Physico-chemical properties of casein micelles in unheated skim milk concentrated by osmotic stressing: Interactions and changes in the composition of the serum phase

Structural dependence of concentrated skim milk curd on micellar restructuring

This study was conducted to establish an understanding of how milk concentration modulates the rennet curd structure. Rennet-induced gelation and renneting under slow acidification achieved using glucono-δ-lactone (GDL) and structural properties of reconstituted skim milk gels at two concentration levels (9 and 25 % total solids) were studied by measuring variations in (a) viscoelastic behaviour, (b) micellar size, charge density, diffusivity, and (c) hydrophobicity using dynamic rheometry, dynamic light scattering and fluorimetry, respectively. Concentrated milk showed a greater estimated hydrodynamic radius of casein micelles, lower zeta (ζ)-potential, ratio of serum to total Calcium (Ca) and charge density and increased surface hydrophobicity, all supporting the view that micellar restructuring particularly sub-particle transfer takes place and contributes to rapid gelation. Moreover, hydrophobic interactions occurred very quickly (within 5 min in combined gels, 10 min for renneting only), demonstrating their pivotal role during the flocculation stage. All gels exhibited a solid viscoelastic character as the elastic modulus (G') was greater than loss modulus (G″) while both G' and tan δ (G''/G') were frequency-dependent. Frequency sweeps classified the concentrated gels into three stiffness categories caused by the level of rennet or GDL as rigid, hard and soft, whereas an increased flow-like behaviour (high tan δ), restricted diffusion and excessive water retention revealed limited structural rearrangements (contraction & macrosyneresis) during curd ageing. Acidification increased the diffusion rate in control curd, thus, enhanced contractive rearrangements, macrosyneresis and curd strength. Findings suggest that micellar restructuring induced by milk concentration is the principal modulator of the curd structure.

Effect of high hydrostatic pressure processing on the rennet coagulation kinetics and physicochemical properties of sheep milk rennet-induced gels

The effects of high hydrostatic pressure on the constituents and coagulation ability and their effect on cheese production of sheep milk have not been studied in detail. The objective of this work was to evaluate the effect of high hydrostatic pressure processing on the coagulation kinetics and physicochemical properties of sheep milk and to explore how such treatment could improve the cheesemaking process. Five batches of milk were tested: 1 untreated control batch and 4 batches each subjected to a different pressure (150, 300, 450, or 600 MPa) for 5 min at 10°C. As treatment pressure increased, values of electrical conductivity and oxidation-reduction potential were found to decrease. However, no significant reduction in pH was recorded. Treatment pressures >300 MPa produced milk with lower lightness (luminosity) and a more yellow and green hue. Pressures >150 MPa resulted in micellar fragmentation, as well as significant increases in particle size, viscosity, and water-holding capacity as a consequence of the denaturing of soluble proteins. High-pressure treatments increased the solubility of colloidal calcium phosphate, leading to a considerable increase in the concentration of minerals in the serum phase. The highest concentrations of calcium and phosphorus in the rennet whey of milk were reached at 300 MPa. Curd coagulation time was reduced by 28% at pressures >300 MPa, and an increase in the curd firming rate was observed. As treatment pressure increased to 450 MPa, the firmness, elasticity, and the percentage creep recovery of gels increased, whereas values of compliance and fracture strain were reduced. Thus, we can conclude that 300 MPa is the optimum treatment pressure for milk intended for cheesemaking by enzymatic coagulation. This pressure produced milk with optimal coagulation kinetics and water-holding properties with the least loss of fat and protein to the whey.

