Soluble protein fractions from pH and heat treated sodium caseinate: physicochemical and functional properties

Plant Protein versus Dairy Proteins: A pH-Dependency Investigation on Their Structure and Functional Properties

Plant proteins are constantly gaining attention as potential substitutes for dairy proteins, due to their suitable functionality and nutritional value. This study was designed to compare the structural and functional responses of different plant protein isolates (soy, pea, lentil, and chickpea) with two commonly used dairy protein (whey protein isolates and sodium caseinate) under different pH treatments (pH 3.0, 5.0, 7.0, and 9.0). The results showed that pH had a different alteration on the structural, surface properties and functional properties of plant and dairy proteins. Plant protein generally possessed a darker color, lower solubility, emulsifying properties, and foaming capacity, whereas their foaming stability and water holding capacity were higher than those of dairy proteins. Soy protein isolates were characterized by its comparable proportion of β-turn and random coils, zeta-potential, emulsifying (30.37 m²/g), and water-holding capacity (9.03 g/g) at alkaline conditions and chickpea protein isolates showed good oil-holding capacity (3.33 g/g at pH 9) among plant proteins. Further analysis confirmed that pH had a greater influence on the structural and functional properties of proteins as compared to protein sources, particularly at acidic conditions. Overall, this study might help processors select the appropriate plant protein as dairy alternatives for their target application in plant-based food products.

Effects of High-Intensity Ultrasound Treatments on the Physicochemical and Structural Characteristics of Sodium Caseinate (SC) and the Stability of SC-Coated Oil-in-Water (O/W) Emulsions

The effects of high-intensity ultrasound treatment (0, 3, 6, 9 min) on physicochemical and structural characteristics of SC and the storage, thermal and freeze–thaw stability of SC O/W emulsions were investigated. The results showed that ultrasound treatment reduced the particle size of SC, although there were no obvious changes in zeta potential, profiles and weights. Ultrasound treatment improved surface hydrophobicity and fluorescence intensity of SC and changed ultraviolet–visible (UV–Vis) spectroscopy but had no influence on the secondary structure of SC. This indicates that ultrasounds might destroy the tertiary structure but leave most of the integral secondary structure. A scanning electron microscope (SEM) also showed that ultrasound-treated SC presented small aggregates and a loose structure. The physicochemical and structural changes of SC benefited the ability of protein adsorbing oil droplets and emulsion stability. Under stresses such as storage, thermal and freeze–thawing, the oil droplets of treated emulsions were still uniform and stable, especially at 6 min and 9 min. Overall, the high-intensity ultrasounds made the SC present small aggregates and a loose structure improving the SC O/W emulsions stability under storage, thermal and freeze–thawing environment and have great potential to stabilize the SC prepared O/W emulsions.

Characterization of Structural Changes of Casein Micelles at Different PH Using Microscopy and Spectroscopy Techniques

This study aimed to evaluate the influence of pH changes on morphometric parameters of casein micelles and a general overview of their conformational structure through microscopy techniques, Raman spectroscopy and multivariate analysis. It was found that casein micelles morphology and protein secondary structure depend strongly upon pH. The changes of arithmetic average roughness (Ra), size, and shape of casein micelles at different pH are properly characterized by atomic force and cryo-transmission electron microscopy. Morphometric changes of casein micelles were correlated correctly with folding and unfolding of casein molecules as evaluated by Raman spectroscopy when the pH was varied. The novelty of this contribution consists in demonstrating that there is a close structure-functionality relationship between the morphometric parameters of proteins and their secondary structure. Knowledge about casein micelles can help improve their use of its diverse applications.

