Gelation properties of salt-extracted pea protein isolate induced by heat treatment: Effect of heating and cooling rate

Gel network formation of a salt-extracted pea protein isolate was studied using dynamic rheological measurements. The gelling point was dependent on heating rate and was unaffected by cooling rate. When both the heating and cooling rates were increased (from 0.5 to 4 °C/min) final G′ value decreased, indicative of decreased gel strength. During the heating phase, the storage modulus and loss modulus fluctuated below 1 Pa at almost constant values with the storage modulus smaller than the loss modulus until the gelling point was reached. The rate of cooling has a greater impact on the development of storage modulus than that of heating. Compared to the gel strength of commercial pea protein isolate (PPIc) and soy protein isolate (SPIc) at the same protein concentration, salt-extracted pea protein isolate (PPIs) was much stronger than PPIc but weaker than SPIc. Careful control of the heating and cooling rates enable maximum gel strength for heat-induced pea protein gel, thus enhancing utilisation of pea protein as an additive in meat food industry.     

盐法提取豌豆分离蛋白凝胶特性及影响因素

采用一种低变性的提取方法--盐法提取豌豆分离蛋白并使用流变仪测定其流变特性,结果表明：盐法豌豆分离蛋白最低凝胶形成质量浓度是5.5 g/100 mL,豌豆分离蛋白凝胶点与蛋白质量浓度无相关性。豌豆分离蛋白凝胶点随加热速率增加呈上升趋势。较高的加热和冷却速率能够增加反应终点储能模量（G’）和耗能模量（G”）值,因而降低凝胶强度。盐法豌豆分离蛋白凝胶韧性随蛋白质量浓度的增加而增大,且蛋白质量浓度与G’、G”值之间存在乘幂规律。tanδ值随蛋白质量浓度升高而降低,当蛋白质量浓度高于5 g/100 mL时,弱凝胶开始形成;当蛋白质量浓度高于7 g/100 mL时,tanδ值几乎保持恒定,表明凝胶弹性在此范围内基本恒定。比较盐法和商业豌豆分离蛋白凝胶性质后发现：盐法豌豆分离蛋白具有更好的凝胶性。     

Roles of components of rice-based fat substitute in gelation

The roles of maltodextrin, protein and fat in the gelation of the rice-based fat substitute were investigated. Rheological properties and gel strength of samples were measured. Results showed that the rising rates of both G′ and G″ of rice starch-based fat substitute were higher than those of rice-based fat substitute when temperature was lower than 35 °C. G′ was equal to G″ at 60 °C for 25% rice-based fat substitute whilst it was at 35 °C for rice starch-based fat substitute. The presence of rice protein and fat increased G′ and G″. Gel strength of rice-based fat substitute was lower than that of starch-based fat substitute at 25% solid content. However, gel strength of a fat substitute increased in the presence of protein. SEM showed that protein particles aggregated to gel. Therefore, protein in rice-based fat substitute gelled firstly, followed by maltodextrin which formed the main matrix of the rice-based fat substitute gel. Fat benefited for gel formation.     

Rheological properties of acid gels prepared from heated milk fortified with whey protein mixtures containing the A,B and C variants of β-lactoglobulin

The effect of fortification of reconstituted skim milk with different levels of a whey protein mixture containing a 1:2 ratio of α-lactalbumin (α-la) and different genetic variants of β-lactoglobulin (β-LG) on the rheological properties of acid milk gels, formed by acidification with glucono-δ-lactone, was investigated. Milk samples were either unheated or heated at 80°C for 30 min before acidification. Acid gels prepared from unheated skim milk had very low G′ values, long gelation times and low gelation pH. Samples prepared from heated milk had markedly higher G′ values, a reduced gelation time and an increased gelation pH. The addition of increasing levels of whey protein mixtures containing β-LG B or β-LG C to the milk prior to heating and acidification caused an almost linear increase in the G′. In contrast, whey protein mixtures containing β-LG A caused a progressive increase in the G′ with added protein levels up to about 0.7% (w/w) but little further change at higher addition levels. A mixture of the A and B variants of β-LG gave an intermediate behaviour between those of the A and B variants. In all samples, the G′ value at 5°C was approximately twice that at 30°C so that the relative differences as a result of the β-LG genetic variants were similar for the two temperatures.     

