Thermal behavior of emulsions manufactured with soy protein ingredients

The conformation, denaturation and aggregation behavior of proteins are important factors which dictate their ingredient functions and applications in formulated food products. The effect of variation in pre-treatment temperature (70–90 °C × 30 s), pH (6.4–7.5) and calcium supplementation (450 and 850 mg/L) on heat coagulation time (HCT at 140 °C) of model emulsions (3.6% (w/v) protein) stabilized with soy protein isolate (SPI) and soy protein hydrolysate (SPH) ingredients was determined. Generally, HCT of emulsions was not significantly affected by alteration of constituent pre-heating temperatures. Model emulsions displayed higher HCTs with increasing pH and lower levels of intrinsic ash content. At both supplementation levels, calcium addition led to decreased HCTs. Supplementation with chloride salts caused a greater decrease in HCT compared to supplementation with citrate salts. Furthermore, soy protein hydrolysis was associated with lower emulsion thermal stability. Results demonstrate that modification of ingredient and manufacturing parameters may be a useful approach for enhancing thermal stability properties of soy protein stabilized emulsions.     

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.     

Catastrophic inversion and rheological behavior in soy lecithin and Tween 80 based food emulsions

Emulsions inversion occurs in many industrial processes and may be influenced by the formulation conditions, composition and emulsification protocols. In this work, the influence of emulsifiers and stirring on catastrophic inversion (O/W to W/O) was evaluated. Emulsions were prepared with different stirring rates, using soy lecithin and Tween 80, at 2 and 5 wt%. The aqueous phase was distilled water with 1 wt% NaCl and the oil phase was soy oil. These emulsions were analyzed by conductivity, stability, microscopy and rheology assays. The most stable emulsions presented inversion with a smaller amount of the external phase. Rheological analysis showed that, with a higher concentration of emulsifier, it is better to use Tween 80 when lower viscosity is desired, while soy lecithin is more appropriate for higher viscosity products. The oscillatory tests showed that while the emulsions prepared using Tween 80 exhibited concentrated solution behavior, those prepared with soy lecithin exhibited strong gel behavior.     

Characterization of soy protein-based films prepared with acids and oils by compression

Soy protein-based films modified with gelatin and plasticized with glycerol were prepared by compression at pH 10. The effect of different contents of lactic acid, epoxydized soybean oil and olive oil on optical, barrier, and mechanical properties was investigated and results were related to changes in the bands obtained by Fourier infrared spectroscopy and thermo-gravimetric analysis. The observed changes indicated interactions of the small molecules of lactic acid with protein and glycerol, which also caused the improvement in the hydrophobic character of the films, maintaining water vapour permeability and good values for puncture and UV barrier properties and, at the same time, decreasing the typical yellowish colour of soy protein based films.     

Cold,gel-like soy protein emulsions by microfluidization: Emulsion characteristics,rheological and microstructural properties,and gelling mechanism

In this paper we reported the rheological and microstructural properties of a kind of novel cold, gel-like soy protein isolate (SPI) emulsions obtained by means of microfluidization. These gel-like emulsions were formed from untreated and preheated (95 °C, 15 min) SPI at a protein concentration of 6% (w/v), and various oil volume fractions (Φ; 0.2–0.6) and NaCl concentrations (0–500 mM). The rheological properties and microstructure were characterized using steady viscosity and dynamic oscillatory measurements, as well as confocal laser scanning microscopy (CLSM). The characteristics (e.g. droplet size distribution and creaming stability) of the emulsions, formed at lower protein concentrations (e.g. 0.5–4.0%), were also characterized, aiming to reveal the mechanism of the gel-like network formation. The dynamic oscillatory data indicated that both untreated and preheated SPI emulsions exhibited gel-like rheological properties, but the specific apparent viscosity (η) and storage modulus (G′) of the latter ones were much higher at a comparable Φ. Both η and G′ progressively increased upon Φ increasing, indicating enhanced inter-droplet interactions. At a given Φ value (0.3), increasing NaCl concentration progressively increased η and G′ of the preheated SPI emulsions, indicating the importance of electrostatic screening for the gel-like network formation. The CLSM analyses confirmed formation of the gel-like network, mainly composed of aggregated oil droplets, which was closely dependent on the Φ and NaCl concentration. The gel-like network was formed by bridging flocculation of oil droplets, mainly through inter-droplet hydrophobic interactions between the proteins adsorbed at the interface. These results suggested that soy proteins exhibit excellent potential to produce cold, gel-like emulsions, especially through a heat pretreatment followed by microfluidization, which might be of vital importance for the development of soy protein-based formulations, especially as carriers for heat-labile ingredients with health effects.     

