Mineral balance in skim-milk and milk retentate: effect of physicochemical characteristics of the aqueous phase

Calcium: A comprehensive review on quantification, interaction with milk proteins and implications for processing of dairy products

Calcium (Ca) is a key micronutrient of high relevance for human nutrition that also influences the texture and taste of dairy products and their processability. In bovine milk, Ca is presented in several speciation forms, such as complexed with other milk components or free as ionic calcium while being distributed between colloidal and serum phases of milk. Partitioning of Ca between these phases is highly dynamic and influenced by factors, such as temperature, ionic strength, pH, and milk composition. Processing steps used during the manufacture of dairy products, such as preconditioning, concentration, acidification, salting, cooling, and heating, all contribute to modify Ca speciation and partition, thereby influencing product functionality, product yield, and fouling of equipment. This review aims to provide a comprehensive understanding of the influence of Ca partition on dairy products properties to support the development of kinetics models to reduce product losses and develop added-value products with improved functionality. To achieve this objective, approaches to separate milk phases, analytical approaches to determine Ca partition and speciation, the role of Ca on protein-protein interactions, and their influence on processing of dairy products are discussed.

The effect of ultrafiltration on the acid gelation properties of protein-standardised skim milk systems

In this study, we investigated the impact of ultrafiltration (UF) on the acid gelation of milk using two protein-standardised milk systems, consisting of either skim milk and retentate (SR) or permeate and retentate (PR), over different seasons in New Zealand. The composition and the physicochemical properties of the two systems before heating were comparable, whereas the levels of heat-induced α-lactalbumin denaturation and the association of the casein micelles with α-lactalbumin were significantly lower in PR than in SR. PR displayed superior acid gelation properties compared with SR, which was most pronounced in the late season. The structural modifications of the whey proteins and casein micelles that were induced by UF and the re-equilibration of calcium in the milk mixtures may have affected the acid gelation properties of the milk by influencing the denaturation and micelle association of the whey proteins. The results suggest that UF has the potential as a tool for tuning the acid gelation properties of milk.

Study on β-Casein Depleted Casein Micelles: Micellar Stability, Enzymatic Cross-Linking, and Suitability as Nanocarriers

β-Casein is an amphiphilic protein and thus considered as multilaterally bound in casein micelles. Its polar molecule part, in particular the phosphoserine residues, can interact electrostatically with colloidal calcium phosphate (CCP) to form nanoclusters and its nonpolar molecule part enhances micellar stability by forming hydrophobic bonds to other caseins. Because cooling weakens hydrophobic interactions, a substantial portion of β-casein can be irreversibly removed from the casein micelle by repeated depletion steps, including cooling and subsequent ultracentrifugation. Although this effect of cooling on the micellar β-casein concentration has been well known for decades, the influence of depletion on the main characteristics of casein micelles has been less investigated yet. Therefore, we aimed to analyze the consequences of β-casein depletion on the stability as well as the functionality of casein micelles to evaluate the suitability of depleted compared to native casein micelles as nanocarriers. Up to 43.2% of the total β-casein was irreversibly sequestered from native casein micelles by repeated cooling and ultracentrifugation steps. Depletion showed no effect on size distribution as well as polydispersity and particle concentration of micelle suspensions as measured via dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA), respectively. Furthermore, the stability of the micelles against ethanol or the chelating agent ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) was not influenced by β-casein depletion. Notwithstanding, depleted micelles were less susceptible to enzymatic cross-linking by microbial transglutaminase (mTG), indicating narrowed water channels due to depletion. Additionally, loading experiments showed that depleted micelles could be loaded with linoleic acid (LA) as intensively as native micelles, whereupon LA displaces up to 81.3% of β-casein from native micelles. Our results confirm that depletion does not enhance the ability of the casein micelle to act as a nanocarrier for hydrophobic substances but could support the understanding of the casein micelle structure. Based on the observed unchanged stability against EGTA, the hindered enzymatical cross-linking, and the efficient displacing of β-casein by LA, we suggest that the major portion of micellar β-casein is hydrophobically incorporated into the micelle structure without impact on the formation of calcium phosphate nanoclusters. The main role of β-casein for the casein micelle structure, therefore, might be to facilitate the high hydration of the interior and thus the high permeability of casein micelles.

