pH-Induced dissociation of bovine casein micelles. II. Mineral solubilization and its relation to casein release

Structure and gelation properties of casein micelles doped with curcumin under acidic conditions

In this study, the ability of micellar casein (MC) to interact with curcumin during acidification and to produce acid gel was investigated. Steady-state fluorescence spectroscopy of curcumin variation and fluorescence quenching of caseins upon binding with curcumin molecules were evidenced. Increasing the temperature from 20 to 35 °C enhanced MC-curcumin interactions as reflected by the increase in the binding constant from 0.6 ± 0.3 × 10(4) to 6.6 ± 0.6 × 10(4) M(-1). From changes in entropy, enthalpy and Gibbs free energy, hydrophobic interactions were proposed as major binding forces. Static fluorescence MC quenching was demonstrated for the MC-curcumin complex during acidification. From pH 7.4 to pH 5.0, the binding site numbers varied in the range from 1.25 ± 0.05 to 1.49 ± 0.05 and the binding constant kb varied from 3.9 ± 0.4 × 10(4) to 7.5 ± 0.7 × 10(4) M(-1). Small angle X-ray scattering profiles demonstrated that the MC internal structure was unchanged upon curcumin binding. The ζ-potential value of curcumin-doped MC indicated that curcumin did not modify the global charge of MC particles. Acid gelation studied by oscillation rheology and static multiple light scattering at 20 and 35 °C led to a similar behavior for native and curcumin-doped MC suspensions. For the first time, it was demonstrated that the colloidal and functional properties of MC were unchanged when doped with curcumin during acidification.

Milk Protein Polymer and Its Application in Environmentally Safe Adhesives

Milk proteins (caseins and whey proteins) are important protein sources for human nutrition; in addition, they possess important natural polymers. These protein molecules can be modified by physical, chemical, and/or enzymatic means. Casein is one of the oldest natural polymers, used for adhesives, dating back to thousands years ago. Research on milk-protein-based adhesives is still ongoing. This article deals with the chemistry and structure of milk protein polymers, and examples of uses in environmentally-safe adhesives. These are promising routes in the exploration of the broad application of milk proteins.

Revisiting the interpretation of casein micelle SAXS data

An in-depth, critical review of model-dependent fitting of small-angle X-ray scattering (SAXS) data of bovine skim milk has led us to develop a new mathematical model for interpreting these data. Calcium-edge resonant soft X-ray scattering data provides unequivocal evidence as to the shape and location of the scattering due to colloidal calcium phosphate, which is manifested as a correlation peak centred at q = 0.035 Å(-1). In SAXS data this feature is seldom seen, although most literature studies attribute another feature centred at q = 0.08-0.1 Å(-1) to CCP. This work shows that the major SAXS features are due to protein arrangements: the casein micelle itself; internal regions approximately 20 nm in size, separated by water channels; and protein structures which are inhomogeneous on a 1-3 nm length scale. The assignment of these features is consistent with their behaviour under various conditions, including hydration time after reconstitution, addition of EDTA (a Ca-chelating agent), addition of urea, and reduction of pH.

Rennet Coagulation and Cheesemaking Properties of Thermally Processed Milk: Overview and Recent Developments

Thermally induced changes in milk proteins and minerals, particularly interactions among caseins and denatured whey proteins, influence important properties of dairy products in both positive and negative ways. Whereas the extensive protein connectivity and increased water-holding capacity resulting from such heat-induced protein modification account for the much desired firmness of acid gels of yogurt, thermal processing, on the other hand, severely impairs clotting and adversely affects the cheesemaking properties of rennet-coagulated cheeses. In technological terms, the principal ongoing challenge in the cheese industry is to take advantage of the water-holding capacity of thermally aggregated whey proteins without compromising the rennetability of cheese milk or the textural and functional attributes of cheese. Including some recent data from the authors' laboratory, this paper will discuss important aspects and current literature on the use of thermally processed milk in the production of rennet-coagulated cheeses and also some of the potential alternatives available for inclusion of whey proteins in cheese, such as the addition of microparticulated whey proteins to cheese milk.

