Stabilization of acidic soy protein-based dispersions and emulsions by soy soluble polysaccharides

Soy soluble polysaccharides (SSPS) are shown to prevent destabilization of soy protein isolate (SPI) dispersions and SPI-based oil-in-water (O/W) emulsions under acidic conditions. Addition of SSPS above a critical concentration (0.25 wt%) increased the stability of 0.50 wt% SPI dispersions against aggregation and phase separation under conditions where SPI would normally precipitate (near its isoelectric point). Though SSPS neutralized SPI surface charge via electrostatic interaction, there was increased stability against aggregation due to steric repulsion. At acidic pH, addition of 1 wt% NaCl electrostatically screened protein–polysaccharide complexation which led to SPI precipitation and sedimentation. However, the order of salt addition had a significant impact on charge screening, with salt added before pH adjustment reducing SPI–SSPS complexation whereas it had less effect when added afterwards. Salt penetration efficacy diminished with decreasing pH. O/W emulsions (5 wt% oil) prepared with 0.50 wt% SPI destabilized at pH 4–5 due to protein aggregation, but addition of ≥0.25 wt% SSPS improved emulsion stability by inhibiting protein–protein interactions thus limiting increases in oil droplet diameter over time. Overall, both dispersion and emulsion stability greatly depended on pH, ionic strength and SSPS concentration. These results demonstrated that SSPS could effectively stabilize acidic SPI dispersions and that SPI–SSPS interactions may be used as a tool to improve the kinetic stability of SPI-based O/W emulsions.     

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.     

Emulsifying properties of proteins extracted from Australian canola meal

Canola protein albumin fraction, globulin fraction, and canola protein isolate (CPI) were compared to commercial soy protein isolate (SPI) in terms of their emulsifying properties at various pH values. The globulin fraction had higher emulsifying capacity (EC), higher emulsifying activity index (EAI), and the droplet size of emulsions it stabilized was consistently smaller irrespective of pH compared to albumin fraction or CPI. In comparison to SPI, globulin fractions also had higher EC at all pH values tested, higher EAI at acidic pH, and smaller or comparable average emulsion droplet size at both pH 4 and 7. The stability of canola protein based emulsions were comparable to those of SPI based emulsions at most pH values (except the emulsion stabilized by the CPI at pH 4), with no significant (p > 0.05) changes in droplet size during storage for up to 7 days at room temperature. These emulsions, however, experienced separation into the emulsion and serum phases after 24 h storage at room temperature with the exception of CPI- and SPI-stabilized emulsions at pH 9. This study demonstrates the comparable emulsifying properties (forming or stabilizing) of some canola proteins to commercially available SPI, suggesting the potential use of canola proteins in food applications.     

Water/Oil Emulsions Prepared by the Membrane Emulsification Method and Their Stability

We developed and tested a simple method to measure dispersed droplet size of W/O emulsions. Then, using a microporous glass membrane treated with oil phase, we produced a W/O emulsion with high water content (40% w/w) at a high emulsification rate by the membrane emulsification method, and assessed its stability. In comparison with emulsions by the stirring methods, variations in dispersed droplet size and viscosity of emulsions by membrane method were small and the emulsions were more stable. Droplet size was not related to the stability of the W/O emulsion prepared by membrane emulsification.     

Enhancement of gel strength by application of thermal treatments in highly flocculated emulsions

Thermal-induced changes of the rheological behavior of flocculated emulsions through the optimization of in situ thermorheological treatments, as well as the effect that several compositional variables (oil fraction, egg yolk concentration or cholesterol reduction level) exert on the enhancement of gel strength, were investigated. With this aim, oil-in-water emulsions were prepared using a spray-dried egg product. Different temperature ramps under oscillatory shear and droplet size distribution measurements were carried out. An increase in temperature produces, first of all, an increase in the storage modulus up to a critical temperature, which depends on heating rate. Subsequently, a dramatic decrease in the viscoelastic functions occurs, due to emulsion breakdown. However, the application of upward/downward temperature cycles, setting the maximum temperature at 67 °C, avoids emulsion breakdown and yield significantly higher values of the rheological functions in comparison to those found with fresh emulsions, in spite of the thermal-induced droplet coalescence observed. The final values of linear viscoelastic functions and droplet size depend on the type of cycles applied, which are based on different heating rates or times at the maximum temperature, and emulsion composition.     