Transmembrane Pressure and Recovery of Serum Proteins During Microfiltration of Skimmed Milk Subjected to Different Storage and Treatment Conditions

Milk pre-processing steps-storage at 4 °C (with durations of 48, 72 or 96 h) and methods for microbiological stabilization of milk (1.4 μm microfiltration, thermization, thermization + bactofugation, pasteurization) are performed industrially before 0.1 µm-microfiltration (MF) of skimmed milk to ensure the microbiological quality of final fractions. The objective of this study was to better understand the influence of these pre-processing steps and their cumulative effects on MF performances (i.e., transmembrane pressure, and transmission and recovery of serum proteins (SP) in the permeate). Results showed that heat treatment of skimmed milk decreased ceramic MF performances, especially after a long 4 °C storage duration (96 h) of raw milk: when milk was heat treated by pasteurization after 96 h of storage at 4 °C, the transmembrane pressure increased by 25% over a MF run of 330 min with a permeation flux of 75 L.h^-1.m^-2 and a volume reduction ratio of 3.0. After 48 h of storage at 4 °C, all other operating conditions being similar, the transmembrane pressure increased by only 6%. When milk was 1.4 µm microfiltered, the transmembrane pressure also increased by only 6%, regardless of the duration of 4 °C storage. The choice of microbiological stabilization method also influenced SP transmission and recovery: the higher the initial heat treatment of milk, the lower the transmission of SP and the lower their recovery in permeate. Moreover, the decline of SP transmission was all the higher that 4 °C storage of raw milk was long. These results were explained by MF membrane fouling, which depends on the load of microorganisms in the skimmed milks to be microfiltered as well as the rate of SP denaturation and/or aggregation resulting from pre-processing steps.

Major Role of Voluminosity in the Compressibility and Sol-Gel Transition of Casein Micelle Dispersions Concentrated at 7 °C and 20 °C

The objective of this work is to bring new information about the influence of temperatures (7 °C and 20 °C) on the equation of state and sol-gel transition behavior of casein micelle dispersions. Casein micelle dispersions have been concentrated and equilibrated at different osmotic pressures using equilibrium dialysis at 7 °C and 20 °C. The osmotic stress technique measured the osmotic pressures of the dispersions over a wide range of concentrations. Rheological properties of concentrated dispersions were then characterized, respectively at 7 °C and at 20 °C. The essential result is that casein micelle dispersions are less compressible at 7 °C than at 20 °C and that concentration of sol-gel transition is lower at 7 °C than at 20 °C, with compressibility defined as the inverse to the resistance to the compression, and that is proportional to the cost to remove water from structure. From our interpretations, these two features were fully consistent with a release of soluble β-casein and nanoclusters CaP and an increased casein micelle hydration and apparent voluminosity at 7 °C as compared with 20 °C.

Effects of milk heat treatment and solvent composition on physicochemical and selected functional characteristics of milk protein concentrate

Milk protein concentrate (MPC) powders (∼81% protein) were made from skim milk that was heat treated at 72°C for 15 s (LHMPC) or 85°C for 30 s (MHMPC). The MPC powder was manufactured by ultrafiltration and diafiltration of skim milk at 50°C followed by spray drying. The MPC dispersions (4.02% true protein) were prepared by reconstituting the LHMPC and MHMPC powders in distilled water (LHMPC_w and MHMPC_w, respectively) or milk permeate (LHMPC_p and MHMPC_p, respectively). Increasing milk heat treatment increased the level of whey protein denaturation (from ∼5 to 47% of total whey protein) and reduced the concentrations of serum protein, serum calcium, and ionic calcium. These changes were paralleled by impaired rennet-induced coagulability of the MHMPC_w and MHMPC_p dispersions and a reduction in the pH of maximum heat stability of MHMPC_p from pH 6.9 to 6.8. For both the LHMPC and MHMPC dispersions, the use of permeate instead of water enhanced ethanol stability at pH 6.6 to 7.0, impaired rennet gelation, and changed the heat coagulation time and pH profile from type A to type B. Increasing the severity of milk heat treatment during MPC manufacture and the use of permeate instead of water led to significant reductions in the viscosity of stirred yogurt prepared by starter-induced acidification of the MPC dispersions. The current study clearly highlights how the functionality of protein dispersions prepared by reconstitution of high-protein MPC powders may be modulated by the heat treatment of the skim milk during manufacture of the MPC and the composition of the solvent used for reconstitution.

