Dairy powder rehydration: influence of protein state, incorporation mode, and agglomeration

A simplified method to study rehydration was used on different dairy powders. The method involved dispersing powder in a stirred vessel equipped with a turbidity sensor. The changes of turbidity occurring during powder rehydration highlighted the rehydration stage, and the influence of the proteins’ state on rehydration was clarified. Casein powders had a quick wetting time but very slow dispersion, making the total rehydration process time-consuming. On the other hand, whey powders were found to have poor wettability but demonstrated immediate dispersion after wetting. Mixing casein (80%) and whey (20%) before spray drying greatly improved rehydration time compared with casein powder; whereas mixing whey powder with casein powder at the same ratio after spray drying caused a dramatic deterioration in the rehydration properties. Moreover, agglomeration was found to significantly improve the rehydration time of whey protein powder and to slow down the rehydration time of casein powder. These opposite effects were related to the rate-controlling stage (i.e., wetting stage for whey protein and dispersion stage for casein).     

Use of a turbidity sensor to characterize micellar casein powder rehydration: influence of some technological effects

A simplified method to study rehydration of dairy powders was developed for native phosphocaseinate powder. The method involved dispersing powder in a stirred vessel equipped with a turbidity sensor under standardized conditions. The changes of turbidity occurring during powder rehydration highlighted several stages. These stages include particles wetting, and then swelling as the water penetrates into the powder bed, followed by a slow dispersion of the particles. With this tool, some technological effects on powder rehydration were analyzed. Ultrafiltrate incorporation to the casein concentrate before spray drying was found to greatly improve the rehydration, whereas mixing ultrafiltrate powder with casein powder after spray drying did not change the rehydration properties. The effect of granulation on powder rehydration stages was also investigated.     

Rehydration of casein powders: effects of added mineral salts and salt addition methods on water transfer

Enrichment of milk with micellar casein decreases water transfer during the rehydration of milk powders. In this study, the effects of the ion environment and the ion addition method on the rehydration kinetics were found to be dependent on the changes in the micellar casein. For example, adding citrate or phosphate solution to the micellar casein suspension before drying considerably increased rehydration rates and this was related to the destruction of the micelle structure. Water transfer in the casein suspension was improved by adding NaCl during rehydration: this effect may be explained by the more hygroscopic nature of NaCl rather than by extensive modification of the micellar structure. The addition of CaCl₂ considerably affected micelle organization and led to the formation of insoluble structures during spray drying.     

Effect of spray freeze drying on the structural modification and rehydration characteristics of micellar casein powders

Micellar casein (MC) is usually spray-dried into powder form for transportation and storage. However, the micellar structure maintained by colloidal calcium phosphate (CCP) and hydrophobic forces leads to poor rehydration ability of MC powders, which limits its potential applications. Here, spray freeze drying (SFD) with controlled droplet size was used to produce MC powders. Their effects on the structure of MC and the subsequent rehydration characteristics including wetting, dispersion and dissolution were investigated. The results showed SFD powders obtained from smaller droplet size caused more than 50% of serum Ca²⁺ and PO₄³⁻ to release from the micellar structure. These powder particles exhibited extremely high porosity (92%) and spherical morphology, which thus greatly shortened their wetting time. Furthermore, the smallest droplets during SFD were believed to produce the MC powders with the quickest dispersion and best solubility, as over 80% of the solids could be completely dissolved in just 15 min.     

The prediction of moisture sorption isotherms for dairy powders

Moisture sorption isotherms were measured for whey protein isolate, high micellar casein and a milk protein concentrate powder. No temperature dependence was observed over the temperature range of 4–37 °C. At 50 °C the powders absorbed less moisture than observed at the lower temperatures. These isotherms were used to predict the isotherms for freeze-dried amorphous lactose/casein/whey protein powders. An isotherm for micellar casein was predicted using a simple additive isotherm model and was used along with isotherms for whey protein and amorphous lactose to predict moisture sorption isotherms for commercial dairy powders. Predicted isotherms compared well with measured isotherms indicating that this simple additive isotherm model is suitable for predicting moisture sorption isotherms of dairy powders. Delayed lactose crystallisation was observed in lactose/whey protein powders when compared to lactose/casein powders over the same water activity range.     