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.     

Rheology and microstructure of myofibrillar protein-plant lipid composite gels: Effect of emulsion droplet size and membrane type

Composite gels were prepared from 2% myofibrillar protein (MP) with 10% imbedded pre-emulsified plant oils (olive and peanut) of various particle sizes at 0.6 M NaCl, pH 6.2. Dynamic rheological testing upon temperature sweeping (20-70 °C at 2 °C/min) showed substantial increases in G′ (elastic modulus) of MP sols/gels with the addition of emulsions, and the G′ increases were inversely related to the emulsion droplet size. Furthermore, gels containing emulsified olive oil had a greater (P < 0.05) hardness than those containing emulsified peanut oil. Regardless of oil types, MP-coated oil droplets exhibited stronger reinforcement of MP gels than Tween 80-stablized oil droplets; the latter composite gels had considerable syneresis. Light microscopy with paraffin sectioning revealed a stable gel structure when filled with protein-coated oil droplets, compared to gels with Tween 80-treated emulsions that showed coalesced oil droplets. These results suggest that rheological characteristics, hardness, texture, and water-holding capacity of MP gels were influenced by type of oils, the nature of the interfacial membrane, and the size of emulsion droplets.     

Comparing the heat stability of soya protein and milk whey protein emulsions

The heat stability of emulsions stabilized by WPC or SPI or mixtures of the two are compared by following the change in oil droplet number during heating, and applying kinetic rate equations to calculate the rate constant (k) for destabilization. SPI emulsions were found to be unstable to heat at pH around the pI, whilst being stable at pH further from the pI. This is related to the pH dependent solubility of soy proteins. This determined that a pH close to the pI (pH 4.5) be used for further studies so as to give a heat labile emulsion. Both WPC and SPI emulsions showed a weak dependence of k on protein concentration at pH 4.5, and an increasing k as the temperature increased. Arrhenius plots for emulsions made with WPC were bilinear, whilst those for SPI followed a single straight line. The change in slope of the Arrhenius plots for the WPC emulsions occurred around 70 °C, lower than would be expected from the denaturation temperature of β-lactoglobulin, the protein that dominates the thermal behaviour of WPC. The activation energies for WPC and SPI emulsions calculated from the slopes of the Arrhenius plots are slightly lower for WPC and considerably lower for SPI than the equivalent values in the literature for these proteins in solution. This, and the apparent lower denaturation temperature of β-lactoglobulin in emulsions, we explain by hypothesizing that the WPC and SPI proteins are already partially denatured by surface adsorption when they are heated, and thus require less energy to denature, and unfold at lower temperatures than native non-adsorbed proteins.     

Influence of different β-glucans on the physical and rheological properties of egg yolk stabilized oil-in-water emulsions

The objective of this study was to obtain additional information on the influence of different β-glucan preparations, i.e. curdlan (CL), barley (BG), oat (OG), and yeast (YG) β-glucans, on the physical and rheological properties of egg yolk stabilized oil-in-water emulsions containing 20% oil. The emulsion without β-glucan (REF) was also prepared as a reference. Addition of CL and OG increased emulsion oil droplet sizes, whereas BG and YG showed no effect. Emulsion microstructures revealed that β-glucans induced flocculation of the oil droplet in the following order: CL > BG ≈ OG > YG. Dynamic oscillatory shear tests indicated that all emulsions exhibited weak gel-like characteristics which were enhanced by β-glucans addition as evidenced by an increase in G′ and a decrease in tan δ values. Flow tests showed that β-glucans enhanced thixotropy and yield stress of the emulsions. Stability tests demonstrated that β-glucans addition improved creaming stability of the emulsions during storage possibly due to an increase in viscosity of the continuous phase and/or a formation of a three-dimensional droplet network. CL exhibited the most pronounced effects on the aforementioned properties of emulsions compared to the other β-glucans tested. YG gave emulsion with higher viscoelastic properties and yield stress but lower stability than those made with BG or OG, indicating complex relationship between rheology and stability of these emulsion systems.     

