Influence of calcium chelators on concentrated micellar casein solutions : from micellar structure to viscosity and heat stability

In practice it is challenging to prepare a concentrated medical product with high heat stability and low viscosity. Calcium chelators are often added to dairy products to improve heat stability, but this may increase viscosity through interactions with the casein proteins. The aim of this thesis was to obtain a better understanding of the influence of different calcium chelators on the physico-chemical properties of casein micelles and the resulting effect on viscosity and heat stability of concentrated micellar casein isolate (MCI) solutions. The calcium chelators disodium uridine monophosphate (Na2UMP), disodium hydrogen phosphate (Na2HPO4), trisodium citrate (TSC), sodium phytate (SP), and sodium hexametaphosphate (SHMP) were studied. Initially, the calcium-binding capacity of the phosphates was investigated and found to be directly related to the amount of charges. The resulting effects on physical changes of casein micelles were subsequently explored before and during heating. The viscosity of the MCI solutions increased upon addition of the calcium chelators, which was attributed to swelling of the caseins at decreasing calcium-ion activity. The calcium chelators induced different changes in turbidity of the MCI solutions, which could be related to the degree of dissociation of the casein micelles. Simulations of the ion equilibria indicated that the extent of casein micelle dissociation followed the calcium-binding capacity of the calcium chelators. Micelle dissociation occurred in the order of SHMP > SP > TSC > Na2HPO4 > Na2UMP. The results on heat stability indicated that the calcium-ion activity and state of the micellar structure before and during heating determined the heat stability of the MCI solutions. Na2UMP was the most effective heat stabilizer, as it bound sufficient free calcium ions to reduce protein aggregation without affecting the micellar structure. SHMP was the least effective heat stabilizer because of heat-induced changes occurring during heating. For polyphosphates, SHMP and SP, it was found that they decreased the isoint of casein by forming direct bindings with the caseins, for which calcium ions were not required. In conclusion, this thesis has provided new insights in the relationships between calcium chelators and their influence on the casein micelle structure and on the physico-chemical properties of concentrated MCI solutions. Also, the practical relevance for the dairy industry was described, demonstrating how different calcium chelators can manipulate the viscosity and heat stability of dairy products. </p

Calcium-binding capacity of organic and inorganic ortho- and polyphosphates

The aim of this research was to determine the calcium-binding capacity of inorganic and organic ortho- and polyphosphates. This calcium-binding capacity can be used to influence the stability of, for example, casein micelles in dairy systems. Four phosphates were selected: disodium uridine monophosphate (Na2UMP, organic orthophosphate), disodium hydrogen phosphate (Na2HPO4, inorganic orthophosphate), sodium phytate (SP, organic polyphosphate), and sodium hexametaphosphate (SHMP, inorganic polyphosphate). Concentrations of up to 100 mmolL-1 phosphate were added to a 50 mmolL-1 CaCl2 solution. The samples were prepared at pH 8.0 and were analyzed before and after sterilization for calcium-ion activity, conductivity, pH, sediment, and turbidity. Both SHMP and SP are strong chelators, as calcium ions bind to these phosphates in the ratio of 3:1 and 6:1, respectively. Calcium ions also strongly bind to Na2HPO4, but in a ratio of 3:2 with insoluble Ca3(PO4)2 complexes as result. The equilibrium position of Na2UMP is not strong towards the chelated complex, and significant levels of free calcium and free phosphate can exist. An equilibrium constant of Lmol-1 was determined for calcium uridine monophosphate (CaUMP) complexes. Both calculation of the equilibrium constant and analysis on the CaUMP precipitate confirmed a reactivity of 1:1 between calcium and Na2UMP. The CaUMP complexes are well soluble at ambient temperature, and insoluble complexes appear after sterilization, because the solubility of CaUMP decreases during heating. Finally, we concluded that the structure of phosphate molecules determines their calcium-binding capacity rather than organic or inorganic origin of phosphate

Effect of calcium chelators on physical changes in casein micelles in concentrated micellar casein solutions

The effect of calcium chelators on physical changes of casein micelles in concentrated micellar casein solutions was investigated by measuring calcium-ion activity, viscosity and turbidity, and performing ultracentrifugation. The highest viscosities were measured on addition of sodium hexametaphosphate (SHMP), because it cross-linked the caseins. For the weak calcium chelator disodium uridine monophosphate (Na2UMP), physical changes in the solutions were negligible. Disodium hydrogen phosphate (Na2HPO4), trisodium citrate (TSC), and sodium phytate (SP) caused similar increases in viscosity, but had different effects on turbidity. The increase in viscosity was attributed to swelling of the casein micelles (i.e., increased voluminosity) at decreasing calcium-ion activity. The major decrease in turbidity was due to dissociation of the casein micelles. The extent of micellar dissociation was dependent on the type and concentration of calcium chelator. It seems that the micelles were dissociated in the order of SHMP = SP > TSC > Na2HPO4 > Na2UM

Effect of calcium chelators on heat coagulation and heat-induced changes of concentrated micellar casein solutions: The role of calcium-ion activity and micellar integrity

There is general consensus that calcium chelators enhance heat stability in milk. However, they increase the heat stability to considerably different extents. For this reason, the effect of various calcium chelators on heat coagulation and heat-induced changes of concentrated micellar casein solutions was investigated by measuring the heat coagulation time (HCT), together with changes in calcium-ion activity, viscosity, turbidity, and zeta potential before and after heating. Surprisingly, the weakest chelator, disodium uridine monophosphate (Na2UMP), gave the most pronounced increase in HCT. Stronger chelators, i.e., disodium hydrogen phosphate, trisodium citrate and sodium phytate, gave a lower HCT than Na2UMP. Sodium hexametaphosphate (SHMP) was the least effective heat stabilizer. Heat-induced changes in SHMP were the major cause for this reduced heat stability effect. Differences in HCT caused by the addition of the various calcium chelators could be attributed to the calcium-ion activity and state of the micellar structure before and during heating

Effect of calcium chelators on physical changes in casein micelles in concentrated micellar casein solutions

The effect of calcium chelators on physical changes of casein micelles in concentrated micellar casein solutions was investigated by measuring calcium-ion activity, viscosity and turbidity, and performing ultracentrifugation. The highest viscosities were measured on addition of sodium hexametaphosphate (SHMP), because it cross-linked the caseins. For the weak calcium chelator disodium uridine monophosphate (Na2UMP), physical changes in the solutions were negligible. Disodium hydrogen phosphate (Na2HPO4), trisodium citrate (TSC), and sodium phytate (SP) caused similar increases in viscosity, but had different effects on turbidity. The increase in viscosity was attributed to swelling of the casein micelles (i.e., increased voluminosity) at decreasing calcium-ion activity. The major decrease in turbidity was due to dissociation of the casein micelles. The extent of micellar dissociation was dependent on the type and concentration of calcium chelator. It seems that the micelles were dissociated in the order of SHMP = SP > TSC > Na2HPO4 > Na2UM