Milk proteins differentiation and competitive adsorption during spray-drying

A non invasive method was developed to differentiate native whey proteins (NWI) and native micellar casein (NMC) at the surface of high protein milk powder. Surface analyses of the powders were performed by X-ray Photoelectron Spectroscopy (XPS). With this tool, it was impossible to differentiate casein and whey proteins by their C_1s, O_1s and N_1s signature; the atomic percentage being similar. But, minerals at the surface of these proteins were significantly different. As a consequence, a calibration curve was obtained with known mixes of both proteins and was used for their differentiation.     

Size of native and heated casein micelles,content of protein and minerals in milk from Norwegian Red Cattle—effect of milk protein polymorphism and different feeding regimes

Milk samples of 59 cows of the Norwegian Red Cattle breed receiving three different supplementary concentrates, were analysed for genotypes of caseins and whey proteins, the content of different milk salts (Ca²⁺, Ca, Mg and citrate), the content of total protein, casein and whey protein and the mean micellar size of native and heated casein micelles. The genotype of α_s1-casein had a statistically significant effect on the content of protein and casein, and the content of whey protein and the casein number were significantly influenced by different feeding regimes, and the content of citrate. The mean size of native and heated casein micelles was significantly influenced by the feeding regimes, genotype of α_s1-casein (native mean size only) and κ-casein, pH and the content of casein, whey protein and casein number. The heat-induced changes in mean micellar size were significantly affected by the calcium ion activity which accounted for approximately 40% of the total variation.     

Deciphering the segregation of proteins in high-protein dairy powders after spray-drying

High-protein dairy powders are ingredients mainly produced by spray-drying, then subjected to aging during transport and storage. They often undergo physicochemical changes at this stage, such as the development of the Maillard reaction, primarily because of their intrinsic chemical properties, but also as a result of nonoptimal storage conditions. Components present at the particle surface are the first to be targeted by moisture and other environmental disruptions. Consequently, the identification, control, and prediction of particle surface components are useful to anticipate the effect of powder aging on product quality. Here, a new diafiltration method is proposed which fractionates proteins from a binary colloidal dispersion of 80% casein micelles and 20% whey proteins, according to their presence at the surface or core of the particle. This method shows that whey proteins are strongly enriched at the particle surface, whereas casein micelles are located at the core of the particles. This protocol also allows the identification of the rehydration kinetics for each rehydrated protein layer of the particle, revealing that 2 distinct forms of swelling occur: (1) a rapid swelling and elution of whey proteins present at the particle surface, and (2) a swelling of casein micelles located below the whey proteins, associated with a slow elution of casein micelles from the particles being rehydrated.     

Isolation of caseins from whey proteins by microfiltration modifying the mineral balance in skim milk

The objective of this work was to study the effect of different salts and salt concentration on the isolation of casein micelles from bovine raw skim milk by tangential flow microfiltration. Tangential flow microfiltration (0.22 μm) was conducted in a continuous process adding a modified buffer to maintain a constant initial sample volume. This buffer contained calcium chloride (CaCl₂), sodium phosphate (Na₂HPO₄), or potassium citrate (K₃C₆H₅O₇) in concentrations ranging from 0 to 100 mM. The concentrations of caseins and whey proteins retained were determined by sodium dodecyl sulfate-PAGE and analyzed using the Scion Image software (Scion Corporation, Frederick, MD). A complete isolation of caseins from whey proteins was achieved using sodium phosphate in the range of 10 to 50 mM and 20 times the initial volume of buffer added. No whey proteins were detected at 50 mM but this was at the expense of low caseins being retained. When lower sodium phosphate concentrations were used, the amount of caseins retained was higher but a small amount of whey proteins were still detected by sodium dodecyl sulfate-PAGE. Among the salts tested, calcium chloride at 50 mM and all volumes of buffer showed the higher retention of casein proteins. The highest casein:whey protein ratio was found at 30 mM CaCl₂, but no complete casein micelle isolation was achieved. Potassium citrate was the most ineffective salt because a rapid loss of caseins and whey proteins was observed at all concentrations and with low quantities of buffer added during the filtration process. Our results show the potential of altering the mineral balance in milk for isolation of casein micelles from whey proteins in a continuous tangential flow microfiltration system.     

