Simultaneous determination of free calcium, magnesium, sodium and potassium ion concentrations in simulated milk ultrafiltrate and reconstituted skim milk using the Donnan Membrane Technique

Modelling ion composition in simulated milk ultrafiltrate (SMUF). I: Influence of calcium phosphate precipitation

“…Na + and K + , were present mainly in the free form. The calculated (DIS model) concentrations of free Ca 2+ , Mg 2+ , Na + and K + ions matched well with reported experimental results (Gao et al, 2009), which were 1.58, 0.53, 17.0 and 38.5 mmol/kg water for the concentration of free Ca 2+ , Mg 2+ , Na + and K + ion, respectively. However, the calculated concentrations of each ion species, such as CaCitrate À , and NaCitrate 2À , were slightly different from literature data (Holt et al, 1981).…”

“…The measured Ca 2+ activity was lower than the calculated result. This was probably because the calcium electrode was influenced by matrix effect (Gao et al, 2009). Instead, the Ca 2+ activity measured by Donnan Membrane Technique was 6.45E-04 (Gao et al, 2009), which was similar to the calculated value, indicating that the DIS model is indeed able to predict the ion composition at t = 0.…”

“…The equilibration time was approximately 8 or 9 days. Free Ca 2+ and Mg 2+ ion concentrations were determined by Donnan Membrane Technique (DMT) and the experimental procedure was described earlier by Gao et al (2009). For the DMT experiment, solid CaCl 2 was added to SMUF to accelerate calcium phosphate precipitation.…”

“…The theory of the Donnan Membrane Technique was described extensively by Gao et al (2009) and Temminghoff, Plette, Van Eck, and Van Riemsdijk (2000). Briefly, the sample solution, called the donor solution is separated from an acceptor solution containing strong electrolytes by a semi-permeable and highly negativelycharged cation exchange membrane.…”

Modelling ion composition in simulated milk ultrafiltrate (SMUF) II. Influence of pH, ionic strength and polyphosphates

“…Free Ca 2+ activity was determined using a calcium ion selective electrode (Orion 97-20, Thermo, Beverly, USA). Free Ca 2+ and Mg 2+ ion concentrations were determined by Donnan Membrane Technique (DMT) as described by Gao et al (2009). The DMT experiment was carried out in triplicate, and the calcium electrode measurement was repeated six times.…”