Pasting and rheological behavior of soy protein-based pudding

Pasting and rheological behavior of pastes made from different commercial soy proteins and starches were investigated for the development of soy protein-based pudding system. Commercial starch Novation^® 2300 and soy protein concentrate Alpha^® 5812 were found to exhibit the most desirable pasting properties. The yield stress values of commercial puddings were in the range of 27.1-59.6 Pa, Consistency index (K) values in the range of 6.57-18.63 (Pa sⁿ) and flow behavior index (n) values in the range of 0.4192-0.6558. The K, and n values for soy protein-based puddings were found to be comparable to those for the commercial puddings. During 2 weeks of refrigeration, both G′ and G″ increased for all the puddings. Neither water separation nor net syneresis were observed for soy protein-based puddings during 2 weeks of storage at 5 °C. Further adjustments of the formulation that was more comparable to the commercial pudding were achieved.     

The evaluation of proteases as coagulants for soy protein dispersions

The ability of different proteases to induce the gelation of soy protein isolate dispersions (5.33% w/w) was studied. The coagulation time and gel firmness were determined using dynamic viscoelastic measurements. Among the six protease tested, papain was the most effective coagulant in terms of gel strength and coagulation speed; the second was alcalase. Degree of hydrolysis (DH), pH and viscosity profiles of soy proteins were tested during the coagulation with different proteases. The result suggested that strong interactions, other than electrostatic interaction, existed between peptides in papain and alcalase induced coagulation. Thermal and pH stability tests indicated that papain was more stable than alcalase in the temperature (60–90 °C) and pH (5.8–7.0) ranges studied. Higher papain dosages within the range of 5–13.3 U/ml resulted in firmer soy protein gels, but concentrations higher than 13.3 U/ml produced weaker gels. With the addition of 0.625 mM cysteine, the soy protein coagulation ability of papain was improved. A soy protein gel formed with 0.025% papain in the presence of 0.625 mM cysteine had about the same strength as that induced by 0.133% papain without cysteine.     

Complexation of curcumin with soy protein isolate and its implications on solubility and stability of curcumin

Curcumin, a natural polyphenolic food colourant, suffers a low bioavailability because of its low solubility and instability in aqueous solutions. Our study demonstrates that the food derived soy protein isolate (SPI) can form a complex with the curcumin. Fluorescence spectroscopy of the SPI–curcumin complex revealed that the complex is formed through hydrophobic interactions. Moreover, curcumin molecules quench the intrinsic fluorescence of SPI upon binding. Upon complexation, curcumin showed increased water solubility. Stability studies by UV spectroscopy showed that >80% of the curcumin was stable in the SPI–curcumin complex when dissolved in water, simulated gastric and intestinal fluids for 12 h, which would provide sufficient time for intestinal absorption. SPI–curcumin complex exhibits enhanced antioxidant activity and is capable of forming foam and emulsion, indicating its possible utilisation in food product formulation. This study suggests that SPI, being an edible protein, could be used as a material to encapsulate water-insoluble bioactive compounds in functional foods.     

Structure-modifying alkaline and acidic pH-shifting processes promote film formation of soy proteins

The effect of pH-shifting, a process that induces the molten globule state in proteins, on the film-forming potential of soy protein isolate (SPI) at different temperatures was investigated. Partial unfolding at pH 1.5 or 12, followed by refolding at pH 7.0, was performed to alter the protein structure. Glycerin-plasticised films were prepared from pH-treated SPI at ambient temperature (20 °C), or by heating at 50, 60, 70, or 80 °C (30 min). Tensile strength (TS), elongation at break (EAB), water vapour permeability (WVP), protein solubility (pH 3–7), and non-participating proteins of films were analysed, and the film microstructures were examined. The pH₁₂-treated SPI spontaneously formed a transparent, slightly yellowish film at 20 °C, which had the greatest EAB, while pH_1.5-treated and native SPIs required preheating at 50 and 70 °C, respectively, to form a film. Heating generally decreased solubility and WVP but increased TS. Films formed from both pH₁₂- and pH_1.5-treated SPIs were more elastic (up to 2-fold greater in EAB, P < 0.05) than the film formed from untreated SPI despite slightly reduced TS and WVP. Electrophoresis revealed disulphide bonds between A and B subunits of glycinin being a dominant force in pH₁₂- and pH_1.5-treated SPI films, while noncovalent forces were abundant in untreated SPI films. The pH₁₂-treated SPI film consisted of more interactive protein strands than other SPI films, which seemed to explain its superior elastic properties.     

