Removal of dark compounds from clarified fruit juices by adsorption processes

To improve the colour of a browned clarified peach juice, an adsorption treatment using different kinds of activated carbon was investigated. The adsorption equilibrium was studied by adsorption isotherms in the range 10-50 degrees C for three types of activated carbon. Absorbance data at 420 nm were used to plot all the isotherms which are correlated reasonably well with the Langmuir and Freundlich isotherms. A kinetic study is also presented where changes with time of th following parameters were analyzed: absorbance at 420 nm, hydroxymethylfurfural (HMF) concentration, soluble solids content and pH of the juice for various amounts of the three activated carbons at different temperatures. Improvements of the juice colour due to the adsorption treatments were detected by a decrease in the absorbance at 420 nm and in the HMF content. Also, it was observed that the pH increased and the soluble solids decreased. The initial rate of the adsorption was very fast, but then it became quite slow and reached equilibrium in an asymptotic form. It was found then, that a contact time of 10-15 min is enough to obtain a suitable juice. The optimum processing temperature was found in the 30-50 degrees C range and the most effective carbon was the granular activated carbon with 2-4 mm particle size. (C) 1998 Elsevier Science Limited. All rights reserved.