Hemp globulin forms colloidal nanocomplexes with sodium caseinate during pH-cycling

Seed from industrial hemp (Cannabis sativa L.) contains around 25% protein (mainly globulins) which is easily digested, but the low solubility of hemp globulins (HG) limits their application in many food systems. In this study, the solubility of HG was improved by blending HG with sodium caseinate (SC) and treating with a pH-cycling process. The pH-cycling involved adjusting the pH to 12 and reacting for 1 hr, followed by neutralisation to pH 7. Nanoparticles composed of HG and SC (Z-average diameter ≈ 130 nm) were formed after the pH-cycling, and the solubility of HG increased to > 80% when there was more than 1% of SC for 1% of HG. These HG|SC nanoparticles were monodisperse (PDI < 0.17) and ζ-potential was ≈ -17 mV. Hydrogen bonding is the main forces that assembles HG|SC nanoparticles because the nanoparticles dissociated by heat treatment (up to 60 °C) or urea, which is an effective hydrogen bond breaker. HG|SC nanoparticles will aggregate irreversibly above 60 °C, possibly due to thiol-disulphide exchange. The nanoparticles were heat-stable as the Z-average diameter was only 229 nm after heating (90 °C, 30 min). N-ethylmaleimide blocked free thiol groups on HG and resulted in less disulphide-linked HG aggregation after pH- cycling, which in turn lead to smaller HG|SC nanoparticles and a bimodal particle size distribution, indicating the importance of disulphide bond for the formation of monodisperse HG|SC nanoparticles. The soluble and heat-stable HG|SC nanoparticles could be used to increase the hemp protein content in beverages and emulsions.

Comparative Study of the Structural and Functional Properties of Membrane-Isolated and Isoelectric pH Precipitated Green Lentil Seed Protein Isolates

The demand for isolated seed proteins continues to increase but functionality in food systems can be greatly dependent on the extraction method. In this work, we report the physicochemical and functional properties of lentil seed proteins isolated using various protocols. Lentil flour was defatted followed by protein extraction using isoelectric pH precipitation (ISO) as well as NaOH (MEM_NaOH) and NaCl (MEM_NaCl) extractions coupled with membrane ultrafiltration. The MEM_NaCl had significantly (p < 0.05) higher protein content (90.28%) than the ISO (86.13%) and MEM_NaOH (82.55%). At pH 3-5, the ISO was less soluble (2.26-11.84%) when compared to the MEM_NaOH (25.74-27.22%) and MEM_NaCl (27.78-40.98%). However, the ISO had higher yield and protein digestibility (48.45% and 89.82%) than MEM_NaOH (35.05% and 77.87%) and MEM_NaCl (13.35% and 77.61%), respectively. Near-UV circular dichroism spectra showed that the MEM_NaOH had loose tertiary conformation at pH 3, 5, 7 and 9 while ISO and MEM_NaCl had more compact structures at pH 7 and 9. The three protein isolates formed better emulsions (lower oil droplet sizes) at pH 7 and 9 when compared to pH 3 and 5. In contrast, foaming capacity was better at pH 5 than pH 3, 7, and 9.

Effect of pH and heat treatment on structure, surface characteristics and emulsifying properties of purified camel β-casein

Oil-in-water emulsions (20%/80%, w/w) were stabilised by two types of β-caseins (1 g/L, w/w) extracted by rennet coagulation from camel and cow's milk, respectively. Both extracts were treated under different ranges of pH (3.0, 6.0 and 9.0) and temperature (25, 65 and 95 °C for 15 min) before emulsification. The emulsifying properties of the proteins were studied by surface and interfacial measurements. Results show that the emulsifying activity (EAI) of camel β-casein is higher than the bovine protein. Yet, both proteins exhibited heat stability and nonsignificant effect of temperature was reported. Conversely, a significant effect of pH on camel β-casein was recorded: at pH 6.0, the lowest values of EAI were measured and explained by the formation of micellar protein structure. Under such conditions, camel β-casein is therefore a novel emulsifying protein with high potential to stabilise oil-in-water interfaces which provides numerous applications for the food chemistry field.

