Modeling the Thin Layer Drying of Date paste of Mozafati (Phoenix dactylifera L.)

Background and objectives: According to FAO statistics, Iran with production about 1023130 million tons date in 2010, is the third largest producer of date fruits in the world after Egypt and Saudi Arabia. However, 30 percent of date product is discarded. Date paste is a by-product that can reduce waste. Moisture and water activity (aw) reduction of date and its paste can increase production efficiency. This research was made experimentally to determine thin layer drying of date paste in different drying conditions and effect of temperature and thickness on time and rate of drying in date paste and finding the best experimental model.
Materials and methods: Date of Mozafati variety, first peeled, destoned and then converted to paste. The prepared date paste was kept in freezer -18 OC and before each test equilibrated with its environment. Thin layer drying characteristics of date paste were determined in hot air dryer. Air temperature was controlled with sensors connected into thermocouple. Weight changes with ±0.1 accuracy recorded with 10 seconds intervals. Date pasts were dried in factorial design experiments using three air temperature levels (70, 80 and 90 oC) and two levels of thickness (3 and 5 mm) with constant hot air velocity (1.5 m / s) in three replications. Eight mathematical models (Lewis, Henderson and Pabis, Page, Modified Page, midilli, Khazaei, Diffusion and Exponential) for describing the hot air drying behavior of date paste were investigated.
Results: temperature of dryer environment is the main and determining factor for drying rate according the findings. By increasing temperature in each thickness, moisture ratio (MR) decrease instantly, so the drying rate increases and drying time decreases. Page and Khazaei models were found to be the most suitable models for describing the drying behavior of the date paste. The drying air temperature had the greatest effect and thickness had the smallest effect on the drying kinetics of date paste. Effective diffusivity of water varied from 1.5517 × 10 -9 to 2.1806 × 10 -10 m2 / s. The activation energy during the studied temperature range calculated 13.166 KJ / mol. The temperature dependence of the diffusivity coefficients was described satisfactorily by a simple Arrhenius-type relationship.
Conclusion: Temperature and thickness are two important factors in date paste drying. The effect of temperature is more significant than thickness. Drying temperature increment had highly impact on the equilibrium moisture content (Me) and drying time in the range of our test.