Emulsification enhances the retention of esters and aldehydes to a greater extent than changes in the droplet size distribution of the emulsion

The physicochemical and organoleptic properties of an emulsion depend on the way the constituents interact with one another to form emulsion droplets, interfacial region and continuous phase. The objective of this work was to evaluate the respective impact of both the emulsification and the modification of the properties of an emulsion, such as the droplet size distribution, on the partition of aroma compounds. The emulsions were prepared with sodium caseinate and the low melting point fraction of anhydrous milk fat (Φ = 0.3). Their volume-surface mean droplet size ranged from 1.8 to 0.3 μm. Results showed that the measured partition coefficients of ethyl pentanoate, isoamyl acetate, hexanal and t-2-hexenal were lower than the calculated ones from values measured separately over continuous and dispersed phases. The droplet size distribution had no significant impact on the partition coefficient of the three esters whereas, for a volume-surface mean diameter below 0.5 μm, the partition coefficients of the two aldehydes were drastically reduced. The greater retention is not related to the sodium caseinate remaining in the continuous phase of the emulsion. The formation of an interfacial area seems to govern the partition of aroma compounds in emulsions.     

Emulsifying properties of pea protein/guar gum conjugates and mayonnaise application

Modified plant protein may be used as a healthy and more functional emulsifier in food products. The objective of this study was to evaluate the emulsifying properties of functionally enhanced pea protein (i.e. pea protein conjugated with guar gum, G-PPI) and its potential application to mayonnaise, compared with unmodified pea protein. Emulsions containing G-PPI were prepared at different pH, salt concentrations, protein concentrations and oil/water ratios. Mayonnaise samples were prepared using the pea proteins or egg yolk powder. Various characteristics of the emulsions, including droplet size, apparent viscosity, viscoelasticity and microstructure, were analysed. The emulsions with G-PPI had significantly increased stability of up to 89.4% and apparent viscosity of up to 48.62 mPa.s. The G-PPI emulsion had a smaller average droplet size of 934.4 nm at pH 7 compared with the PPI emulsion (stability 62.7%, apparent viscosity 22.8 mPa.s and droplet size 1664.8 nm). The pH, NaCl concentration, protein concentration and oil/water ratio greatly affected the emulsifying properties. The G-PPI mayonnaise at higher protein concentrations (6 or 8%) exhibited excellent emulsifying and rheological properties. The modified pea protein through the green modification process with natural polysaccharides could be used as a safe and functional emulsifier in different emulsified foods.     

Stability and Oil Migration of Oil‐in‐Water Emulsions Emulsified by Phase‐Separating Biopolymer Mixtures

Oil‐in‐water (O/W) emulsions with varying concentration of oil phase, medium‐chain triglyceride (MCT), were prepared using phase‐separating gum arabic (GA)/sugar beet pectin (SBP) mixture as an emulsifier. Stability of the emulsions including emulsion phase separation, droplet size change, and oil migration were investigated by means of visual observation, droplet size analysis, oil partition analysis, backscattering of light, and interfacial tension measurement. It was found that in the emulsions prepared with 4.0% GA/1.0% SBP, when the concentration of MCT was greater than 2.0%, emulsion phase separation was not observed and the emulsions were stable with droplet size unchanged during storage. This result proves the emulsification ability of phase‐separating biopolymer mixtures and their potential usage as emulsifiers to prepare O/W emulsion. However, when the concentration of MCT was equal or less than 2.0%, emulsion phase separation occurred after preparation resulting in an upper SBP‐rich phase and a lower GA‐rich phase. The droplet size increased in the upper phase whereas decreased slightly in the lower phase with time, compared to the freshly prepared emulsions. During storage, the oil droplets exhibited a complex migration process: first moving to the SBP‐rich phase, then to the GA‐rich phase and finally gathering at the interface between the two phases. The mechanisms of the emulsion stability and oil migration in the phase‐separated emulsions were discussed.     