Ultrasonic processing influences rheological and optical properties of high-methoxyl pectin dispersions

The effect of high-intensity ultrasound on the rheological and optical properties of high-methoxyl pectin dispersions was studied. Pectin solutions (1.15 wt% pectin, 41.4 wt% sucrose) were treated with high-intensity ultrasound at intensity levels ranging from 0 to 40 W cm⁻² for various times (0–60 min). Samples were adjusted to pH 1.5 to initiate gelation and their dynamic rheological properties (G′, G″) were recorded as a function of time using a rotational rheometer. Ultrasonically pretreated pectin dispersions formed weaker gels with increasing sonication power and time. After 4 h, log G′ of the pectin dispersion that was pretreated at the highest intensity level (40 W cm⁻² and 30 min) was approximately five times lower than log G′ of the untreated dispersion. The change in phase angle [arctan (G′/G″)] with time indicated that the rate of gelation decreased as ultrasonic intensity and application time increased. The turbidity of ultrasonically pretreated pectin dispersions decreased by 50% yielding more transparent gels. Results were attributed to an overall reduction in the average molecular weight of pectin due to cavitational effects. A power law model was fitted to the flow curves of ultrasonically pretreated pectin dispersions to determine both flow behavior index n and consistency coefficient K. With increased sonication power and application time, n increased from 0.6 to 0.97 indicating that the flow behavior changed from viscoelastic to Newtonian.     

Gelling properties of protein fractions and protein isolate extracted from Australian canola meal

Gelling properties of canola albumin and globulin fractions, and canola protein isolate (CPI) were examined in this study. The effects of pH and salt concentration on canola protein gelling properties were studied primarily by means of dynamic oscillatory rheology and gel texture analysis. The findings were supported by confocal laser scanning microscopy (CLSM) images of the gels, isoelectric point, and solubility measurement data. All canola proteins showed typical heat-set gel protein profiles. Gels formed at higher pH had better gelling properties including higher overall resistance to deformation (G*), higher gel elasticity (low tan δ ), higher fracture stress and firmness, and denser gel microstructure. Isoelectric points of canola proteins used in this study were in the range of pH 3.0–4.7 where low protein solubility was observed. The albumin fraction was able to form a very weak gel at pH 4, whereas the globulin fraction and CPI precipitated due to loss of protein surface charge. The effects of NaCl on gelling were protein sample dependent. The presence of NaCl negatively affected gelling properties of albumin and globulin fractions, with decreases in overall resistance to deformation (G*), and fracture stress and firmness, but positively affected CPI gels in the same aspects. The elasticity (tan δ) of all canola protein gels remained constant in the presence of NaCl. Frequency sweep analysis revealed that the albumin fraction and CPI formed weak gels, whereas the globulin fraction formed a strong gel. Strain sweep analysis further confirmed that the globulin fraction formed a stronger gel with a critical strain of at least 10%. This study demonstrates the high potential of canola proteins, particularly the globulin fraction, as a prospective gelling agent.     

Thermo-mechanical properties of egg albumen–cassava starch composite films containing sunflower-oil droplets as influenced by moisture content

The effect of moisture content on the thermo-mechanical and structural properties of egg albumen–cassava starch composite films containing sunflower oil droplets was studied using dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Composite films were prepared by cold gelation, dried in a moisture controlled incubator (83.5%RH) at 25 °C for 8 days and aged at different relative humidity at room temperature (21 ± 1 °C) for 7 days to obtain composite films with moisture contents of 4%, 7%, 11%, 17% and 46% (dry weight basis). In DMA thermograms the magnitude of G′ and G″ increased with increasing temperature in high-moisture samples, decreased and then again gradually increased for intermediate-moisture samples, and decreased in low moisture samples. DSC thermograms indicated two distinct peaks (at 49–53 °C and 79.8 to 132.4 °C) which were attributed to phase transitions and protein denaturation. SEM images indicated that the microstructure of the composite matrix changed with moisture content and heating temperature. Our study confirms that moisture content plays a key role in the thermo-mechanical properties and microstructure of egg albumen–cassava starch composite films containing sunflower oil.     

