A study on the mechanisms of calcium-induced gelation in skim milk : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Albany, New Zealand

Thumbnail Image
Open Access Location
Journal Title
Journal ISSN
Volume Title
Massey University
The Author
The destabilisation and aggregation of milk proteins is the first step towards the gelation of milk. The addition of calcium to milk is known to destabilise milk proteins and may result in gelation on heating. However, the mechanisms involved in gelation induced by heating calcium-added milk was not well understood. Therefore, this project aimed to determine the fundamental mechanisms involved in the development of a calciuminduced skim milk gel. Skim milk was selected as the model system and gelation was induced in-situ by heating the calcium-added skim milk at the rheometer. The changes in the storage modulus, G′, were monitored to study the development of the gel network. This project examined the impact of the following factors on the rheological properties of a calciuminduced skim milk gel: the type of soluble calcium salt added (calcium chloride, calcium lactate, calcium gluconate, calcium lactobionate and calcium iodide), pH, holding temperature during gelation (70°C to 90°C), preheat treatment, ionic strength (by addition of sodium chloride) and the contribution of casein and whey proteins. A higher calcium ion activity (aCa 2+), which indicated a higher calcium ion (Ca2+) concentration, and a lower pH favoured the formation of a stronger gel. An increase in ionic strength by addition of sodium chloride decreased the final G′ of the calciuminduced skim milk gel due to reduced calcium bridging and increased hydration repulsion. A higher heating temperature also resulted in gels with higher final G′ due to more frequent particle collisions. Casein micelles and whey proteins were both responsible for the structure of the gel network. The contribution of whey proteins towards the gel network was dependent on if they were denatured prior to heating, on the concentration of calcium ions available, and on the ratio between the casein and whey proteins present. At lower added calcium concentrations (10 mmol L-1) where the available calcium ions were limited, interactions and aggregation amongst denatured whey proteins via hydrophobic and disulphide bonds may have resulted in the formation of a stronger gel. However, at higher added calcium concentrations (20 and 40 mmol L-1), where sufficient calcium ions may be available for binding, interactions between casein and calcium dominated over the self-aggregating effect of denatured whey proteins. In conclusion, the results demonstrated that the final gel properties of a calcium-induced skim milk gel were dependent on the net effect of all the factors involved in the stability and interactions of the milk proteins, including the calcium salt concentration, pH, preheat treatment, ionic strength, and the protein composition in solution. These findings provide alternative methods for texture modification in milk.
Figures re-used wth permission.
Gelation, Skim milk, Calcium