Thermodynamic limitations for pigment adsorption during the bleaching of triglyceride oils

Abstract

Triglyceride oils are complex multi-component mixtures, so that the adsorption of chlorphylls is limited by co-adsorption of other components. Efforts to evaluate the effect of co-adsorbates on the capacity of adsorbents for a chlorophyll model compound were undertaken for two-component systems by means of the Ideal Dilute Solution Theory, (IDST). Single-component isotherms were measured for adsorption of the model compounds protoporphyrin IX dimethyl ester (PPIX), tripalmitin, trilinolein, stearic acid and linolenic acid onto both acid-activated clay (ABE) and an experimental adsorbent (EXP1) in a non-interacting solvent (CH2Cl2). Adsorbent affinities, as qualitatively measured by the initial slope of the isotherms, were in the order porphyrin (pKb = 4.8) » fatty acid (pKb ∼ 20.0) > triglyceride (pKb ∼ 20.5) for EXP1, while for ABE the affinities were in the order porphyrin » fatty acid, triglyceride. These affinities roughly correlate with the adsorbate basicity so that the mechanism of adsorption is acid-base reaction. Isotherms for binary mixtures of PPIX with the other adsorbates were calculated from single-component isotherm data by using IDST. Calculated isotherms for the adsorption of low concentrations of prophyrin in the presence of higher concentrations of triglyceride or fatty acid dramatically demonstrate the ability of these co-adsorbates to suppress the adsorption of porphyrin. Thus, competitive adsorption has a predominant effect on the behavior of adsorbents in the edible oil application. Improvements in adsorbent selectivity or the removal of co-adsorbing components are necessary to substantially improve bleaching performance.