Supersaturation of calcium citrate as a mechanism behind enhanced availability of calcium phosphates by presence of citrate

Abstract

Dissolution of amorphous calcium phosphate (ACP) in aqueous citrate at varying pH has been studied with perspective of increasing availability of calcium from sidestreams of whey protein, lactose and/or cheese production or on development of new functional foods. ACP formed as an initial precipitate in 0.10 mol L equimolar aqueous calcium chloride, sodium citrate, and sodium hydrogenphosphate was used as model for mineral residues formed during milk processing. Upon acidification of the ACP suspension by hydrochloric acid decreasing pH from 6.5 to 4.5, the transformations of ACP occurred through an 8 h period of supersaturation prior to a slow precipitation of calcium citrate tetrahydrate. This robust supersaturation, which may explain increased availability of calcium phosphates in presence of citrate, presented a degree of supersaturation of 7.1 and was characterized by precipitation rates for 0.10 mol L equimolar aqueous calcium chloride, sodium hydrogencitrate, and sodium hydrogenphosphate with pH 5.5, and for 0.10 mol L equimolar aqueous calcium chloride, sodium hydrogencitrate, and sodium dihydrogenphosphate with pH 4.1, with a degree of supersaturation of 2.7. The crystallization processes were similar according to Avrami's model with a half-life for precipitation of approximately 5 h independent of the degree of supersaturation. Ion speciation based on measurement of pH, and total concentrations of calcium, phosphate and citrate, and of conductivity and calcium ion activity during precipitation indicates a low driving force for precipitation with calcium citrate complex dominating at pH 5.5 and calcium hydrogencitrate complex dominating at pH 4.1. Calcium hydrogencitrate is suggested to be the species involved in the crystal growth followed by solid state transformation to calcium citrate tetrahydrate.

Lime Juice Enhances Calcium Bioaccessibility from Yogurt Snacks Formulated with Whey Minerals and Proteins

“…Lime juice reduced the concentration of free calcium both in the initial and final digestion. Calcium binding to organic molecules like hydroxycarboxylates prevents calcium precipitation, but it also reduces spontaneous diffusion caused the decrease of free calcium content [35]. During intestinal digestion, free calcium decreased in all cases, but most in the presence of WMC and less significantly in the presence of WPI.…”

“…When the pH was adjusted to 7.0, the turbidity of solutions without lime, i.e., WMC, WMC/WPI and WMC/WPH increased by a factor of 400 as seen in Figure 6d. However, pH had little effect on turbidity of solutions with lime juice even after 48 h. Garcia, Vavrusova and Skibsted [35] also found the precipitation was not immediate when conditions favored the formation of calcium citrate. WMC as calcium phosphate has low solubility under neutral condition, but can be solubilized by addition of citric acid and citrate from lime juice leading to supersaturation of calcium citrate.…”

“…The effect of citric acid on the solubility of calcium salts in dairy products appears to be important for the transformation of calcium forms during digestion. This transformation between amorphous calcium phosphate and precipitated calcium citrate is related to the feature of calcium citrate as an improver of calcium absorption, since the long lag phase of calcium citrate precipitation leads to adequate absorption of calcium in the intestine [35]. Y (0.035 ± 0.001 mmol/g DM) also had higher final soluble calcium than Y/I (0.023 ± 0.000 mmol/g DM) and Y/H (0.030± 0.001 mmol/g DM).…”

Biguanide-Based Synthesis of 1,3,5-Triazine Derivatives with Anticancer Activity and 1,3,5-Triazine Incorporated Calcium Citrate Nanoparticles

“…Unfortunately, the obtained product was large precipitate particles of 2a as confirmed by SEM and FTIR, and the desired CaCittriazine NPs was not observed, since there was no IR signal characteristic to calcium citrate, i.e., 3400 cm −1 O-H stretching or peaks in the carbonyl group stretching region ( Figure 3c). This was due to the basicity of 2a and sodium citrate [32]. In alkali condition, calcium ions have more affinity towards hydroxide than citrate ions, thus allowing the possibility to form calcium hydroxide instead of calcium citrate precipitate [33].…”

“…The pH of citric acid solution was adjusted to pH 6.0 using 1:2 mole ratio of citric acid to NaOH resulting in citrate ions in the divalent form [35]. Both the selected pH value and the divalent form of citrate ions have been reported to allow the formation of calcium citrate [32]. The FTIR spectrum of product obtained from using citric acid with NaOH exhibited characteristic peaks of both calcium citrate (3452, 1543, 1429 and 1081 cm −1 ) and 2a (2920, 2851, 1570, 817, 804 and 720 cm −1 ) as depicted in Figure 3d.…”