Aqueous Solubility of Calcium l-Lactate, Calcium d-Gluconate, and Calcium d-Lactobionate: Importance of Complex Formation for Solubility Increase by Hydroxycarboxylate Mixtures

Among the calcium hydroxycarboxylates important for cheese quality, d-lactobionate [Ksp = (7.0 ± 0.3) × 10–3 mol3 L–3] and l-lactate [Ksp = (5.8 ± 0.2) × 10–3 mol3 L–3] were found more soluble than d-gluconate [Ksp = (7.1 ± 0.2) × 10–4 mol3 L–3], as indicated by the solubility products determined electrochemically for aqueous 1.0 M NaCl at 25.0 °C. Still, solubility of calcium l-lactate increases by 45% in the presence of 0.50 M sodium d-gluconate and by 37% in the presence of 0.50 M sodium d-lactobionate, while solubility of calcium d-gluconate increases by 66 and 85% in the presence of 0.50 M sodium l-lactate and 0.50 M sodium d-lactobionate, respectively, as determined by complexometric titration. Sodium l-lactate and sodium d-gluconate have only little influence on solubility of calcium d-lactobionate. The increased solubility is described quantitatively by calcium binding to d-gluconate (K1 = 14 ± 3 mol–1 L) in 1.0 M NaCl at 25 °C, d-lactobionate (K1 = 11 ± 2 mol–1 L), and l-lactate (K1 = 8 ± 2 mol–1...