Oats protein isolate: Thermal,rheological, surface and functional properties

Oat protein isolate (OPI) was extracted in 0.015 N NaOH and acetylated or succinylated. The thermal analysis of the isolate showed a glass transition (T_g) at 43.4 °C and ΔC_p of 0.102 J/g/°C. The positive net charge of OPI and the positive or neutral charge of the modified OPI were apparent from the free capillary zone electrophoresis (FZCE) profiles. Acetylation significantly lowered foaming and emulsifying properties of OPI, while succinylation showed the highest foaming capacity, foam stability, and emulsion stability. Acetylated OPI showed the highest surface hydrophobicity compared to the other samples, while OPI was the most soluble of all. The water holding capacity of all samples analyzed was the same except for acetylated-crosslinked (ACXL). The surface tension test confirmed that unmodified and modified OPI possessed surface activity and the equilibrium surface tensions decreased sharply with increasing protein concentration and leveled off to a constant value. The elastic modulus, G′, for the acetylated OPI suspension exhibited the highest value, while the G′ of the crosslinked (XLOPI) had the lowest. The plateau of G′, was 2961 Pa, 920 Pa, 223 Pa, 41 Pa, and 1.8 Pa for the ACOPI, ACXL, SOPI, and XL, respectively.     

Characterization and acid-induced gelation of butter oil emulsions produced from heated whey protein dispersions

Whey protein isolate was dispersed at 4% or 8% (w/v) and heated at neutral pH to produce protein polymers. Butter oil, up to 20%, was homogenized in heated whey protein dispersions at pressure ranging from 10 to 120 MPa. Emulsion gelation was induced by acidification with glucono-δ-lactone. Whey protein polymers produced finely dispersed emulsions with fat droplet diameter ranging from 340 to 900 nm. Homogenization pressure was the main factor influencing droplet size. At low fat volume fraction, the emulsions exhibited Newtonian behaviour. As fat content increased, shear thinning behaviour developed as a result of depletion flocculation. Emulsion consistency index increased with protein and fat concentrations. Increasing homogenization pressure had no effect on Newtonian emulsions but promoted flocculation and significantly increased the consistency of high fat emulsions. Protein concentration was the main factor explaining emulsion gel hardness and syneresis. Syneresis decreased with increasing fat content in the gel.     

Preparation and rheological properties of a dairy dessert based on whey protein/potato starch

Gels were prepared by heating different mixtures of whey protein concentrates (WPC) potatoes and ι-carrageenan. The influence of these ingredients on the strength of the obtained gels was investigated. The rheological properties of these gels were also measured during cooling from 90 to 10 °C at variable frequencies and strains. Analysis of variance showed a highly significant (P<0.0001) relationship between the concentrations of the different ingredients and the gel strength. The storage modulus (G^′) was generally higher than the loss modulus (G″) at different temperatures. Generally, the log (G^′) and log (G″) increased with the increase in the applied frequency, which suggested a weak gel entanglement network. Desserts were also prepared using 4% WPC and 3% potato starch (PS), 0.1% ι-carrageenan, 10% sucrose, 3% milk fat and 3% cocoa powder, by heating at 100, 110 or 120 °C for 30 min, packaged hot and stored for 28 days at room temperature (20 ± 5 °C). Samples of fresh and stored desserts were taken at 0, 7, 14, 21, 28 days of storage and their gel strengths were measured. Also, the effect of heat treatment during preparation on G^′, G″ and phase angle (δ) of the different desserts was followed. The effects of the heating time and temperature on the gel strength and rheological properties were relatively small.     

