Stability of oil‐in‐water emulsions as influenced by thermal treatment of whey protein dispersions or emulsions

Properties of whey protein concentrate stabilised emulsions were modified by protein and emulsion heat treatment (60–90 °C). All liquid emulsions were flocculated and the particle sizes showed bimodal size distributions. The state and surface properties of proteins and coexisting protein/aggregates in the system strongly determined the stability of heat‐modified whey protein concentrate stabilised emulsions. The whey protein particles of 122–342 nm that formed on protein heating enhanced the stability of highly concentrated emulsions. These particles stabilised protein‐heated emulsions in the way that is typical for Pickering emulsions. The emulsions heated at 80 and 90 °C gelled due to the aggregation of the protein‐coated oil droplets.     

Physicochemical properties and issues associated with trypsin hydrolyses of bovine casein-dominant protein ingredients

Milk protein concentrate (MPC) and sodium caseinate (NaCas) were hydrolysed using the enzyme trypsin and the subsequent physical properties of the two ingredients were examined. Trypsin hydrolysis was carried out at pH 7 and at 45 °C on 11.1% (w/w) protein solutions. Heat inactivation of trypsin was carried out when the degree of hydrolysis reached either 10 or 15%. Size-exclusion chromatography and electrophoresis confirmed a significant reduction in protein molecular weight in both ingredients. However, whey proteins in MPC were more resistant to trypsin hydrolysis than casein. Oil-in-water emulsions were prepared using intact or hydrolysed protein, maltodextrin, and sunflower oil. Protein hydrolysis had a negative effect on the subsequent physical properties of emulsions, compared with non-hydrolysed proteins, with a larger particle size (only for NaCas stabilised emulsions), faster creaming rate, lower heat stability, and increased sedimentation observed in hydrolysed protein emulsions.     

The effect of protein profile and preheating on denaturation of whey proteins and development of viscosity in milk protein beverages during heat treatment

The effect of preheat temperature (63 or 77 °C for 30 s; final heat 120 °C for 30 s) and casein to whey protein ratio on the physical characteristics of 3.3%, w/w, dairy protein beverages was investigated. Dispersions preheated at 77 °C had lower viscosity than dispersions preheated at 63 °C. Casein‐containing dispersions had significantly lower levels of α‐lactalbumin denaturation than whey protein‐only dispersions. A higher proportion of casein improved the thermal stability of protein dispersions. Overall, alteration of preheat temperature and casein to whey protein ratio can influence dairy beverage quality, with increasing levels of casein reducing physical changes due to heat treatment.     

High pressure versus heat treatments for pasteurisation and sterilisation of model emulsions

Heat treatments can have considerable influence on the droplet size distribution of oil-in-water emulsions. In the present study, high-pressure (HP) pasteurisation and sterilisation were evaluated as alternatives for heat preservation of emulsions. HP conditions used were 600 MPa, 5 min, room temperature and 800 MPa, 5 min, 80 °C initial temperature, 115 °C maximum temperature for HP pasteurisation and HP sterilisation respectively. The effects on droplet size of these conditions were compared to heat treatments for whey protein isolate (WPI) and soy protein isolate (SPI) emulsions at two pH values and two ionic strengths. For WPI, also the effect of protein in the bulk phase was evaluated.Both HP and heat pasteurisation treatments resulted in similar or slightly decreased average droplet sizes compared to the untreated samples. For neutral SPI emulsions, heat sterilisation increased the average droplet size from 1.6 μm to 43.7 μm, while HP sterilisation resulted only in a small increase towards an average droplet size of 2.1 μm. The neutral WPI emulsions, except those with a high ionic strength, gave similar results with respect to the droplet size, showing that for neutral pH WPI or SPI emulsions HP sterilisation is preferable above heat sterilisation. Concerning the low pH WPI emulsions, the droplet sizes were unaffected after both heat and HP sterilisation.Industrial relevanceHeat pasteurisation and sterilisation are effective treatments to preserve food products that are based on emulsions with respect to microbial safety. However, heat treatments can negatively affect emulsion stability. Currently, in addition to high pressure at room temperature, high-pressure treatments at elevated temperature received a great deal of interest to achieve sterilised products. This study evaluated the effects of both heat and high-pressure pasteurisation and sterilisation on droplet size of whey protein isolate and soy protein isolate emulsions. It was shown that for pasteurisation treatments, both heat and high pressure have minor effects on the droplet size of the emulsion. However, for sterilisation purposes high-pressure treatment is preferable for emulsion at neutral pH. High-pressure sterilisation can therefore be interesting alternatives to heat treatments to preserve emulsion stability.     

