Viscoelastic characterization of fluid and gel like food emulsions stabilized with hydrocolloids

Food emulsions exhibit a great diversity of rheological characteristics; hydrocolloids are usually added to deal with creaming instability. Viscoelastic measurements provide information about the microstructure of the system. The objectives of this work were: a) to determine the viscoelastic behavior of two different low in fat oil-in-water food emulsions: a gel like and a fluid type emulsions stabilized with hydrocolloids (gellan gum and xanthan-guar mixtures respectively) b) to model and predict the mechanical relaxation spectrum for both emulsions and continuous aqueous phases. Low-in-fat oil-in-water emulsions (20 g/100 g) were prepared using sunflower oil and Tween 80 (1 wt.%). Fluid emulsions containing xanthan and guar gums were formulated using a synergistic ratio 7:3, with total hydrocolloid concentration ranging between 0.5 to 2 wt%. The aqueous phases contained NaCl (2 wt.%) and acetic acid (2 wt.%). The effect of hydrocolloids was studied using oscillatory measurements (G’ and G” vs. frequency) within the linear viscoelastic range previously determined by stress-sweeps. Time-Concentration Superposition principle was applied to find the master curves that describe the mechanical spectra of the viscoelastic materials. Superposition allows to obtain a wide spectrum of nearly ten decades of frequencies in emulsions containing xanthan–guar mixtures, whereas gellan gum systems did not show a significant frequency displacement. Viscoelastic behavior of the systems was satisfactorily modeled using Baumgaertel-Schausberger-Winter (BSW) equation. This empirical model was used to predict the mechanical relaxation spectrum for both emulsions and continuous aqueous phases. Validation of the predicted spectra was carried out through creep compliance data for emulsion-filled gels and steady-state flow curves for emulsions containing xanthan–guar mixtures.     

The interplay between diverse oil body extracts and exogenous biopolymers or surfactants

Hazelnuts, sesame seeds and soybeans were selected as three diverse sources of oil bodies. Application of aqueous extraction and centrifugation steps resulted in concentrated oil body creams that were studied for their physical stability after dilution to a series of 5.0 wt.% oil-in-water emulsions incorporating sodium caseinate (1.0 wt.%), Tween 80 (1.0 wt.%) or xanthan gum (0.1 wt.%). In terms of aggregation/coalescence and creaming, the stability of the oil body based emulsions was ruled to a large extent by the initial natural oil droplet size and the presence of co-extracted exogenous proteins and secondarily by the added biopolymers and the surfactant. More specifically, soybean oil bodies exhibited the highest physical stability, even though incorporation of Tween 80 into all three oil body emulsions improved the stability against aggregation/coalescence, while xanthan gum was an effective stabilizer against creaming.     

Effect of xanthan and guar gums on the formation and stability of soy soluble polysaccharide oil-in-water emulsions

The incorporation of relevant amounts of non-adsorbing hydrocolloids to oil-in-water (O/W) emulsions is a suitable alternative to reduce creaming. The effect of incorporating xanthan gum (XG) or guar gum (GG) in soy soluble polysaccharide (SSPS) stabilized oil-in-water (O/W) emulsions was studied. The emulsions contained 6 wt.% of SSPS, 20 wt.% Perilla seed oil (PSO), an omega-3 vegetable oil, and variable amounts of XG or GG ranging from 0.03 to 0.3 wt.%. The presence of minute amounts of XG or GG in fresh emulsions significantly decreased the emulsion droplet size (EDS) although such low concentrations did not provide enough continuous phase viscosity to arrest creaming. Emulsion microstructure indicated the presence of flocculation even at high concentrations of XG or GG caused by a depletion mechanism. All emulsions with XG or GG exhibited pseudoplastic behavior while the control emulsions showed an almost Newtonian behavior. Emulsion droplet polydispersion generally decreased with increase in the continuous phase viscosity indicating the importance of continuous phase viscosity in the dissipation of shear energy throughout the emulsion during homogenization. The characteristics of the emulsions were closely related to the rheological changes of the continuous phase.     

