Compositional changes of casein micelles induced by calcium or chelatant addition at threefold and natural casein concentration

Gastric aggregation of micellar casein powders induced by high hydrostatic pressure: Effect of serum Ca2+ level

Micellar casein (MC) has a unique gastric colloidal behavior in response to Ca2+ cross-linking, and its aggregation properties are closely related to pepsin and gastric acid. In this study, MC with different levels of colloidal calcium phosphate (CCP) was obtained by high hydrostatic pressure (HHP) at different pressures, followed by spray drying to obtain the powders. Different amounts of calcium chloride (exogenous Ca2+) were added to MC powders prior to in vitro simulated digestion to investigate the effect of exogenous serum Ca2+ levels on the aggregation behavior and the structure change of curds generated in gastric tract. The results revealed that HHP induced the emergence of more Ca2+-binding sites, thus Ca2+ was more likely to bind to MC matrix with low CCP levels. Meanwhile, high serum Ca2+ level provided more opportunities to form aggregates. The Highest pressure (500 MPa) with the highest Ca2+ level (5 mM) caused the lowest solubility aggregates, which were only 30% at the end of gastric digestion (120 min), half of the control sample (0 MPa with 0.15 mM Ca2+). The results of wide-angle X-ray scattering / small-angle X-ray scattering suggested that both pepsin and gastric acid-induced aggregation via Ca2+ as a bridge. For pepsin, Ca2+ cross-linked between para-κ-casein; For gastric acid, Ca2+ recombined phosphorylation sites and caused cross-linking of casein subunits.

Engineering artificial casein micelles for future food: Is casein phosphorylation necessary?

Industrial-scale production of recombinant proteins for food products may become economically feasible but correct post-translational modification of proteins by microbial expression systems remains a challenge. For efficient production of hybrid products from bovine casein and recombinant casein, it is therefore of interest to evaluate the necessity of casein post-translational phosphorylation for the preparation of hybrid casein micelles and study their rennet-induced coagulation. Our results show that dephosphorylated casein was hardly incorporated into artificial casein micelles but was capable of stabilising calcium phosphate nanoclusters with an increased size through adsorption on their surface. Thereby, dephosphorylated casein formed larger colloidal particles with a decreased hydration. Furthermore, the presence of increasing amounts of dephosphorylated casein resulted in increasingly poor rennet coagulation behaviour, where dephosphorylated casein disrupted the formation of a coherent gel network by native casein. These results emphasise that post-translational phosphorylation of casein is crucial for their assembly into micelles and thereby for the production of dairy products for which the casein micelle structure is a prerequisite, such as many cheese varieties and yoghurt. Therefore, phosphorylation of future recombinant casein is essential to allow its use in the production of animal-free dairy products.

Gastrointestinal digestibility of micellar casein dispersions: Effects of caprine vs bovine origin, and partial colloidal calcium depletion using in vitro digestion models for the adults and elderly

In vitro coagulation and digestion of caprine and bovine micellar casein concentrate (MCC) with or without partial colloidal calcium depletion (deCa) were studied under simulated adult and elderly conditions. Gastric clots were smaller and looser for caprine than bovine MCC, and were further looser with deCa and under elderly condition for both caprine and bovine MCC. Casein hydrolysis and concomitant formation of large peptides was faster for caprine than bovine MCC, and with deCa and under adult condition for caprine and bovine MCC. Formation of free amino groups and small peptides were faster for caprine MCC, and with deCa and under adult condition. Upon intestinal digestion, proteolysis occurred rapidly, and was faster under adult condition, but showed less differences with increasing digestion between caprine and bovine MCC, and with and without deCa. These results suggested weakened coagulation and greater digestibility for caprine MCC and MCC with deCa under both conditions.

Cold ultrafiltered or microfiltered milk retentates: A systematic comparison of the effects of compositional differences on their gelation functionality

The colloidal stability of casein micelles suspensions prepared using ultrafiltration (UF) and microfiltration (MF) was studied by testing acid- and rennet-induced destabilization. Skim milk and 4× (based on volume reduction) concentrates were obtained by processing under similar conditions, at temperatures below 10°C. Concentrates were subjected to different levels of diafiltration (DF), resulting in samples with comparable casein volume fractions but different amounts of proteins and ions in the serum phase. The novelty of the work is the systematic comparison of MF and UF concentrates of similar history. More specifically, concentrates similar in ionic composition but with or without serum proteins were compared, to evaluate whether whey proteins and β-casein depletion from the micelles will play a role in the processing properties, or whether these are affected solely by the ionic balance. Microfiltered micelles' apparent diameter decreased by about 50 nm during the specific hydrolysis of κ-casein by chymosin, whereas those in skim milk control showed a decrease of about half that size. All concentrates subjected to extensive DF showed smaller hydrodynamic diameters, with reductions of ∼18 and 13 nm for MF and UF, respectively. Highly diafiltered UF retentates showed a delayed onset of rennet-induced gelation, due to low colloidal calcium, compared with other samples. Low-diafiltered samples showed weak storage modulus (∼1 Pa) after 60 min of onset of gelation. In addition, onset pH increased with diafiltration to ∼5.8 for UF and ∼6 for MF in high-diafiltered samples. These results clearly demonstrated that the functional properties of casein micelles change during membrane concentration, and this cannot be solely attributed to changes in ionic equilibrium.