Binding of added strontium by milk proteins under native conditions was investigated using pectin of various degrees of esterification. The partition of strontium, as well as cesium and europium, in aqueous two-phase milk-pectin system, is discussed in terms of membraneless dialysis and described by Donnan equilibria, and compared with the distribution between cation exchanger and milk, artificial milk serum, or pectin solutions. When going from pectin with a degree of esterification D.E.=93.2 to 61.4, the distribution of strontium decreases for about 40% in favor of pectin phase and this was explained by a relatively lower degree of dissociation of free carboxyle groups of pectin. The low-molecular fraction of added strontium in milk was assessed from Dowex 50×8 sorption data to be 31%, and that of cesium and europium 58% and 40%, respectively. However, distribution ratio of strontium and europium in milk/pectin system is much higher than it would correspond to the ion exchanger adsorption data.
Authors:F. Macášek, P. Gerhart, A. Malovíková, and A. Čelková
Partition of native sodium, potassium, magnesium, calcium and radioisotopes of cesium, strontium and curopium was investigated in the aqueous liquid-liquid systems formed after agitation of fresh pasteurized skim bovine milk with 4% w/w water solution of pectin of various degree of esterification (60–93%). The partition of the ions in the membraneless dialysis was described by Donnan equilibria and ion-exchange in the macro- and microheterogenous systems and, within uncertainty of results, does not depend on the degree of esterification. Strong negative non-ideality of Sr and Eu in milk phase is attributed to binding with proteins, casein particles in particular, and complexation with low molecular ligands. Separation factor =D(Sr)/D(Ca) is 0.70±0.06, in favor of strontium concentration in pectin phase. While >96% of cesium is diffusible to pectin phase, only 43–56% (depending on physico-chemical model of dialysis) of strontium behaves in that way, and in original milk phase the percent of strontium cationic form may be as low as 13%.