In vitro digestion of purified β-casein variants A(1), A(2), B, and I: effects on antioxidant and angiotensin-converting enzyme inhibitory capacity

Genetic polymorphisms of bovine milk proteins affect the protein profile of the milk and, hence, certain technological properties, such as casein (CN) number and cheese yield. However, reports show that such polymorphisms may also affect the health-related properties of milk. Therefore, to gain insight into their digestion pattern and bioactive potential, β-CN was purified from bovine milk originating from cows homozygous for the variants A(1), A(2), B, and I by a combination of cold storage, ultracentrifugation, and acid precipitation. The purity of the isolated β-CN was determined by HPLC, variants were verified by mass spectrometry, and molar extinction coefficients at λ=280nm were determined. β-Casein from each of the variants was subjected to in vitro digestion using pepsin and pancreatic enzymes. Antioxidant and angiotensin-converting enzyme (ACE) inhibitory capacities of the hydrolysates were assessed at 3 stages of digestion and related to that of the undigested samples. Neither molar extinction coefficients nor overall digestibility varied significantly between these 4 variants; however, clear differences in digestion pattern were indicated by gel electrophoresis. In particular, after 60min of pepsin followed by 5min of pancreatic enzyme digestion, one ≈4kDa peptide with the N-terminal sequence (106)H-K-E-M-P-F-P-K- was absent from β-CN variant B. This is likely a result of the (122)Ser to (122)Arg substitution in variant B introducing a novel trypsin cleavage site, leading to the changed digestion pattern. All investigated β-CN variants exhibited a significant increase in antioxidant capacity upon digestion, as measured by the Trolox-equivalent antioxidant capacity assay. After 60min of pepsin + 120min of pancreatic enzyme digestion, the accumulated increase in antioxidant capacity was ≈1.7-fold for the 4 β-CN variants. The ACE inhibitory capacity was also significantly increased by digestion, with the B variant reaching the highest inhibitory capacity at the end of digestion (60min of pepsin + 120min of pancreatic enzymes), possibly because of the observed alternative digestion pattern. These results demonstrate that genetic polymorphisms affect the digestion pattern and bioactivity of milk proteins. Moreover, their capacity for radical scavenging and ACE inhibition is affected by digestion.

Effect of calcium addition and pH on yield and texture of Minas cured cheese

Milk calcium concentration is a factor related to cheese texture, an important rheological property of cheese quality as perceived by consumers. This study aimed to evaluate the effect of different pH conditions (5.8 and 6.6) and calcium addition (0, 150, 300 ppm of CaCl2), on yield and nutrient retention of the clots obtained and on the texture of Minascured cheese. Clots were analyzed for wet and dry yield, percentage content and retention of protein, fat and calcium. The texture of the cheese was evaluated by instrumental and sensorial tests. No differences were observed on the wet and dry yields, or on the protein content, fat and calcium retention of clots produced in the different experimental conditions. The instrumental evaluation showed that calcium addition significantly influenced the texture of cheeses, regardless of the pH of milk clotting. The sensory panel did not find a difference in the hardness ofcheeses produced at the same pH of milk clotting in function of CaCl2 addition. There was no difference in the texture of Minascured cheese due to the calcium addition to milk for dairy product consumers, which brings a new perspective on manufacture for cheese markers.

The structure of the casein micelle of milk and its changes during processing

The majority of the protein in cow's milk is contained in the particles known as casein micelles. This review describes the main structural features of these particles and the different models that have been used to define the interior structures. The reactions of the micelles during processing operations are described in terms of the structural models.