Stability of flocculated particles in concentrated and high hydrophilic solid layer-by-layer (LBL) emulsions formed using whey proteins and gum Arabic

The objective of the present study was to investigate flocculation in layer-by-layer (LBL) emulsion systems with high total solids content and deflocculation at various pH conditions, and the effects of whey protein isolate (WPI) concentration and total solids content on the stability of LBL emulsions. WPI (1.96% (1WPI) or 10.71% (10WPI), w/w in water) was prepared in water and high-pressure homogenized with sunflower oil (10%, w/w, of total emulsion). Gum Arabic (0.15%, w/w, in total emulsion) was added to assemble electrostatically on WPI at oil particle interfaces at pH 3.5 using aqueous citric acid (10% w/w) forming LBL emulsion. The ζ-potential measurements showed charge reversal upon addition of gum Arabic solution into single layer (SL) emulsion confirming the formation of LBL interface. Trehalose:maltodextrin mixture (1:1, w/w, total emulsion, 28.57% (28) or 57.14% (57), w/w, in water) was used in the continuous phase. The high total solids content of the system results in depletion flocculation of the particles leading to bridging flocculation without coalescence as deflocculation into individual particles occurred with increasing pH from pH 3.5 to pH 6.5 in 10WPI systems. Deflocculation was evident in 10WPI-28 and 10WPI-57 as found from a decreased ζ-average diameter and visually under microscope. Coalescence was observed in 1WPI systems. Viscosity of the systems was significantly (P < 0.05) increased with higher total solids content. Accelerated destabilization test showed that systems at higher WPI and total solids contents exhibited the highest stability against creaming. Deflocculation in LBL systems can be controlled by pH while high solids in the aqueous phase provide stability against creaming.     

Influence of sodium dodecyl sulfate on the thermal stability of bovine serum albumin stabilized oil-in-water emulsions

The influence of an anionic surfactant (sodium dodecyl sulfate, SDS) on the thermal stability of emulsions stabilized by a globular protein (bovine serum albumin, BSA) was examined. 1 wt% n-hexadecane oil-in-water emulsions (0.5 wt% BSA, 100 mM NaCl, 20 mM imidizole, pH 7.0) were held isothermally at temperatures from 30 to 90 °C for 30 min. Extensive droplet flocculation was only observed in emulsions held at 90 °C, which lead to rapid creaming instability. The thermal stability of emulsions heated at 90 °C could be greatly improved by adding SDS at surfactant-to-protein molar ratios (R) greater than 5 (∼0.01 wt% SDS). On the other hand, adding surfactant (0<R<600) to the emulsions after heating could not prevent extensive droplet aggregation. Adding SDS to emulsions prior to heating may have improved their thermal stability by increasing the electrostatic repulsion between the lipid droplets or by increasing the denaturation temperature of the adsorbed proteins. These results may have important applications in the development of heat-stable emulsions.     

Characterization of emulsion stabilization properties of quince seed extract as a new source of hydrocolloid

The capability of seed extracts in stabilizing emulsions has particularly received interest in recent years. Upon soaking quince seeds into water, biopolymers inside the seeds are extracted to water, forming mucilage. This study investigates the physical stability, rheology and microstructure of oil (sunflower oil) in water emulsions, stabilized by 2% (w/v) whey protein isolate with varying concentrations of xanthan and quince seed gum. Quince seed gum resulted in emulsions with smaller low-shear viscosities and shear thinning capabilities compared to the same concentrations of xanthan. Quince seed gum emulsions with concentrations ≤ 0.1 (w/v), displayed rapid creaming due to bridging flocculation. Despite the difference in apparent viscosities, for gum concentrations < 0.2 (w/v), both gums demonstrated comparable stability with xanthan gum in general yielding marginally more stable emulsions. Gum concentrations > 0.3 (w/v) resulted in physically stable emulsions even after 5 months. Overall, quince seed gum displayed significant emulsification and stabilization properties.     

Physical and oxidative stability of whey protein oil-in-water emulsions produced by conventional and ultra high-pressure homogenization: Effects of pressure and protein concentration on emulsion characteristics

Oil-in-water pre-emulsions (15% sunflower + 5% olive oils) obtained by colloid mill homogenization (CM) at 5000 rpm using whey protein isolate at different levels (1, 2 and 4%) were stabilized by ultra high-pressure homogenization (UHPH, 100 and 200 MPa) and by conventional homogenization (CH, 15 MPa). Emulsions were characterized for their physical properties (droplet size distribution, microstructure, surface protein concentration, emulsifying stability against creaming and coalescence, and viscosity) and oxidative stability (hydroperoxide content and thiobarbituric acid reactive substances, TBARs) under light (2000 lux/m² for 10 days). UHPH produced emulsions with lipid droplets of small size in the sub-micron range (100–200 nm) and low surface protein with unimodal distribution when produced at 4% whey proteins and 200 MPa. All emulsions exhibited Newtonian behavior (n ≈ 1). Long term physical stability against creaming and coalescence was observed in UHPH-emulsions, compared to those obtained by CM and CH. However, CH emulsions were highly stable against creaming (days) in comparison to the CM emulsions (hours). UHPH resulted in emulsions highly stable to oxidation compared to CM and CH treatments, especially when 100 MPa treatment was applied.Industrial relevanceIn the food, cosmetic and pharmaceutical sectors, industrial operators are currently interested in developing encapsulating systems to delivery bioactive compounds, which are generally hydrophobic, unstable and sensitive to light, temperature or/and oxygen. Ultra high-pressure homogenization is capable of producing stable submicron emulsions (< 1 μm) with a narrow size distribution, inducing more significant changes in the interfacial protein layer thus preventing droplet coalescence and also inhibit lipid oxidation. The present study suggests that emulsions produced by whey protein (4%) treated by ultra high-pressure homogenization have a good physical stability to flocculation, coalescence and creaming and also high stability to lipid oxidation, opening a wide range of opportunities in the formulation of emulsions containing bioactive components with lipid nature.     

