Protein–protein interactions of a whey–pea protein co-precipitate

The aim of this study was to investigate the mechanisms behind protein–protein interactions in a co-precipitate of whey protein isolate (WPI) and pea protein isolate (PPI). A co-precipitate and blend, consisting of 80% WPI and 20% PPI, were compared. Covalent disulphide interactions were studied by blocking of free thiols with N-Ethylmaleimide (NEM), while electrostatic interactions were studied in systems with 0.5 m NaCl and hydrophobic interactions with 0.2% SDS. Protein solubility, stability and secondary, tertiary and quaternary protein structures were analysed. Co-precipitation did not introduce different protein–protein interactions than the direct blending of proteins. SDS affected solubility (P < 0.05), secondary and tertiary structure. However, the effects of NEM and NaCl were significant greater (P < 0.05) on the same parameters and thermal stability, especially when combined (P < 0.01). Thus, the protein–protein interactions in a whey–pea co-precipitate and protein blend consisted of disulphide bonds and electrostatic interactions.     

Isolation and characterisation of κ-casein/whey protein particles from heated milk protein concentrate and role of κ-casein in whey protein aggregation

Milk protein concentrate (79% protein) reconstituted at 13.5% (w/v) protein was heated (90 °C, 25 min, pH 7.2) with or without added calcium chloride. After fractionation of the casein and whey protein aggregates by fast protein liquid chromatography, the heat stability (90 °C, up to 1 h) of the fractions (0.25%, w/v, protein) was assessed. The heat-induced aggregates were composed of whey protein and casein, in whey protein:casein ratios ranging from 1:0.5 to 1:9. The heat stability was positively correlated with the casein concentration in the samples. The samples containing the highest proportion of caseins were the most heat-stable, and close to 100% (w/w) of the aggregates were recovered post-heat treatment in the supernatant of such samples (centrifugation for 30 min at 10,000 × g). κ-Casein appeared to act as a chaperone controlling the aggregation of whey proteins, and this effect was stronger in the presence of α_S- and β-casein.     

Rheology of galactomannan–whey protein mixed systems

The rheological behaviour of whey protein/galactomannan mixtures in aqueous solutions was studied under gelling conditions of the protein component, at neutral pH and at a pH close to the protein isoelectric point. The presence of the neutral polysaccharide had significant effects on the formation and viscoelastic behaviour of the cured gels. This effect was dependent on the structural organisation of the protein network. At pH 7 the galactomannan had a general positive effect on WPI gel formation. It is suggested that under these conditions, the protein network forms a continuous phase that accommodate the polysaccharide chains, acting as a filler of the protein network. The minimum protein concentration for gelation to occur, the gelation temperature and time all decrease in the presence of the galactomannan. Under pH conditions near the whey protein isoelectric point, different effects were observed as a result of the galactomannan addition. At low WPI concentration, the galactomannan had a detrimental influence on the protein network formation, but a negligible effect or even a positive influence on the gelation process at higher concentrations.     

Gel mechanical properties of milk whey protein–dextran conjugates obtained by Maillard reaction

The effect of Maillard reaction on the mechanical properties of whey protein isolate (WPI) heat-induced gels was evaluated. WPI and dextran (15–25 kDa) conjugates were obtained by controlled dry heating during storage at 60 °C and 63% relative humidity for 2, 5 and 9 days. Changes in browning intensity and content of free amino groups were used to estimate the Maillard reaction. A decrease in free amino groups of WPI was observed when increasing polysaccharide concentration and reaction time. An increase in both a* and b* CIE Lab colour parameters indicated the development of a reddish-brown colour, typical of the Maillard reaction. Uniaxial compression and stress relaxation tests were performed to measure the mechanical properties of mixed and conjugate gels. Maillard reaction significantly modified the mechanical properties of WPI/DX gels, and even prevented fracture when conjugate gels were subjected to 80% deformation in uniaxial compression test.     

Pilot-scale formation of whey protein aggregates determine the stability of heat-treated whey protein solutions—Effect of pH and protein concentration

Denaturation and consequent aggregation in whey protein solutions is critical to product functionality during processing. Solutions of whey protein isolate (WPI) prepared at 1, 4, 8, and 12% (wt/wt) and pH 6.2, 6.7, or 7.2 were subjected to heat treatment (85°C × 30 s) using a pilot-scale heat exchanger. The effects of heat treatment on whey protein denaturation and aggregation were determined by chromatography, particle size, turbidity, and rheological analyses. The influence of pH and WPI concentration during heat treatment on the thermal stability of the resulting dispersions was also investigated. Whey protein isolate solutions heated at pH 6.2 were more extensively denatured, had a greater proportion of insoluble aggregates, higher particle size and turbidity, and were significantly less heat-stable than equivalent samples prepared at pH 6.7 and 7.2. The effects of WPI concentration on denaturation/aggregation behavior were more apparent at higher pH where the stabilizing effects of charge repulsion became increasingly influential. Solutions containing 12% (wt/wt) WPI had significantly higher apparent viscosities, at each pH, compared with lower protein concentrations, with solutions prepared at pH 6.2 forming a gel. Smaller average particle size and a higher proportion of soluble aggregates in WPI solutions, pre-heated at pH 6.7 and 7.2, resulted in improved thermal stability on subsequent heating. Higher pH during secondary heating also increased thermal stability. This study offers insight into the interactive effects of pH and whey protein concentration during pilot-scale processing and demonstrates how protein functionality can be controlled through manipulation of these factors.     