Modelling of rheological,microbiological and acidification properties of a fermented milk product containing a probiotic strain of Lactobacillus paracasei

The simultaneous effect of fermentation temperature (FT, 36.7–43.4°C), milk total solid level (TS, 11.3–14.7%, w/v) and total inoculum concentration (TI, 2.16–3.84 v/v) on the acidification process and the rheological properties of fermented milk products with Lactobacillus paracasei ssp paracasei B117, Lactobacillus delbrueckii ssp bulgaricus Y 6.15 and Streptococcus thermophilus Y 4.10 was explored by means of response surface methodology. Maximum storage modulus (G′_max), minimum loss tangent (tan δ_min), rate of gelation (I_E) and onset of gelation were the rheological parameters studied. Maximum acidification rate (V_m), time at which maximum acidification rate was observed (T_m), and time to reach the end of fermentation (T_e) characterized the kinetics of acidification, whereas the increase in the number of the three bacteria at the end of fermentation was chosen as the microbiological parameter of the system. The growth/survival of microorganisms and the organic acid profile during cold storage as well as the overall product acceptability by a consumer panel were also assessed. TS strongly affected G′_max and tan δ_min; high TS resulted in large increase in G′_max and decrease in tan δ_min. Increasing fermentation temperature gave a decrease in the onset of gelation, V_m, T_m and T_e, and an increase in the gelation rate (I_E). Under conditions of relatively low FT (37–40°C), high TS (about 14%) and high TI (3–4%), relatively high gelation and low acidification rates were observed, fermentation took a longer time to finish, but the formed gels were firmer, showing higher G′_max and lower tan δ_min values. Low FT (36–38°C) enabled higher increase in the number of L. paracasei B 117. The probiotic strain showed good compatibility with the S. thermophilus Y 4.10 and L. bulgaricus Y 6.15, and satisfactory levels of all bacteria were found during fermentation and storage at 4°C for 21 days. No major differences in lactic and uric acid contents were seen between the control (probiotic strain-free product) and the probiotic fermented milk, whereas the latter contained slightly higher amounts of citric, pyruvic and orotic acids. Moreover, the probiotic fermented milk was graded by the consumer panel with a similar acceptability score as the control product.     

Physicochemical and textural properties of heat-induced pea protein isolate gels

Chemical and thermal properties of pea protein isolates (laboratory prepared or native; PPIn and commercial; PPIc) and textural properties of heat-set gels obtained from pea protein isolates were compared with homologous soy protein isolates (laboratory prepared, or native; SPIn and commercial; SPIc). The protein banding pattern resulting from electrophoresis separation confirmed the presence of predominant storage proteins of pea and soy seeds in the respective protein products. PPIc and SPIc had lower nitrogen solubility than their native counterparts, likely due to their denaturated state which was further confirmed by the absence of distinct endotherms in these commercial materials compared to the laboratory prepared ones. Addition of NaCl at 1.0–2.0% (w/w) to PPIn and SPIn slurries increased thermal transition temperatures for both proteins.     

Effect of genetic polymorphism of milk proteins on rheology of acid-induced milk gels

Individual milk samples from 80 cows in mid-lactation of the Swedish Red and Swedish Holstein breeds with known protein genotypes of β- and κ-casein and β-lactoglobulin were analysed for acid coagulation properties. Glucono-δ-lactone (1.5%) was added to defatted, heated (90–95 °C) samples and rheological properties of the gels were measured using a Bohlin VOR Rheometer. Coagulation time (CT) and curd firmness after 4, 8 and 10 h (G₄′, G₈′, and G₁₀′ were registered for each sample. Milk protein composition was analysed by reversed phase HPLC. Concentration of β-lactoglobulin in milk was found to be an important factor for the variation in CT and G′. The A allele of β-lactoglobulin was associated with higher concentrations of β-lactoglobulin in milk compared with B. When no adjustment for β-lactoglobulin concentration was made, there was a significant overall effect of β-lactoglobulin genotype on acid coagulation, where the AA and AB genotypes were associated with better curd firmness compared with BB, whereas at equal β-lactoglobulin concentrations a tendency in the opposite direction was found with a significant and positive effect of BB compared with AB. Lactose concentration of milk had a positive effect on acid coagulation and was shown to improve G′ in milk with low β-lactoglobulin concentrations.     