Thermal behavior of concentrated oil-in-water emulsions based on soybean oil and palm kernel olein blends

Droplet size distribution and thermal behavior of concentrated oil-in-water emulsions based on soybean oil (SBO)/palm kernel olein (PKO) blends were investigated. The emulsions were prepared using 70% (wt./wt.) oil blends of SBO/PKO as dispersed phases and stabilized by egg yolk. An increase in PKO level (0–40% wt./wt.) in the oil dispersed phase volume fraction caused significant increases (p < 0.05) in volume-weighted mean diameter (d_4,3). The DSC data suggested that crystallization of the emulsions was induced by a ‘template effect’ of yolk constituents via a surface heterogeneous nucleation. Emulsions with 0–20% (wt./wt.) PKO levels in the dispersed phase demonstrated a good cool–heat stability even after three successive thermal cycles (from 50 °C to ?70 °C at 10 min/°C). After the first thermal cycle, emulsions with 30% and 40% PKO levels in the oil dispersed phase were destabilized due to strong coalescence and crystallized via volume-surface heterogeneous nucleation. The unstable emulsions were attributable to high level of saturated triacylglycerols from PKO, with high droplet size characteristic, causing them to be more prone to partial coalescence.     

Effect of processing conditions and composition on sodium caseinate emulsions stability

Many food products such as ice cream, yoghurt, and mayonnaise are some examples of emulsion-based food. The physicochemical properties of emulsions play an important role in food systems as they directly contribute to texture, sensory and nutritional properties of food. One of the main properties is stability which refers to the ability of an emulsion to resist physical changes over time. The aim of the present work was to analyze the effect of processing conditions and composition on sodium caseinate (NaCas) emulsions stability. The main destabilization mechanisms were identified and quantified. The relationship between them and the factors that influence them were also investigated. Emulsions stabilized with NaCas were prepared using an ultrasound liquid processor or a high pressure homogenizer. Stability of emulsions was followed by a Turbiscan (TMA 2000) which allows the optical characterization of any type of dispersion. The physical evolution of this process is followed without disturbing the original system and with good accuracy and reproducibility. To further describe systems, droplet size distribution was analyzed with light scattering equipment. The main mechanism of destabilization in a given formulation depended on different factors such as NaCas concentration, droplet size or processing conditions. The rate of destabilization was markedly lower with addition of sugar or a hydrocolloid to the aqueous phase. Xanthan (XG) and locust bean (LBG) gums produced an increase in viscosity of the continuous phase and structural changes in emulsions such as gelation. Sugars interacted with the protein decreasing particle size and increasing emulsion stability. The stability of caseinate emulsions was strongly affected not only by the oil-to-protein ratio but also by processing conditions and composition of aqueous phase. The structure of the protein and the interactions protein–sugar or the presence of a hydrocolloid played a key role in creaming and flocculation processes of these emulsions.     

Effect of cooling rate on lipid crystallization in oil-in-water emulsions

The effect of cooling rate on the destabilization of oil-in-water (o/w) emulsions was studied as a function of oil content (20% and 40% o/w), homogenization conditions, and crystallization temperatures (10, 5, 0, ?5 and ?10 °C). The lipid phase was a mixture of anhydrous milk fat and soybean oil, and whey protein was used as the emulsifier. Differential scanning calorimetry was used to analyze the crystallization and melting behaviors; while a vertical scan macroscopic analyzer measured the physicochemical stability. Slow cooling rate increased the stability of emulsions with 20% oil. In addition, slow cooling promoted the onset of crystallization and delayed crystal growth. These effects were more significant in emulsions formulated with 20% oil and formulated under processing conditions that resulted in bigger droplet sizes (～0.9 μm).     

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).     