Structural characterization and functional properties of CNPP, a byproduct formed during CPP preparation

Casein nonphosphopeptide (CNPP), a byproduct formed during the preparation of casein phosphopeptide (CPP), is often discarded on a large scale. Although our previous studies have demonstrated the ameliorative effect of CNPP on muscle wasting disorders, its structure-function mechanism is still unclear. Therefore, considering the great influence of structural characteristics on function, this study aims to explain the potential mechanism by characterizing the physicochemical and functional properties of CNPP. The results of structural characterization indicated that CNPP was of low molecular weight and composed of the complete range of amino acids; it was particularly rich in leucine. Compared with casein, CNPP had a lower molecular size and total/free sulfhydryl content (reduced 2.44 and 2.02 µmol/g in CNPP, respectively). Additionally, Fourier transform infrared spectroscopic analysis revealed that enzymatic hydrolysis caused protein unfolding, and the content of β-turns and random coils reached 50.20% and 10.67%, respectively. Fluorescence-dependent detection of CNPP indicated a reduction of spectral intensity and the occurrence of a red shift. The changes in the structure of CNPP significantly affected its functional characteristics. CNPP has better solubility, foaming, and digestion properties than those of casein and whey protein. Specifically, the foam stability and emulsification properties decreased in the order of casein > CNPP > whey protein. The present study can provide a substantial basis for future application of CNPP as a functional ingredient against sarcopenia.

Comparison of legume and dairy proteins for the impact of Maillard conjugation on nanoemulsion formation, stability, and lutein color retention

While dairy proteins have traditionally been used to stabilize nanoemulsions, there is a trend towards plant-based formulations. Additionally, both types of protein are poorly soluble near their isoelectric point. The main goals of this research were to develop and characterize Maillard conjugates from pea protein (PPI) or caseinate and dextran, and to evaluate the physical stability of nanoemulsions made with such emulsifiers at various ionic strengths, pH = 4.6, and temperatures during storage, as well as lutein color retention over storage. Protein conjugates formed nanoemulsions with diameters of 125 ± 12 nm (PDI = 0.13 ± 0.00) and 269 ± 36 nm (PDI = 0.76 ± 0.42) (pH = 7) for caseinate and PPI, respectively. Conjugation improved the physical stability (droplet size) of emulsions at the isoelectric point, during storage at 4-55 °C, and in ionic solutions. Lutein color degradation was better associated with particle size than conjugation and was lowest for PPI-stabilized emulsions. This study suggests that Maillard conjugation could improve PPI emulsification properties.

Interfacial characteristics, colloidal properties and storage stability of dairy protein-stabilized emulsion as a function of heating and homogenization

This research investigated the influence of processing history on physicochemical properties of dairy protein-stabilized emulsions. Emulsions were heated (UHT) either before or after a single homogenization (UHTSH, SHUHT) or homogenized both before and after heating (double homogenization, DHUHT). The results demonstrated that UHT treatment increased the protein load at the oil/water interface while homogenization prior to UHT (SHUHT) inhibited displacement of protein by surfactant molecules, and this emulsion exhibited higher interfacial protein coverage and wider size distribution compared to the emulsion produced by UHTSH. The use of the double homogenization with UHT resulted in emulsion droplets with the smallest average size and lowest concentration of unabsorbed protein. However, no difference in the protein load in a specific area was noticed between emulsions produced by DHUHT and SHUHT. When changes of surface tension at the air/water interface were measured using a drop tensiometer, SHUHT emulsion showed the fastest decrease of surface tension due to the occurrence of a lower level of surfactant displacement where more surfactant was available for fast adsorption. Emulsions prepared with DHUHT or UHTSH decreased the surface tension in a slower speed than SHUHT. During storage, partial coalescence of emulsion droplets was observed for emulsions produced with single homogenization, regardless of whether this was carried out before or after heating. Double homogenization formed more stable emulsions than single homogenization. This work clearly showed that it is possible to tailor physico-chemical functionalities of dairy protein-based emulsions by controlling the interactions between proteins or with surfactants during processing.

This research investigated the influence of processing history on physicochemical properties of dairy protein-stabilized emulsions.

