Abstract:
The minerals in milk constitute a small part (8-9 g L-₁) existing in dynamic equilibrium between the protein micellar and serum phase. However they play extremely significant roles in the stability and structure of the casein micelles. The objective of this research was to investigate the mineral equilibrium in milk samples with added calcium or EDTA, and to relate this equilibrium to the physico-chemical properties of these systems. The calcium concentration was adjusted by the addition of different amount of calcium chloride (CaCl₂) (up to ~45 mM) or ethylenediaminetetraacetic acid (EDTA) (up to 50 mM). All the samples were equilibrated at pH 6.66±0.01. The results showed that the addition of up to ~45 mM CaCl2 did not affect the casein micelle integrity. Upon adding up to 13.31 mM CaCl₂, ~75% added divalent calcium ions (Ca2+) formed micellar calcium phosphate with the inorganic phosphate (Pi) in the serum. This resulted in the decrease in the viscosity and particle size of the milk samples. SAXS experiments showed that the casein micelle fine structure (colloidal calcium phosphate and sub-micelle) was affected by the addition of CaCl₂. In addition, the aggregation temperature of the milk samples decreased form 85±1°C to 53±1°C when ~45 mM CaCl₂ was added. Conversely the addition of EDTA resulted in the release of Ca²+, Pi and caseins into the serum phase. When 5 mM EDTA was added to milk, ~15% of Ca²+, ~18% of Pi and ~5% of casein were removed from the micelles; further addition of EDTA (20 mM EDTA) resulted in the dissociation of ~80% of the colloidal Ca²+, ~90% of colloidal Pi and ~90% of casein from the micellar phase. These findings were also in agreement with SAXS measurements on milk samples with added EDTA, which showed the dissociation of the casein micelle and the dissolution of the CCP. The mineral analysis performed on these systems was compared to a modified theoretical model derived from first principles of the chemistry of the milk components and were found to be in very good agreement. Further, milk powders made by freeze-drying the milks with added CaCl₂ or EDTA showed similar properties to the original milks. It is believed that the findings of this thesis do not only contribute to the understanding of the mineral equilibrium and the micelles structure in milk, but can be also give an insight into dairy product formulations, such as calcium fortification of milks or the addition of emulsifying salts in processed cheese.