Progressive freeze concentration of whey protein–sucrose–salt mixtures

Progressive freeze concentration of whey protein solutions is evaluated. Since solutions in industry are more complex, the effect of the addition of sodium chloride and sucrose on the inclusion behaviour is studied as well. Using a progressive freeze concentrator solutions of whey protein and mixtures of whey protein and/or sucrose and/or sodium chloride were freeze concentrated. At an initial concentration of 4%(w/w), whey proteins were not included in the ice fraction. At higher concentrations the inclusions are caused by the increase in viscosity in the boundary layer, impeding mass transfer. The addition of sucrose caused a similar effect. Presence of sodium chloride causes inclusions through the occurrence of a zone where the solution is locally super-cooled and leads to the formation of dendritic ice which encapsulates pockets of solution in the ice layer. Mixtures of both sucrose and sodium chloride gave no additive effect on solute inclusion but just a concurrent effect.     

Co-precipitation of whey and pea protein – indication of interactions

The aim was to optimise the yield of co-precipitation of whey protein isolate (WPI) and pea protein isolate (PPI) and compare co-precipitates and protein blends with respect to solubility. The yield of co-precipitates was tested with different protein ratios of WPI and PPI in combination with different temperatures and acid precipitation (pH 4.6). The highest precipitation yield was obtained at protein ratios WPI < PPI, high temperature and alkaline protein solvation. The solubility was measured by an instability index and absorption spectroscopy of re-suspended precipitated proteins at pH 3, 7 and 11.5. Co-precipitates had significantly lower solubility than protein blends. Protein ratios WPI > PPI, low precipitation temperature and high pH showed the highest solubility. Differences in protein composition between co-precipitates and protein blends were observed with SDS-PAGE and matrix-assisted laser desorption ionisation time-of-flight, and indicated different protein–protein interaction in samples, which needs further investigations.     

Quinoa protein–chitosan–sunflower oil edible film: Mechanical,barrier and structural properties

Quinoa protein extracts (Q) were prepared and alkalised at pH 8 and 12 (Q-8 and Q-12). Qs were mixed with chitosan (CH) to form Q/CH mixtures. The optimal proportion of the mixtures was determined by the formation of coacervates. All the films were obtained by solution casting. From the optimal Q/CH mixture and the addition of three different concentrations of sunflower oil (SO) 2.9, 3.8 and 4.7 g/100 mL, and the optimal proportion of SO g/100 mL was selected based on the mechanical and barrier properties of the films. The CH, Q/CH and Q/CH/SO optimal blend films were characterised by FTIR, X-ray diffraction, and SEM. The physicochemical properties of the films were also evaluated. The 0.1 Q-8/CH blend was selected due to its high degree of interaction between the quinoa proteins and CH. The optimum concentration of SO used in the Q-8/CH/SO film was 2.9 g/100 mL. The addition of SO to the film improved the water-vapour permeability (WVP) as a result of hydrophobic interactions and the presence of clusters of hydrophobic masses on the surfaces of these films but reduced the film’s tensile strength and oxygen permeability due to the formation of micropores and microfractures detected by SEM.     

Development and characterization of an edible chitosan–whey protein nano composite film for chestnut (Castanea mollissima Bl.) preservation

Chitosan (CHI) and whey protein are usually used to prepare edible films for food preservation. However, the composite film composed of the two components does not yield satisfactory properties for chestnut preservation. In this study, nano-cellulose and cinnamaldehyde (CMA) were added to CHI and whey protein, creating a new composite film with strong water retention, bacteriostatic, and mechanical properties. The water vapor permeability (WVP) of the film decreased by 21.61% with the addition of 0.5% (w/v) nano-cellulose, and 23.02% with the addition of 0.3% (w/v) CMA. Furthermore, water solubility (WS) decreased 22.05%, and the density of the film was significantly improved with the addition of 0.3% (w/v) CMA. The optimized formula of the film was CHI 2.5% (w/v), whey protein 3.0% (w/v), nano-cellulose 0.5% (w/v), CMA 0.3% (w/v), and pH 3.8, as determined by orthogonal testing L9(3⁴), with fuzzy comprehensive assessment, of WVP, WS, tensile strength, and elongation at break. The film clearly inhibited the growth of E. coli, S. aureus, and Chinese chestnut fungus, destroying the mycelial structure of the fungus. In addition, coating effectively reduced the weight loss, mildew rate, and calcification index during 16 days of storage of chestnuts at 25 °C.     

