Chemical and physical stability of protein and gum arabic-stabilized oil-in-water emulsions containing limonene

ABSTRACT:  An important flavor component of citrus oils is limonene. Since limonene is lipid soluble, it is often added to foods as an oil-in-water emulsion. However, limonene-containing oil-in-water emulsions are susceptible to both physical instability and oxidative degradation, leading to loss of aroma and formation of off-flavors. Proteins have been found to produce both oxidatively and physically stable emulsions containing triacylglycerols. The objective of this research was to determine if whey protein isolate (WPI) could protect limonene in oil-in-water emulsion droplets more effectively than gum arabic (GA). Limonene degradation and formation of the limonene oxidation products, limonene oxide and carvone, were less in the WPI- than GA-stabilized emulsions at both pHs 3.0 and 7.0. These data suggest that WPI was able to inhibit the oxidative deterioration of limonene in oil-in-water emulsions. The ability of WPI to decrease oxidative reactions could be due to the formation of a cationic emulsion droplet interface at pH 3.0, which can repel prooxidative metals, and/or the ability of amino acids in WPI to scavenge free radical and chelate prooxidative metals.     

Inhibition of citral degradation in model beverage emulsions using micelles and reverse micelles

Citral is a flavour component that is widely used in the beverage, food, and fragrance industries. Citral chemically degrades over time in aqueous solutions due to acid catalysed and oxidative reactions, leading to loss of desirable flavour and the formation of off-flavours. We examined the influence of surfactant micelles (Tween 80) in the aqueous phase and reverse micelles (polyglycerol polyricinoleate, PGPR) in the oil phase on the oil–water partitioning and chemical degradation of citral in medium chain triglyceride oil-in-water emulsions. The percentage of citral in the aqueous phase of the emulsions increased with increasing Tween 80 concentration, which was attributed to its incorporation within surfactant micelles. The rate of citral degradation decreased as the Tween 80 concentration was increased from 1% to 5% w/w in both aqueous solutions and in emulsions, suggesting that citral was protected from degradation once it was incorporated into micelles. The presence of reverse micelles (5% or 10% w/w PGPR) in the oil droplets decreased the percentage of citral present within the aqueous phase of the emulsions, suggesting that citral was preferentially incorporated into the reverse micelles. In addition, the presence of reverse micelles increased the chemical stability of citral, possibly because a greater fraction remained within the oil droplets. These results show that micelle or reverse-micelle structures may be used to improve the chemical stability of citral in beverage emulsions.     

Design of interfacial films to control lipid oxidation in oil-in-water emulsions

In oil-in-water (O/W) emulsions, the droplets covered by native proteins are more prone to oxidation than droplets covered by surfactants. We attempted in this work to improve the barrier properties of protein-stabilized interfacial layers by controlled modifications of their composition and structure. Native bovine β-lactoglobulin (BLG) or β-casein (BCN), partially aggregated BLG and mixtures of the proteins with dilauroyl phosphatidylcholine (DLPC) were used to prepare emulsions and reconstituted Langmuir–Blodgett films. Lipid oxidation in the emulsions, as evaluated from oxygen uptake and formation of conjugated dienes, propanal and hexanal was roughly unmodified with aggregated BLG and DLPC–BLG mixtures and even favored with DLPC–BCN mixtures. The reconstituted phospholipid/protein interfacial layers presented interfacial heterogeneity evidenced by atomic force microscopy (AFM). This indicates that the structural homogeneity of the interface could be a key factor in controlling lipid oxidation.     

Ability of flaxseed and soybean protein concentrates to stabilize oil-in-water emulsions

The ability of flaxseed protein concentrate (FPC) to stabilize soybean oil-in-water emulsion was compared with that of soybean protein concentrate (SPC). The stability of emulsions increased with increase in protein concentration. The FPC-stabilized emulsions had smaller droplet size and higher surface charge, but worse stability at the same protein concentration compared to SPC-stabilized emulsions. Oil-in-water emulsions stabilized by both proteins were diluted and compared at different pH values (3–7), ionic strength (0–200 mM NaCl) and thermal treatment regimes (25–95 °C for 20 min). Considerable emulsion droplet flocculation occurred around iso-electric point of both proteins: FPC (pH 4.2) and SPC (pH 4.5). FPC and SPC-stabilized emulsions remained relatively stable against droplet aggregation and creaming at NaCl concentration below 100 and 50 mM, respectively. The emulsions stabilized by both proteins were fairly stable within these thermal processing regimes. FPC appears to be less effective as an emulsifier compared to SPC due to its lower emulsion viscosity. Hence, FPC could be more effective in emulsions that are fairly viscous.     