超声改性大豆分离蛋白与大豆可溶性多糖复合乳化体系的冻融稳定性研究

本实验针对不同超声功率改性的大豆分离蛋白与大豆可溶性多糖形成的复合乳液的冻融稳定性进行研究, 揭示乳液冻融稳定机理与形成乳液复合物结构特性之间的构效关系。对2 次冻融循环处理前后乳液油滴进行共聚焦 观察,研究等温结晶固脂含量、油脂被乳化量的变化和作为乳化剂的大豆分离蛋白不同超声处理（0、200、300、 400、500 W）下二级结构的变化,进而分析其与乳液冻融稳定性的关系。结果表明：乳液经2 次冻融循环处理后 随着超声功率的增加聚结程度降低,400 W超声处理的大豆分离蛋白与大豆可溶性多糖复合乳液最为稳定;等温 结晶条件下不同乳液固脂含量增加速率不同,但最终平衡时总含量相同;油脂被乳化量发生不同程度的变化;不 同超声处理改变了大豆分离蛋白的二级结构,400 W超声处理的大豆分离蛋白无规卷曲结构含量最高。说明不同 超声改性的大豆分离蛋白与大豆可溶性多糖会形成不同结构的复合物,影响了乳液的冻融稳定性,初步明确了 适当的超声处理能够改善大豆分离蛋白的空间结构,促进其与大豆可溶性多糖分子的键合,进而影响大豆分离蛋 白-多糖界面结构特性和乳化体系的冻融稳定性。     

Emulsifying characterization of hens egg yolk proteins in oil-in-water emulsions

Emulsifying properties of egg yolk as a function of pH and oil volume were studied. Egg yolk proteins formed larger emulsion particles at pH 3 and the mean droplet size of the emulsions was decreased with increasing pH. A linear relationship between turbidity and mean droplet size of egg yolk emulsions could not be obtained. This may be due to the floculation of the emulsions. Egg yolk proteins formed thicker multilayers at low oil volume, however total protein adsorption ratio against original proteins was 55–65%, independent to protein and oil concentration. Electrophoretic analysis of the egg yolk emulsion revealed that the main components to adsorb at the interface was glanular lipovitellins, even though its emulsifying property was lower than that of plasma because of poor solubility at low ionic strength (0.1 M NaCl) at pH 7. These results indicate that the main contributor for egg yolk emulsion is granules and it can affect the emulsifying properties of egg yolk at different pH values.     

大分子蛋白界面修饰改善乳液特性研究进展

蛋白界面修饰指在乳液的乳化后阶段直接针对界面蛋白进行的修饰技术,其通过改变界面蛋白膜厚度或提升其强度从而改善乳液理化特性,并可一定程度规避乳化前预修饰蛋白对于乳液特性的某些不良影响,但是截至目前,对于蛋白界面修饰领域仍缺乏深入研究及系统总结。基于此,本文综述了目前研究较为广泛的几种蛋白界面修饰方法,包括酶促交联修饰、多糖修饰、多酚修饰以及氧化修饰,同时阐述了各种方法的作用机制以及对乳液特性的改善效果,并对蛋白界面修饰的未来发展方向作出展望,以期为乳液品质的进一步提高提供理论参考与技术支持。     

大豆分离蛋白-高酯柑橘果胶-没食子酸复合Pickering乳液制备及其稳定性分析

以大豆分离蛋白、高酯柑橘果胶、没食子酸为原料,制备一种蛋白质-多糖-多酚复合物,利用单因素试验、正交试验优化复合物制备条件,并通过流变特性、粒径及分布、Zeta电位、乳液稳定性等分析手段对Pickering乳液稳定性能进行表征。结果表明：在pH 4.5、温度35 ℃、没食子酸含量40 mg时复合乳液的吸光度达到最大值3.082,此时大豆分离蛋白-高酯柑橘果胶-没食子酸结合最紧密;当油相体积分数为0.7时,Pickering乳液弹性和黏性最好,形成的较好凝胶网络结构,此时的电位为（－54.08±2.74）mV,平均粒径为（220.36±7.13）nm;与25 ℃常温贮存相比,Pickering乳液在4 ℃冷藏析乳现象更弱,油滴粒径变化更小,更有利于维持乳液稳定性;随热处理温度升高,乳液析乳情况逐渐增强,油相体积分数为0.7和0.8时,液滴粒径受温度变化不明显;冷冻会破坏复合物形成的界面层,随着油相体积分数升高和冷冻时间的延长乳液析油现象明显导致稳定性大大降低;随着pH值升高,析油现象逐渐明显,当乳液体系pH值接近4时,乳滴粒径最小且分布相对均匀;高浓度的盐离子会破坏复合物结合的紧密程度,液滴发生聚集,乳液析乳情况明显,稳定性下降。     

