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

Abstract

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 (R 2 = 0.995; MRE = 6.71%) compared to MW drying (R 2 = 0.94; MRE = 18.4%)

Effect of Substitution of Sugars by Date Powders Variety H’lowa on the Quality of the Soft Drinks

“…Dates are dried at 80-90°C and the drying process was continued until the pulp moisture content did not decrease significantly with increasing drying time and/or the colour change is not visible to the naked eye ( Figure 1). The moisture content thus obtained was considered as equilibrium moisture content [13,14]. …”

“…The aptitude of the date to undergo a complementary thermal drying for obtaining powders was already underlined [14,21]. Then, the comprehension of the behavior of the powder is imperative to lead a final product with best physical quality.…”

Kinetic studies for microwave-assisted drying of Oraby date slices

“…On increasing microwave output power level, excessive browning of the nectarine slices (burning) occurred. Benamara, et al, (2009) reported that the MW drying could be considered instantaneous, but it involves a few scorched spots on pulp pieces which was explained by either the non-uniformity of the initial date pulp color or to the inadequacy of the chosen power. In practice, the temperature in the later stages of MW and hot air drying can easily involve caramelization and scorching in high sugar material (Zhang, et al,2006).…”

“…In practice, the temperature in the later stages of MW and hot air drying can easily involve caramelization and scorching in high sugar material (Zhang, et al,2006). Benamara, et al, (2009) stated that scorching limits the date pulp equilibrium moisture content to 7% (d.b) in MW drying against less than 5% for hot air drying. Nonetheless, to reach 7% moisture content, the drying time was reduced by about 95% when the microwave drying was used.…”

“…The authors found that osmotically and microwave pretreated samples suppressed browning compared to the untreated samples. Benamara, et al, (2009) indicated 95% of time saving by using microwave drying as a pretreatment of date pulp followed by convective drying. Al-Awaadh, et al, (2015) reported that drying affected Sukkari date fruit color and texture.…”