Diffusing wave spectroscopy of gelling food systems: The importance of the photon transport mean free path (l*) parameter

The gelation of milk proteins by the addition of 1.5% (w/w) glucono-δ-lactone (GDL) as well as the effect of the addition of pectin to low pH oil-in-water emulsions stabilized with whey protein isolate were measured in situ using Diffusing Wave Spectroscopy (DWS). For both systems, large changes were observed in the 1/l* parameter before any marked increase in particle size, indicating a partial correlation between particles which increased as the pH dropped (for the milk case) and with each addition of pectin (for the emulsion case). Theoretical calculations of l* were made assuming no correlation between particles (S(q)=1) and compared to the experimentally-obtained 1/l* values. Without interparticle interactions the value of 1/l* remained essentially constant throughout. This indicates that the change observed experimentally is primarily due to a change in S(q) and that 1/l* is a good indication of system order. Finally, it is noted that the opacity of a gel is mainly determined by the initial properties of the dispersed particles.     

Impact of whey protein – Beet pectin conjugation on the physicochemical stability of β-carotene emulsions

The aim of the present study was to investigate the impact of whey protein isolate (WPI)-beet pectin conjugation on the physical and chemical properties of oil-in-water emulsions incorporating β-carotene within the oil droplets. Covalent coupling of WPI to beet pectin was achieved by dry heating of WPI-beet pectin mixtures of different weight ratios at 80, 90, 100 °C and 79% relative humidity for incubation times ranging from 1 to 9 h. It was confirmed by SDS-polyacrylamide gel electrophoresis that WPI covalently linked to beet pectin. The physical and chemical stability of β-carotene emulsions was characterized by droplet size and distribution, transmission profiles using novel centrifugal sedimentation technique, microstructure and β-carotene degradation during the storage. Compared with those stabilized by WPI alone and unheated WPI-beet pectin mixtures, β-carotene emulsions stabilized by WPI-beet pectin conjugates had much smaller droplet sizes, more homogenous droplet size distribution, less change in centrifugal transmission profiles and obviously improved freeze–thaw stability, indicating a very substantial improvement in the physical stability. Rheological analysis exhibited that emulsions stabilized by WPI-beet pectin conjugates changed from a shear thinning to more like Newtonian liquid compared those with WPI alone and unheated WPI-beet pectin mixtures. Degradation of β-carotene in emulsion during storage was more obviously retarded by WPI-beet pectin conjugate than WPI and unheated WPI-beet pectin mixture, probably due to a thicker and denser interfacial layer in emulsion droplets. These results implied that protein–polysaccharide conjugates were able to improve the physical stability of β-carotene emulsion and inhibit the deterioration of β-carotene in oil-in-water emulsions.     

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.     

The effect of pectin concentration and degree of methyl-esterification on the in vitro bioaccessibility of β-carotene-enriched emulsions

Soluble fibers, like pectin, are known to influence the physicochemical processes during the digestion of dietary fat and may therefore affect the absorption of lipophilic micronutrients such as carotenoids. The objective of the current work was to investigate whether the pectin concentration and degree of methyl-esterification (DM) influence the bioaccessibility of carotenoids loaded in the oil phase of oil-in-water emulsions. The in vitro β-carotene bioaccessibility was determined for different oil-in-water emulsions in which 1 or 2% citrus pectin with a DM of 99%, 66% and 14% was present. Results show that pectin concentration and DM influence the initial emulsion properties. The most stable emulsions with the smallest oil droplets (D(v,0.9) of 15–16 μm) were obtained when medium or high methyl-esterified pectin was present in a 2% concentration while gel-like pectin structures (D(v,0.9) of 114 μm), entrapping oil droplets, were observed in the case where low methyl-esterified pectin was present in the aqueous emulsion phase. During in vitro stomach digestion, these gel-like structures, entrapping β-carotene loaded oil droplets, significantly enlarged (D(v,0.9) of 738 μm), whereas the emulsion structure could be preserved when the medium or high methyl-esterified pectin was present. Initial emulsion viscosity differences, due to pectin concentration and especially due to pectin DM, largely disappeared during in vitro digestion, but were still significant after the stomach digestion phase. The observed differences in emulsion structure before and during in vitro digestion only resulted in a significant difference between emulsions containing low methyl-esterified pectin (β-carotene bioaccessibility of 33–37%) and medium/high methyl-esterified pectin (β-carotene bioaccessibility of 56–62%).     

