TY - JOUR

T1 - Hydrates of calcium citrate and their interconversion in relation to calcium bioaccessibility

AU - Liu, Xiao-Chen

AU - Kirkensgaard, Jacob J. K.

AU - Skibsted, Leif H.

PY - 2021

Y1 - 2021

N2 - Calcium citrate tetrahydrate (CCT) and hexahydrate (CCH) precipitates from aqueous solutions of CaCl2 and sodium citrate above and below the transition temperature of 52 °C, respectively. The CCT, the dihydrate (CCD) and anhydrate (CCA) as obtained by a stepwise dehydration of solid CCH have enthalpy of dehydration of ΔH0CCH to CCT = 43.6, ΔH0CCT to CCD = 43.8, and ΔH0CCD to CCA = 88.1 kJ∙mol−1 as measured by DSC. WAXS measurements demonstrate a stepwise decrease in unit cell size upon dehydration, and a stronger binding of the two first water compared to additional. The increasing negative enthalpy of dissolution, as calculated from the temperature dependence of solubility (10–90 °C), +21 kJ∙mol−1 (CCH), –20 kJ∙mol−1 (CCT), –22 kJ∙mol−1 (CCD), and –40 kJ∙mol−1 (CCA) shows along the series of hydrates with increasing solubility, enthalpy–entropy compensation with an isoequilibrium temperature of 49 °C. Conversion of CCD and CCA in aqueous solutions yields the more soluble CCT, not the stable CCH in agreement with Ostwald's stage law, increasing calcium bioaccessibility under physiological conditions in intestines.

AB - Calcium citrate tetrahydrate (CCT) and hexahydrate (CCH) precipitates from aqueous solutions of CaCl2 and sodium citrate above and below the transition temperature of 52 °C, respectively. The CCT, the dihydrate (CCD) and anhydrate (CCA) as obtained by a stepwise dehydration of solid CCH have enthalpy of dehydration of ΔH0CCH to CCT = 43.6, ΔH0CCT to CCD = 43.8, and ΔH0CCD to CCA = 88.1 kJ∙mol−1 as measured by DSC. WAXS measurements demonstrate a stepwise decrease in unit cell size upon dehydration, and a stronger binding of the two first water compared to additional. The increasing negative enthalpy of dissolution, as calculated from the temperature dependence of solubility (10–90 °C), +21 kJ∙mol−1 (CCH), –20 kJ∙mol−1 (CCT), –22 kJ∙mol−1 (CCD), and –40 kJ∙mol−1 (CCA) shows along the series of hydrates with increasing solubility, enthalpy–entropy compensation with an isoequilibrium temperature of 49 °C. Conversion of CCD and CCA in aqueous solutions yields the more soluble CCT, not the stable CCH in agreement with Ostwald's stage law, increasing calcium bioaccessibility under physiological conditions in intestines.

KW - Calcium citrate

KW - Dehydration

KW - Dissolution

KW - DSC

KW - DVS

KW - Entropy-enthalpy compensation

KW - Ostwald's stage law

KW - WAXS

U2 - 10.1016/j.foodres.2020.109867

DO - 10.1016/j.foodres.2020.109867

M3 - Journal article

C2 - 33648185

AN - SCOPUS:85095981045

VL - 140

JO - Food Research International

JF - Food Research International

SN - 0963-9969

M1 - 109867

ER -