Drying a moist material and decreasing the water activity mean evaporation of bound water from in- side the solid material into the atmosphere. Breaking water bonds, releasing, and transferring heat con- nected to phase change require energy. Drying can be done with different types of drying energy: con- vective (warm air), contact (cooled surface), radiative (infrared rays), and excitation (microwave) energies.
With convective drying, the heated air low in mois- ture content meets the wet material and as a result, the moisture moves onto the surface of the mate- rial and then into the drying air. Tasks of the warm air are to transfer heat to the material being dried to establish the drying potential and to transfer moisture into the air. For contact drying, the heat expanded by conduction from the cooled surface of the material evaporates the moisture. With infrared drying, the heat spreads from a radiating body—which can be a spot lamp, a piece of heated metal, or ceramic—
directly to the material being dried. This method can be well-applied using vacuum drying for very small or chopped material (Szab´o, 1987). For heat ex- change by excitation, materials consisting of highly polarized molecules absorb the energy of excitation, resulting in heat necessary for drying the material.
Using this method, liquids, pastes, and highly milled materials can be handled quickly and without a deteri- oration of the product. Vacuum drying can be used for heat-sensitive materials with low moisture content. In
84 Part I: Processing Technology a vacuum with no transferring medium, convective
heat exchange cannot be applied.
Moisture Transport in Solid Material
The phenomenon of drying is similar regardless of the drying method. This section deals with convec- tive drying, the most widely used method in the fruit processing industry. The wet material (fruit) is placed in an air space with relative moisture content lower than the ERH of the material; moisture is transferred from the solid material (fruit) into the drying medium (air space).
Mass flow of the moisture (qm in kg/s) isqm=
y(Ys−Yg) A, where y is the material exchange factor at the gaseous side (kg/m2s),Ys,Yg are the absolute vapor content of the air at the surface of the material and in the air, respectively, (kg/kg),Ais the surface area (m2).
Simultaneously, the moisture content of the ma- terial is decreased. The water moves from the solid (fruit) and changes to vapor either inside or on the surface of the solid material. This vapor moves to the surface and goes into the air. In certain materials, such as gels, moisture transport is caused by diffusion flow of the water in the given material. This diffusion flow is initiated by the moisture difference of the material (Barta et al., 1990). Most foods are capillary-colloidal porous materials in which simultaneous liquid–vapor transport can occur. The character and direction of this transport depend on the texture, shape, and re- lationship of capillaries and pores. The vapor pro- duced by water evaporation in the capillary-porous structure flows by diffusion to the surface. The so- called Knudsen flow in the micro-capillaries can be several orders of magnitude larger than Poiseuille flow in macro-capillaries. In foods, the conduction
form of energy mentioned above occurs together with diffusion and moisture transport that is a function of the type of material and circumstances. For industrial calculations, the various forms of water transport can be handled together by means of an effective apparent diffusion parameter. Using the average apparent dif- fusion parameter (De, m2/s) the mass flow (qm, kg/s) of the moisture is in a stationary state:
qm=csDedX
dz A, (5.2)
whereAis the surface area perpendicular to the direc- tion of the moisture transport (m2),csis the concen- tration of the solid material (kg/m3),zis the length in direction of the moisture transport (m), andXis the moisture content of the material, i.e., the amount of water related to 1 kg dry material.
The relationship above can be derived from Fick’s law. In a non-stationary state, the material equation written for water results in a second order, non- linear, parabolic differential equation, which can be given together with the initial and boundary condi- tions (e.g., material exchange on the surface). The moisture distribution along the length can be de- termined at an arbitrary drying period (Gion, 1986, 1988; K¨ormendy, 1985; Mohr, 1984).
Drying Procedure
At steady-state conditions (constant temperature, air flow rate, and air moisture content), the experimen- tal results of drying are plotted by time. Generally, the moisture content (X) related to dry material is shown as a function of time (t). This is presented in Figure 5.2.
This plot shows a typical case where the mois- ture from the solid material evaporates first from the moisture layer on the surface and decreases
9 8 7 6 5 4 3 2 1 0 0
10 20 30 40 50 60 70
t (min)
x (kg/kg)
Figure 5.2. Drying curve of a wet material.