Emulsifying properties of collagen fibers: Effect of pH,protein concentration and homogenization pressure

The emulsifying properties of collagen fibers were evaluated in oil-in-water (O/W) emulsions produced under different conditions of pH, protein content and type of emulsification device (rotor–stator and high-pressure homogenizer). The stability, microstructure and rheology of the O/W emulsions were measured. The phase separation and droplet size of the emulsions prepared using the rotor–stator device (primary emulsion) decreased with protein concentration and reduction in pH, allowing the production of electrostatically stable emulsions at pH 3.5. In contrast, emulsions at higher pH values (4.5, 5.5 and 7.5) showed a microscopic three-dimensional network responsible for their stability at protein contents higher than 1.0% (w/w). The emulsions at pH 3.5 homogenized by high pressure (up to 100 MPa) showed a decrease in surface mean diameter (d₃₂) with increasing pressure and the number of passes through the homogenizer. These emulsions showed droplets with lower dispersion and d₃₂ between 1.00 and 4.05 μm, six times lower than values observed for primary emulsions. The emulsions presented shear-thinning behavior and lower consistency index and viscosity at higher homogenization pressures. In addition, the emulsions showed a less structured gel-like behavior with increase in homogenization pressure and number of passes, since the pressure disrupted the collagen fiber structure and the oil droplets. The results of this work showed that the collagen fiber has a good potential for use as an emulsifier in the food industry, mainly in acid products.     

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.     

Steam-exploded camellia (Camellia oleifera Abel.) seed protein improves the stability of camellia seed oil emulsions

We aimed here to investigate the ability of steam-exploded camellia seed protein (SECSP) in stabilising steam-exploded camellia seed oil-in-water emulsion in comparison with soy protein isolate (SPI). Emulsions with different SECSP and SPI contents (0.5, 1, 1.5 and 2 g 100 mL⁻¹) were formulated and the effect of protein concentration on emulsion droplet size, zeta (ζ)-potential, physical stability and bioaccessibility was examined. The stability of emulsion increased with increasing protein concentration. SECSP emulsion was characterised by smaller droplet sizes, higher surface charge and better stability than SPI emulsion. Furthermore, SECSP emulsion exhibited good digestive stability. SECSP emulsion showed creaming only at protein concentrations above 0.5 g 100 mL⁻¹ after 28 days of storage, while SPI emulsion showed stratification and flocculation during the same storage period. Hence, steam explosion pretreatment physical modification of camellia seed protein (CSP) provides a novel approach of utilising proteins for emulsions stabilisation, and digestive stability improvement under gastrointestinal conditions.     

Flocculation and coalescence in water-in-oil emulsions stabilized by paraffin wax crystals

Water-in-mineral oil emulsions were prepared with small amounts of paraffin wax (0–2% w/w) added to the continuous phase, either by addition of pre-crystallized wax to the emulsion prior to emulsification or via subsequent quench-cooling of wax crystals in situ. Stability of the emulsions was examined using pulsed NMR droplet-size analysis, sedimentation and microscopy. Both pre and post-crystallized wax decreased the degree of droplet coalescence, however, emulsions made with post-crystallized wax were more stable over a 10-day period. Microscopy showed that visible crystals were strictly associated with droplets and droplet clusters indicating an affinity of the crystals to the interface. The incorporation of as little as 0.125% wax resulted in a notable decrease in emulsion sedimentation. After 24 days of storage, samples prepared with post-crystallized wax showed no sedimentation or flocculation, unlike pre-crystallized samples which were still somewhat destabilized despite the presence of as much as 2% wax. From these findings, rapid crystallization of wax in the continuous phase of a water-in-oil emulsion following emulsification is an effective means of enhancing long-term stability.