heat-sensitive foods.

Vacuum Band and Vacuum Shelf Dryers. A food slurry is spread or sprayed onto a steel belt (or band) that passes over two hollow drums within a vacuum chamber at 1—70 mmHg. The food is dried by the first steam-heated drum, and then by steam-heated coils or radiant heaters located over the band. The dried food is cooled by the second water-

Figure 10. Drum dryers: (a) single drum; (b) double drum. Source: Courtesy of APV Mitchell Ltd.

cooled drum and removed by a doctor blade. Vacuum shelf dryers consist of hollow shelves in a vacuum chamber. Food is placed in thin layers on flat metal trays that are carefully made to ensure good contact with the shelves. A particular vacuum of 1-70 mmHg is drawn in the chamber and steam or hot water is passed through the shelves to dry the food.

Rapid drying and limited heat damage to the food make both methods suitable for drying heat-sensitive foods. However, care is necessary to prevent the dried food from burning onto trays in vacuum shelf dryers, and shrinkage reduces the contact between the food and heated surfaces of both types of equipment. Both have relatively high capital and operating costs and low production rates.

Vacuum-band shelf dryers are used to produce puff-dried foods. Explosion puff drying involves partially drying food to a moderate moisture content and then sealing it into a pressure chamber. The pressure and temperature in the chamber are increased and then instantly released. The rapid loss of pressure causes the food to expand and develop a fine porous structure. This permits faster final drying and rapid rehydration. Sensory and nutritional qualities are well retained. The technique was first applied commercially to breakfast cereals and now includes a range of fruit and vegetable products.

600 Chocolate Recipes

600 Chocolate Recipes

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