Heat stability of oil-in-water emulsions formed with intact or hydrolysed whey proteins: influence of polysaccharides

The heat stability of emulsions (4 wt% corn oil) formed with whey protein isolate (WPI) or extensively hydrolysed whey protein (WPH) products and containing xanthan gum or guar gum was examined after a retort treatment at 121 °C for 16 min. At neutral pH and low ionic strength, emulsions stabilized with both 0.5 and 4 wt% WPI (intact whey protein) were stable against retorting. The amount of β-lactoglobulin (β-lg) at the droplet surface increased during retorting, especially in the emulsion containing 4 wt% protein, whereas the amount of adsorbed α-lactalbumin (α-la) decreased markedly. Addition of xanthan gum or guar gum caused depletion flocculation of the emulsion droplets, but this flocculation did not lead to their aggregation during heating. In contrast, the droplet size of emulsions formed with WPH increased during heat treatment, indicating that coalescence had occurred. The coalescence during heating was enhanced considerably with increasing concentration of polysaccharide in the emulsions, up to 0.12% and 0.2% for xanthan gum and guar gum, respectively; whey peptides in the WPH emulsions formed weaker and looser, mobile interfacial structures than those formed with intact whey proteins. Consequently, the lack of electrostatic and steric repulsion resulted in the coalescence of flocculated droplets during retort treatment. At higher levels of xanthan gum or guar gum addition, the extent of coalescence decreased gradually, apparently because of the high viscosity of the aqueous phase.     

Improvement in emulsifying properties of soy protein isolate by conjugation with maltodextrin using high‐temperature,short‐time dry‐heating Maillard reaction

Soy protein isolate (SPI)–maltodextrin (MD) conjugates were synthesised using Maillard reaction under high‐temperature (90, 115 and 140 °C), short‐time (2 h) dry‐heating conditions. The loss of free amino groups in proteins and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS‐PAGE) profile confirmed that SPI‐MD conjugates were formed and higher dry‐heated temperatures could increase the glycosylation degree. The emulsifying properties of SPI and SPI‐MD conjugates were evaluated in oil‐in‐water emulsions. The emulsions stabilised with SPI‐MD conjugates synthesised at 140 °C exhibited higher emulsifying stability and excellent storage stability against pH, ionic strength and thermal treatment compared with those synthesised at 90 °C, 115 °C and SPI stabilised emulsions. This might be due to a greater proportion of conjugated MD in SPI‐MD conjugates synthesised at 140 °C because of the higher glycosylation degree, and more conjugated MD on the droplet surface could provide steric effect and enhance the stability of the droplets in the emulsions.     

Rheological properties and structure of inulin–whey protein gels

The rheological properties, structure and synergistic interactions of whey proteins (1–7%) and inulin (20% and 35%) were studied. Gelation of whey proteins was induced with Na⁺. Inulin was dissolved in preheated whey protein solutions (80 °C, 30 min). Inulin gel formation was strongly affected by whey proteins. The presence of whey proteins at a level allowing for protein gel network formation (7%) significantly increased the G′ and G″ values of the gels. Scanning electron micrographs showed a thick structure for the mixed gel. Whey proteins at low concentrations (1–4%) were not able to form a gel; further, these low concentrations partly or wholly impaired formation of a firm inulin gel. Although interactions between inulin and whey proteins may be concluded from hydrophobicity measurements, the use of an electrophoretic technique did not show any inulin–whey protein complexes.     

Characterisation of heat-set milk protein gels

Milk protein solutions [10% protein, 40/60 whey protein/casein ratio containing whey protein concentrate (WPC) and low-heat or high-heat milk protein concentrate (MPC)] containing fat (4% or 14%) and 70–80% water, form gels with interesting textural and functional properties if heated at high temperatures (90 °C, 15 min; 110 °C, 20 min) without stirring. Adjustment of pH before heating (HCl or glucono-δ-lactone) produces soft, spoonable gels at pH 6.25–6.6, but very firm, cuttable gels at pH 5.25–6.0. Gels made with low-heat MPC, WPC and low fat gave some syneresis; high-fat gels were slightly firmer than low-fat gels. Citrate markedly reduced gel firmness; adding calcium had little effect on firmness, but increased syneresis of low-heat MPC/WPC gels. The gels showed resistance to melting, and could be boiled or fried without flowing. Microstructural analysis indicated a network structure of casein micelles and fat globules interlinked by denatured whey proteins.     

