‘New views’ on structure–function relationships in milk proteins

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.

Strong Uranium(VI) Binding onto Bovine Milk Proteins, Selected Protein Sequences, and Model Peptides

Hexavalent uranium is ubiquitous in the environment. In view of the chemical and radiochemical toxicity of uranium(VI), a good knowledge of its possible interactions in the environment is crucial. The aim of this work was to identify typical binding and sorption characteristics of uranium(VI) with both the pure bovine milk protein β-casein and diverse related protein mixtures (caseins, whey proteins). For comparison, selected model peptides representing the amino acid sequence 13-16 of β-casein and dephosphorylated β-casein were also studied. Complexation studies using potentiometric titration and time-resolved laser-induced fluorescence spectroscopy revealed that the phosphoryl-containing proteins form uranium(VI) complexes of higher stability than the structure-analog phosphoryl-free proteins. That is in agreement with the sorption experiments showing a significantly higher affinity of caseins toward uranium(VI) in comparison to whey proteins. On the other hand, the total sorption capacity of caseins is lower than that of whey proteins. The discussed binding behavior of milk proteins to uranium(VI) might open up interesting perspectives for sustainable techniques of uranium(VI) removal from aqueous solutions. This was further demonstrated by batch experiments on the removal of uranium(VI) from mineral water samples.

Effect of Xanthan Gum on the Rheological Behavior and Microstructure of Sodium Caseinate Acid Gels

The aim of this work was to study the effect of xanthan gum (XG) on the gelation process of bovine sodium caseinate (NaCAS) induced by acidification with glucono-δ-lactone (GDL) and on the mixed acid gel microstructure. Before GDL addition, segregative phase separation was observed in all the NaCAS-XG mixtures evaluated. The gelation process was analyzed by using a fractional factorial experimental design. The images of the microstructure of the mixed acid gels were obtained by conventional optical microscopy and the mean diameter of the interstices was determined. Both the elastic character and the microstructure of the gels depended on the concentrations of XG added. As XG concentration increased, the kinetics of the gelation process was modified and the degree of compactness and elasticity component of the gel network increased. The microstructure of gels depends on the balance among thermodynamic incompatibility, protein gelation and NaCAS-XG interactions.

Fluorescent carbon dots from milk by microwave cooking

Fluorescent carbon dots were synthesized from milk by microwave cooking and used for two-photon excitedin vitrocell imaging.

High pressure control of protein structure and functionality

Purpose

– This article aims to consider the use of high pressure processing in order to gain functional advantages through proteins structure control. High pressure processing has been used to produce high-quality food with extended shelf life and could also be used to modify foods functionality.

Design/methodology/approach

– The effect of high pressure on protein structure and functionality is looked at and comparisons are made with heat effect in places. β-lactoglobulin and whey proteins are mainly taken as examples.

Findings

– A controlled partial protein unfolding through mild high pressure processing could lead to a range of intermediate molecular structures. These are distinct from the native and completely unfolded structure and have been referred to as molten globules. The partly unfolded molecular states, hence, are postulated to have increased functionality and could be interesting for the food industry.

Originality/value

– The opportunity and challenges represented by these theoretical elements are discussed. In particular, the effect of protein concentration and aggregation is emphasised.

Interaction of curcumin with phosphocasein micelles processed or not by dynamic high-pressure

The binding of curcumin to native-like phosphocaseins (PC) dispersed in simulated milk ultrafiltrate at pH 6.6 was assessed by fluorescence spectrophotometry. Curcumin binds to native-like PC micelles with ∼1 binding site per casein molecule, and a binding constant of 0.6-5.6 × 10(4)M(-1). Dynamic high pressure (or ultra-high pressure homogenisation, UHPH) at 200 MPa did not affect the binding parameters of curcumin to processed PC. UHPH-processing of PC dispersions at 300 MPa was followed by a slight but significant (p=0.05) increase in the binding constant of curcumin to processed PC, which may result from the significant UHPH-induced dissociation of initial PC micelles into neo-micelles of smaller sizes, and from the corresponding 1.5-2-fold increase in micelle surface area. PC-curcumin complexes were resistant to pepsin but were degraded by pancreatin, providing the possibility of a spatiotemporally controlled release and protection of bound biomolecules. UHPH-processed PC did not induce TC7-cell damage or major inflammation as assessed by LDH release or IL-8 secretion, respectively, compared with native-like PC. PC micelles could provide a valuable submicron system to vectorise drugs and nutrients.