Effects of soy protein hydrolysis and polysaccharides addition on foaming properties studied by cluster analysis

The objective of the work was to study foaming properties (foam overrun, drainage rate and collapse stability) of soy protein and their hydrolysates as affected by polysaccharides. As starting material a sample of commercial soy protein isolate was used (SP) and hydrolysates of 0.4, 5.0 and 5.2% degree of hydrolysis (DH) were produced by an enzymatic reaction. The polysaccharides added were xanthan, λ and κ-carrageenan, guar, locust bean gum and hydroxypropylmethylcelluloses as surface-active polysaccharides.     

Stabilization of milk proteins in acidic conditions by pectic polysaccharides extracted from soy flour

Pectic polysaccharides were extracted from soy flour at either room temperature (SPRT) or 121°C (SPH), and their abilities to stabilize milk proteins in acidic conditions were evaluated. Both SPRT and SPH were found to contain proteinaceous components that were difficult to dissociate from polysaccharide components using size exclusion chromatography, whereas the molar mass of the former was approximately twice that of the latter. Due to the higher molar mass, SPRT was expected to provide stronger steric effects to prevent aggregation between milk proteins in acidic conditions than SPH. Alkaline treatment of SPRT for breaking O-linkages between AA and monosaccharide residues decreased its molar mass by approximately 160 kDa, indicating that they contained naturally occurring conjugates of pectic and proteinaceous moieties. Particle size distributions in simulated acidified milk drink samples containing 0.2% SPRT or SPH showed monomodal distributions with median diameters of around 1.2 μm at pH 4. The presence of large protein aggregates (～5 μm) was detected at 0.2% SPRT and pH 3.2, 0.6 to 0.8% SPRT and pH 4, or 0.2% SPH and pH 3.4. The presence of excess polysaccharide molecules unbound to proteins was detected at 0.2% SPRT and pH 3.2 to 3.4, 0.4 to 0.8% SPRT and pH 4, 0.2% SPH and pH 3.4 to 3.6, and 0.4 to 0.8% SPH and pH 4. The present results suggest that molecular characteristics of pectic polysaccharides vary depending on extraction conditions and hence their functional behavior.     

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

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

Gelation behaviour and rheological properties of acid-induced soy protein-stabilized emulsion gels

The protein concentration dependence on the rheological properties of acid-induced gels formed with unheated and heated soy protein-stabilized emulsions (UHSPE and HSPE) was investigated at different acidification temperatures. Pre-heat treatment on soy protein solutions resulted in a higher storage modulus (G′) and a shorter gelation time (t_gel) of acid-induced emulsion gels. A maximum in tan δ was observed in the UHSPE gels but no maximum was detected in the HSPE gels. Increasing the acidification temperature decreased the G′ and t_gel. The dependence of the G′ on the protein concentration (c) can be scaled with a power law: G′ ∼ c^A. The exponent (A) increased with pre-heat treatment and acidification temperature. The experimental data.fitted the fractal scaling model (G′ ∼ φ^A) and the simple time- scaling model above very well for the acid-induced soy protein-stabilized HSPE gels with varying oil volume fraction. The large deformation and fracture properties were significantly affected by soy protein concentration, pre-heat treatment, acidification temperature and volume fraction of oil droplets (p < 0.05).     

Rheological properties of acid-induced soy protein-stabilized emulsion gels in the absence and presence of N-ethylmaleimide

The storage modulus (G′) and fracture properties of the non-treated and NEM-treated emulsion gels were investigated in the absence and presence of unadsorbed soy protein aggregates (USPA). In the absence of USPA, a decrease in the G′ of emulsion gels was observed after NEM treatment. However, in the presence of USPA, the addition of NEM only slightly decreased the G′ (p < 0.05). For both non-treated and NEM-treated emulsions, a ∼63-folds higher G′ value was obtained after the USPA addition. These results revealed the presence of sulphydryl group – disulfide bond interchange reactions at ambient temperature and under acidic conditions. In the absence of USPA, the sulphydryl group – disulfide bond interchange reactions was the main interactions responsible for the higher G′ values of non-treated emulsion gels, but for the emulsions with USPA presented, the large quantity of non-covalent interactions (e.g. hydrophobic group & hydrogen bonds) is the main interactions responsible for the aggregation and gelation of emulsion droplets. In the presence of USPA, the sulphydryl group – disulfide bond interchange reactions formed in the non-treated emulsion gels did not increase the final G′ but increased the stability of network. A power law relation was observed between the USPA concentration and the final G′, as well as between the oil volume fraction and the fracture stress/strain.     

