Optimization of Anthocyanin Extraction from Roselle (Hibiscus sabdariffa) Calyces: RSM, Kinetic Modelling, Mass Transfer and Thermodynamic Studies

Roselle calyces (Hibiscus sabdariffa) are becoming very important in the food and beverage industry, especially because of the presence of anthocyanin which is an antioxidant responsible for their red colour. The effect of processing parameters such as contact time, temperature and calyx-water ratio on the anthocyanin content of roselle calyces extract was studied and optimized along with evaluation of kinetic models, mass transfer and thermodynamic parameters. Extraction kinetics for anthocyanin were obtained at different time (5, 10 and 15 min), temperature (30, 50, 75 and 100 °C) and calyx-water mass ratio (1:50, 1:20 and 1:10). The maximum anthocyanin yield was obtained at 15 min; 100 °C and ratio of 1:10. The data obtained were fitted to 6 different extraction models and the ones that best suited the data were Weibull type, Peleg and Pseudo-second-order with Adj. R2 of 0.98, 0.99 and 0.99 respectively. The data obtained were used to calculate the kinetic, mass transfer and thermodynamic parameters. The kinetic variables were also related to the fractional extraction or conversion model. The fractional extraction increased with increased temperature and calyx-water. The effective diffusion coefficient ranged between 1.04×10-11 to 1.48×10-11 m2/s. The mass transfer coefficient calculated ranged between 1.62×10-8 and 11.02×10-8 (m/s), Biot number ranges from 25 to 168. The thermodynamic properties: Activation energy ranged from 15.7 to 16.4 kJmol-1; the enthalpy from 36.60 to 58.30 kJmol-1; the entropy from 88 to 147 JK-1mol-1, and the Gibbs free energy from -5.80 to-11 kJmol-1. The extraction process was observed to be endothermic, feasible and spontaneous.