Physico-Chemical and Rheological Properties of Date Fruits Extract During Concentration

Sewi date extract was extracted at high pressure and temperature and then concentrated under vacuum to 72.5 °Brix. Some physical, chemical and rheological properties of date extract were studied during concentration steps.
Moisture, acidity and reducing sugars contents gradually decreased during concentration, while total soluble solids (TSS), carbonyl compounds and pH values increased. Specific heat, thermal conductivity and thermal diffusion values decreased with increasing the TSS content. Also, colour attributes of date extract was found to be effected by concentration process. Whereas, lightness (L*), yellowness (b*), chroma, hue angle and whiteness index values decreased, while redness (a*) values varied during concentration.
Rheological parameters were measured by Brookfield digital viscometer at temperature of 5-70°C. Three rheological models (Bingham, IPC Paste and Power law) were used to calculate the relationship between shear stress and shear rate. The flow behaviour of date extract changed during concentration from shear thickening behaviour at 7.5 to 40 °Brix (n > 1) to shear thinning (pseudoplastic) behaviour at concentration higher than 40 °Brix (n < 1). The plastic viscosity, yield stress, consistency coefficient and 10 rpm viscosity values gradually increased during concentration. The effect of temperature on viscosity was very well correlated with the Arrhenius equation (r2 > 0.90).