The effect of ultrasound on casein micelle integrity

Samples of fresh skim milk, reconstituted micellar casein, and casein powder were sonicated at 20 kHz to investigate the effect of ultrasonication. For fresh skim milk, the average size of the remaining fat globules was reduced by approximately 10 nm after 60 min of sonication; however, the size of the casein micelles was determined to be unchanged. A small increase in soluble whey protein and a corresponding decrease in viscosity also occurred within the first few minutes of sonication, which could be attributed to the breakup of casein-whey protein aggregates. No measurable changes in free casein content could be detected in ultracentrifuged skim milk samples sonicated for up to 60 min. A small, temporary decrease in pH resulted from sonication; however, no measurable change in soluble calcium concentration was observed. Therefore, casein micelles in fresh skim milk were stable during the exposure to ultrasonication. Similar results were obtained for reconstituted micellar casein, whereas larger viscosity changes were observed as whey protein content was increased. Controlled application of ultrasound can be usefully applied to reverse process-induced protein aggregation without affecting the native state of casein micelles.     

Influence of NaCl and CaCl2 on Cold-Set Gelation of Heat-denatured Whey Protein

Heat‐denatured whey‐protein isolate (HD‐WPI) solutions were prepared by heating a 10 wt% WPI solution (pH 7) to 80 °C for 10 min and then cooling it back to 30 °C. Cold‐set gelation was initiated by adding either NaCl (0 to 400 mM) or CaCl₂ (0 to 15 mM). Both salts increased the turbidity and rigidity of the HD‐WPI solutions. Gelation rate and final gel strength increased with salt concentration and were greater for CaCl₂ than NaCl at the same concentration because the former is more effective at screening electrostatic interactions and can form salt bridges.     

Disruption and sedimentation of casein micelles and casein micelle isolates under high-pressure homogenization

Native casein micelles were isolated from raw skim milk by ultrafiltration (< 30 kDa) or microfiltration (< 0.2 μm) and subjected to high-pressure homogenization (HPH) at 100, 200, 250, 300, and 350 MPa. Of particular interest was the effect of HPH on casein micelle size in solutions varying in ionic strength (0, 5, 10, and 15 mM CaCl₂) and micelle size populations. Particle size distribution reflected an initial decrease in micelle diameter in all samples at 100 MPa. In samples containing 10 and 15 mM CaCl₂, there was an abrupt increase in particle size and subsequent casein precipitation followed by sedimentation upon centrifugation at elevated pressures (300 and 350 MPa). The amount of sedimentable casein protein increased as CaCl₂ concentration (10 and 15 mM) and pressure (300 and 350 MPa) increased as determined by UV absorbance of sample supernatant. SDS-PAGE indicated extensive micellar disruption at elevated pressures (300 and 350 MPa) and confirmed that the sedimented portion of the samples contained casein proteins and minimal amounts of whey proteins. Results indicated that through HPH treatment casein micelle size can be modified based on CaCl₂ concentration and pressure applied. Based on these findings, HPH in combination with an appropriate suspending medium has the ability to modify micelles to a desired size for a number of potential applications.Industrial relevanceThe modification of structure-function properties of the casein micelle from bovine milk by using high-pressure homogenization is relevant in (1) the development of new ingredients to change rheological/textural properties of dairy based foods, and (2) the discovery of new and/or improved functionalities for protein quaternary structures.     