Effect of high-methoxy pectin on properties of casein-stabilized emulsions

We report on the effect of high-methoxy pectin on the stability and rheological properties of fine sunflower oil-in-water emulsions prepared with α_s1-casein, β-casein or sodium caseinate. The aqueous phase was buffered at pH 7.0 or 5.5 and the ionic strength was adjusted with sodium chloride in the range 0.01–0.2 M. Average emulsion droplet sizes were found to be slightly larger at the lower pH and/or with pectin present during emulsification. Analysis of the serum phase after centrifugation indicated that some pectin becomes incorporated into the interfacial layer at pH 5.5 but not at pH 7.0. This was also supported by electrophoretic mobility measurements on protein-coated emulsion droplets and surface shear viscometry of adsorbed layers at the planar oil–water interface. A low pectin concentration (0.1 wt%) was found to give rapid serum separation of moderately dilute emulsions (11 vol% oil, 0.6 wt% protein) and highly pseudoplastic rheological behaviour of concentrated emulsions (40 vol% oil, 2 wt% protein). We attribute this to reversible depletion flocculation of protein-coated droplets by non-adsorbed pectin. At ionic strength below 0.1 M, the initial average droplet sizes, the creaming behaviour, and the rheology were found to be similar for emulsions made with either of the individual caseins (α_s1 and β) or with sodium caseinate. At higher ionic strength, however, whereas emulsions containing β-casein or sodium caseinate were stable, the corresponding α_s1-casein emulsions exhibited irreversible salt-induced flocculation which was not inhibited by the presence of the pectin.     

Utilisation of pectin coating to enhance spray-dry stability of pea protein-stabilised oil-in-water emulsions

In this study, development of pea (Pisum sativum) protein stabilised dry and reconstituted emulsions is presented. Dry emulsions were prepared by spray-drying liquid emulsions in a laboratory spray-dryer. The effect of drying on the physical stability of oil-in-water emulsions containing pea protein-coated and pea protein/pectin-coated oil droplets has been studied. Oil-in-water emulsions (5 wt.% Miglyol 812 N, 0.25 wt.% pea protein, 11% maltodextrin, pH 2.4) were prepared that contained 0 (primary emulsion) or 0.2 wt.% pectin (secondary emulsion). The emulsions were then subjected to spray-drying and reconstitution (pH 2.4). The stability of the emulsions to dry processing was then analysed using oil droplet size, microstructure, Zeta potential, and creaming measurements. Obtained results showed that the secondary emulsions had better stability to droplet aggregation after drying than primary emulsions. To interpret these results, we propound that pectin, an anionic polysaccharide, formed a less charged protective layer around the protein interfacial film surrounding the oil droplets that improved their stability to spray-drying mainly by increasing steric effects.     

Multilayer emulsions as delivery systems for controlled release of volatile compounds using pH and salt triggers

Multilayer oil-in-water (M-O/W) emulsions were compared to primary oil-in-water (P-O/W) emulsions as carriers for volatile organic compounds (VOCs) under various environmental conditions (pH and salt). The M-O/W emulsion consisted of soy oil coated with β-lactoglobulin (βLG) and pectin layers. The release of VOCs with different physiochemical properties from aqueous solutions and emulsion systems was measured using static and dynamic headspace methods. The partition coefficients (K) calculated by the phase ratio variation (PRV) method, showed different volatile release profiles between the emulsion types. An increase in VOC release was found for the unstable P-O/W emulsion at pH 5, whereas M-O/W emulsions were stable at the same pH and retained the hydrophobic VOCs. Hydrophobic interactions and hydrogen bonds with the secondary dense layer of pectin may be responsible for the improved retention. Increasing pH and ionic strength acts as a VOC release trigger to detach the pectin from the interface. The release rates from initial dynamic curves support the results under equilibrium conditions. The results of this study demonstrate the capability of using M-O/W emulsions for controlled release of VOCs, as well as an alternative system to create stable emulsions with similar VOC release profiles.     

