Modifying the physicochemical properties, solubility and foaming capacity of milk proteins by ultrasound-assisted alkaline pH-shifting treatment

This study investigated the effects of different treatment of alkaline pH-shifting on milk protein concentrate (MPC), micellar casein concentrate (MCC) and whey protein isolate (WPI) assisted by the same ultrasound conditions, including changes in the physicochemical properties, solubility and foaming capacity. The solubility of milk proteins had a significant increase with gradual enhancement of ultrasound-assisted alkaline pH-shifting (p < 0.05), especially for MCC up to 99.50 %. Also, treatment made a significant decline in the particle size of MPC and MCC, as well as the turbidity of the proteins (p < 0.05). The foaming capacity of MPC, MCC, and WPI was all improved, especially at pH 11, and at this pH, the milk protein also showed the highest surface hydrophobicity. The best foaming capacity at pH 11 was the result of the combined effect of particle size, potential, protein conformation, solubility, and surface hydrophobicity. In conclusion, ultrasound-assisted pH-shifting treatment was found to be effective in improving the physicochemical properties and solubility and foaming capacity of milk proteins, especially MCC, with promising application prospect in food industry.

Protein microparticles visualize the contact network and rigidity onset in the gelation of model proteins

Protein aggregation into gel networks is of immense importance in diverse areas from food science to medical research; however, it remains a grand challenge as the underlying molecular interactions are complex, difficult to access experimentally, and to model computationally. Early stages of gelation often involve protein aggregation into protein clusters that later on aggregate into a gel network. Recently synthesized protein microparticles allow direct control of these early stages of aggregation, decoupling them from the subsequent gelation stages. Here, by following the gelation of protein microparticles directly at the particle scale, we elucidate in detail the emergence of a percolating structure and the onset of rigidity as measured by microrheology. We find that the largest particle cluster, correlation length, and degree of polymerization all diverge with power laws, while the particles bind irreversibly indicating a nonequilibrium percolation process, in agreement with recent results on weakly attractive colloids. Concomitantly, the elastic modulus increases in a power-law fashion as determined by microrheology. These results give a consistent microscopic picture of the emergence of rigidity in a nonequilibrium percolation process that likely underlies the gelation in many more systems such as proteins, and other strongly interacting structures originating from (bio)molecules.

Gluconic acid as a chelator to improve clarity of skim milk powder dispersions at pH 3.0

Clear acidic protein beverages have a niche market. Acidification of skim milk powder (SMP) dispersions to pH 3.0 using citric acid (CA) lowers turbidity but the dispersion remains translucent. The present study aimed at comparing physicochemical properties of 5% w/v SMP dispersions acidified to pH 3.0 using chelating gluconic acid (GA) and CA and non-chelating hydrochloric acid. GA was the most effective in reducing the dispersion turbidity to 394 NTU at pH 3.0, which was further reduced to 248 NTU after heating at 90 °C for 2 min resulting in transparent dispersions. The better chelating ability of GA than CA was supported by the higher extent of dissolved CCP in serum phase. The aggregation of dissociated caseins was not observed for the GA treatment based on transmission electron microscopy. The findings from this study may be used to produce clear casein-based protein beverages.

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.

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.

High pressure homogenization to improve the stability of casein - hydroxypropyl cellulose aqueous systems

The effect of high pressure homogenization on the improvement of the stability hydroxypropyl cellulose (HPC) and micellar casein was investigated. HPC with two molecular weights (80 and 1150 kDa) and micellar casein were mixed in water to a concentration leading to phase separation (0.45% w/v HPC and 3% w/v casein) and immediately subjected to high pressure homogenization ranging from 0 to 300 MPa, in 100 MPa increments. The various dispersions were evaluated for stability, particle size, turbidity, protein content, and viscosity over a period of two weeks and Scanning Transmission Electron Microscopy (STEM) at the end of the storage period. The stability of casein-HPC complexes was enhanced with the increasing homogenization pressure, especially for the complex containing high molecular weight HPC. The apparent particle size of complexes was reduced from ~200nm to ~130nm when using 300 MPa, corresponding to the sharp decrease of absorbance when compared to the non-homogenized controls. High pressure homogenization reduced the viscosity of HPC-casein complexes regardless of the molecular weight of HPC and STEM imagines revealed aggregates consistent with nano-scale protein polysaccharide interactions.