Emulsifying properties of soy whey protein isolate–fenugreek gum conjugates in oil-in-water emulsion model system

Soy whey protein isolate (SWPI)–fenugreek gum conjugates were prepared by Maillard type reactions in a controlled dry state condition (60 °C, 75% relative humidity for 3 days) to improve emulsification properties. SDS-PAGE electropherogram showed that conjugation formed high molecular weight products with the disappearance of 7S fraction, acidic subunits of the 11S fractions and protein band at molecular weight 21 and 24 kDa. However, the amount of protein at molecular weight 30 kDa remained unchanged. The protein solubility of SWPI–fenugreek gum conjugates improved as compared to SWPI and SWPI–fenugreek gum non-conjugated mixture especially at isoelectric point of protein when assessed in the pH range 3–8 at 22 °C. In comparison to SWPI, fenugreek gum and non-conjugated SWPI–fenugreek gum, SWPI–fenugreek gum conjugates had better emulsifying properties near the isoelectric pH of protein. Emulsification at near the isoelectric pH of protein was chosen as at this pH the proteins are prone to aggregate, which could destabilize the emulsion. Heating solutions of the conjugates prior to emulsification further improved their emulsification properties. The conjugates also showed a better emulsifying property at high salt concentration as compared to SWPI alone.     

Properties of konjac glucomannan–whey protein isolate blend films

Edible composite packaging has been developed by blending biocomponents for specific applications, aiming to take advantage of complementary functional properties or to overcome their respective flaws. The aim of this work was to study the effect of incorporation of whey protein isolate (WPI) on the properties of konjac glucomannan (KGM) based films. Five aqueous solutions of KGM and/or WPI were prepared by casting and solvent evaporation of 1:0, 0.8:3.4, 0.6:3.6, 0.4:3.8 and 0:4.2 g KGM:g WPI/100 g solution. Glycerol (Gly) was used as a plasticizer at 1.5 and 1.8 g/100 g solution. The result showed that incorporated WPI proportionally increased transparency of KGM-based films. An increase in proportion of WPI resulted in decreased tensile strength and elastic modulus as well as improved flexibility. The incorporation of WPI into the KGM matrix led to an increase in water insolubility which enhanced product integrity and water resistance. Nevertheless, WPI did not improve water vapor barrier of KGM–WPI films. WPI and blend film with the highest concentration of WPI could be heat sealed at 175 °C. Overall, the range of Gly in this study did not apparently affect properties of the films.     

Effects of ultrasonic treatment on the gel properties of microbial transglutaminase crosslinked soy,whey and soy–whey proteins

Food Science and Biotechnology - This paper studied the influences of diverse ultrasonic power treatments on the physico-chemical properties of soy–whey mixed protein induced by microbial...     

Functional properties and Angiotensin-I converting enzyme inhibitory activity of soy–whey proteins and fractions

Soy proteins when prepared to high purity can confer good functional properties and the whey by-product is a potential source for bioactivity. In this study, we determined the protein, moisture, fiber, solubility, foaming, emulsion properties, as well as Angiotensin-I converting enzyme (ACE-I) inhibitory activity of prepared soy–whey proteins and its fractions. The soy–whey proteins were fractionated into < 5, > 5, > 10, and > 50 kDa using ultrafiltration. The expanded AACC methods were used to determine protein, moisture, and fiber analyses of the whey and its fractions. Solubility method was conducted to determine the protein solubility profile of the soy–whey and its fractions at varying pHs. Turbidimetric method was used to evaluate emulsifying activity (EA) and emulsion stability (ES). There were significant differences observed in moisture, protein and salt contents between unfractionated, > 50 kDa and smaller sized fractions. No significant differences were observed with phytic acid and total dietary fiber contents among all samples. The unfractionated whey protein and > 50 kDa fraction showed better solubility than other fractions. Unfractionated whey protein had the highest foam capacity (42.7 mL) while the fraction > 5 kDa showed the greatest foaming stability (46 min). The highest emulsion activity (0.33 ± 0.1) and stability (825.1 ± 45.1) was obtained with the > 50 kDa fraction while the unfractionated whey protein had the highest ACE-I inhibition activity. The findings indicate that soy–whey protein fraction (> 50 kDa) had good solubility, emulsion activity and stability, while the unfractionated whey protein exhibited the strongest ACE-I inhibition percentage (19%).     