Nanostructures and polymorphisms in protein stabilised lipid nanoparticles,as food bioactive carriers: contribution of particle size and adsorbed materials

Eight oil-in-water emulsions were prepared using melt high-pressure homogenisation (HPH) at 300 or 1200 Bar. The emulsions produced from lipid phase (20%) were composed by palm oil alone or in mixture with α-tocopherol at 4:1 weight ratio, and an aqueous phase containing whey proteins alone or in mixture with phospholipids. The resulting nanoemulsions (fat droplet size ranging from 200-500 nm) presented different stability against aggregation and coalescence, fat crystallinity and polymorphisms in relation to different degrees of α-tocopherol encapsulation and protection against chemical degradation. Protein stabilised emulsions were monomodal, while emulsions stabilised by proteins and lecithins were slightly bimodal. Application of an isothermal treatment (4 °C for 2 hours) to these emulsions showed crystallization peaks located at longer time values in smaller particle size emulsions, while in the presence of added α-tocopherol average particle size values were higher and crystallization was not observed in 2 hours storage. Study of fat polymorphisms performed after 12 hours storage at 4 °C revealed the formation of 2L structures with coexistence of α, β’ and β forms in all of the emulsions. Increasing HPH from 300 to 1200 Bar favoured development of β structure (4.5 A^-1) in α-tocopherol added emulsions, with the presence of one extra peak β structure evolved at 3.9 A^-1 only in emulsions containing lecithins. α-tocopherol addition decreased in 2L structures (by approx. 40-50%). The formation of lipid nanoparticles with decreasing size values (increasing HPH parameters) was accompanied by increased long-term stability against aggregation and coalescence, but increased vitamin degradation (up to 15 wt% for 1200 bar). Degradation of α-tocopherol after 2 months storage at 4 °C was lower for nanoparticles stabilised by whey proteins alone (21 and 33%, respectively) than for nanoparticles stabilised by whey proteins in mixture with phospholipids and presenting higher size values (44 and 52%, respectively), where β polymorphs were more evolved.     

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.     

Influence of EDTA and citrate on thermal stability of whey protein stabilized oil-in-water emulsions containing calcium chloride

The influence of calcium ions and chelating agents on the thermal stability of model nutritional beverages was examined. Oil-in-water emulsions (6.94% (w/v) soybean oil, 0.35% (w/v) WPI, 0.02% (w/v) sodium azide, 20 mM Tris buffer, 0–10 mM CaCl₂, and 0–40 mM EDTA or citrate, pH 7.0) were stored at temperatures between 30 and 120 °C for 15 min. The particle size, particle charge, creaming stability, rheology, and free-calcium concentration of the emulsions were then measured. In the absence of chelating agents, appreciable droplet aggregation occurred in emulsions held at temperatures from 80 to 120 °C, which led to increased emulsion particle diameter, shear-thinning behavior, apparent viscosity, and creaming instability. Addition of chelating agents to the emulsions prior to heating decreased, but did not prevent, droplet aggregation in the emulsions. EDTA was more effective than citrate in decreasing droplet aggregation. Heat treatment increased the amount of chelating agents required to prevent droplet aggregation in the emulsions. Free-calcium concentration and droplet surface potential was independent of heat-treatment temperature, indicating that the performance of the chelating agents in binding calcium ions was not affected by the heat treatment. It was suggested that increased hydrophobic attractive interactions between the droplets occurred during heating, which induced droplet aggregation.     

Performance of whey protein hydrolysate–maltodextrin conjugates as emulsifiers in model infant formula emulsions

Model infant formula emulsions containing 15.5, 35.0 and 70.0 g L⁻¹ protein, soybean oil and maltodextrin (MD), respectively, were prepared. Emulsions were stabilised by whey protein hydrolysate (WPH) + CITREM (9 g L⁻¹), WPH + lecithin (9 g L⁻¹) or WPH conjugated with MD (WPH–MD). All emulsions had mono-modal oil droplet size distributions post-homogenisation with mean oil droplet diameters (D_4,3) of <1.0 μm. No changes in the D_4,3 were observed after heat treatment (95 °C, 15 min) of the emulsions. Accelerated storage (40 °C, 10 d) of unheated emulsions resulted in an increase in D_4,3 for CITREM (2.86 μm) and lecithin (5.36 μm) containing emulsions. Heated emulsions displayed better stability to accelerated storage with no increase in D_4,3 for CITREM and an increase in D_4,3 for lecithin (2.71 μm) containing emulsions. No increase in D_4,3 over storage was observed for unheated or heated WPH–MD emulsion, indicating its superior stability.     