Stability and rheology of concentrated O/W emulsions based on soybean oil/palm kernel olein blends

Droplet size distribution and rheological properties of egg yolk-stabilized emulsions were studied before and after storage (25 °C, 30 days). The dispersed phase (70%) of the emulsions was based on soybean oil (SBO) and 10–40% palm kernel olein (PKO) replacements of SBO. Replacement of PKO resulted in a significant increase in droplet mean diameters and a decrease in rheological properties of the emulsions. All emulsion exhibited a gel-like characteristic with storage modulus higher than loss modulus and tan δ greater than 0.3. Significant increase (p < 0.05) was found for droplet mean diameters and rheological properties of the emulsions after storage. Emulsion with fully SBO and the highest PKO replacement (40%) were found to be the most unstable, which was ascribed to a strong flocculation. With 10–30% PKO replacements, the emulsions displayed a better stability after storage, most probably promoted by significant content of short-medium chain fatty acids in PKO.     

Physical characteristics of submicron emulsions upon partial displacement of whey protein by a small molecular weight surfactant and pectin addition

O/W emulsions (6 wt.% olive oil) were prepared at pH 3.3 using different WPI:Tween 20 weight ratios (1:0, 3:1, 1:1, 1:3, 0:1) at 1 wt.% total concentration. The emulsion droplet size was found to decrease with an increase in Tween 20. A minimum droplet size of d_3,2 300 nm was found for Tween systems alone, similar to that found (360 nm) for a 1:1 WPI:Tween 20 combination (p < 0.05). This specific composition showed a value for the interfacial tension close to that of Tween 20 alone. However, the emulsions presented low stability regardless of the WPI:Tween 20 ratio. To increase their stability, pectin was added, in various concentrations (0.2, 0.4 and 0.6 wt.%), using the Layer by Layer technique. In the presence of pectin, the ζ-potential of the oil droplets became negative; indicating that negatively charged pectin was absorbed onto the positively-charged droplet surface forming a secondary layer. The additional layer resulted in a wide range of emulsion stability. For all pectin concentrations, the 1:1 ratio of WPI:Tween 20 showed the highest stability. In most emulsions, extensive aggregation of oil droplets was observed, and their viscosity increased. Insufficient amounts of pectin to form the secondary layers led to bridging flocculation phenomena of oppositely charged pectin and proteins, leading to aggregation of the oil droplets. The higher the concentration of pectin, the greater the stability of the emulsion due to higher viscosity. All in all, the addition of a second layer consisting of pectin can be used to increase the stability of an emulsion containing emulsion droplets in the sub-micron range.     

Inhibition of droplet flocculation in globular-protein stabilized oil-in-water emulsions by polyols

The influence of neutral cosolvents (polyols) on the stability of hydrocarbon oil-in-water emulsions stabilized by a globular protein was investigated. Glycerol (0–40 wt%) and sorbitol (0–35 wt%) were added to n-hexadecane oil-in-water emulsions stabilized by β-lactoglobulin (β-lg, pH 7.0, 150 mM NaCl), either before or after incubation at 30 °C for 24 h. The stability of the emulsions to flocculation and creaming improved when neutral cosolvents were added, with the effectiveness of the cosolvents depending on their type, concentration and time of addition. Emulsion stability was better for sorbitol than glycerol, improved with increasing cosolvent concentration, and was better when the cosolvents were added immediately after homogenization than when they were added 24 h later. The influence of the cosolvents on emulsion stability is interpreted in terms of their effect on the conformation and interactions of the adsorbed proteins, as well as on the droplet–droplet collision frequency. This study has implications for the development of protein stabilized oil-in-water emulsions for utilization in industrial products.     

Ultra-High Pressure Homogenization enhances physicochemical properties of soy protein isolate-stabilized emulsions

The effect of Ultra-High Pressure Homogenization (UHPH, 100–300 MPa) on the physicochemical properties of oil-in-water emulsions prepared with 4.0% (w/v) of soy protein isolate (SPI) and soybean oil (10 and 20%, v/v) was studied and compared to emulsions treated by conventional homogenization (CH, 15 MPa). CH emulsions were prepared with non-heated and heated (95 °C for 15 min) SPI dispersions. Emulsions were characterized by particle size determination with laser diffraction, rheological properties using a rotational rheometer by applying measurements of flow curve and by transmission electron microscopy. The variation on particle size and creaming was assessed by Turbiscan® analysis, and visual observation of the emulsions was also carried out. UHPH emulsions showed much smaller d_3.2 values and greater physical stability than CH emulsions. The thermal treatment of SPI prior CH process did not improve physical stability properties. In addition, emulsions containing 20% of oil exhibited greater physical stability compared to emulsions containing 10% of oil. Particularly, UHPH emulsions treated at 100 and 200 MPa with 20% of oil were the most stable due to low particle size values (d_3.2 and Span), greater viscosity and partial protein denaturation. These results address the physical stability improvement of protein isolate-stabilized emulsions by using the emerging UHPH technology.     

