Calcium phosphate precipitation from whey permeate

Currently the New Zealand Dairy Industry manufactures a very successful milk calcium product known as ALAMINTM by the precipitation of calcium phosphates (CaP) from various heated whey permeate streams. The main scientific objective of the study described in this thesis was to seek a detailed insight into the fundamental chemistry of the calcium phosphate precipitation process from whey permeates. To achieve this scientific objective, the project involved the use of a combination of the theoretical, computer-based speciation calculations, experimental bench-scale calcium phosphate precipitation and application of a large suite of chemical, spectroscopic and microscopic instrumental techniques for analysing the as received whey permeates and commercial ALAMINTM samples, as well as products of bench-scale precipitation reactions from whey permeate and laboratory-prepared solutions modelling whey permeate. An extensive literature search of calcium phosphate precipitation was also carried out. Equilibrium Speciation Calculations which predicted precipitation and ion speciations as a function of pH in a complex ionic medium modelling various whey permeate media were used to investigate the influence of several components, i.e. lactate, citrate, sulphate,Mg2+ and Ca:P molar ratio on the extent of precipitation of calcium phosphate solids in the system. The ion speciation program also modelled the effect of seasonal variations in the whey permeate by varying the concentrations of certain components while keeping others constant. In general, the fundamental component influencing yield of calcium phosphate solids in the permeate was found to be the free Ca2+ ion, the concentration of which was sensitive to pH, citrate, lactate, and sulfate concentration as well as Ca:P molar ratio. To provide an experimental dimension to these ion speciation studies, a simulated lactic whey ultrafiltrate (SLWUF) buffer which mimicked lactic whey permeate was developed and its precipitation chemistry studied under pH-stat control at pH 6.8 and 25°C. In conjunction with the pH-stat studies, Ca ISE studies were also undertaken to provide additional scientific data on the systems. In general, the experimental work effectively confirmed most of the trends predicted by the ion speciation calculations with regard to the effects on free calcium ion by other components in the simulated buffer. The most important finding on the chemistry of calcium phosphate precipitation from the simulated lactic whey ultrafiltrate buffer was that the precipitation product proceeds via the well documented phase transformation process from an initially amorphous structurally illdefined solid through to the final microcrystalline non-stoichiometric hydroxyapatite. Bench-scale precipitation work which incorporated laser light scattering analysis, Ca ISE, FTIR, TEM and ICP-OES/AAS analysis executed on the actual whey permeates effectively proved that calcium phosphate precipitation from the (heated) whey permeate exhibited a similar phase transformation chemistry to that observed with the simulated lactic whey ultrafiltrate (SLWUF) buffer. Agitation rate data and laser light scattering of precipitation from lactic whey permeates coupled with a Log-Normal distribution analysis of the Malvern (Mastersizer) particle size data revealed an increase of aggregation with time and with increases in agitation rate. The Log-Normal analysis suggested that 2-3 distinct particle populations were present throughout the precipitation reaction. Other characterisation work on the commercial ALAMIN™ solids proved that it was a poorly crystalline insoluble non-stoichiometric hydroxyapatite material which co-deposits with a small amount of citrate and protein. This agrees with the precipitation work conducted on precipitates from SLWUF buffers which also show co-deposition with citrate. An additional finding was that lactic whey permeates exhibited less buffering capacity than sulfuric acid whey permeates due to the presence of higher concentrations of citrate in the latter. Concerns about inclusions of calcium sulfate solid phases in the commercial sulfuric acid whey-derived ALAMIN™ were found to be unnecessary due to the experimental confirmation that these phases were not present to any great extent in the final product, a fact backed up by theoretical speciation calculations and ICP-OES analysis. In general, this study illustrates the successful application of both theoretical speciation calculations and experimental work to make sense of the chemistry occurring in a naturally complex system which is potentially of great value to the New Zealand Dairy Industry