Effect of low-frequency ultrasound on the particle size, solubility and surface charge of reconstituted sodium caseinate

Low-frequency sonication (20 kHz) was applied to sodium caseinate suspensions (4%, 7% and 10% protein concentrations) at pH 4.0, 4.6, 6.7 and 9.0. Particle size, zeta potential and solubility analysis were used to evaluate the physical changes of the sodium caseinate suspensions before and after the application of ultrasound. At pH 6.7 the particle size remained between 5 and 7 µm for all concentrations before and after sonication (15-400 J/mL), resulting in no significant change (p > 0.05). Similarly, sonication did not significantly (p > 0.05) affect the solubility at pH 6.7. At this pH, the initial solubility was high at 94-98% (w/w) before sonication. At pH 9.0 for 4% and 7% concentrations, suspensions became more negatively charged and the initial particle size increased to 78-82 µm. In the presence of larger suspensions, the application of ≥15 J/mL reduced the particle size to less than 2 µm. By contrast to pH 6.7, the solubility at pH 9.0 for 4% and 7% protein suspensions reached 99% before and after sonication. Viscosity was the highest (80 mPa.s at 15 sec^-1) for a 10% protein concentration at pH 9.0. As the protein concentration of the sodium caseinate suspensions decreased from 10% to 4% at pH 9.0, the viscosity of the suspensions also decreased. However, application of low-frequency ultrasound had no effect on the viscosity of the sodium caseinate suspensions. Due to the absence of large insoluble aggregates in reconstituted sodium caseinate suspensions, the overall effect of low-frequency sonication were largely insignificant at native pH and only became evident at outlier pH values when the casein proteins associate.

Effect of Ohmic heating on functionality of sodium caseinate - A relationship with protein gelation

Sodium caseinate (NaCAS) is widely used in the food industry to provide nutritional and functional benefits. This work deals with the effects of applying moderate electric fields (MEF) of different intensity - ranging from 2 V·cm^-1 to 17 V·cm^-1 - on the physical and functional properties of NaCAS solutions during Ohmic heating (OH) at 95 °C. Self-standing gels were produced regardless the heating technique applied (i.e. conventional or OH), and these gels were much more prone to physical rupture when compared with the ones produced from unheated NaCAS. Interestingly, OH treatment formed gels with lower values of strain at rupture and water holding capacity than unheated samples; this pattern was not observed for gels obtained through the conventional heating treatment (at 0 V·cm^-1). These effects may be linked with disturbances of the distribution of random coil structures and enhanced solubility of NaCAS at its isoelectric point, reducing aggregation and impairing the development of a more compact protein network. Results show that OH presents potential to be used as volumetric heating tool for NaCAS solubilization and for the production of distinctive acidified systems.

Complexation of sodium caseinate with gum tragacanth: Effect of various species and rheology of coacervates

We investigated complex coacervation of sodium caseinate/Astragalus rahensis (A.r) as a function of pH with light scattering, spectrophotometry, and viscosity measurements. Interestingly, sodium caseinate/A.r displayed five structural transitions; pH 7.00 to pH ∼5.40: no interaction occurred, pH ∼5.40 to pH ∼4.80: initiation of the formation of primary soluble complexes, pH ∼4.80 to ∼4.30: formation of interpolymer complexes, pH ∼4.30 to ∼4.02: optimum coacervation and pH ∼4.02 to ∼2.50: suppression of coacervation. In addition, rheological properties of sodium caseinate/A.r coacervates were studied at various pH values. A much higher storage modulus (G') than loss modulus (G″) for all sodium caseinate/A.r coacervates suggests the formation of highly interconnected gel-like network structures with mainly elastic behaviour. Moreover, sodium caseinate/A.r coacervates at all pH values exhibited a shear thinning behaviour across the entire shear rate range investigated. Effects of different species of gum tragacanth on the interactions with sodium caseinate have been scarcely studied. Our study showed that systems containing various species (A.r, soluble fraction of A.r and Astragalus gossypinus (A.g)) had different critical pH values and particle sizes during complex coacervation, which could be due to different ratio of soluble to insoluble fractions and uronic acid content of various species.