Interfacial properties of deamidated wheat protein in relation to its ability to stabilise oil-in-water emulsions

Isolated wheat protein (IWP) is an acidic deamidated wheat protein. The deamidation process enhances the protein solubility at pHs greater than 6, and therefore its potential ability to act as a food emulsifier. The interfacial properties and the mechanism by which this protein stabilises oil-in-water emulsions were investigated by measuring the protein's absorbed layer thickness on latex particles, its interfacial rheology, and the colloidal and thermal stability of IWP stabilised emulsions. IWP forms a relatively thick interfacial layer of 18 nm upon adsorption onto latex beads, suggesting that the protein adsorbed with the long axis perpendicular to the surface, i.e. end-on, at a full protein coverage. The interfacial rheology measurement showed that IWP formed a relatively weak fluid-like interface. Similar to other protein emulsifiers, the colloidal stability of IWP emulsions is provided largely through electrostatic repulsion. Although IWP emulsions were sensitive to salt induced flocculation, the presence of excess protein in the aqueous phase (e.g. 4 wt%) was able to reduce the effect of salt screening (50 mM CaCl₂) on a 25 wt% oil-in-water emulsion completely. The emulsions underwent minimal coalescence when droplets were in close contact, e.g. flocculated, because the interfacial layer of IWP provides a barrier to droplet coalescence, even in high salt environments. IWP emulsions were resistant to thermal treatment with no changes in particle size observed when the emulsions were heated (up to 90 °C for 20 min) in the absence or the presence of 150 mM NaCl. The heat stability of IWP emulsions is thought to arise from the structure of IWP at the interface. A lack of free cysteines combined with few hydrophobic regions meant that there were minimal interactions between protein molecules adsorbed onto the same droplet or on neighbouring droplets. The unique interfacial properties of IWP, e.g. its physical layer thickness and the structure provide enhanced stability for emulsions against coalescence and heating.     

Mannans as stabilizers of oil-in-water beverage emulsions

The stabilizing effect of spruce galactoglucomannan (GGM) on a model beverage emulsion system was studied and compared to that of guar gum and locust bean gum galactomannans, konjac glucomannan, and corn arabinoxylan. In addition, guar gum was enzymatically modified in order to examine the effect of the degree of polymerization and the degree of substitution of galactomannans on emulsion stability. Use of GGM increased the turbidity of emulsions both immediately after preparation and after storage of up to 14 days at room temperature. GGM emulsions had higher turbidity than the emulsions containing other mannans. The initial turbidity increased with increasing GGM content, but after 14 days storage at room temperature, the turbidity was the highest for GGM/oil ratio of 0.10:1 when ethanol-precipitated GGM was used. Increasing the storage temperature to +45 °C led to rapid emulsion breakdown, but a decrease in storage temperature increased emulsion stability after 14 days. Confocal microscopy showed that the average particle size in the bottom part of GGM emulsions stored for 14 days was smaller than 1 μm. A low degree of polymerization and a high degree of substitution of the modified galactomannans were associated with a decrease in emulsion turbidity.     

Physicochemical properties of whey protein isolate stabilized oil-in-water emulsions when mixed with flaxseed gum at neutral pH

Concentrations ranging from 0% to 0.33% (w/v) of gum (Emerson and McDuff) were added to the emulsions at pH 7. Particle size distribution, viscosity, ζ-potential, microstructure, and phase separation kinetics of the emulsions were observed. Both polysaccharides and protein coated droplets are negatively charged at this pH, as shown by ζ-potential measurements. At all the concentrations tested, the addition of gum did not affect significantly (p < 0.05) the apparent diameter of the emulsion droplets. At low concentrations (gum  0.075% (w/v)), no visual phase separation was observed and the emulsion showed a Newtonian behaviour. However, at concentrations above the critical concentration of gum, depletion flocculation occurred: when 0.1 flaxseed gum was present, there was visual phase separation over time and the emulsion exhibited shear-thinning behaviour. These results demonstrate that flaxseed gum is a non-interacting polysaccharide at neutral pH; it could then be employed to strengthen the nutritional value of some milk-based drinks, but at limited concentrations.     