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.

Study of acid milk coagulation by an optical method using light reflection

A turbidimetric method, based on light reflection, was used to study acid coagulation of reconstituted skim milk at low temperature. Capillary viscosimetry, gelograph and laser granulometer techniques were also employed. Acidification of milk was produced by hydrolysis of glucono-δ-lactone. The general shape of the turbidimetric signal as a function of pH or time can be divided into three stages: a lag phase followed by a significant decrease and then a final rise. Two factors have a great influence on the development of milk turbidity, pH and temperature. Dynamic viscosity measurements can be related to the turbidimetric signal while laser granulometrie measurements cannot be correlated with changes in turbidity: the micelle size distribution remains constant until the first signs of gelation. As previous work showed, dynamic viscosity diminishes with acidification until a particular pH is reached (pH 5·9 at 15°C and pH 5·75 at 20°C). We have related this latter period to the turbidimetric lag phase. As milk turbidity became lower than its initial value (pH 5·75 at 15°C and pH 5·55 at 20°C), dynamic viscosity increased significantly. The release of material from micelles (β-casein and Ca) could explain this phenomenon. In the same way, further increase of turbidity at a particular pH value (pH 5·3 at both 15 and 20°C) coulcl be partly due to the reincorporation of soluble casein monomers in the micelle framework. As the onset of gelation was approached, turbidity still increased as a result of gel network formation.

Mineral and protein equilibria between the colloidal and soluble phases of milk at low temperature

Mechanisms controlling exchange and partition of minerals and β-casein between micellar and soluble phases during cooling have been studied using suspensions of milk proteins of varying concentrations in the presence of aqueous phases having varying mineral compositions. In these studies the final equilibrium between Ca and phosphate was found to be regulated by formation of Ca phosphates of varying solubilities. The effect of the total casein level on mineral and protein equilibria was also studied. In milk, part of the β-casein content is bound to micelles by hydrophobic bonds; the proportion present in this form was increased by spontaneous micellar demineralization during cooling as well as by addition of a complexant to milk. At low temperatures, an equilibrium between the micellar and the monomeric states was reached which depends upon the total casein level and on the hydrophobic-bonded β-casein content. Not all of these hydrophobic bonds were broken when milk was cooled.

Effect of milk protein standardization, by ultrafiltration, on the manufacture, composition and maturation of Cheddar cheese

Skim milks were pre-acidified to pH 6·4 and concentrated by ultra-filtration to give retentates with protein levels of 210 g/1. Retentates were blended with skim milk and cream to give standardized milks with protein levels ranging from 30 to 82 g/1. These were used for the manufacture of Cheddar cheese in conventional equipment. Increasing milk protein level resulted in reduced gelation times, increased curd firming rates and a decrease in the set-to-cut time when cutting at equal firmness values (i.e. elastic modulus, G′, ∼ 16 Pa). As the curd firming rates increased with milk protein level, it became increasingly difficult to cut the curd cleanly, without tearing, before the end of the cutting cycle. Reflecting the tearing of curd, and consequent curd particle shattering, fat losses in the running wheys were greater than those predicted on the basis of volume reduction (due to ultrafiltration) for milks with protein levels > 50 g/1. Reduction of setting temperatures, in the range 31–27 °C, and the level of added rennet brought the set-to-cut times and curd firming rates of concentrated milks closer to those of the control milk. While increasing milk protein level in the range 30–70 g/1 had little effect on cheese composition, it resulted in slower proteolysis and maturation.

