SOLAR DRYING KINETICS OF DATE PALM FRUITS ASSUMING A STEP-WISE AIR TEMPERATURE CHANGE

The effect of drying using a step-wise temperature change was studied considering the case of indirect solar drying of the date palm fruit (Phoenix dactylifera L.). The followed procedure consists of building drying kinetics by stages of temperatures resulting from drying, in constant conditions, of the same variety of dates from Algerian and Tunisian origin. A law of daily temperature variation prevailed by 60°C, was deduced from a statement of temperature collected on a laboratory solar dryer prototype. Two drying curve equation models were used and some comparisons were discussed. The results obtained for dates from the two origins highlighted different response times by changing the air temperature and showed the possibility of reaching a fruit with standard moisture content in only one day of drying on the basis of initial water contents ranging from 0.40 to 0.65. This moisture range is in practice allotted to rehydrated dates by water immersion in order to enhance their quality. Experiments conducted in a laboratory solar drier under temperatures oscillating around 50°C and 60°C led to the same end up regarding the drying time ensuring a visually appreciable fruit quality. Results obtained by a simple sensorial test revealed a better quality of date fruits treated by solar drying comparing to those issued from industrial heat treatment units