Lipid and water crystallization in protein-stabilised oil-in-water emulsions

Phase and state transitions occurring during freezing and thawing of oil-in-water emulsions with different water phase formulations, interfacial compositions and two lipid types were studied as crucial factors affecting emulsion stability. Emulsions containing 0–40% (w/w) sucrose in the water phase at pH 7, and 10, 20, 30, 40% (w/w) dispersed lipid phase (sunflower oil, SO or hydrogenated palm kernel oil, HPKO) with whey protein isolate, WPI, or sodium caseinate, NaCAS, (protein:lipid = 1:10 and 2:10) as emulsifier were prepared. Phase/state behaviour of the continuous and dispersed phases was determined by differential scanning calorimetry (DSC). Emulsion stability and morphology were derived from DSC data, gravitational separation and particle size analysis during 4 freeze-thaw cycles. Systems were stable when only lipid crystallization occurred. DSC data showed that lipid crystallization prior to water crystallization (i.e. emulsions containing HPKO) caused destabilisation at low sucrose concentrations (0, 2.5 and 5% w/w). Emulsions were stable if the dispersed oil phase crystallized after the dispersing water phase (i.e. emulsions containing SO). A concentration of sucrose ≥10% (w/w) in the aqueous phase gave stable emulsions. At 10:1 lipid to protein ratio, WPI showed better stabilising properties than NaCAS at 2.5 and 5% (w/w) sucrose. Double concentration of WPI (lipid:protein = 10:2) at 0% (w/w) sucrose significantly improved systems stability, whereas no positive effect was observed when the concentration of NaCAS was increased. From morphology study, in addition to lipid destabilisation, thickening and flocculation caused instability of the systems. These were extensive in systems containing WPI and were ascribed to interactions between whey proteins during thermal cycling.     

饮料乳化香精的稳定性研究

本文简要介绍了乳化香精的制备技术,综述了乳化香精的稳定性问题,详细阐述了控制乳化香精稳定性的主要措施.     

On the stability of oil-in-water emulsions to freezing

Oil in water emulsions (40 wt%) were prepared from a homologous series of n-alkanes (C10–C18). The samples were temperature cycled in a differential scanning calorimeter (two cycles of 40 °C to −50 °C to 40 °C at 5 °C min⁻¹) and in bulk (to −20 °C). The emulsions destabilized and phase-separated after freeze–thaw if the droplets were solid at the same time as the continuous phase and were more unstable if a small molecule (SDS or polyoxyethylene sorbitan monolaurate) rather than a protein (whey protein isolate or sodium caseinate) emulsifier was used. The unstable emulsions formed a self-supporting cryo-gel that persisted between the melting of the water and the melting of the hydrocarbon phase. Microscopy provides further evidence of a hydrocarbon continuous network formed during freezing by a mechanism related to partial coalescence which collapses during lipid melting to allow phase separation.     

Oxidative stability of oil-in-water emulsions stabilised with protein or surfactant emulsifiers in various oxidation conditions

Many studies have investigated the effect of emulsifiers on the oxidative stability of oil-in-water (O/W) emulsions. A better oxidative stability of surfactant-stabilised O/W emulsions as compared to protein-stabilised emulsions has been recently shown in conditions when the major part of the emulsifier is adsorbed at the oil-water interface and oxidation is induced by iron−ethylenediaminetetraacetic acid (EDTA) complex. In this work, the contribution of the interfacial layer to the oxidation of emulsified lipids is investigated under various incubation conditions, involving different oxidation mechanisms. O/W emulsions were formulated at pH 6.7 with limited amounts of emulsifiers in the aqueous phase. Emulsions were incubated either at 33 °C without initiator at 25 °C in the presence of iron/ascorbate, metmyoglobin or 2,2′-azobis(2-amidinopropane)-dihydrochloride (AAPH). Oxygen uptake and volatile compound formation confirmed that protein-stabilised emulsions are less oxidatively stable than Tween 20-stabilised ones. This work also shows complex oxidative interrelationships between oxidation initiator and certain proteins, such as β-casein and bovine serum albumin.     

