A sequential method to estimate equilibrium Point and diffusion coefficients of bioactive compounds during solid–liquid extraction

Abstract

The aim of this study was to develop and validate a simple method to estimate both the equilibrium points and diffusion coefficients of bioactive compounds during solid–liquid extraction. The method is based on the sequential application of two straight-line fits, where the equilibrium point is first estimated with a semi-empirical equation and the diffusion coefficients are later evaluated with an analytical solution of Fick’s second law for short times. Estimated equilibrium points and diffusion coefficients were also compared with those simultaneously obtained using a nonlinear regression procedure. The hereby described method was applied to the analysis of total phenolic compounds (TPC), total flavonoids (TFL) and their total antioxidant capacity (TAC) during the aqueous extraction of green tea at three different temperatures (50, 65 and 80°C). The amount of extracted bioactive compounds at equilibrium were in the ranges of 114–122g GAE/kg dry matter (d.m.) for TPC, 68–89g catechin/kg d.m. for TFC and 178–192g Trolox/kg d.m. for their TAC. On the other hand, diffusion coefficients for TPC, TFC and their related TAC were in the range of 5.8–14.3×10−11, 2.8–4.5×10−11 and 1.7–12.6×10−11m2/s, respectively. The use of higher temperatures increased the equilibrium point as well as the diffusion coefficient values (p<0.05). Results demonstrated that the proposed methodology can be applied for the accurate description of experimental solid–liquid extraction curves allowing a precise yet uncomplicated estimation of mass transfer properties.