Critical phosphate salt concentrations leading to altered micellar casein structures and functional intermediates

We investigated the effect of different phosphate salts on the structural integrity of micellar casein (MC) at pH 7.0. With the increase of salt concentration, a reduction in turbidity was observed for the MC solutions, and it was modeled using an exponential decay function. The inflection point of the model was defined as the first critical salt concentration (C*), and it is suggested that the salt concentration initiates the disintegration of MC. For linear polyphosphates, C* decreased with the number of phosphate groups. Apparent viscosity (η_app) of MC solutions increased with the increase of salt concentration, and they recorded a peak while the turbidity decreased to a minimum. The salt concentration that resulted in the highest η_app was identified as the second critical salt concentration (C**). It is hypothesized that the interactions among protein species present in the mixtures are at an optimum state at C**. Both C* and C** were found to be dependent on the MC concentration. The work presented herein supports an understanding of the concentration effect of phosphate salts on MC for structuring dairy products.     

Comparison of casein micelles in raw and reconstituted skim milk

During the manufacture of skim milk powder, many important alterations to the casein micelles occur. This study investigates the nature and cause of these alterations and their reversibility upon reconstitution of the powders in water. Samples of skim milk and powder were taken at different stages of commercial production of low-, medium-, and high-heat powders. The nature and composition of the casein micelles were analyzed using a variety of analytical techniques including photon correlation spectroscopy, transmission electron microscopy, turbidity, and protein electrophoresis. It was found that during heat treatment, whey proteins are denatured and become attached to the casein micelles, resulting in larger micelles and more turbid milk. The extent of whey protein attachment to the micelles is directly related to the severity of the heat treatment. It also appeared that whey proteins denatured during heat treatment may continue to attach to casein micelles during water removal (evaporation and spray-drying). The process of water removal causes casein and Ca in the serum to become increasingly associated with the micelles. This results in much larger, denser micelles, increasing the turbidity while decreasing the viscosity of the milk. During reconstitution, the native equilibrium between colloidal Ca and serum Ca is slowly reestablished. The reequilibration of the caseins and detachment of the whey proteins occur even more slowly. The rate of reequilibration does not appear to be influenced by shear or temperature in the range of 4 to 40°C.     

Effect of partial whey protein depletion during membrane filtration on thermal stability of milk concentrates

Membrane filtration technologies are widespread unit operations in the dairy industry, often employed to obtain ingredients with tailored processing functionalities. The objective of this work was to better understand the effect of partial removal of whey proteins by microfiltration (MF) on the heat stability of the fresh concentrates. The micellar casein concentrates were compared with control concentrates obtained using ultrafiltration (UF). Pasteurized milk was microfiltered (80 kDa polysulfone membrane) or ultrafiltered (30 kDa cellulose membrane) without diafiltration (i.e., no addition of water) to 2× and 4× concentration, based on volume reduction. The final concentrates showed no differences in pH, casein micelle size, or mineral concentration in the serum phase. The micellar casein retentates (obtained by MF) showed a 20 and 40% decrease in whey protein concentration compared with the corresponding UF milk protein concentrates for 2× and 4× concentration, respectively. The heat coagulation time decreased with increasing protein concentration, regardless of the treatment; however, MF retentates showed a higher thermal stability than the corresponding UF controls. The average diameter for casein micelles increased after heating in UF but not MF concentrates. The turbidity (measured by light scattering) increased after heating, but to a higher extent for UF retentates than for MF retentates at the same protein concentration. It was concluded that the reduced amount of whey protein in the MF retentates caused a significant increase in the heat stability compared with the corresponding UF retentates. This difference was not due to ionic composition differences or pH, but to the type and amount of complexes formed in the serum phase.     

