Drying and browning of date pulp during hot air and microwave drying

The present work is a part of our scientific project about the valorisation of the common dates grown in southern Algeria. The principal aim was to study the drying ability of the fruit pulp with the view to produce food powders, which can easily take the place of many synthetics ingredients (white sugar, colorant like caramel) in many food preparations. The specific structure of the date pulp was also described: presence of two edible constitutive tissues (outside pigmented and inside white) that can influence the technological proprieties (as drying) of the whole fruit. The pigmented and white part weights were significantly different ( p ≤ 0.05). Results reveal the preponderance of white part, which can favourably influence the heat processing such as drying since the coloured pulp is already pre-browned compared with the white part. Hot air (60°C) and microwave (MW) (350W) drying kinetics of date ( Phoenix dactylifera L.) pulp pieces from Mech-Degla variety were investigated. Colour change (browning) was also analyzed during these drying processes using absorbance measurement at 420nm of the hydro alcoholic extract from pulp pigmented part. The latter has a heterogeneous initial colour in the same fruit. Three shades can be noticed: yellow, beige and brown related to the optic densities (at 420nm) of 0.92, 1.5 and 1.93 respectively. The minimal moisture contents reached by means of MW and hot air drying are about 8 (during 5 min) and 5 % dry basis (during 165 min) respectively. On the other hand, the MW drying could be considered instantaneous but it involves a few scorched spots on pulp pieces what may be due to the non uniformity of the initial date pulp colour or to the inadequacy of the chosen power. In addition, the applied model strongly fit the experimental data for convective air drying (R² = 0.995; MRE = 6.71%) compared to MW drying (R² = 0.94; MRE = 18.4%)