Aluminium and calcium ions binding to pectin in sugar beet juice: Model of electrical double layer

Abstract

In sugar industry, there is a problem with the presence of undesirable macromolecules such as pectins in sugar beet juice. Removal of these compounds is done mostly by CaO. Calcium may cause undesirable process of alkalization of soil in the proximity of sugar factory. The theoretical basis of new juice purificatin method based on the application of Al 2 (SO 4) 3 , CaSO 4 and their mixtures are presented. Two model solutions of pectin (0.1%, w/w) are investigated using method of measuring zeta potential. Pure salts Al 2 (SO 4) 3 and CaSO 4 , showed better binding with pectin than mixtures. Amount of all studied pure salts and mixtures of Al 3+ and Ca 2+ were significantly less (142-710 mg/g pectin) than the average amount of CaO used in classical process (about 9 g/g pectin). Mechanism of discharge of pectin macromolecules in the presence of mixtures of these ions using a model of double electric layer is suggested.

Comparison of methods of zeta potential and residual turbidity of pectin solutions using calcium sulphate/aluminium sulphate as a precipitant

“…The amount of the CaSO 4 /Al 2 (SO 4 ) 3 mixture needed to bring the zeta potential to zero for pectin P1 and P2 was 465 mg•dm -3 (705 mg•g p -1 ) and 420 mg•dm -3 (585 mg•g p -1 ), respectively. These quantities are slightly larger than the required amount of pure compound CaSO 4 examined in the previous work under the same conditions (7). However, with regards to the magnitude of the concentration of the binary solution CaSO 4 /Al 2 (SO 4 ) 3 , the residual turbidity of the solutions reached a minimum value at the concentration of 400 mg•dm -3 (550 mg•g p -1 ), for pectin P1 and 350 mg•dm -3 (490 mg•g p -1 ), for pectin P2.…”

“…However, the quantities of used limestone, in this stage of purification of sugar beet juice are very high, and range from 1 % to 3 % w/w, calculated on the beet. In order to reduce this amount, many compounds with divalent and trivalent cations that chemically cause discharge of undesirable macromolecules of sugar beet juice have been studied (5)(6)(7)(8)(9). Of the total of non-sucrose matter contained in the sugar beet juice, approximately 60% are pectins.…”

“…On the other hand, Ca ions with weaker binding effect, particularly in the form of CaSO 4 , in hydrophilic pectin macromolecules cause significant decrease of hydration. This is, in addition to the charge neutralization, a precondition for more rapid coagulation and precipitation (7,8,10,12).…”

CaO&CaSO4 and CaO&Al2(SO4)3 as Pectin Precipitants–Model of Overlapping Diffuse Layers

“…By comparing the binding efficiency of the CaSO 4 and CaCl 2 with pectins under the same experimental conditions, it can be seen that the Ca 2+ cations bounded with SO 4 2anions have no better affinity to complexation than Ca 2+ cations bounded with Clanions [16]. In the previous works [17,18], the amounts of Al 2 (SO 4 ) 3 and CaSO 4 , which were used to precipitate pectin, individually or in mixtures, were significantly lower compared with the amount of conventional coagulant CaO. Even better results were achieved using CaSO 4 in combination with cationic polyelectrolytes.…”

“…From the Stern layer to the diffuse layer, the charge density of cations gradually decreases, but that of anion increases. Within the diffuse layer there is a notional boundary in which the ions form a stable entity [17,18]. The potential at this boundary (shear plane) is easily measurable size and it is known as the Zeta potential (ζ).…”