The effect of copper ions, aluminium ions and their mixtures on separation of pectin from the sugar beet juice

Abstract

In sugar industry there is a problem of the presence of undesirable macromolecules compounds such as pectin in sugar beet juice. The affinity of calcium ions commonly used in the sugar industry for the removal of pectin from the sugar beet juice is relatively small. Coagulation and precipitation of pectin can be performed by process of discharging that is chemically induced. Compounds with di- and trivalent cations such as pure CuSO4, Al2(SO4)3 or their mixtures can be applied for clarification of pectin colloidal systems. According to data from the order of pectin selectivity to divalent metal ions, Cu2+ ions are the first order of ion binding. Also, aluminum sulfate is commonly used in the waste water treatment. Two model solutions of pectin whose concentration corresponds to the concentration of these macromolecules in sugar beet juice (0.1% w/w) are investigated. Using a method of measuring zeta potential, it was proven for both investigated pectin that fewer quantities of Cu2+ compared to the values of Al3+ are needed to reach zero zeta potential. In all the investigated coagulants and their mixtures, zeta potential has changed the sign. In experiments with mixtures has been shown that pure salts showed better coagulation properties. The reduced strength of binding of cations in the case of most of the applied mixture of Cu2+ and Al3+, can be explained by the mutual competition of these ions for the adsorption site (COO- groups) on the surface of macromolecules. Mixture with approximately equal shares of ions Cu2+ and Al3+ had the most unfavorable coagulation ability (ion antagonism). Mechanism of discharge as well as the model of double electric layer surrounding pectin macromolecules in the presence of mixtures of Cu2+ and Al3+ are suggested. However, due to possible undesirable effects of CuSO4 on food processing, Al2(SO4)3 is proposed instead of traditional coagulant CaO, not only because of lower consumptions of coagulants but owing to protection of the environment.