0 0 1 2 3 4 5 6 7
10 20 30 40 50 60 70
t (min)
(dx/dt) × 103
Figure 5.3. Drying rate curve as a function of time.
0 0 1
1 2
2 3
3 4
4 5
5 6
6 7
7 8
x (kg/kg)
(dx/dt) ×103
Figure 5.4.Drying rate curve as a function of the moisture content.
continuously until water evaporates from the inside of the solid material. It can be seen in the figure that variations in the drying rate depend on time and mois- ture content of the fruit product. This change can be seen better if the drying curve is differentiated and a drying flow rate curve is derived. Drying rate can be presented as a function either of the drying period (Fig. 5.3), or the moisture content of the material (Fig. 5.4).
Curves for the drying rate and drying flow rate can be divided into several parts. These parts are the re- sult of the inner mechanism of drying and of changes occurring during drying. In the first step of drying, temperature equalization and moisture transport oc- cur. In the next step, which is the constant rate period, there is a constant moisture flow to the surface, there- fore, the surface is always wet. The average moisture measured at drying of the surface is the so-called critical moisture content. The drying rate decreases after reaching the critical moisture content. Drying stops and the drying rate becomes equal to zero when the average moisture content reaches the equilibrium
moisture content related to the relative vapor content of the air. Figure 5.5 shows a curve for the average temperature of the material.
At the initial period of drying, the temperature of the material reaches the temperature of a wet thermometer. The temperature does not change in the constant rate period until reaching the criti- cal moisture content. The temperature of the ma- terial increases in the falling rate period and be- comes equal to the temperature of the drying air when drying stops. Dimensions of the material being dried are of primary importance in drying technology (Figure 5.6).
Linear variation of the size of the material changes the drying period to the second power. Increasing the drying temperature, and therefore the drying rate, the drying period is shortened and the capacity of the equipment is raised. This method is useful only in the constant rate period because the higher tem- perature of drying air does not result in significant increases in the temperature of the material. The in- crease in the drying rate is hindered by some stresses
86 Part I: Processing Technology
Figure 5.5. Average temperature of drying material as a function of time.
10 1
20 2
30 3
40 4
50 5
60 6
70 80
7 8
0 0
t (min) 1= 2,5 mm
1= 5 mm 1= slice thickness of drying material
x (kg/kg)
Figure 5.6. Effect of the size of material on the drying process.
in the material, by precipitation of the solute salts on the surface and crust formation. The intensive air ventilation enhances the moisture transport to the sur- face; however, it can result in crust formation (Barta et al., 1990). Managing the drying process takes into account the following aspects:
rhigh temperature and intensive air ventilation at the initial period
rmechanical removal of the surface moisture layer rtemperatures ensuring a low drying rate for a long
period at the end of drying, “quiet” ventilation.
Effect of the Drying Air Characteristics on the Drying Process
It is necessary to know the factors determining the quality of the finished product, which can help to es- tablish the parameters of the drying procedure. The concept of optimum drying must include the concept
of economy, or the optimal application of heat used for drying. The external factors influencing drying are the following: temperature, moisture content, flow rate, direction of the drying air, and drying period.
These factors must fit the properties of the material being dried (variety, water content, dimensions) and the methods of preparation (Kilpatrick et al., 1955;
Lazar and Farkas, 1971; Lozano et al., 1983; Van Arsdel, 1973; Ratti, 1991). The most important fac- tor is thetemperature of the applied air. Under the same conditions, the lower the temperature used for drying, the better the quality of the product. Since an increase in temperature increases the rate of some chemical reactions and changes the original composi- tion and properties, it is advisable to limit fruit tem- peratures to 60◦C during the period of falling rate.