Partition and digestive stability of α-tocopherol and resveratrol/naringenin in whey protein isolate emulsions

Co-encapsulation of multiple bioactive components is an emerging field that shows promise as an approach to develop functional foods. Hydrophobic components are generally dissolved in the inner oil phase of protein-stabilised emulsions. Some components may co-adsorb to oil droplet surfaces, due to the ligand-binding properties of proteins. In this study, α-tocopherol and resveratrol/naringenin were co-encapsulated in emulsions stabilised by whey protein isolate (WPI). α-Tocopherol was totally encapsulated and its partitioning inside oil droplets was about 3.3 times that bound by free WPI in the aqueous phase. The total encapsulation efficiency for resveratrol or naringenin was 52% and 58%, respectively. Addition of resveratrol improved digestive stability of α-tocopherol, but naringenin did not. Co-encapsulation with α-tocopherol had no significant influence on the digestive stability of resveratrol/naringenin. The data gathered here should be useful for the delivery of bioactive components with different solubilities.     

Influence of milk pre-heating conditions on casein–whey protein interactions and skim milk concentrate viscosity

The viscosity of concentrates (50–55% total solids) prepared from skim milk heated (5 min at 80 or 90 °C) at pH 6.5 and 6.7 was examined. The extent of heat-induced whey protein denaturation increased with increasing temperature and pH. More denatured whey protein and κ-casein were found in the serum phase of milk heated at higher pH. The viscosity of milk concentrates increased considerably with increasing pH at concentration and increasing heating temperature, whereas the distribution of denatured whey proteins and κ-casein between the serum and micellar phase only marginally influenced concentrate viscosity. Skim milk concentrate viscosity thus appears to be governed primarily by volume fraction and interactions of particles, which are governed primarily by concentration factor, the extent of whey protein denaturation and pH. Control and optimization of these factors can facilitate control over skim milk concentrate viscosity and energy efficiency in spray-drying.     

Gelation of mixed systems whey protein concentrate–gluten in acidic conditions

Gels of whey protein concentrate (WPC)–gluten were prepared by heating WPC–gluten dispersions (10% whey protein/0–5–10% gluten protein, w/w; pH 3.75 or 4.2). Gels were characterized through solubility assays in different extraction solutions, measures of water-holding capacity (WHC), firmness, elasticity and relaxation time, and light microscopy. Differential scanning calorimetry (DSC) of WPC–gluten dispersions was also performed. Gluten increases the firmness and elasticity of gels, mainly at pH 4.2. The WHC also increases with gluten content, being higher at pH 3.75 than at pH 4.2. Solubility assays indicate that electrostatic forces, hydrophobic and H bindings would be involved in maintaining the gel structure of WPC gels at pH 3.75 and 4.2, whereas in mixed gels of WPC–gluten, the principal forces responsible for the maintenance of the gel structure at these pHs would be hydrophobic and H bindings, and in gels prepared at pH 4.2 also disulfide bonds, but in a minor extent. The presence of gluten shifts the apparent transition temperature for whey protein denaturation towards lower temperatures. Gels with gluten present a smooth network with gaps and a more elastic appearance, as observed by light microscopy.     

Fabrication and evaluation of physicochemical properties of probiotic edible film based on pectin–alginate–casein composite

The objective of this study was to develop a casein-based edible film for the entrapment of probiotic Enterococcus faecium Rp1. Casein, pectin, sodium alginate and glycerol were used to prepare the film. In this study, the physicochemical and morphological properties of casein-based edible film and its impact on the stability of probiotic were evaluated. Surface morphology and properties of the film were tested using a scanning electron microscope, fluorescence microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy and X-ray diffraction. Probiotic-incorporated casein-based edible film showed significant improvement in the antimicrobial and antioxidant properties and enhanced the structural, optical and thermal properties. Furthermore, the film was found to be desirable to carry probiotics, with the viability of 10⁷ CFU mL⁻¹ rate up to 30 days of storage at 4 °C. Hence, the current study suggests a probiotic-incorporated casein-based edible film for active packaging of food products.     

Effects of protein interactions on properties and microstructure of zein–gliadin composite films

Zein and gliadin are both readily dissolved in aqueous ethanol and have good film-forming property. This article describes an attempt to improve the flexibility of zein films by the addition of gliadin to the zein film-forming solution. The properties of zein–gliadin composite films, i.e., color, transparency, moisture content, water solubility, water vapor permeability, dynamic contact angle which in turn affected the mechanical property, water resistance and glass transition temperature of films were investigated. The contents of second structure were characterized via Fourier transform infrared spectroscopy (FTIR), whereas morphology of films was examined by scanning electron microscopy (SEM). It was observed that the addition of gliadin enhanced the strain at break of zein–gliadin composite films as a result of the increase in the content of α-helix, β-turn structures and decrease in the level of β-sheet structure. The water resistance of films decreased with the content of gliadin increasing. Morphology of composite films showed that gliadin and zein organized a homogeneous material. This work opens a new perspective for zein in flexible food package.