Antioxidant and emulsifying properties of potato protein hydrolysate in soybean oil-in-water emulsions

The efficacy of a previously developed antioxidative potato protein hydrolysate (PPH) for the stabilisation of oil droplets and inhibition of lipid oxidation in soybean oil-in-water (O/W) emulsions was investigated. Emulsions (10% lipid, pH 7.0) with PPH-coated oil droplets were less stable than those produced with Tween 20 (P < 0.05). However, the presence of PPH, whether added before or after homogenisation with Tween 20, retarded emulsion oxidation, showing reduced formation of peroxides up to 53.4% and malonaldehyde-equivalent substances up to 70.8% after 7-d storage at 37 °C (P < 0.05), when compared with PPH-free emulsions. In the emulsions stabilised by PPH + Tween 20, 8–15% of PPH was distributed at the interface. Adjustment of the pH from 3 to 7 markedly increased ζ-potential of such emulsions (P < 0.05). Inhibition of lipid oxidation by PPH in soybean O/W emulsions can be attributed to both chemical and physical (shielding) actions.     

Impact of surfactants on the lipase digestibility of gum arabic-stabilized O/W emulsions

The bioavailability of lipids from an emulsion can be controlled and regulated by the property of the stabilizing interfacial layer. Here we evaluate how low-molecular weight surfactants including hexadecyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Tween 80 (T80) influence the interfacial behavior of lipase and bile extract on the surface of lipid droplets stabilized with gum arabic (GA). The lipolysis behavior was influenced by surfactant type and concentration. The results showed that anionic SDS could completely displace GA from droplet surface. Cationic CTAB might either adsorb onto existing GA layers or displace GA, whereas non-ionic T80 could co-adsorb with GA on the interface. When the concentration of surfactants was much higher than the critical micelle concentration (CMC), all the surfactants would form a dense adsorption layer on the droplet interface to prevent lipase from the direct contact with lipids. A considerable amount of surfactant in the aqueous phase may also compete with the bile salt and lipase, thus leading to suppressed digestion of lipids. Ionic surfactants would denature the lipase resulting in reduced enzyme activity, and T80 micelles may interact with the lipase, hindering their adsorption onto the droplet interface as well. These results were confirmed both by the digestion model and interfacial techniques. The results provided guidance for the development of emulsion-based delivery systems for functional lipid foods.     

Structure and stability of heat-treated concentrated dairy-protein-stabilised oil-in-water emulsions: A stability map characterisation approach

Emulsion instabilities such as depletion flocculation, coalescence, aggregation and heat-induced protein aggregation may be detrimental to the production of sterilised food emulsions. The type and the amount of protein present in the continuous phase and at the oil–water interface are crucial in the design of emulsions with appropriate stability. In this study, four oil-in-water model emulsion systems (pH 6.8–7.0) were formulated, characterised and categorised according to the potential interactions between protein-coated or surfactant-coated emulsion droplets and non-adsorbed proteins present in the continuous phase. The heat stability, the creaming behaviour and the flow behaviour of the model emulsions were influenced by both the emulsifier type and the type of protein in the continuous phase. The results suggest that this stability map approach of predicting droplet–droplet, droplet–protein and protein–protein interactions will be useful for the future design of heat-stable emulsion-based beverages with good creaming stability at high protein concentrations.     

Heat-induced changes in oil-in-water emulsions stabilized with soy protein isolate

The physicochemical properties of soy proteins stabilized oil-in-water emulsions were studied after heating at two different temperatures, 75 and 95 °C. The effect of changing the order of the process (heating the solution before emulsification, or heating the emulsion) was also studied. The heating temperatures were chosen as they are known to selectively cause denaturation of the two major proteins present in the soy protein isolate: β-conglycinin and glycinin. The thermal transitions observed for soy proteins adsorbed at the interface were different from those measured in protein solutions, suggesting that some changes occur in the structure of the soy proteins upon adsorption on the oil droplet. Heating induces aggregation of the oil droplets, as shown by an increase of the particle size and the bulk viscosity of the emulsions, with a more prominent effect after heating at 95 °C. Transmission electron microscopy observations clearly demonstrate that heating induces the formation of large protein aggregates at the interface. In addition, the composition of the protein present at the interface changes depending on the order of heating and homogenization. While heating the solutions before emulsification results in all the protein subunits to be present at the interface in an aggregated form, when heating is applied after emulsification, a portion of the α and the α′ subunit of β-conglycinin as well as the acidic subunits of glycinin remain unadsorbed.     