Effect of Ultra-high Temperature Treatment on the Enzymatic Cross-linking of Micellar Casein and Sodium Caseinate by Transglutaminase

ABSTRACT: It was found that ultra-high temperature (UHT) treatment of sodium caseinate and native whey protein-depleted micellar casein drastically increases the protein polymerization effect of an enzymatic treatment by microbial transglutaminase (TG). As a result the concentration of the isopeptide ε-(γ-glutamyl)lysine was increased significantly in UHT-treated micellar casein solutions after TG incubation compared with the unheated casein solution. Sodium caseinate was more susceptible to the cross-linking reaction as compared with the native casein micelles. The results demonstrate that the protein structure significantly affects the TG cross-linking reaction. The effect of an UHT treatment was considered to be related to a better TG accessibility due to a more open casein micelle structure and to the inactivation of a TG inhibitor substance. The results demonstrate that an unidentified component in the natural milk serum inhibits the TG reaction. The thermal inactivation of a TG inhibitor is the dominant effect explaining the improved cross-linking of UHT-treated casein micelles as well as sodium caseinate.     

Effect of calcium chelators on physical changes in casein micelles in concentrated micellar casein solutions

The effect of calcium chelators on physical changes of casein micelles in concentrated micellar casein solutions was investigated by measuring calcium-ion activity, viscosity and turbidity, and performing ultracentrifugation. The highest viscosities were measured on addition of sodium hexametaphosphate (SHMP), because it cross-linked the caseins. For the weak calcium chelator disodium uridine monophosphate (Na₂UMP), physical changes in the solutions were negligible. Disodium hydrogen phosphate (Na₂HPO₄), trisodium citrate (TSC), and sodium phytate (SP) caused similar increases in viscosity, but had different effects on turbidity. The increase in viscosity was attributed to swelling of the casein micelles (i.e., increased voluminosity) at decreasing calcium-ion activity. The major decrease in turbidity was due to dissociation of the casein micelles. The extent of micellar dissociation was dependent on the type and concentration of calcium chelator. It seems that the micelles were dissociated in the order of SHMP ≥ SP > TSC > Na₂HPO₄ > Na₂UMP.     

Protein and Salt Effects on Ca2+-Induced Cold Gelation of Whey Protein Isolate

Increasing whey protein concentration (from 6 to 10% w/v) decreased gel opacity but increased gel strength and water-holding capacity (WHC). Increasing CaCl₂, concentration (from 5 to 150 mM) increased gel opacity and gel strength at the high protein concentration (i.e., 10%). However, it lowered gel strength at protein concentration > 10%. Young's modulus and distance to fracture values indicated that gels were most rigid at 30 mM CaCl₂, at which point the extent of aggregation (measured by turbidity) was the highest. Increasing CaCl₂ concentration from 5 to 150 mM slightly affected the WHC of Ca²⁺-induced gels. Protein concentration was the major factor in determining fracture properties and WHC.     

A novel technique for determining glass–rubber transition in dairy powders

A novel rheological technique is described, for determining the glass–rubber transition temperature (T_gr) of spray dried dairy powders. The approach involves constant rate heating of powder under compression and measurement of changes in either gap distance (Method 1) or normal force (Method 2). Significant increases in the rate of change of these parameters was shown to correspond with T_gr. The techniques were applied to skim milk, micellar casein and whey permeate powders and a range of fat-enriched micellar casein powders. T_gr temperatures, so obtained, were compared with glass transition temperatures (T_g) determined by Differential Scanning Calorimetry (DSC). Methods 1 and 2 gave predictions for non-fat dairy powders of T_g endset (T_ge) with SEP of 8.8 and 4.4 °C, respectively. These novel techniques provide an accurate means of determining glass transitions in dairy powders, including high protein and fat-containing powders, whose relaxation properties can be difficult to measure by DSC.