Study of chemical stability of lemon oil components in sodium caseinate–lactose glycoconjugate-stabilized oil-in-water emulsions using solid-phase microextraction-gas chromatography

A method was developed to quantify lemon oil components and their degradation products in oil-in-water emulsions.

Thermal protection of β-carotene in re-assembled casein micelles during different processing technologies applied in food industry

β-Carotene is a carotenoid usually applied in the food industry as a precursor of vitamin A or as a colourant. β-Carotene is a labile compound easily degraded by light, heat and oxygen. Casein micelles were used as nanostructures to encapsulate, stabilise and protect β-carotene from degradation during processing in the food industry. Self-assembly method was applied to re-assemble nanomicelles containing β-carotene. The protective effect of the nanostructures against degradation during the most common industrial treatments (sterilisation, pasteurisation, high hydrostatic pressure and baking) was proven. Casein micelles protected β-carotene from degradation during heat stabilisation, high pressure processing and the processes most commonly used in the food industry including baking. This opens new possibilities for introducing thermolabile ingredients in bakery products.

Actinidin enhances gastric protein digestion as assessed using an in vitro gastric digestion model

Consumption of kiwifruit has long been claimed anecdotally to assist in gastric digestion. This has generally been assumed to be due to the presence of the proteolytic enzyme actinidin; however, there is little published evidence supporting this assumption. This paper reports the findings of an in vitro study that examined the effect of kiwifruit proteases (actinidin) on the digestion of a range of common food proteins under simulated gastric conditions. An extract from green kiwifruit containing actinidin was prepared. Several protein sources derived from soy, meat, milk, and cereals were incubated in the presence or absence of the kiwifruit extract using an in vitro digestion system consisting of incubation with pepsin at pH 1.9, simulating gastric digestion in humans. The digests were subjected to gel electrophoresis (SDS-PAGE). For some protein sources, simulated digestion in the presence of kiwifruit extract resulted in a substantially greater loss of intact protein and different peptide patterns from those seen after digestion with pepsin alone. As an example, the addition of actinidin extract enhanced the digestion of alpha-, beta-, and kappa-caseins in sodium caseinate by 37, 33, and 48%, respectively. Under simulated gastric conditions, kiwifruit extract containing actinidin enhanced the digestion of some, but not all, food proteins over and above that found with pepsin alone.

Micellization of casein-graft-dextran copolymer prepared through Maillard reaction

Casein is almost insoluble at around pH 4.6, which is its isoelectric point (pI). Grafting copolymer, casein-g-dextran, was prepared through the Amadori rearrangement of the Maillard reaction. The copolymer has a reversible pH sensitive property: micellization at the pI of casein forming a casein core and dextran shell structure and dissociation when pH differs from the pI. The micelles produced at pH 4.6 have a spherical shape and their size is dependent on the Maillard reaction: reaction time, molar ratio of casein to dextran, and molecular weight of dextran used. Typically, the hydrodynamic diameter of the micelles is about 100 nm and the critical micelle concentration is about 10 mg/L. The micelles are very stable in aqueous solution and can be stored as lyophiled powder. The micelles are able to encapsulate hydrophobic compounds such as pyrene.

Properties of Sodium Caseinate Film-Forming Dispersions and Films

The flow properties of sodium caseinate (NaCAS) film-forming dispersions were studied as a function of protein and glycerol concentration. Apparent viscosity vs. concentration profiles showed a disruption at about 10% (wt/wt) NaCAS. This was interpreted in terms of protein-water and protein-protein interactions. The presence of glycerol in the NaCAS film-forming suspensions reduced the viscosity of aqueous NaCAS as a consequence of decreasing protein-protein and protein-solvent interactions through the hydrogen bonding formation between the glycerol, the polypeptide chain of NaCAS, and water molecules. For higher concentration of glycerol, viscosity of NaCAS dispersions increased as a result of increasing total solid content. The glass transition temperature for solid films of 10% (wt/wt) NaCAS decreased with the glycerol content. The glycerol-protein interactions reduced the protein-protein interaction and, thus, increased the intermolecular spacing, leading to higher protein chain mobility and water vapor permeability.