Quantitative multivalent binding model of the structure, size distribution and composition of the casein micelles of cow milk

Non-Covalent Interaction of Folic Acid and 5-Methyltetrahydrofolate with Caseinates Improves the Folates Stability Studied by Multi-Spectroscopic Analysis and Molecular Docking

Folates, a crucial B-group vitamin, serve as a significant functional food supplement. Nevertheless, considerable obstacles persist in improving folates stability in liquid products. In this study, folic acid (FA) and 5-methyltetrahydrofolate (MTFA), two approved sources of folates, were encapsulated with sodium caseinate (NaCas) to enhance their stability. The protective effect of NaCas on folate molecules was investigated using experimental and computational methods. Meanwhile, the influence of divalent calcium ion (Ca2+) on the properties of the NaCas-MTFA complex was examined to evaluate the potential application of calcium 5-methyltetrahydrofolate (CaMTFA). Fluorescence tests showed both folates had static quenching behavior and bound to NaCas with a binding constant of 104-105 M-1. Hydrophobic interactions were crucial in NaCas-FA complex formation, while hydrogen bonding drove NaCas-MTFA binding. The encapsulation of caseinate notably slowed down the degradation of folates under both light and dark conditions. Moreover, the addition of a low concentration of Ca2+ did not adversely impact the binding mechanism of the NaCas-MTFA complex or the degradation curve of MTFA. The results of this study could serve as a valuable resource for the utilization of caseinates in incorporating folates, specifically MTFA, in the creation of natural liquid dietary supplements.

Invited review: Modeling milk stability

Novel insights into the stability of milk and milk products during storage and processing result from describing caseins near neutral pH as hydrophilic, intrinsically disordered, proteins. Casein solubility is strongly influenced by pH and multivalent ion binding. Solubility is high at a neutral pH or above, but decreases as the casein net charge approaches zero, allowing a condensed casein phase or gel to form, then increases at lower pH. Of particular importance for casein micelle stability near neutral pH is the proportion of free caseins in the micelle (i.e., caseins not bound directly to nanoclusters of calcium phosphate). Free caseins are more soluble and better able to act as molecular chaperones (to prevent casein and whey protein aggregation) than bound caseins. Some free caseins are highly phosphorylated and can also act as mineral chaperones to inhibit the growth of calcium phosphate phases and prevent mineralized deposits from forming on membranes or heat exchangers. Thus, casein micelle stability is reduced when free caseins bind to amyloid fibrils, destabilized whey proteins or calcium phosphate. The multivalent-binding model of the casein micelle quantitatively describes these and other factors affecting the stability of milk and milk protein products during manufacture and storage.

Structure of biomimetic casein micelles: Critical tests of the hydrophobic colloid and multivalent-binding models using recombinant deuterated and phosphorylated β-casein

Milk contains high concentrations of amyloidogenic casein proteins and is supersaturated with respect to crystalline calcium phosphates such as apatite. Nevertheless, the mammary gland normally remains unmineralized and free of amyloid. Unlike κ-casein, β- and αS-caseins are highly effective mineral chaperones that prevent ectopic and pathological calcification of the mammary gland. Milk invariably contains a mixture of two to five different caseins that act on each other as molecular chaperones. Instead of forming amyloid fibrils, several thousand caseins and hundreds of nanoclusters of amorphous calcium phosphate combine to form fuzzy complexes called casein micelles. To understand the biological functions of the casein micelle its structure needs to be understood better than at present. The location in micelles of the highly amyloidogenic κ-casein is disputed. In traditional hydrophobic colloid models, it, alone, forms a stabilizing surface coat that also determines the average size of the micelles. In the recent multivalent-binding model, κ-casein is present throughout the micelle, in intimate contact with the other caseins. To discriminate between these models, a range of biomimetic micelles was prepared using a fixed concentration of the mineral chaperone β-casein and nanoclusters of calcium phosphate, with variable concentrations of κ-casein. A biomimetic micelle was also prepared using a highly deuterated and in vivo phosphorylated recombinant β-casein with calcium phosphate and unlabelled κ-casein. Neutron and X-ray scattering experiments revealed that κ-casein is distributed throughout the micelle, in quantitative agreement with the multivalent-binding model but contrary to the hydrophobic colloid models.