Ultrasound effects on the assembly of casein micelles in reconstituted skim milk

Reconstituted skim milks (10 % w/w total solids, pH 6·7-8·0) were ultrasonicated (20, 400 or 1600 kHz at a specific energy input of 286 kJ/kg) at a bulk milk temperature of <30 °C. Application of ultrasound to milk at different pH altered the assembly of the casein micelle in milk, with greater effects at higher pH and lower frequency. Low frequency ultrasound caused greater disruption of casein micelles causing release of protein from the micellar to the serum phase than high frequency. The released protein re-associated to form aggregates of smaller size but with surface charge similar to the casein micelles in the original milk. Ultrasound may be used as a physical intervention to alter the size of the micelles and the partitioning of caseins between the micellar and serum phases in milk. The altered protein equilibria induced by ultrasound treatment may have potential for the development of milk with novel functionality.

Postprandial blood cell transcriptomics in response to the ingestion of dairy products by healthy individuals

The aim of this intervention study was to measure genome-wide postprandial gene expression in human blood cells after the ingestion of a single serving of milk, to identify the downstream physiological processes regulated by the differentially expressed genes, and to use this gene expression signature as a reference to compare it with the response following the ingestion of a transformed dairy product, namely, yogurt. We conducted a randomized, controlled, single-blinded, crossover study on six healthy male individuals. After an overnight fast, 540 g of milk or yogurt was ingested by the subjects. Blood samples were collected before (0 h) and after (2 h/4 h/6 h) ingestion, and the blood cell transcriptome was analyzed using a linear kinetic analysis that increases the statistical power of the study. The differentially expressed transcripts identified after the ingestion of milk (575 transcripts) and yogurt (625 transcripts) modulated similar biological processes. In particular, genes involved in protein biosynthesis and mitochondrial activities followed biphasic kinetics being down-regulated at 2 h and more pronouncedly up-regulated at 6 h. The opposite kinetics were observed for inflammatory and apoptotic processes during the same time frame. The human blood cell transcriptome appeared to be specifically modulated by specific nutrients present in bovine milk, a property that was further modified when milk was fermented to yogurt. The coordinated changes in postprandial expression of genes involved in basic biological processes suggest that postprandial blood cell transcriptomics may allow insight into the nutritional effects of selected foods in the prevention or development of chronic metabolic and inflammatory disorders.

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.

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.

Formation of acid-heat-induced skim milk gels in the pH range 5·0–5·7: effect of the addition of salts and calcium chelating agents

The effects of pH and added salts or chelating agents on the gel strength and dynamic rheological properties of acid–heat-induced gels made from reconstituted skim milk (200 g solids/l) were investigated. Gel strength increased as pH was lowered between 5·75 and 5·15 except in the range pH 5·45–5·25 where a local maximum in gel strength was obtained at pH 5·35. Gel characteristics were affected by addition of salts or chelating agents but each of their effects was different, depending on the final pH of the milk gel. The addition of CaCl2 or chelating agents (Na2HPO4, disodium citrate or the disodium salt of EDTA) which affected micellar calcium phosphate, non-sedimentable casein and Ca2+ activity in different ways all resulted in decreased gel strength at pH 5·5. The addition of CaCl2 or MgCl2 caused a decrease in tanδ (ratio of the viscous modulus G″ to the elastic modulus G′) whereas disodium citrate or the disodium salt of EDTA addition caused an increase and Na2HPO4 addition did not cause a change. The addition of NaCl (up to 50 mM), which causes an increase in ionic strength but has no effect on non-sedimentable casein and Ca2+ activity, decreased gel strength but did not change tanδ. The addition of a range of other salts (KCl, NH4Cl, NaSCN, NaNO3 or Na2SO4) also decreased gel strength at pH 5·5.

