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Partial Calcium Depletion during Membrane Filtration Impacts Gelation of Reconstituted Milk Protein Concentrates
Solubilization of colloidal calcium phosphate (CCP) from casein micelles during membrane filtration (e.g. through acidification) may affect the structural organization of these protein particles. The aim of this study was to investigate the effects of addition of glucono delta lactone (GDL) to skim milk during membrane filtration on the structural changes of the casein micelles, by studying their functionality after reconstitution of the final powders. In particular, the renneting behavior of the casein micelles was examined, as renneting is affected by both the calcium equilibrium and their supramolecular structure. Milk protein concentrate (MPC) powders were manufactured in duplicate, either by ultrafiltration (65% protein, MPC 65) or by ultrafiltration followed by diafiltration (80% protein, MPC80), using pasteurized skim milk, either at the native milk pH (~ pH 6.6), or after addition of GDL to pH 6.0, followed by spray drying. The amount of total calcium for the MPC80 without and with GDL varied with a significant difference (p<0.05) from 18449.5±265 to 15954.5±271 (μg/g), respectively. Samples were reconstituted at a 3.2% (w/w) protein to compare their gelation behavior between treatments. Both reconstituted MPC 65 and MPC 80 treated with GDL showed significantly increased amounts of soluble calcium (p<0.05) and non sedimentable caseins compared to their respective controls, as measured by ion chromatography and SDS-PAGE electrophoresis, respectively. The primary phase of rennet gelation was not significantly different (p<0.05) between treatments, as measured by the amount of caseino-macropeptide released, using reverse phase-high performance liquid chromatography (RP-HPLC). Rheological measurements were carried out using a controlled stress rheometer on the reconstituted samples immediately after addition of rennet, both before and after dialysis against skim milk, to ensure similar serum composition for all samples. While reconstituted samples before dialysis showed no gelation (defined as tan d = 1), only control MPC 65 and 80 showed gelation after serum re-equilibration. It was concluded that the gelation properties of reconstituted MPC powders were negatively affected by the presence of soluble casein, and positively affected by the amount both soluble and insoluble calcium present after reconstitution. This work, testing the renneting behavior of various reconstituted MPC samples, clearly demonstrated that decrease in pH to 6.0 during membrane filtration affects the structure of casein micelles with important consequences to their processing functionality.
Keywords:
Milk protein concentrate, Calcium depletion, Rennet Coagulation