Oil globule microstructure of protein/polysaccharide or protein/protein bilayer emulsions at various pH

The microstructure of oil droplets of bi-layer emulsions was studied as a function of pH (i.e. 7, 5, and 3) using scanning electron microscopy. The bi-layer emulsions consisted of a primary emulsion: 5 wt% soybean oil (SBO) in a 1% protein (nonfat dry milk) aqueous solution. The secondary layer was ι-carrageenan, high- (HMp), low (LMp)-methoxyl pectin, or gelatin. The secondary emulsions consisted of 2.5% SBO, 0.5% protein, and 0.2% polysaccharide or protein. Gelatin secondary emulsions were stable at pH 7 with defined droplets and became unstable at pH 5 and 3. The destabilization mechanisms for these emulsions at pH 5 and 3 were different as observed with the SEM: at pH 5 there is complete aggregation of protein due to their proximity to the isoelectric point; and at pH 3 the droplets are perfectly separated, suggesting that at this pH, when the net charge is positive, the destabilization is mainly due to depletion flocculation. HMp secondary emulsions shift from being stable (individual droplets) at pH 3 to being unstable at pH 7 where an extensive webbing is observed between the droplets at this pH value. The ι-carrageenan secondary emulsions are stable at each pH and the individual droplet microstructure is minimally altered as the pH changes. LMp secondary emulsions shift from being stable at pH 7 with individual droplets observed in the SEM micrographs to being unstable at pH 3 where extensive webbing is observed in the SEM micrographs.     

Crosslinking of interfacial layers in multilayered oil-in-water emulsions using laccase: Characterization and pH-stability

The enzymatic crosslinking of polymer layers adsorbed at the interface of oil-in-water emulsions was investigated. A sequential two step process, based on the electrostatic deposition of pectin onto a fish gelatin interfacial membrane was used to prepare emulsions containing oil droplets stabilized by fish gelatin-beet pectin membranes (citrate buffer, 10 mM, pH 3.5). First, a fine dispersed primary emulsion (5% soybean oil (w/v), 1% (w/w) gelatin solution) (citrate buffer, 10 mM, pH 3.5) was produced using a high pressure homogenizer. Second, a series of secondary emulsions were formed by diluting the primary emulsion into pectin solutions (0 - 0.4% (w/w)) to coat the droplets. Oil droplets of stable emulsions with different oil droplet concentrations (0.1%, 0.5%, 1.0% (w/v)) were subjected to enzymatic crosslinking. Laccase was added to the fish gelatin-beet pectin emulsions and emulsions were incubated for 15 min at room temperature. The pH- and storage stability of primary, secondary and secondary, laccase-treated emulsions was determined. Results indicated that crosslinking occurred exclusively in the layers and not between droplets, since no aggregates were formed. Droplet size increased from 350 to 400 nm regardless of oil droplet concentrations within a matter of minutes after addition of laccase suggesting formation of covalent bonds between pectin adsorbed at interfaces and pectin in the aqueous phase in the vicinity of droplets. During storage, size of enzymatically treated emulsions decreased, which was found to be due to enzymatic hydrolysis. Results suggest that biopolymer-crosslinking enzymes could be used to enhance stability of multilayered emulsions.     