Depletion flocculation and thermodynamic incompatibility in whey protein stabilised O/W emulsions

The stability of whey protein stabilised emulsions, containing methylcellulose added after emulsification in their bulk phase, was investigated. The phase diagram of the ternary system whey proteins/methylcellulose/water was first established and used to identify the conditions permitting polymer phase separation within the emulsion bulk phase. Emulsions containing a whey protein and methylcellulose concentration in the bulk phase below and above the phase separation threshold could therefore be prepared. Below the phase separation threshold, the creaming rate of the oil droplets was faster than the one predicted by the Stokes equation, due to methylcellulose-induced depletion flocculation. Above the phase separation threshold, the destabilisation of the emulsion involved different mechanisms, depending on the emulsifier adsorbed at the O/W interface. In the case of Tween 40 stabilised droplets, depletion flocculation led to a complete creaming of the fat globules while phase separation led to the formation of two polymer-rich phases, namely a protein-rich phase at the bottom of the tube and a methylcellulose-rich phase above. In the case of whey protein stabilised droplets, phase separation between bulk whey proteins and methylcellulose occurred, and the fat globules were entrapped in the protein-rich phase. These results permitted to describe the destabilisation mechanisms of both Tween 40 and whey protein stabilised emulsions in the presence of unadsorbed polysaccharide. They could be used to better understand the destabilisation processes arising in food emulsions, especially in those emulsions containing whey proteins, small surfactant molecules and polysaccharides.     

Fortification of dairy products with vitamin D3

The purpose of this study was to analyse emulsions for vitamin D₃ delivery in yoghurt and sour cream. Oil‐in‐water emulsions stabilised by whey proteins alone and by whey proteins plus carboxymethylcellulose were used. No change in vitamin D₃ added to the yoghurt and sour cream in the form of both emulsions was observed after storage at 7 days in light and 14 days in dark at 4 °C. The results of bioavailability tests, using rats, for vitamin D₃ from the fortified emulsions and yoghurt indicated that it is feasible to use stabilised emulsions as delivery systems of vitamin D₃ in fortified products.     

Effect of dry heat stabilisation on the functional properties of rice bran proteins

Rice bran was stabilised by dry heat method at 120 °C for 10–60 min, and then, protein was extracted from stabilised rice bran using weak alkali method. The storage characteristics of stabilised rice bran and the influences of dry heat pretreatment on the physicochemical properties of rice bran protein isolate were also evaluated. The results indicated that dry heat pretreatment could not only prevent rancidity of rice bran effectively, but also improve some functional properties of rice bran proteins, such as emulsifying properties, oil holding capacity, and water holding capacity. However, foaming properties and protein solubility were slightly destroyed because of heating. Rice bran was pretreated at 120 °C for 10 or 20 min and then extracted at pH 9.5, and the protein yields were 50.09% and 46.98%, respectively. Therefore, the dry heat treatment at 120 °C for 10 or 20 min was a suitable alternative process in stabilisation of rice bran.     

Preparation,characterisation and selected functional properties of hydrolysed sodium caseinate–maltodextrin conjugate

Sodium caseinate was hydrolysed to a limited, moderate or extensive degree. The hydrolysates were conjugated with maltodextrin by a Maillard‐type reaction by dry‐heat treatment at 60 °C and 79% relative humidity for 2 or 4 days. Conjugates were characterised by SDS–PAGE and gel permeation chromatography. In comparison with the hydrolysates themselves, the conjugated hydrolysates had improved solubility, particularly around the isoelectric pH of the protein. The emulsifying properties of these conjugates were assessed in oil‐in‐water (o/w) emulsions; on emulsion formation, each conjugate‐stabilised emulsion had lower mean fat globule size than the corresponding hydrolysate‐stabilised emulsion. After storage for 7 days under accelerated shelf life testing conditions, the limited and moderate hydrolysate conjugate–stabilised emulsions had improved storage stability compared with hydrolysate‐stabilised emulsions; however, further research is required to optimise the hydrolysate fraction prior to conjugation for the production of novel low molecular weight emulsifiers.     

