Literature Review
2.2 Review on Drying Kinetics Studies of the Thin Layer Solar and Open Sun Drying Processes of Food and Agricultural Products
2.2.2 Drying of fruits
Akpinar (2008) investigated the drying kinetics of white mulberry dried in a forced convection solar dryer and the open sun with natural convection. The moisture content of the mulberry was reduced to 0.17 g water/g dry matter from the average initial moisture content of 4.55 g water/g dry matter in 104 h and 152 h in the solar dryer drying and the open sun drying, respectively. The drying process of the white mulberry took place in the falling rate period, and the Logarithmic and the Verma et al. models satisfactorily described the thin layer drying behaviour of the white mulberry. The investigator also determined the diffusivity coefficient which was 3.56 × 10−9 m2/s and 2.4 × 10−9 m2/s for the solar drying and the open sun drying, respectively.
Togrul and Pehlivan (2002) developed a mathematical model of the thin layer solar drying process of apricots dried in a forced convection solar dryer attached to a conical concentric solar air heater. The moisture content of the SO2 pre - treated apricots was reduced to 18% (w.b.) from its initial moisture content of 77.8% (w.b.) in 68‒78 h in the solar dryer while it took 112 h in the direct sun drying. The drying experiments were carried out at different
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mass flow rates of drying air (50, 60 and 70 kg/h). The maximum drying air temperature (80
°C) was recorded at the air heater outlet at the mass flow rate of 50 kg/h. The drying process of the apricots took place in the falling rate period, and the Logarithmic model adequately described the drying process of the product. The investigators also obtained the relationship of the constants and the coefficient of the best model with the drying air temperature, velocity, and the relative humidity.
Yaldyz et al. (2001) studied the thin layer drying kinetics of pre - treated sultana grape and the effect of air velocity and the temperature on the drying kinetics. The drying experiments were carried out in the same dryer used by Yaldyz and Ertekyn (2001). The grapes were pre- treated in a mixture of potassium carbonate, olive oil, and water. The product was dried at different velocities of air (0.5, 1.0 and 1.5 m/s). The drying rate was high at the air velocity of 1 m/s compared to other velocities during the first 34 h of the drying period. Afterwards, the drying rate increased at the air velocity of 1.5 m/s. The experimental results showed that the drying process of the sultana grapes occurred in the falling rate period, and the two - term model satisfactorily described the drying behaviour of this product. The coefficients and the constants of the best model were expressed as the function of the drying air velocity.
Midilli and Kucuk (2003a) presented the mathematical modelling of the thin layer drying process of the shelled and unshelled pistachios. The pistachios were dried in a forced convection solar dryer at the drying air temperature ranging from 40 ºC to 60 °C in a fixed air velocity of 1.23 m/s and also in the direct sun. Eight thin layer drying models were applied to determine the best model for representing the drying kinetics of the pistachios. They reported that the Logarithmic model adequately represented the drying behaviour of the shelled and unshelled pistachios in the forced convection solar drying. For the natural sun drying, the Two - term model satisfactorily described the drying behaviour of both the shelled and unshelled pistachios. The investigators also expressed the constant and the coefficients of the suitable models in terms of the drying air temperature.
Lahsasni et al. (2004) studied thin layer drying characteristics of prickly pear peel dried in an indirect - type forced convection solar dryer integrated with an auxiliary heater. The effect of the drying air temperature and the mass flow rate on the drying rate of the product were investigated. The drying air temperature and the mass flow rate varied between 50 C and 60
C and 0.0277 m3/s and 0.0833 m3/s, respectively. The drying rate increased with increase in the drying air temperature and the mass flow rate, and the drying process of the prickly pear peel occurred in the falling period. The effect of the drying air temperature on the drying rate
was more than the mass flow rate. The Midilli - Kucuk model satisfactorily described the drying process of this product.
El - Beltagy et al. (2007) dried pre - treated strawberries of different shapes (whole, half, quarter and 3 mm discs) in an indirect - type active solar dryer to study its drying behaviour. They reported that the drying process of the strawberry comprised both the constant and falling rate periods. The drying time decreased with the change in the shape from whole, half, quarter to 3 mm discs. The chemical composition of the strawberry was not affected by the pre - treatment. The Newton or exponential model suitably described the drying process of the product.
Koua et al. (2009) investigated and modelled the thin layer drying behaviour of plantain banana, mango, and cassava dried in a direct - type natural convection solar dryer. The drying processes of all these products comprised of a short constant rate period followed by a long falling period. The Henderson and Pabis model adequately represented the drying behaviour of all the products. They also determined the effective diffusivity and observed that its value decreased with increase in the initial moisture content of the products. The effective diffusivities of the plantain banana, mango, and cassava varied from 1.44 × 10−9 to 1.30 ×10−9 m2/s, 1.29 ×10−9 to 1.18 ×10−9 m2/s, and1.23 × 10−9 to 1.59 × 10−9 m2/s, respectively.
Dissa et al. (2011) determined the solar drying characteristic of two varieties of mangoes (Amelie and Brooks) dried in a direct - type passive solar dryer. They also estimated the effective diffusivity and the efficiency of the drying system and studied the effect of the tray position on the drying rate. The effective diffusivity did not change much with the varieties of mango but it decreased with advancing of the drying days. The tray positions significantly affected the drying rate and the effective diffusivity. Higher drying rate of the product was observed in the top tray which was exposed to the solar radiation. The thermal efficiency of the dryer decreased with progressing of the drying days. The Amelie variety dried more rapidly than the Brooks. The drying processes of these varieties of the mangoes comprised of a short constant rate period followed by a long falling rate period. The Two - term and the Approximations of the diffusion models accurately predicted the drying characteristics of the products.
Doymaz (2005) studied the open sun drying kinetics of figs. The figs were dried in the ambient temperature range from 35 ºC to 47 °C, and the moister content of the product was reduced to 25% (w.b.) from its initial moisture content of 74% (w.b.) in 80 h. The drying process of the figs occurred in the falling rate period. The Verma et al. model adequately
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described the drying process of the figs, and the effective moisture diffusivity of this fruit was 2.47 × 10−10 m2/s.
Togrul and Pehlivan (2004) studied the drying kinetics of apricot, grapes, figs, peaches, and plums dried in the direct sun in the ambient temperature range from 27 °C to 43 °C. The apricot was treated with SO2 and NaHSO3, and the remaining products were dried without pre- treatment. The pre - treated apricot dried faster than the other fruits, and the peach dried faster than the grape, fig, and plum. The drying processes of all the fruits took place in the falling rate period. The diffusion model was the best model to describe the drying processes of the SO2 treated apricot and figs. The modified Henderson and Pabis model adequately described the drying processes of the apricot without pre - treatment, grapes, and plums, and the Verma et al. was the best model for the peaches.