Effects of the proteinaceous moiety on the emulsifying properties of sugar beet pectin

The role of the proteinaceous moiety in emulsifying was investigated using pectin from sugar beet as a model polysaccharide. Physicochemical and macromolecular characteristics of sugar beet pectin were examined with or without an enzymatic modification using multiple acid-proteinases. The enzymatic modification decreased the total protein content from 1.56±0.15% to 0.13±0.02% by the Bradford method without significant change in ferulic acid or most constitutional sugars. It also decreased the weight-average molecular weight (M_w) from 517±28 to 254±20 kg/mol and the z-average root-mean-square radius of gyration from 43.6±0.8 to 35.0±0.6 nm. Emulsifying properties of the polysaccharide with or without the enzymatic modification were evaluated by emulsion droplet size and creaming stability of O/W emulsions (pH 3.0) containing 15 w/w% middle-chain triglyceride and 1.5 w/w% sugar beet pectin as main constituents. The modification increased the average diameter (d_3,2) of emulsion droplets from 0.56±0.04 to 3.00±0.25 μm immediately after the preparation, suggesting a decrease in the emulsifying activity. It caused the creaming of the emulsions during incubation at 60 °C, which was in line with the finding that macroscopic phase separation occurred only in the presence of the modified pectin after storage at 20 °C for a day, suggesting a decrease in the emulsion stabilizing ability. The modification also decreased significantly the amount of the pectin fraction that adsorbed onto the surface of emulsion droplets from 14.58±2.21% to 1.22±0.03% and the interfacial concentration of the polysaccharide from 1.42±0.23 to 0.45±0.05 mg/m², where the proteinaceous materials in the pectin molecules activated the oil-water interface. Results from the present study suggest an important role of the proteinaceous moiety to explain the emulsifying properties of sugar beet pectin as in the case of gum arabic and soy soluble polysaccharide.     

Bubble stability in the presence of oil-in-water emulsion droplets: Influence of surface shear versus dilatational rheology

We discuss the stability of bubbles to coalescence when undergoing a pressure drop and their stability to disproportionation under quiescent conditions, studied using previously established ‘single bubble layer experimental’ techniques, focussing on the effects on stability of the inclusion of a low volume fraction (0.25%) of stable oil droplets. Detailed measurements of the surface dilatational elasticity (?_s) and surface shear viscosity (η_s) of systems in the presence and absence of oil droplets have been performed. The surface rheology and stability have been measured as a function of adsorption time and pH, between pH 4.5 and 7, by including glucono-δ-lactone (GDL) as an acidification agent. Commercial sodium caseinate (SC) and purified β-lactoglobulin (β-L) were used at 1 wt% bulk concentration as bubble stabilizing agents. The emulsion oil droplet phase was n-tetradecane (TD) or 1-bromohexadecane (BHD), with a mean droplet size of (d₄₃) = 0.59 and 0.67 μm, respectively. The emulsion droplets were completely stable to coalescence and did not enter or spread at the air–water (A–W) interface. With SC at all pH values the values of η_s were markedly higher in the presence of TD droplets than in their absence, but particularly when the pH was lowered to pH ≤ 5.5 such that the SC started to aggregate. The increase in η_s correlated with the increase in coalescence stability under the same conditions. With neutrally buoyant BHD droplets the increase in η_s was not as great, but η_s was still significantly higher than in the absence of droplets, indicating that the rise to, and packing of, TD droplets at the A–W interface due to gravity was not solely responsible for their observed effects. The values of ?_s did not increase much at all for either β-L or SC as the pH was lowered and/or TD droplets were added, except at very low pH values, when the effects with SC were obscured by the tendency for the bulk SC to gel. In agreement with the relatively insignificant changes in ?_s as the pH was lowered or droplets were added, the resistance to disproportionation of bubbles did not change very much either.     