Effects of protein genetic variants on their phosphorylation levels, milk composition, milk proteome, and milk coagulation ability in Chinese Holstein bovine milk

Milk samples were collected from 3625 Chinese Holstein cows to assess the effects of κ-casein (κ-CN) and β-lactoglobulin (β-LG) genetic variants on its milk coagulation properties. The results show that Chinese Holstein cows have a higher frequency of the κ-CN AA and AB variants, and β-LG of the AB and AA variants. Of these, κ-CN B variants, the β-LG AA and BB variants were more frequent in milk showing good coagulation. The effects of the genetic variants on milk composition, milk proteome, and protein phosphorylation sites were studied. The results showed that higher concentrations of protein and dry matter were found in κ-CN BE variant. Moreover, large variations in milk proteome among different κ-CN and β-LG variants were observed. Highly phosphorylated for κ-CN, especially Ser97, was observed in cows with the κ-CN BE variant, but no effect of β-LG variants on phosphorylation site was found. Of the various factors examined, variation of κ-CN phosphorylation sites Ser97 may be the most important in affecting casein structure and milk coagulation ability. Some milk protein contents were found to be negative factors for milk coagulation. In summary, this study showed that κ-CN genetic variants contained different milk compositions and phosphorylation site Ser97 influenced milk coagulation.

Influence of calcium concentration on the re-assembly of sodium caseinate into casein micelles and on their renneting behavior

Inducing the spontaneous aggregation from casein molecules (i.e. αs1, αs2, β, and κ-casein) into re-assembled casein micelles (RCMs) through the addition of salts as an alternative to native casein micelles, has garnered increasing attention in recent years. In this investigation, re-assembled casein micelles were generated by adding varying amounts of calcium, phosphate, and citrate ions to a sodium caseinate dispersion. The formed micelles were further characterized in terms of particle size, optical density, and partitioning of calcium ions and caseins. Besides, their small-angle X-ray scattering (SAXS) profiles and renneting properties were evaluated. The observations revealed that the particle size and optical density of RCMs increased with the continuous addition of salts, while the micellar yield improved and could exceed 85 %. Moreover, the quantity of individual casein molecules that contributed to the creation of micelles was in concordance with their level of phosphorylation (i.e. αs2-casein > αs1-casein > β-casein > κ-casein). Mineral analysis results and SAXS scattering profiles confirmed that the added calcium ions acted as cross-linkers and participated in the construction of calcium phosphate nanoclusters. The renneting ability of RCMs was primarily dependent upon the colloidal calcium content per gram of micellar casein.

A mini-review about direct steam heating and its application in dairy and plant protein processing

The world's requirement for plant protein consumption is increasing. However, their application in different foods is limited due to their low techno-functionality. Heating is the most widely used method to improve the functionality of proteins. Compared to indirect tubular or plate heating methods, direct steam injection heating (DSIH) can heat the sample much faster, thus modifying the structure and functionality of protein differently. It is used in the sterilization of milk to minimize the heat-induced denaturation of whey proteins and the loss of volatiles. By contrast, its application in producing plant protein ingredients is seldom. This review summarizes recent research using DSIH to process dairy- and plant-based proteins and proposes future research perspectives. DSIH is a promising technique for producing functional protein ingredients. It is of particular interest to overcome the techno-functional hurdles of plant protein blends using DSIH to improve their behavior in different food matrices.