Molecular water motions of skim milk powder solutions during acidification studied by 17O and 1H nuclear magnetic resonance and rheology

The molecular motion of water was studied in glucono-δ-lactone-acidified skim milk powder (SMP) solutions with various pH values and dry matter contents. NMR relaxometry measurements revealed that lowering the pH in SMP solutions affected 17O and 1H T2 relaxation rates almost identically. Consequently, the present study indicates that the proteins present in the samples do not affect the 1H relaxation behavior markedly, even at relatively high SMP concentrations (15-25%). Comparison of rheological measurements and NMR measurements suggested that the collapse of κ-casein during acidification could contribute to the initial decrease in 17O and 1H relaxation rate in the pH range between 6.6 and 5.5 for 15% SMP and in the pH range between 6.6 and 5.9 for 25% SMP. However, below pH 5.5 the viscosity and 17O and 1H NMR relaxation rates did not correlate, revealing that the aggregation of casein micelles, which increases viscosity below pH 5.5, does not involve major repartitioning of water.

Rennet-induced aggregation of heated pH-adjusted skim milk

Heated (20-100 °C/0-30 min) skim milks (pH 6.5-7.1) were diluted in buffer (pH 7.0). Rennet was added, and the particle size with time was measured. For all samples, the size initially decreased (lag phase) and then increased (aggregation phase). Milks heated at ≤60 °C had short lag phases and rapid aggregation phases regardless of pH. Milks heated at >60 °C at pH 6.5 had long lag phases and slow aggregation phases. As the pH increased, the lag phase shortened and the aggregation phase accelerated. The aggregation time was correlated with the level of whey protein associated with the casein micelles and with the level of κ-casein dissociated from the micelles. Heated milks formed weak gels when renneted. It is proposed that the milks heated at low pH have whey proteins associated with the casein micelles and that these denatured whey proteins stabilize the micelles to aggregation by rennet and therefore inhibit gelation. In the milks heated at higher pH, the whey proteins associate with κ-casein in the serum and, on rennet treatment, the κ-casein-depleted micelles and the serum-phase whey protein/κ-casein complexes aggregate; however, the denatured whey proteins stabilize the aggregates so that gelation is still inhibited.

The pH induced sol-gel transition in skim milk revisited. A detailed study using time-resolved light and x-ray scattering experiments

We present a detailed study of the evolution of the size, structure and stability of casein micelles upon acidification of skim milk typically applied in yogurt-making processes using a combination of time-resolved light and small-angle X-ray scattering experiments. While most of the available light scattering studies on casein acidification have been restricted to transparent and therefore highly diluted samples, we now profit from a newly developed multiangle 3D light scattering instrument, which allows for time-resolved measurements in highly turbid samples. Our experiments clearly demonstrate the presence of two parallel pH-dependent processes, micellar reassembly and aggregation. Using a systematic investigation of the effect of casein concentration, acidification rate, and ionic strength, we are able to decouple these two processes and obtain detailed information about the pH-induced restructuration of the casein micelle structure that occurs prior to destabilization. Moreover, our experiments also unambiguously demonstrate that these micellar reassembly processes are highly concentration dependent, and that typical light scattering studies conducted under highly diluted conditions are resulting in findings that may not be relevant for the situation encountered in industrial processes at higher concentrations. Experiments conducted with covalently cross-linked micelles, where the pH-induced reassembly has been suppressed, further confirm our findings.

Structural changes of milk components during acid-induced coagulation kinetics as studied by synchronous fluorescence and mid-infrared spectroscopy

Dynamic oscillatory experiments and front-face synchronous fluorescence spectroscopy and mid-infrared (mid-IR) spectroscopy have been used to investigate structure evolution, at the macroscopic and molecular levels, during milk acidification kinetics. The studies were performed using skim milk, at two different temperatures (30 °C and 40 °C), to which was added glucono-δ-lactone (GDL) to generate different structural changes in casein micelles and gels. Synchronous fluorescence spectra were recorded in the 250-500 nm excitation wavelength range using an offset of 80 nm between the excitation and emission monochromators for each system during the 300 min acidification kinetics. The change in the fluorescence intensity at 281 nm reflects the pH-induced physicochemical changes of casein micelles and, in particular, structural changes in the micelles in the pH range 5.5-5.0. Regarding mid-infrared spectroscopy, the region located between 1700 and 1500 cm(-1), corresponding to the amide I and II bands, and the 1500-900 cm(-1) region, called the fingerprint region, were considered for the characterization of milk coagulation kinetics. Changes in the absorbance at 1063 cm(-1) as a function of pH for kinetics recorded at 30 °C and 40 °C reflected pH-induced phosphate dissolution in the pH range 5.5-5.0. Compared to rheometry, which reveals microstructure changes only in the gel state, spectroscopic methods make it possible to monitor molecular structure changes in micelles throughout the acidification processes.