A 43Ca and 31P NMR study of the calcium and phosphate equilibria in heated milk solutions

Casein micelle suspensions, colloidal Ca phosphate-free casein solution and simulated milk ultrafiltrate (SMUF) were studied using high-resolution 43Ca and 31P NMR in the temperature range 4–64 °C. Only one 43Ca and one 31P signal was obtained for each solution. Signal intensities and line widths varied with the environment and were affected by the redistribution of Ca and P during heating and cooling. Based upon the temperature-dependent broadening of the 43Ca signal, five different Ca environments were discerned in the heated milk fractions. The Ca state, induced by heating of a casein-containing milk salt solution, differed from both the state of Ca present in micellar colloidal Ca phosphate and from the state of Ca induced by the heating of SMUF.

Chemical characterization of milk concentrated by ultrafiltration

Whole milks concentrated 1·5–4-fold and acidified and citrated milks concentrated 2·8-fold by ultrafiltration at 50 °C were analysed for chemical changes relevant to further processing, storage or nutrition. Fat and protein were entirely retained in the concentrate. The retention of water-soluble vitamins, Ca, Mg, phosphate and trace minerals depended on the proportion bound to the protein. Ascorbic acid was rapidly destroyed during concentration. Because of the differential retention of nitrogenous components, protein comprised a progressively higher proportion of the total N as the milk became more concentrated. No denaturation of whey protein or disruption of casein micelles was detected during concentration of whole milk, but some solubilization of the casein occurred after citration. Reduction of fat globule size occurred early in the concentration process, damage to the fat globule membrane was indicated and the milk became more susceptible to lipolysis. Apart from a tendency for preacidified or precitrated concentrates to gel, no change in the susceptibility of the milks to heat damage was detected.

Observations on the heat-induced salt balance changes in milk I. Effect of heating time between 4 and 90°C

Milk permeate was separated at various temperatures by means of a hollow fibre ultrafiltration unit coupled to a stainless steel heat exchanger. Milk samples conditioned at 4°C were heated to 20, 40, 60, 80, 85 or 90°C prior to ultrafiltration. Ca, P, Mg, Na, K and citrate concentrations were measured in the permeate samples. Ca and P contents of the permeate decreased as the temperature increased. The pH was measured after cooling the permeate to room temperature. Smaller losses of Mg and citrate were also observed with increase in temperature. Na and K levels were not affected. A two-step time-concentration relationship was apparent for the species under study. An initial sharp decrease in concentration occurred in the first minute of holding time and was followed by a slower reaction. The possible occurrence of a two-step mechanism in the heat-induced salt balance changes is discussed. Dicalcium phosphate precipitation is believed to be coupled with tricalcium citrate precipitation upon heating.

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.

The minerals of milk

The salt of milk constitutes a small part of milk (8-9 g.L(-1)); this fraction contains calcium, magnesium, sodium and potassium for the main cations and inorganic phosphate, citrate and chloride for the main anions. In milk, these ions are more or less associated between themselves and with proteins. Depending on the type of ion, they are diffusible (cases of sodium, potassium and chloride) or partially associated with casein molecules (cases of calcium, magnesium, phosphate and citrate), to form large colloidal particles called casein micelles. Today, our knowledge and understanding concerning this fraction is relatively complete. In this review, the different models explaining (i) the nature and distribution of these minerals (especially calcium phosphate) in both fractions of milk and (ii) their behaviour in different physico-chemical conditions, are discussed.

Effect of temperature on the salt balance of milk studied by capillary ion electrophoresis

Many inorganic species, such as calcium, phosphate and magnesium, are in equilibrium between the liquid and colloidal phases of milk and hence are of importance with respect to the coagulation properties of milk. Capillary ion electrophoresis makes possible the determination of anions and cations in less than 6 min. The soluble phase of milk was obtained by ultrafiltration and samples had to be diluted 250-fold before analysis. Cold storage increased soluble calcium and phosphate concentrations, and warm-up of the milk restored the initial ionic equilibria. More drastic heat treatments (80-90 degrees C) caused precipitation of tricalcium phosphate and calcium citrate.