Contribution of okra extracts to the stability and rheology of oil-in-water emulsions

Three okra polysaccharide extracts were isolated and studied in terms of their composition and their capacity to affect the rheology and stability of emulsions. HBSS (hot buffer soluble solids, extracted at 70 °C, pH = 5.2) comprised of charged (zeta potential −21.5 mV) polysaccharides sizing between 5 kDa (d ∼ 3 nm) and 50 kDa (d ∼ 200 nm), and a population of very large molecules (MW >> 1.4 MDa). Upon addition in Tween 20-stabilized emulsions, HBSS caused flocculation and enhanced creaming at low concentrations (0.125%), while at higher concentrations (1.25%–2.50%) it drastically reduced creaming due to its increase of the continuous phase viscosity.     

Rheological properties and stability of oil-in-water emulsions containing tapioca maltodextrin in the aqueous phase

The present research focuses on the effect of the concentration and dextrose equivalent (DE) values of tapioca maltodextrin in the aqueous phase on rheological behavior and stability of oil-in-water emulsions prepared with Tween80. The critical flocculation concentrations (CFCs) of oil-in-water emulsions containing tapioca maltodextrin with DE of 16 (DE16), 12 (DE12) and 9 (DE9) were 11%, 9% and 7% (w/w) respectively, as revealed by transmittance measurement. Coalescence was observed as maltodextrin concentration increased above the CFC. The rheological parameters of flow behavior index (n) and consistency index (k) have been well-described by the Herschel–Bulkley model. The relative consistency index (k_relative) increased markedly when the concentration of maltodextrin exceeded the CFC because of depleting flocculation. The consistency index (k_emulsion) and yield stress (τ₀) of emulsions containing tapioca maltodextrin increased with increasing maltodextrin concentration or decreasing DE. The emulsions containing maltodextrin showed Newtonian flow behavior when the maltodextrin concentration was below the CFC. At maltodextrin concentrations above the CFC, emulsions containing maltodextrin exhibited shear thinning behavior. An increase in the maltodextrin concentration resulted in a decrease in the n_emulsion until maltodextrin concentration reached 20% (w/w) for DE9, DE12 and 25% (w/w) for DE16. Further increase in the maltodextrin concentration resulted in an increased the n_emulsion because of predominant influence of the continuous phase.     

Influence of xanthan gum on the formation and stability of sodium caseinate oil-in-water emulsions

Studies have been made of the changes in droplet sizes, surface coverage and creaming stability of emulsions formed with 30% (w/w) soya oil, and aqueous solution containing 1 or 3% (w/w) sodium caseinate and varying concentrations of xanthan gum. Addition of xanthan prior to homogenization had no significant effect on average emulsion droplet size and surface protein concentration in all emulsions studied. However, addition of low levels of xanthan (≤0.2 wt%) caused flocculation of droplets that resulted in a large decrease in creaming stability and visual phase separation. At higher xanthan concentrations, the creaming stability improved, apparently due to the formation of network of flocculated droplets. It was found that emulsions formed with 3% sodium caseinate in the absence of xanthan showed extensive flocculation that resulted in very low creaming stability. The presence of xanthan in these emulsions increased the creaming stability, although the emulsion droplets were still flocculated. It appears that creaming stability of emulsions made with mixtures of sodium caseinate and xanthan was more closely related to the structure and rheology of the emulsion itself rather than to the rheology of the aqueous phase.     

Albumin causes a synergistic increase in the antioxidant activity of green tea catechins in oil-in-water emulsions

Model oil-in-water emulsions containing epicatechin (EC) and epigallocatechin gallate (EGCG) showed a synergistic increase in stability in emulsions containing added albumin. EGCG showed a stronger synergy (35%) with ovalbumin than did EC. Oxidation of the oil was monitored by determining peroxide values and hexanal contents. The effect of bovine serum albumin (BSA) on model oil-in-water emulsions containing each of the green tea catechins [epicatechin gallate (ECG), EGCG, EC and epigallocatechin (EGC)] was studied during storage at 30 °C. The green tea catechins showed moderate antioxidant activity in the emulsions with the order of activity being ECG ≈ EGCG > EC > EGC. Although BSA had very little antioxidant activity in the absence of phenolic antioxidants, the combination of BSA with each of the catechins showed strong antioxidant activity. BSA, in combination with EC, EGCG or EGC, showing the strongest antioxidant activity with good stability after 45 days storage. Model experiments with the catechins stored with BSA in aqueous solutions confirmed that protein–catechin adducts with antioxidant activity were formed between the catechins and protein. The antioxidant activity of the separated protein–catechin adducts increased strongly with storage time and was stronger for EGCG and ECG than for EC or EGC.