Fabrication and characterization of filled hydrogel particles based on sequential segregative and aggregative biopolymer phase separation

In this study, filled hydrogel particles were created based on the ability of proteins and ionic polysaccharides to phase separate through both aggregative (complexation) and segregative (incompatibility) mechanisms. At pH 7, a mixture of 3% (w/w) high-methoxy pectin and 3% (w/w) sodium caseinate phase separated through a segregative mechanism. Following centrifugation, the phase separated system consisted of an upper pectin-rich phase and a lower casein-rich phase. Casein-coated lipid droplets added to the phase separated pectin/caseinate system partitioning into the lower casein-rich phase. This was attributed to a reduction in the unfavorable osmotic stress in this phase associated with biopolymer depletion. When shear was applied this system formed an oil-in-water-in-water (O/W₁/W₂) emulsion consisting of oil droplets (O) contained within a casein-rich watery dispersed phase (W₁) suspended in a pectin-rich watery continuous phase (W₂). Acidification of the O/W₁/W₂ system from pH 7–5 promoted adsorption of pectin around the casein-rich W₁ droplets, resulting in the formation of filled hydrogel particles (d = 3–4 μm) that remained stable to aggregation or dissociation when stored for 24 h at ambient temperature. These particles may be useful as encapsulation and delivery systems for lipophilic components in the food, cosmetics and pharmaceutical industries.     

Improved emulsion stabilizing properties of whey protein isolate by conjugation with pectins

Functional properties of glyco-protein conjugates of the anionic polysaccharide pectin with whey protein isolate, obtained by dry heat treatment at 60 °C for 14 days, have been investigated in O/W emulsions containing 20% (w/w) soybean oil and 0.4% (w/w) protein both at pH 4.0 and 5.5. Emulsion stabilizing properties of mixtures and conjugates were compared at five protein to pectin weight ratios by determining changes in droplet size distribution and extent of serum separation with time. The results indicated that the dry heat-induced covalent binding of low methoxyl pectin to whey protein, as shown by SDS-PAGE, led to a substantial improvement in the emulsifying behaviour at pH 5.5, which is near the isoelectric pH of the main protein β-lactoglobulin. At pH 4.0, however, a deterioration of the emulsifying properties of whey protein was observed using either mixtures of protein and pectin or conjugates.The observed effects could be explained by protein solubility and electrophoretic mobility measurements. The protein solubility at pH 5.5 was hardly changed using mixtures of protein and low methoxyl pectin or conjugates, whereas at pH 4.0 it was decreased considerably. Electrophoretic mobility measurements at pH 5.5 revealed a much more pronounced negative charge on the emulsion droplets in the case of protein–pectin conjugates, which clearly indicated that conjugated pectin did adsorb at the interface even at pH conditions above the protein's iso-electric point. Hence, the improved emulsifying properties of whey protein isolate at pH 5.5 upon conjugation with low methoxyl pectin may be explained by enhanced electrosteric stabilization.Comparing two different commercial pectin samples, it was clearly shown that the dextrose content during dry heat treatment of protein–pectin mixtures should be as low as possible since protein–sugar conjugates not only resulted in increased brown colour development, but also gave raise to a largely decreased protein solubility which very badly affected the emulsifying properties.     

A novel improvement in whey protein isolate emulsion stability: Generation of an enzymatically cross-linked beet pectin layer using horseradish peroxidase

Extensive research has indicated that the electrostatic attraction between polysaccharides and proteins on the oil-water interface can improve the stability of emulsions. However, this electrostatic effect will be weakened or even eliminated as the solution pH or ionic strength of emulsions change, resulting in the shedding of the polysaccharide layer. We prepared primary oil-in-water emulsions at pH 7.0 using whey protein isolate (WPI) as an emulsifier and then beet pectin was added to form secondary emulsions. After the pH of emulsions was adjusted to 4.0 to promote electrostatic attraction between the beet pectin molecules and the protein-coated droplets, horseradish peroxidase was added to generate a cross-linked beet pectin coating. Results show that stable emulsions coated with WPI and cross-linked beet pectin interfaces could be formed. The sensitivity of the emulsions to the environmental stresses of pH changes, ions addition, thermal processing and freezing was also characterized in this work. Our results support the view that cross-linked beet pectin improves the stability of emulsions and is superior to simple deposition on the surface of lipid droplets. The interfacial engineering technology used in this study could be used to create food emulsions with improved stability to environmental stresses.     