Use of surface plasmon resonance (SPR) to study the dissociation and polysaccharide binding of casein micelles and caseins

Tests were made to determine whether surface plasmon resonance (SPR) could be used as a technique to study the dissociation properties of bovine casein micelles or of sodium caseinate and the interactions between these protein particles and different polysaccharides. Surfaces of bound micelles or caseinate were made, and the changes in refractive index of these layers were used to define changes in the structures of the chemisorbed material. The technique appears to have some potential for studying details of the dissociation of casein micelles and of the binding of different polysaccharides to caseins.

Theoretical model for calculating ionic equilibria in milk as a function of pH: comparison to experiment

Acidification is fundamental for the processing of milk into cheeses, caseins, and fermented dairy products. It is established that a pH decrease changes the ionic equilibria of milk, inducing the solubilization of micellar calcium phosphate. This study aimed to present a theoretical model calculating ionic equilibria in milk as a function of pH. From the pH and total concentrations of minerals and caseins, the model calculated the concentrations of all ionic species and their partition between micellar and aqueous phases of milk. As the pH decreased, the minerals present in the micellar phase were gradually displaced into the aqueous phase. The calculated concentrations of minerals were in a good agreement with the experimental ones determined from acidified milk. A very satisfactory accuracy of the calculations, estimated by a root-mean-square error (RMSE) value of 5% for Ca and P(i) and a slope of the plot close to a unit, was obtained. The model is proposed for the simulation of ionic equilibria and the partition of salts between the aqueous and micellar phases of milk and dairy formulations during acidification.

The influence of milk composition on pH and calcium activity measured in situ during heat treatment of reconstituted skim milk

The pH and calcium activity of reconstituted skim milk solutions (9–21% w/w milk solids non-fat) on heating and after cooling were studied as a function of milk pH prior to heating (pH 6·2–7·2 at 25°C) and added calcium complexing agents (phosphate or EDTA). The pH decreased as the temperature was raised from 25 to 90°C and the magnitude of the pH decrease was greater with increase in initial pH at 25°C before heating or milk concentration. The pH decrease on heating from 25 to 90°C in skim milk solutions with added calcium complexing agents was lower than that of milk without the addition of these salts. The calcium activity decreased on heating from 25 to 60°C. The magnitude of the change decreased with increase in initial pH at 25°C before heating and milk concentration. The decrease in calcium activity on heating from 25 to 60°C for skim milk solutions with added calcium complexing agents was lower than that of milk solutions without the addition of calcium complexing agents. The changes in pH and calcium activity on heating milk were largely reversible after cooling the milk. The results suggested that the pH and calcium activity at high temperatures are a function of the milk composition. Knowledge of the initial pH prior to heating alone is not sufficient for predicting the changes that occur during heating.

Effect of gel firmness at cutting time, pH, and temperature on rennet coagulation and syneresis: an in situ 1H NMR relaxation study

The objective of this study was to monitor rennet-induced milk gel formation and mechanically induced gel syneresis in situ by low-field NMR. pH, temperature, and gel firmness at cutting time were varied in a factorial design. The new curve-fitting method Doubleslicing revealed that during coagulation two proton populations with distinct transverse relaxation times (T2,1=181, T2,2=465 ms) were present in fractions (f1=98.9%, f2=1.1%). Mechanical cutting of the gel in the NMR tube induced macrosyneresis, which led to the appearance of an additional proton population (T2,3=1500-2200 ms) identified as whey. On the basis of NMR quantification of whey water the syneresis rate was calculated and found to be significantly dependent on pH and temperature.