Microbial cells as colloidal particles: Pickering oil-in-water emulsions stabilized by bacteria and yeast

Thermally-inactivated baker's yeast (Saccharomyces cerevisiae) and lactic acid bacteria (Lactobacillus acidophilus and Streptococcus thermophilus) were used to generate and stabilize model oil-in-water (O/W) emulsions containing up to 80 wt.% dispersed oil. With optimized compositions, cell-covered dispersed oil droplets were stable against droplet coalescence and bulk phase separation for over four months. From a textural perspective, these emulsions were self-supporting and exhibited a mayonnaise-like consistency. The microbial cells acted as Pickering-type stabilizers by residing at the oil–water interface. The three-phase contact angle of the yeast at the oil–water interface measured using confocal microscopy was 30 ± 9°, demonstrating its ability to stabilize O/W emulsions. These microbial cells may be used in the design of processed food emulsions with an ‘all-natural’ designation as well as for the replacement of common synthetic surfactants to permit clean label declarations.     

Gel-like pea protein Pickering emulsions at pH 3.0 as a potential intestine-targeted and sustained-release delivery system for β-carotene

In recent years, there is increasing interest in the development of food-grade Pickering emulsions as promising delivery systems for bioactive compounds. Our previous work reported that most of the proteins in pea protein isolate (PPI) at pH 3.0, present in the nanoparticle form, can effectively perform as a kind of food-grade Pickering stabilizers for oil-in-water emulsions (LWT, 2014). The purpose of this study was to further report that PPI-stabilized emulsions at pH 3.0 exhibited great potential to act as intestine-targeted and sustained-release delivery systems for β-carotene. The emulsions were produced by microfluidization at a specific protein concentration of 6.0% (w/v) and varying oil fractions (ϕ) of 0.2–0.6. The results indicated that increasing ϕ was favorable for the gel-like network strengthening of these emulsions. The gel formation was largely related to the droplet flocculation as well as inter-floc attractive interactions. The in vitro simulated digestion results showed that the release of β-carotene during the intestinal digestion of these emulsions could be well modulated by altering ϕ. The gel-like emulsion at ϕ = 0.6 exhibited much lower release of β-carotene, but higher stability towards degradation during the digestion, than that at ϕ = 0.3. The findings provide important information not only for the design of novel delivery systems for lipophilic bioactive components, but also for the development of plant protein-based formulations.     

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.     

Characterisation of fish oil emulsions stabilised by sodium caseinate

Fish oil emulsions varying in sodium caseinate concentration (25% w/w oil and 0.1–1.0% w/w protein, giving oil-to-protein ratios of 250–25) were investigated in terms of their creaming stability, rheological properties, the mobility of oil droplets and the oil/protein interaction at the interface. The presence of excessive protein in an emulsion (i.e., at 1% w/w) caused the aggregation of oil droplets through depletion flocculation, resulting in low creaming stability and high low-shear viscosity. At a lower protein concentration (0.1% w/w), when protein was limited, the emulsion droplets were stabilised by bridging flocculation and showed good stability to creaming. Shear-thinning behaviour was observed for both flocculated emulsions. A reduction in the low-shear viscosity and a Newtonian flow was obtained for the emulsion containing an intermediate concentration of protein (0.25% w/w). At this concentration, there was relatively little excess unadsorbed protein in the continuous phase; thus the emulsion was most stable to creaming. NMR was used to characterise these emulsion systems without dilution. Shorter T₂ values (by low-field ¹H NMR), for the emulsions containing both high (1% w/w) and low (0.1% w/w) amounts of protein, indicated increased restricted mobility of oils, caused by depletion or bridging flocculation. The line broadening in oil signals in the high-field NMR spectra (¹H, ¹³C) indicated increased interaction between oil molecules and proteins at the interface with increasing protein concentration in emulsions. In addition, ³¹P NMR spectra, which reflect the mobility of the casein component only, showed increased line broadening, with reduction in protein content due to the relatively higher proportion of the protein being adsorbed to the interface of the oil droplets, compared to that in the continuous phase (i.e., as the oil-to-protein ratio was increased). The T₂ values of resonances of the individual groups on oil molecules, obtained using high-field ¹H NMR, reflected their different environments within the oil droplet.     

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.