Multinuclear NMR study of the pH dependent water state in skim milk and caseinate solutions

The effects of decreasing pH and micellar calcium concentrations of reconstituted skim milk and caseinate solution were studied by 1H and 17O NMR spectroscopy. The proton transverse relaxation rate 1/T2 of skim milk decreased as the pH decreased, reaching a minimum at pH 5·3. However, as the pH fell sodium caseinate solution showed a continuous increase in 1/T2, with no minimum. Analysis of proton relaxation as a function of the interpulse time in the CPMG (Carr-Purcell-Meiboom-Gill) sequence demonstrated that both the proton exchange mechanism and ‘bound’ water contributed to proton relaxation in skim milk. The study of 17O relaxation rate as a function of pH confirmed the change in protein hydration upon acidification. Increasing the amount of EDTA showed that the proton transverse relaxation rate of skim milk decreased until a plateau was reached when the micellar calcium was totally solubilized. With excess EDTA the relaxation rates of skim milk and caseinate solution were identical. A strong correlation was also found between the pH dependent relaxation rate and the solubilization of micellar phosphorus as detected by 31P NMR. Together, these results suggested that aggregation of caseins by calcium and colloidal calcium phosphate is mainly responsible for the excess hydration in skim milk micelles.

Electroacoustic determination of size and zeta potential of casein micelles in skim milk

Measurements of the zeta potential and particle size of casein micelles in skim milk suspensions at natural and lower pH have been made using the technique of electroacoustics. This technique requires no dilution or change of environment of the casein micelles. The zeta potential obtained at natural pH for a commercial skim milk suspension was −18 mV; it became less negative with decreasing pH. The median particle size observed at natural pH for a commercial skim milk suspension was 0·2 εm, in good agreement with previously reported values. The particle size increased as the pH was decreased.

Sonodisruption of re-assembled casein micelles at different pH values

Casein solutions with different pH values were sonicated at a frequency of 35kHz and increasing acoustic powers. As the sonication power increased, turbidity of solutions and particle diameter decreased at any given pH value, suggesting particles disruption due to the ultrasonic treatment. The magnitude of decrease in re-assembled micelles diameter was greater at a higher pH, indicating an interaction between pH and sonication power in sonodissociation. This interaction is attributed to a looser structure of micelles at higher pH values which increases the efficiency of ultrasonic disruption and not directly to the increased cavitation efficiency. We argue that increased cavitation efficiency with increasing sonication power, which enhances shear forces is the most likely reason for sonodisruption of re-assembled casein micelles.

Monitoring the chemical and textural changes during ripening of Iranian White cheese made with different concentrations of starter

The effect of the concentration of starter inoculated to milk on the composition, free tyrosine-tryptophan content, microstructure, opacity, and fracture stress of Iranian White cheese (IWC) was studied during 50 d of ripening in brine. Three treatments of cheese were made using 1-fold (IWC1S), 2-fold (IWC2S), and 4-fold (IWC4S) concentrations of a direct-to-vat mesophilic mixed culture containing Lactococcus lactis ssp. cremoris and Lactococcus lactis ssp. lactis as starter. As ripening progressed, moisture and protein contents of the treatments continuously decreased, whereas their total ash, salt, and salt in moisture contents increased. Fat content and pH of cheeses remained stable during ripening. The pH of cheese milk at the time of renneting, which decreased by increasing the concentration of starter (6.57, 6.49, and 6.29 for IWC1S, IWC2S, and IWC4S, respectively), significantly affected most of the chemical characteristics and opacity of cheese. Lower pH values at renneting decreased moisture and ash contents, whereas cheese protein content increased. The concentration of free tyrosine-tryptophan in curd increased at first 29 d but decreased between d 29 and 49 of aging. The changes observed in cheese whiteness followed the changes in moisture content of the treatments. As the concentration of starter inoculated to milk increased, the value of fracture stress at a given ripening time significantly decreased, leading to a less resistant body against applied stress. A similar trend was also observed for fracture strain during cheese ripening. The micrographs taken by scanning electron microscopy provided a meaningful explanation for decrease in the value of fracture stress. As the cheese ripening progressed or the concentration of starter increased, the surface area occupied by the protein fraction in cheese microstructure decreased, leading the way to lower the force-bearing component in cheese texture.