Relative viscoelasticity of soy protein hydrolysate and polysaccharides mixtures at cooling conditions analyzed by response surface methodology

The objective of this work was to study the relative viscoelasticity of soy protein hydrolysate and polysaccharides mixtures at cooling conditions analyzed by response surface methodology.Systems of soy protein hydrolysate (HSP) of 4% degree of hydrolysis, a hydroxypropylmethylcellulose (E4M) and kappa-carrageenan (κC) were made with concentrations conformed by Doehlert matrix as experimental design used.The samples were subjected to dynamic rheological studies with a control stress rheometer, Paar Physica MCR 300, with a program with a heating and a cooling period. At the end of the cooling at 10 °C the relative viscoelasticity (tan δ) was evaluated from these measurements.To relate the relative viscoelasticity with the components of systems and their concentrations at cooling conditions the response surface methodology was used to obtain this information.The results obtained indicate that E4M promoted in general a decrease of relative viscoelasticity only in the combined systems. When E4M was in combination with HSP, two regions in the plot with the lowest tan δ can be possible to obtain. One of them was at lower HSP and E4M concentrations and the other at the HSP and E4M highest concentrations. In similar way, when E4M was in combination with κC an increase of relative viscoelasticity was observed at the lowest E4M and κC concentrations and other region was found at the highest E4M and κC concentrations.In other hand, κC would enhance a higher relative viscoelasticity, however, when this polysaccharide was used in combination with hydrolyzed soy protein and/or E4M, a decrease of relative viscoelasticity was observed in the mixed systems.It can be concluded that E4M is the principal component which determines high viscoelastic characteristics in combination with hydrolyzed soy proteins and κC at 10 °C.     

Extreme pH treatments enhance the structure-reinforcement role of soy protein isolate and its emulsions in pork myofibrillar protein gels in the presence of microbial transglutaminase

Alkali (pH₁₂) and acid (pH_1.5) pH-treated soy protein isolates (SPI) were incorporated (0.25–0.75% protein) into sols of myofibrillar protein (MP, 3%, in 0.6 M NaCl at pH 6.25) with or without 0.1% microbial transglutaminase (TG) to investigate the potential as meat processing ingredients. Static and dynamic rheological measurements showed significant enhancements of MP gelling ability by the inclusion of pH_1.5-treated as well as preheated SPI (90 °C, 3 min). A 7-h incubation with TG accentuated the gel-strengthening effect by these SPI samples. The B subunit in 11S of SPI was the main component manifesting structure reinforcement in the mixed gels. The MP gelling properties were also greatly improved (P < 0.05) by the addition of 10% canola oil emulsions stabilized by pH-treated SPI. The principal force in the MP gels incorporated with pH-treated SPI was hydrophobic patches; in the presence of TG, cross-linking of previously dissociated A and B subunits of 11S was also intimately involved.     

Effect of soy milk powder addition on staling of soy bread

Effect of soy milk components (soluble fibre (SF), insoluble fibre (ISF), soy protein) on physicochemical properties (crust and crumb colour, water activity, total moisture content, “freezable” water (FW), “unfreezable” water (UFW), amylopectin recrystallisation (ARC), stiffness and firmness) of soy breads stored for 7 days was studied. By the end of storage ISF additions significantly increased ARC (from 0.01 to 0.57 W/g), whereas SF additions (0.30 W/g) retarded staling with respect to soy flour bread (0.39 W/g). Principal Component Analysis (PCA) of all the different treatments and formulations indicated that SMP addition resulted in the lowest firmness and least amylopectin retrogradation at the end of storage, likely due to the synergistic effect of soluble fibre, partly denatured soy proteins and lipid content of this ingredient.     

Interactions between soy protein isolate and xanthan in heat-induced gels: The effect of salt addition

The influence of xanthan and/or KCl addition on the properties of heat-induced soy protein isolate (SPI) gels at pH 3.0 was studied. Changes in protein solubility and subunit composition as well as in the mechanical properties, microstructure and water holding capacity of the gels were determined. The effect of KCl addition on each biopolymer solution was also investigated. The results indicated that SPI–xanthan gels prepared without KCl were mainly stabilized by non-covalent (H-bonding and hydrophobic) and SS bond interactions, whereas in gels containing KCl, electrostatic interactions were also involved in maintaining the gel structure. The β-7S subunit was probably the fraction electrostatically linked to the xanthan. The different values found for the mechanical properties after the addition of xanthan and/or KCl, were associated with the coarseness of the gel. Xanthan and KCl probably showed a synergistic effect on the Young modulus because KCl induced a conformation transition of the xanthan molecules.