Sodium caseinate hinders chymosin-induced aggregation of caseins in concentrated milk: The role of soluble caseins and calcium ions

Concentrated casein micelle suspensions show an altered balance between the colloidal and soluble phases compared with native skim milk. The objective of this research was to probe the role of such a change on the chymosin-driven destabilization of casein micelles. Skim milk was gently concentrated by osmotic stressing to increase the volume fraction of the micelles while maintaining a constant ionic composition. In situ turbidity measurements demonstrated that the secondary stage of gelation was hindered in the concentrated suspensions. Addition of ionic calcium overcame this inhibition, whereas restoring the original concentration by redilution did not. This work clearly demonstrated that calcium plays a major role in decreasing electrostatic repulsion in casein micelles, but also showed the importance of noncolloidal proteins in altering the gelling functionality of casein micelles in concentrated milk. Additional calcium induces aggregation of these soluble caseins, restoring gelation in concentrates.

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.

Acid induced gelation behavior of skim milk concentrated by membrane filtration

This work reports a detailed study of the effect of ultrafiltration (UF) and diafiltration (DF) on the acid-induced gelation behavior of fresh milk retentates (2× and 4×). Concentrates were heated at 80°C for 15 min, and compared to unheated samples. The use of extensive DF caused a significantly greater amount of protein (both caseins and whey proteins) in the supernatant fraction, compared to UF retentates at the same concentration, both in unheated and heated samples. DF retentates showed higher pH of gelation, compared to the corresponding UF retentates. The development of tan δ is reported for the first time as a function of colloidal calcium release, and the protein gelation behavior discussed in light differences in composition of the soluble fraction. The results demonstrate how processing history can affect compositional changes and the gelation behavior of fresh milk retentates. Membrane filtration is a widespread unit operation in the dairy industry, employed either to prepare fresh concentrates for further processing, or ingredients with specific functional properties. This work describes in detail the effect of processing history during membrane filtration on the rheological properties of acid induced gels and will help in optimizing formulations and prepare the right ingredients for the right application. It will also be possible to determine new ways to define processing quality of the milk protein concentrates, as it relates to their ability to form texture in fermented dairy products.

Preparation of konjac glucomannan/casein blending gels optimized by response surface methodology and assessment of the effects of high-pressure processing on their gel properties and structure

BACKGROUND

In order to improve the compatibility of polysaccharide-protein mixtures and enhance their performance, a response surface methodology was used to optimize the preparation conditions of konjac glucomannan/casein blend gel. Moreover, the effects of high-pressure processing (HPP) on the gel properties and structure were also investigated.

RESULTS

The optimal preparation parameters were a temperature of 60 °C, a total concentration 40 g kg^-1 , and a dietary alkali concentration 5 g kg^-1 . Under these conditions, the experimental value of hardness was 38.7 g, which was close to the predicted value. HPP increased gel hardness by 161-223% and led to a more compact structure at 200-600 MPa/10 min, while a hardness increase of ∼120% and damaged structure were observed at 600 MPa/30 min. Fourier transform infrared spectroscopy showed that noncovalent interactions are likely the most important factor in the modification of gel hardness; indeed, hydrogen bonding interactions in the gels are enhanced when subjected to HPP, but are weakened at 600 MPa/30 min.

COUCLUSION

Protein-polysaccharide complexes with excellent properties could be obtained through this method, with broad application prospects in the food industry. © 2018 Society of Chemical Industry.

Rennet-induced gelation of concentrated milk in the presence of sodium caseinate: differences between milk concentration using ultrafiltration and osmotic stressing

Concentrating milk is a common unit operation in the dairy industry. With the reduction of water, the particles interact more frequently with each other and the functionality of the casein micelles may depend on the interactions occurring during concentration. The objective of this research was to investigate the effect of concentration on the renneting properties of the casein micelles by comparing 2 concentration methods: ultrafiltration and osmotic stressing. Both methods selectively concentrate the protein fraction of milk, while the composition of the soluble phase is unaltered. To evaluate possible differences in the rearrangements of the casein micelles during concentration, renneting properties were evaluated with or without the addition of soluble caseins, added either before or after concentration. The results indicate that casein micelles undergo rearrangements during concentration and that shear during membrane filtration may play a role in affecting the final properties of the milk.