Texture of Low-Fat Iranian White Cheese as Influenced by Gum Tragacanth as a Fat Replacer

The effect of different concentrations of gum tragacanth on the textural characteristics of low-fat Iranian White cheese was studied during ripening. A batch of full-fat and 5 batches of low-fat Iranian White cheeses with different gum tragacanth concentrations (without gum or with 0.25, 0.5, 0.75, or 1 g of gum/kg of milk) were produced to study the effects of fat content reduction and gum concentration on the textural and functional properties of the product during ripening. Cheese samples were analyzed with respect to chemical, color, and sensory characteristics, rheological parameters (uniaxial compression and small-amplitude oscillatory shear), and microstructure. Reducing the fat content had an adverse effect on cheese yield, sensory characteristics, and the texture of Iranian White cheese, and it increased the instrumental hardness parameters (i.e., fracture stress, elastic modulus, storage modulus, and complex modulus). However, increasing the gum tragacanth concentration reduced the values of instrumental hardness parameters and increased the whiteness of cheese. Although when the gum concentration was increased, the low-fat cheese somewhat resembled its full-fat counterpart, the interaction of the gum concentration with ripening time caused visible undesirable effects on cheese characteristics by the sixth week of ripening. Cheeses with a high gum tragacanth concentration became very soft and their solid texture declined somewhat.

Influence of Calcium Salt Supplementation on Calcium Equilibrium in Skim Milk During pH Cycle

Calcium is a mineral essential for humans, especially for bone constitution. Yet most of the worldwide population does not satisfy their Ca needs. Hence, Ca supplementation is of major importance, even in western countries where some specific populations at risk do not satisfy the recommended daily intake of Ca. More than 70% of dietary Ca comes from dairy products. Calcium supplementation of naturally Ca-rich sources such as skim milk is then of special interest. To our knowledge, few data are available concerning milk Ca (MC) supplementation of milk, particularly when followed by pH cycle. In this paper, MC supplementation is studied and compared with Ca chloride (CC) supplementation as a well-known source of Ca. The effect of Ca salt supplementation followed by pH cycle was studied in reconstituted skim milk. Calcium supplementation was carried out with CC and MC at 25 mmol of Ca/kg of skim milk. Ionized Ca concentration and turbidity variations were followed in situ by Ca ion selective electrode and turbidimetry using light reflection. From normalized data on ionized Ca concentration and turbidity vs. pH, it appeared that hysteresis areas were smaller for CC-supplemented milk, whereas unsupplemented milk and MC-supplemented milk behaved similarly. For these 3 dairy systems, pH cycles to pH 5.0 led to a larger hysteresis area than pH cycles to pH 5.5. The shrinkage of the hysteresis area could be interpreted as a reinforcement of casein micelles with Ca ions over the pH cycle.

Micellar casein gelation at high sucrose content

This article investigates the effect of sucrose addition on the formation of casein gels by acidification and/or renneting of pure micellar casein. Gelation kinetics and gel properties were followed by rheological methods, and microscopy and syneresis measurements were used to obtain a more complete characterization of the structures formed. Sucrose content has been identified as a key parameter for controlling the kinetics of aggregation and the strength of the final gels. Results have shown that the effect of sucrose on gelation can vary such that effects can be completely reversed depending on the gelation route used. During acid gelation, addition of up to 30% (wt/wt) sucrose causes gels to form more rapidly and at higher pH values, and to have higher viscoelastic moduli and a more homogeneous microstructure than those without sucrose. By contrast, gels formed by renneting in the presence of sucrose are weaker and have longer gelation times. It is proposed that sucrose reduces solvent quality and causes the collapse of the "hairy" kappa-casein brush on the surface of the casein micelles. This may explain why sucrose increases the possibility of gel formation during acidification and reduces the degree of kappa-casein hydrolysis during renneting.

Growth kinetics and fractal dimensions of casein particles during acidification

Small angle static light scattering was used to study the effect of milk dilution in permeate on the mechanism of acid-induced aggregation of casein particles. Growth kinetics of casein aggregates during acidification was characterized by the succession of four populations of particles. The first one corresponded to casein particles ranging from 0.1 to 1 microm, with a mean value of 0.3 microm. The second population, from 1 to 10 microm, was quickly replaced by a third population, from 10 to 100 microm, which gave rise to the last population measurable, from 100 to 1000 microm. The angular dependence of static light scattering from about 0.01 to 50 degrees was used to determine the fractal dimension (D) of pH-induced casein aggregates. With the formation of about 10-microm aggregates, fractal structures appeared. The D values, determined from double logarithmic plots of intensity versus scatteringvector resulted in values between 1.85 and 2.03.