However, in the constant rate period, where the mass transfer rate is in equilibrium with the heat transfer, 80–85◦C can be achieved. The temperature of the drying air is generally 20–25◦C higher than that of the material. In the case of drying at excessively high
temperatures, the food temperature increases due to the warm air. Then the evaporation of the surface moisture occurs so quickly that there is not enough time to remove moisture from the inside. In this case, undesirable changes occur on the surface of the ma- terial. Internal water evaporation becomes more and more difficult except at higher temperatures. Because of the very high temperature, the material becomes too dry and fragile. Thicker plant sections dried in this way develop a hard and strong crust while the inside remains wet, contributing to a product that deteriorates easily. The crust formation also causes shrinkage inappropriate to the amount of water re- moved, and a resistance to the forces directed in- ward. The relatively soft inner parts move to the outer crust and the product becomes hollow (Lozano et al., 1983; Barta et al., 1990). The moisture gradient be- tween the inner and outer parts can be described as a counter flowing concentration gradient. The origi- nally uniform distribution of solutes is more diluted inside than on the surface layer, which has less mois- ture. This may cause an inward diffusion of solutes and undesirable reactions. During drying, the tem- perature used must match the physical and chemical properties of the material being dried. Generally, the starting temperature is high, gradually decreased, and then held constant for a longer period. Themoisture content of the airused for drying is also an impor- tant factor in the drying process. Vapor absorptivity of the air is a function of the temperature (e.g., it is doubled by increasing the temperature by 15◦C). Un- der the same conditions (temperature, flowing rate, etc.), the more moisture that is absorbed by the dry- ing air, the less the moisture content at the inlet of the dryer. Therefore, drying becomes more economical if the outgoing air removes as much moisture as possi- ble. The drying rate—as was mentioned earlier—can be controlled by the appropriate temperature setting.
In order to prevent deterioration, the air moving out of the dryer is mixed with fresh air at a ratio suit- able for re-application. Therefore, the same, climate- independent air is used (dryers do not work generally in climatized rooms) with appropriately chosen rel- ative moisture content. In this way, at least partly, the heat of the used air is also utilized. Theflow- ing rate of the airused for drying is important both for its appropriate usage and for the quality of the finished product. The higher the air temperature and its decrease, the faster the drying if there is no crust formation. The flow rate of air must be set, so both roles of air can be fulfilled (to evaporate the moisture on the surface and to remove the internal moisture).
Both must occur at a given temperature interval. At 1 m/s of air flowing rate, the drying rate is double that of air at rest and at a 2 m/s flow rate, drying is about three times quicker. The flowing rate of the air is hin- dered by the capacity of the food for fluidization. At high air velocities, e.g., above 10 m/s, the drying air can take the material along.The flowing direction of the aircan also vary in dryers. If the fresh air en- tering the drier meets the raw material first, then the airflow and the drying are called direct airflow and direct drying, respectively. This is less economical, but not harmful. If the fresh air comes in contact with the driest material and the used air with the newest material (or the wettest solid), the flowing of air and drying are called counter flowing air and drying, re- spectively. Direct flow is most commonly applied for drying products that are more sensitive. The finished product is the most sensitive to further drying. When the moisture content of the material being dried is lower, deterioration caused by heat occurs more eas- ily. Drying air is less harmful if it has the highest possible vapor content and the lowest temperature.
These properties are features of used air; therefore, it is desirable that the most used air is directed to the finished product. If the flowing direction of the air is perpendicular to the direction of the motion of the material, the flowing and drying are called cross- flowing and drying, respectively. The drying period is determined both by the raw material and the dry- ing air according to the viewpoints discussed above.
The upper limit of the drying period is a function of the economical factors (more efficient usage of the dryer) and the risk of deterioration of the material be- ing dried. The drying period for vegetables and fruits cannot be longer than 6–7 h and moreover, with ap- propriate controls cannot exceed 3 h. Of course, this is highly dependent on the type of dryer. The lower limit of the drying period is influenced by the char- acter, species, and quality of the product. Taking into account the risk of drying too fast, a period shorter than 2 h is not advisable for a standard dryer. Shape, surface area, and thickness have a significant effect on the drying rate. The thinner the material, the larger the surface area, and the faster the movement of mois- ture from the inside. This happens in such a way that the drying rate curve reaches the breaking point only at the end of drying. Therefore, sensitive materials can be dried at a higher temperature in thin-bedded layers, because the drying period is shorter. This can be done in a thin-layered columnar dryer. The very small sphere-like drops of spray drying are highly effective.
88 Part I: Processing Technology