Characterization of cold-set gels produced from heated emulsions stabilized by whey protein

This paper reports the cold gelation of preheated emulsions stabilized by whey protein, in contrast to, in previous reports, the cold gelation of emulsions formed with preheated whey protein polymers. Emulsions formed with different concentrations of whey protein isolate (WPI) and milk fat were heated at 90 °C for 30 min at low ionic strength and neutral pH. The stable preheated emulsions formed gels through acidification or the addition of CaCl₂ at room temperature. The storage modulus (G′) of the acid-induced gels increased with increasing preheat temperature, decreasing size of the emulsion droplets and increasing fat content. The adsorbed protein denatures and aggregates at the surface of the emulsion droplets during heat treatment, providing the initial step for subsequent formation of the cold-set emulsion gels, suggesting that these preheated emulsion droplets coated by whey protein constitute the structural units responsible for the three-dimensional gel network.     

Effect of heat treatment on the properties of soy protein‐stabilised emulsions

The effect of heat treatment on the properties of soy protein‐stabilised emulsions was investigated. Emulsions were prepared with unheated and heat‐treated soy protein (NSP and HSP) dispersions. Heating on soy protein dispersions at 95 °C for 30 min resulted in smaller average oil droplet size, lower tendency for oil droplet flocculation, higher protein adsorption and lower viscosity. The properties of emulsions were significantly influenced by the protein concentration. The sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS–PAGE) profiles showed that the heat treatment on soy protein dispersions increased the protein adsorption at O/W interface. The viscosity of all samples at low shear rate was inversely proportional to the d₃₂, suggesting a positive relation to the total interfacial area per unit volume. Emulsions showed shear‐thinning behaviour. The relaxation time was found to increase with aqueous phase viscosity determined by the Cross viscosity model.     

Comparison of properties of oil-in-water emulsions stabilized by coconut cream proteins with those stabilized by whey protein isolate

Coconut cream protein (CCP) fractions were isolated from coconuts using two different isolation procedures: isoelectric precipitation (CCP1-fraction) and freeze–thaw treatment (CCP2-fraction). The ability of these protein fractions to form and stabilize oil-in-water emulsions was compared with that of whey protein isolate (WPI). Protein solubility was a minimum at ∼pH 4, 4.5 and 5 for CCP1, CCP2, and WPI, respectively, and decreased with increasing salt concentration (0–200 mM NaCl) for the coconut proteins. All of the proteins studied were surface active, but WPI was more surface active than the two coconut cream proteins. The two coconut cream proteins were used to prepare 10 wt% corn oil-in-water emulsions (pH 6.2, 5 mM phosphate buffer). CCP2 emulsions had smaller mean droplet diameters (d₃₂ ≈ 2 μm) than CCP1 emulsions (d₃₂ ≈ 5 μm). Corn oil-in-water emulsions (10 wt%) stabilized by 0.2 wt% CCP2 and WPI were prepared with different pH values (3–8), salt concentrations (0–500 mM NaCl) and thermal treatments (50–90 °C for 30 min). Considerable droplet flocculation occurred in the emulsions near the isoelectric point of the proteins: CCP2 (pH ∼ 4.3); WPI (pH ∼ 4.8). Emulsions with monomodal particle size distributions, small mean droplet diameters, and good creaming stability could be produced at pH 7 for WPI, but CCP2 produced bimodal distributions at this pH. The CCP2 and WPI emulsions remained relatively stable to droplet aggregation and creaming at NaCl concentrations ⩽50 and ⩽100 mM, respectively. In the absence of salt, both CCP2 and WPI emulsions were quite stable to thermal treatments (50–90 °C for 30 min).     

Secondary adsorption of milk proteins from the continuous phase to the oil–water interface in dairy emulsions

Low protein surface concentration emulsions are susceptible to secondary protein adsorption where protein moves from the continuous phase to the existing, oil–water interface. The resulting increase in protein surface concentration can greatly alter emulsion properties. Butteroil was emulsified with whey proteins and the emulsion was combined with a solution of dissolved skim milk powder (SMP), producing mixes with fat and protein levels representative of ice cream. The primary adsorbed layer was modified by heating the whey protein solution prior to emulsion formation (70°C, 80°C, 90°C), by heating the emulsion (70°C, 80°C, 90°C) or by pH adjustment of the emulsion (6–8). Modifications of the SMP solution included heat treatment (80°C, 95°C) or sugar addition with or without κ-carrageenan. The effect of addition of SMP solution on the protein surface concentration and shear stability of the diluted emulsions was determined. Addition of untreated solution to the control, heated or pH adjusted emulsions greatly reduced shear destabilization and increased the protein surface concentration. Addition of heat treated or sugar containing SMP solution to the control emulsion produced the same result. However, sugar and carrageenan in the mix maintained the susceptibility to partial coalescence and reduced the secondary adsorption of caseins and whey proteins.