Acid-induced gelation of milk protein concentrates with added pectin: Effect of casein micelle dissociation

The dynamics of the formation of the acid gel network for mixtures of milk protein concentrate (MPC) and low methoxyl amidated (LMA) pectin were studied using rheological measurements. The results as a function of pectin content and casein micelle integrity, from neutral pH to approximately pH 4.2, together with the microstructural changes observed in some of these systems, are presented.The gelation profiles of a mixture of 4% w/v MPC and LMA pectin (0–0.075% w/v) after the addition of 1.2% w/v glucono-δ-lactone showed a gradual decrease in the shear modulus with the incorporation of pectin. The effects of casein micelle integrity on casein–pectin interactions were studied, by preparing MPC dispersions containing various levels of micellar casein. A gradual change in the shear modulus, from a disrupting effect of pectin added to MPC, in which the casein micelles are intact, to a clear synergistic effect of pectin added to dissociated casein systems, was found in the acid-induced milk gels.     

Colloidal stability and interactions of milk-protein-stabilized emulsions in an artificial saliva

Oil-in-water emulsions (20 wt% soy oil) with lactoferrin or β-lactoglobulin (β-lg) as the interfacial layer were prepared using a two-stage valve homogenizer. At pH 6.8, lactoferrin produces a stable cationic emulsion whereas β-lg forms an anionic emulsion. These emulsions were mixed with an artificial saliva that contained a range of mucin concentrations and salts. Negatively charged mucin was shown to interact readily with the positively charged lactoferrin-stabilized emulsion droplets to provide a mucin coverage of approximately 1 mg/m². As expected, the negatively charged β-lg-stabilized emulsion droplets had lower mucin coverage (0.6 mg/m² surface load) under the same conditions. The β-lg-stabilized emulsions were stable but showed depletion flocculation at higher mucin levels (≥1.0 wt%). In contrast, lactoferrin-stabilized emulsion droplets showed considerable aggregation in the presence of salts but in the absence of mucin. This salt-induced aggregation was reduced in the presence of mucin, possibly because of its binding to the positively charged lactoferrin-stabilized emulsion droplets and thus a reduction in the positive charge at the lactoferrin-coated droplet surface. However, at higher mucin concentration (≥2.0 wt%), lactoferrin-stabilized emulsions also showed droplet aggregation.     

Encapsulation and Oxidative Stability of PUFA-Rich Oil Microencapsulated by Spray Drying Using Pea Protein and Pectin

This study aimed at evaluating the potential of pectin combination with pea protein isolate (PPI) in the microencapsulation of polyunsaturated fatty acids (PUFA)-rich oil by spray drying, in order to maximize encapsulation efficiency and minimize lipid oxidation. The feed emulsions used for particle production consisted of PUFA-rich oil droplets coated by either PPI (primary emulsion) or PPI–pectin (secondary emulsion). Dry emulsions characteristics and oxidative stability of microencapsulated oil as a function of relative humidity (RH; from 11 % to 75 %) were determined. Scanning electron microscopy (SEM) images revealed considerable structural changes. Oil droplets retained their shape upon drying and reconstitution. However, a shift in oil droplet size upon reconstitution indicated that oil droplet coalescence occurred within the process. Oxidation of microencapsulated oil in secondary emulsion was delayed from that of primary emulsion but followed the same pattern with regards to humidity. A high rate of oxidation was found for intermediate RH conditions (33 % and 57 % RH). The lowest rate of oxidation as followed by hydroperoxide and thiobarbituric acid reactive substances values was found at 75 % RH, a condition that is likely to diverge significantly from the monolayer moisture value. The oxidative stability of encapsulated oil was influenced by both physical state of the emulsions and the different constituents at the oil-in-water interface with PPI–pectin secondary emulsion giving the best protection of the oil.     