Fabrication, characterization, and potential applications of re-assembled casein micelles

Re-assembled casein micelles (rCMs), were formulated in the 1970s as a model system to understand native casein micelles (nCMs) in milk. These early works allowed an understanding of the critical factors involved in the formation of rCMs, such as minerals (citrate, phosphate, and calcium), casein type (αs-, β-, and κ-casein) and the extent of their phosphorylation. rCMs were also used to understand the effect of treatments such as ethanol, high hydrostatic pressure and heating on the stability and integrity of the micelles. More recently, the applications of rCMs have been investigated, these include their use as a nanocarrier of bioactive molecules and as electrode-bound substrates to monitor chymosin activity by electrochemistry, to cite a few. Moreover, the potential to use rCMs in both food and non-food applications remains to be fully exploited. The advantage of choosing rCMs over nCMs as an encapsulant and a lucrative food ingredient is due to their more efficient preparation and being free from impurities. In this review, we report on the formulation of rCMs, their physico-chemical properties and their behavior under different physico-chemical treatments, along with the applications and challenges of rCMs in food systems and their industrial production as a dairy ingredient.

Are casein micelles extracellular condensates formed by liquid-liquid phase separation?

Casein micelles are extracellular polydisperse assemblies of unstructured casein proteins. Caseins are the major component of milk. Within casein micelles, casein molecules are stabilised by binding to calcium phosphate nanoclusters and, by acting as molecular chaperones, through multivalent interactions. In light of such interactions, we discuss whether casein micelles can be considered as extracellular condensates formed by liquid-liquid phase separation. We analyse the sequence, structure and interactions of caseins in comparison to proteins forming intracellular condensates. Furthermore, we review the similarities between caseins and small heat-shock proteins whose chaperone activity is linked to phase separation of proteins. By bringing these observations together, we describe a regulatory mechanism for protein condensates, as exemplified by casein micelles.

α-casein micelles-membranes interaction: Flower-like lipid protein coaggregates formation

BACKGROUND

Environmental conditions regulate the association/aggregation states of proteins and their action in cellular compartments. Analysing protein behaviour in presence of lipid membranes is fundamental for the comprehension of many functional and dysfunctional processes. Here, we present an experimental study on the interaction between model membranes and α-casein. α-casein is the major component of milk proteins and it is recognised to play a key role in performing biological functions. The conformational properties of this protein and its capability to form supramolecular structures, like micelles or irreversible aggregates, are key effectors in functional and pathological effects.

METHODS

By means of quantitative fluorescence imaging and complementary spectroscopic methods, we were able to characterise α-casein association state and the course of events induced by pH changes, which regulate the interaction of this molecule with membranes.

RESULTS

The study of these complex dynamic events revealed that the initial conformation of the protein critically regulates the fate of α-casein, size and structure of the newly formed aggregates and their effect on membrane structures. Disassembly of micelles due to modification in electrostatic interactions results in increased membrane structure rigidity which accompanies the formation of protein lipid flower-like co-aggregates with protein molecules localised in the external part.

GENERAL SIGNIFICANCE

These results may contribute to the comprehension of how the initial state of a protein establishes the course of events that occur upon changes in the molecular environment. These events which may occur in cells may be essential to functional, pathological or therapeutical properties specifically associated to casein proteins.

Effect of Glycerol, Calcium and Transglutaminase Post-Treatment on the Properties of Regenerated Fibers from Rennet-Treated Casein Micelles

Regenerated fibers can be prepared from a cooled solution of renneted casein micelles in a wet spinning process. For better handling and stability of the fiber, plasticizers, network modifiers or cross-linkers are used in the production process. For that reason, fibers with different glycerol and calcium content are prepared in this study and subsequently treated with the enzyme transglutaminase before being characterized after air drying. In addition to the swelling behavior in NaOH, H2O, simulated milk ultrafiltrate buffer as well as HCl, the mechanical properties of the fibers are investigated, taking into account their microscopic fine structure. Transglutaminase-treated fibers show sigmoidal absorption curves for all solvents and reach higher equilibrium swelling percentages than untreated fibers. When the calcium content in the coagulation bath is increased from 50 mM to 100 mM, more stabilizing calcium bridges lead to a denser fiber structure that swells more slowly in all solvents considered. With increasing glycerol content, the flexibility of the fibers increases, as indicated by the decrease in elastic moduli, and a fine structure in the sub-µm range becomes visible. The fibers also demonstrate lower elastic moduli when post-treated with transglutaminase. Besides the higher casein content due to the transglutaminase treatment, this could also contribute to the higher equilibrium swelling percentages compared to the untreated fibers.