Kinetic modeling of the ultrasound-assisted extraction of polyphenols from Picea abies bark

In this paper, the kinetics of polyphenols extraction from spruce bark (Picea abies) under ultrasounds action was investigated. Studies were performed in order to express the effect of some specific parameters (as: ultrasounds, surface contact between solvent and solid, extraction time and temperature) on the total phenolic content (TPC). Experiments were performed in the presence and absence of ultrasounds, using different contact surfaces between solvent and solid, for times from 5 to 75 min and temperatures of 318, 323 and 333 K. All these factors have a positive influence on the process, enhancing the extraction rate by recovering higher amounts of polyphenols. The process takes place in two stages: a fast one in the first 20–30 min (first stage), followed by a slow one approaching to an equilibrium concentration after 40 min (second stage). In these conditions, the second-order kinetic model was successfully developed for describing the mechanism of ultrasound-assisted extraction of polyphenols from P. abies bark. Based on this model, values of second-order extraction rate constant (k), initial extraction rate (h), saturation concentration (C_s) and activation energy (E_a) could be predicted. Model validation was done by plotting experimental and predicted values of TPC’s, revealing a very good correlation between the obtained data (R² > 0.98).