The stabilizing behaviour of soybean soluble polysaccharide and pectin in acidified milk beverages

The mechanisms of stabilization of soybean soluble polysaccharide (SSPS) and high methoxyl pectin (HMP) in acidified milk drinks were studied focusing on the differences in behaviour between the two polysaccharides. The changes in casein micelles size during acidification with glucono-δ-lactone or by direct acidification were measured using light scattering. When HMP was added to skim milk before acidification, pectin adsorbed on the surface of the casein micelles via electrostatic interactions and prevented casein aggregation. Results suggested that adsorption of pectin occurred from the beginning of acidification and somewhat affected the rearrangement of casein micelles in the pH range between 5.8 and 5.0. On the other hand, SSPS, at concentrations up to 2% (w/w), did not interact with caseins at pH >4.6. At pH <4.2 SSPS showed better stabilizing properties than HMP. In addition, between pH 4.2 and 3.2, SSPS-stabilized acid dispersions were not affected by pH, while dispersions homogenized with pectin showed a size distribution that depended on pH. The differences in structure between SSPS and HMP account for the unique functionalities of the two polysaccharides in acid milk systems.     

pH-dependent characteristics of gel-like emulsion stabilized by threadfin bream sarcoplasmic proteins

Threadfin bream sarcoplasmic proteins (TBSP) were identified by 2-dimensional polyacrylamide gel electrophoresis and the major proteins were likely be pyruvate kinase, creatine kinase, aldolase, and carbonic anhydrase. Emulsifying and rheological properties of emulsion stabilized by TBSP at pH 3, 7.5, and 12 were investigated. Surface hydrophobicity of TBSP increased with pH. Emulsifying activity index was highest at pH 3 followed by pH 12 and pH 7.5. The mean droplet diameter (d_3,2) was in the order of pH 7.5 > pH 3 > pH 12 at oil volume fraction ranging from 0.1 to 0.5. A decrease of oil volume fraction resulted in a reduction of oil droplet diameter. At oil volume fraction of 0.5, TBSP-stabilized emulsion at pH 7.5 and 12 exhibited gel-like characteristics, while liquid emulsion was observed at pH 3, based on visual observation and dynamic rheological analysis. Storage modulus (G′) values of emulsion at all studied pH values increased with temperature, implying the development of protein gel network upon heating. The gel-like emulsion at pH 12 absorbed more protein and water than at pH 7.5. Protein retention and water holding capacity of emulsion increased with oil volume fraction. Hydrophobic interactions between proteins and oil droplets were likely responsible for the development of a gel-like emulsion.     

Factors influencing the production of o/w emulsions stabilized by β-lactoglobulin–pectin membranes

A primary emulsion was prepared by homogenizing 10 wt% corn oil with 90 wt% aqueous β-lactoglobulin solution (0.5 wt% β-lg, pH 3 or 7) using a two-stage high-pressure valve homogenizer. This emulsion was mixed with aqueous pectin (citrus, 59% DE) stock solution (2 wt%, pH 3 or 7) and NaCl solution to yield secondary emulsions with 5 wt% corn oil, 0.225 wt% β-lactoglobulin, 0.2 wt% pectin and 0 or 100 mM NaCl. The final pH of the emulsions was then adjusted (3–8). Primary and secondary emulsions were ultrasonically treated (30 s, 20 kHz, 40% amplitude) to disrupt any flocculated droplets. Secondary emulsions were more stable than primary emulsions at intermediate pHs. Secondary emulsions prepared at pH 7 had smaller particle diameters (0.35 to 6 μm) than those prepared at pH 3 (0.42 to 18 μm) across the whole pH range studied, and also had smaller diameters than the primary emulsions (0.35 to 14 μm). Ultrasound treatment reduced the particle diameter of both primary and secondary emulsions and lowered the rate of creaming. The presence of NaCl screened the charges and thus the electrostatic interaction between biopolymer molecules and primary emulsion droplets. Secondary emulsions were more stable to the presence of 100 mM NaCl at low pHs (3–4) than primary emulsions. This study shows that stable emulsions can be prepared by engineering their interfacial membranes using the electrostatic interaction of natural biopolymers, especially at intermediate pHs where proteins normally fail to function.