Dynamics of well-folded and natively disordered proteins in solution: a time-of-flight neutron scattering study

Casein proteins belong to the class of natively disordered proteins. The existence of disordered biologically active proteins questions the assumption that a well-folded structure is required for function. A hypothesis generally put forward is that the unstructured nature of these proteins results from the functional need of a higher flexibility. This interplay between structure and dynamics was investigated in a series of time-of-flight neutron scattering experiments, performed on casein proteins, as well as on three well-folded proteins with distinct secondary structures, namely, myoglobin (alpha), lysozyme (alpha/beta) and concanavalin A (beta). To illustrate the subtraction of the solvent contribution from the scattering spectra, we used the dynamic susceptibility spectra emphasizing the high frequency part of the spectrum, where the solvent dominates. The quality of the procedure is checked by comparing the corrected spectra to those of the dry and hydrated protein with negligible solvent contamination. Results of spectra analysis reveal differences in motional amplitudes of well-folded proteins, where beta-sheet structures appear to be more rigid than a cluster of alpha-helices. The disordered caseins display the largest conformational displacements. Moreover their global diffusion rates deviate from the expected dependence, suggesting further large-scale conformational motions.

Functional Improvement of Milk Whey Proteins Induced by High Hydrostatic Pressure

High pressure is emerging as a new processing technology that produces particular changes in the molecular structure of proteins and thus gives rise to new properties inaccessible via conventional methods of protein modification. This review deals with the main effects of high hydrostatic pressure on the physicochemical characteristics of milk whey proteins and how modifications in their structural properties contribute to functionality. In this paper the mechanism underlying pressure-induced changes in ss-lactoglobulin, a-lactabumin, and bovine serum albumin is explained, and related to functional properties such as gel-forming ability, emulsifying activity, or foaming capacity. The possibility of using high pressures to favor chemical reactions of proteins with other food components, such as carbohydrates, to produce novel molecules with new food uses is also considered.

Low-resolution 1H spin-spin relaxation of n-decane/water emulsions stabilized by beta-casein

A low-resolution 1H NMR relaxometry study on the dynamics of an n-decane/water emulsion stabilized by beta-casein is presented. Spin-spin (transverse) relaxation time constants (T2) were used to assess relative mobilities of emulsion components, by a selective deuteration procedure. Data analysis allowed the emulsion investigated to be described by a heterogeneous collection of dynamically distinct populations. A major population of n-decane molecules presented an average mobility that very nearly approached that of pure solvent, which is compatible with its occurrence in the emulsion continuous microphase. beta-Casein molecules displayed a prevalent population with significantly decreased mobility as compared to the free protein in solution, which is in accordance with the protein location at the oil/water interface. Also, a major H2O population with significantly lower average T2 as compared to the pure liquid was detected and has been assigned to interfacial water.

A Study of β-Casein Tertiary Structure by Intramolecular Crosslinking and Mass Spectrometry

The objective of this study was to obtain experimental evidence to extend the discussion on the 3-D structure of beta-casein (beta-CN). The approach involved the preparation of homobifunctional crosslinkers, bis(sulfosuccinimidyl) derivatives of dicarboxylic acids of several lengths, which specifically react with primary amines of lysinyl residues or the N-terminal in the protein. The intramolecular crosslinks formed were determined by enzymatic digestion and by matrix-assisted laser desorption and ionization time-of-flight mass spectrometry combined with comparison against the theoretical digestion patterns. This procedure allowed the measurement of distances between the crosslinked residues. Ten different masses arising from 8 different specific intramolecular crosslinks were identified. Of these, 5 crosslinks were in good agreement with a published model (Kumosinski et al., 1993). Two other crosslinks each connected 2 residues that are much closer together, according to the model, than the maximum length of the crosslink. However, one of the crosslinks apparently connected 2 residues that are predicted by the model to be 16.7 A farther apart than the crosslink's stretched length. This disparity might be explained by structural flexibility. The structure expressed by the model is probably one of several energetically favorable conformations of the beta-CN molecule, whose structure is best described as rheomorphic rather than either a fixed structure or a random coil.

Dependence of the Interfacial Behavior of β-Casein on Phosphoserine Residues

The role of the phosphoserine residues on the dynamical and structural properties of beta-casein was studied by molecular dynamics of the protein in water/lipid interfacial regions. The initial protein structure adopted in the modeling was that proposed for bovine beta-casein A2, where the five phosphoserine residues, originally present in its primary structure, were partially or totally substituted by serine residues. The simulations revealed a dependence of the interfacial behavior of beta-casein on the phosphorylation grade. When only partially dephosphorylated, the protein showed a similar behavior as that observed for the original beta-casein reported in previous work. During dynamics, the protein migrated from the aqueous environment towards the lipid medium, and remained attached to the interface separating both media. Quite different was the dynamics of the totally dephosphorylated beta-casein, that did not perceive the interface and immersed incessantly into lipid medium. The results suggest that the phosphoserine residues appear to be, in fact, intrinsically related to the mechanisms of beta-casein emulsion stabilization.