168 JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 1, 2002 © 2002 Institute of Food Technologists Food Engineering and Physical Properties JFS: Food Engineering and Physical Properties Drying and Rehydrating Kinetics of Green and Red Peppers F. KAYMAK-ERTEKIN ABSTRACT: The effects of air temperature, air velocity, and pretreatments (blanching, sulphiting, and sodium chlo- ride dipping) on drying kinetics of green- and red-pepper slices were investigated. Drying experiments were per- formed in a fluidized bed dryer. In the falling rate period, moisture transfer from peppers was described by applying the unsteady state Fickian diffusion model, and the apparent moisture diffusion coefficients (D a ) were calculated. The effect of temperature on D a could be interpreted according to Arrhenius law. Drying rate and therefore D a values were found to be affected by pretreatments. Rehydration rates of dried peppers at 25 and 45 °C were also determined and found to be independent of drying conditions and rehydration temperature. Predrying treatments were found to improve partly the rehydration characteristics of peppers. Keywords: diffusion coefficient, drying, moisture transfer, rehydration rate, water absorbtion Introduction D RYING IS ONE OF THE OLDEST METHODS KNOWN FOR THE preservation of vegetables, which has a wide production po- tential in Turkey. The drying of food materials by the use of heated air has advantages on quality control, on achieve- ment of hygienic conditions, and on reduction of product loss. However, apart from the high-energy consumption re- quired for drying with heated air and shrinkage of the prod- uct, insufficiency in the rehydration ability of the dried prod- uct and undesired changes in the quality characteristics of the food are the other main disadvantages of this process. Therefore, reduction of the production costs by increasing the drying efficiency and improving the quality characteris- tics of dried products have been the subjects of many projects (Alzamora and Chirife 1980; Mazza and LeMaguer 1980; Mazza 1983). Although the most common types of hot-air dryers used for commercial vegetable dehydration are tray and tunnel dryers, drying in fluidized beds offers an interesting alterna- tive. A fluid bed dryer permits gentle and very efficient dry- ing to a very low moisture content due to uniform tempera- ture distribution and a large exchange area between solid and gas (Nargal and Ooraikul 1996). Various mathematical models describing the drying mechanism have been suggest- ed for the optimization of the process and the design of ef- fective dryers (Parry 1985; Mulet and others 1989; Tong and Lund 1990; Karathanos and others 1990; Kiranoudis and oth- ers 1990, 1992a, 1992b; Vagenas and Marinos-Kouris 1991; Gekas and Lamberg 1991; Magee and Wilkinson 1992). A review of literature showed that most of the drying pro- cesses are carried out at high air velocities. So external resis- tance to mass transfer is neglected, and the resistance of the solid is assumed to control the process. The unsteady state Fickian diffusion model can be applied (Mazza and LeMaguer 1980; Mulet and others 1987), and under these conditions, the rate of drying is affected by the air tempera- ture, the moisture content of the solid, and the structure of the solid. The effect of air velocity on drying kinetics has been ex- amined by various researchers such as Mulet and others (1987), Berna and others (1990, 1991), and Rossello and oth- ers (1992) in the drying of various food products. They stated that this effect was negligible at high air velocities and the critical air velocity value at which drying rate is not affected was found to be 1 to 1.5 m/s. Magee and Wilkinson (1992) have stated that a constant drying rate period was not ob- served in the drying of potatoes, and the process was con- trolled by diffusion mechanism. It is expressed that apparent diffusivities D a , obtained from the model where external re- sistance is neglected, is affected by air velocity and the slice thickness. It needs to be emphasized that the effect of air ve- locity is less than the effect of slice thickness. By using an empirical mass transfer model in the drying of onions and peppers, Kiranoudis and others (1992a) have proved that the air velocity does not significantly affect the drying rate. The parameters obtained from the model have been affected considerably by air temperature and slice thickness. Mazza (1983) has investigated the effect of pretreatments on the drying of carrots and stated that blanching increases the drying rate, whereas treatments with starch and sulphite solutions do not have a significant effect on drying rate. Dip- ping in sodium-chloride solution prior to drying decreases the drying rate (Speck and others 1977; Mazza 1983; Magee and Wilkinson 1992). The vapor pressure of the solid de- creases with increasing salt concentration. This leads to a de- crease in the driving force for mass transfer and inhibits the drying rate. Rapid and complete rehydration is an important property of dried products. Rehydration capacity is affected signifi- cantly by drying conditions, pretreatments prior to drying, and textural characteristics of dried products (Neumann 1972; Mazza and LeMaguer 1980; Mazza 1983). Drying can diminish the osmotic properties of cell walls. Therefore, in- crease in water absorption and volume occur due to the swelling of hydrophilic materials such as starch, cellulose, and pectic materials (Neumann 1972; Horn and Sterling 1982). Change of these materials from amorphous phase to crystal form, especially during storage, decreases consider- ably the rehydration properties of dried products. Mazza (1983) and Jayaraman and others (1990) pointed out that pretreatments such as blanching in water and sul- phiting, prior to drying, increase rehydration capacity of