Rheological properties of stirred yoghurt as affected by gel pH on stirring,storage temperature and pH changes after stirring

The rheological properties of stirred yoghurt were studied as a function of the delay between milk heat-treatment and inoculation (0, 1 and 2 days), of pH in the acid gel on stirring (4.4, 4.7, and 5.0), of the storage temperature (4, 12, and 20 °C) for 24 h following stirring and of over-acidification (allowed or inhibited). At low pH values, the gels exhibited higher elastic modulus (G′) and fracture strength. They yielded stirred yoghurts with higher G′ and viscosity, and higher increase in G′ and viscosity during storage (“rebodying”). Rebodying was only partially explained by over-acidification and cooling. Changing the storage temperature had no impact on the evolution of G′ after stirring; hydrophobic interactions were therefore probably not involved in rebodying. Electrostatic interactions seemed to play a major role in rebodying, as pH on stirring was the significant factor.     

Gelation properties of salt soluble meat protein and soluble wheat protein mixtures

Salt soluble meat proteins (SSMP) and commercially available soluble wheat proteins (SWP) were characterised by SDS-polyacrylamide gel electrophoresis, differential scanning calorimetry (DSC) and small and large deformation testing. DSC scans indicated transitions similar to those of native actomyosin for the salt soluble meat extract whereas SWP did not indicate any transitions between 20 and 120 °C. Small deformation tests on SWP indicated a G′/G″ crossover gelation temperature of 90 °C and weak gels as judged by frequency sweeps. In contrast, SSMP gelled at 40 °C and formed strong gels on heating to 90 °C. However, on autoclaving at 120 °C, 20% SWP in distilled water produced strong elastic gels with little syneresis, compared with the more brittle gels produced with 20% (w/w) SSMP as indicated by large deformation testing. Mixtures of the two proteins in the ratio SSMP/SWP (15:5) gave strong elastic gels similar to the SWP gels. Even the presence of very small amounts of SWP in the mixture, e.g. SSMP/SWP 20:1 trebled the elastic modulus compared with a SSMP gel and reduced syneresis. This was probably due to the close association of SWP with actomyosin strands as viewed by transmission electron microscopy. However, increased levels of SWP in the mixture, for example SSMP/SWP 10:10 ratio, resulted in the separation of the two protein phases as shown by phase contrast microscopy, and consequently led to lower G′ values in the mixed gels. The addition of 20 mM chloride salts showed that potassium reduced the shear modulus, sodium had no effect and calcium enhanced the shear modulus for SWP gels formed at 120 °C. In contrast, SSMP gels were stronger in the presence of potassium, followed by sodium and calcium.     

Functional properties of yellow field pea (Pisum sativum L.) seed flours and the in vitro bioactive properties of their polyphenols

Functional and bioactive properties of yellow field pea (Pisum sativum L) seed flour, protein isolate (PPI), two high fibre products (Centara III, Centara IV), and one high fibre–starch ingredient (Uptake 80), were determined. The whole seed flour had superior water and oil absorption capacities but the high fibre flours had significantly higher (p < 0.05) swelling ability. Centara IV and Uptake 80 had the highest gel clarity while Centara IV gel was the most resistant to freeze–thaw treatment. Polyphenolic constituents were extracted singly or sequentially with aqueous methanol and acetone; the whole pea seed flour and the pea protein isolate had significantly more polyphenolic constituents than the fibre products, which also resulted in higher in vitro antioxidant activities (trolox equivalent antioxidant capacity and diphenyl-picryl hydrazyl free radical scavenging ability). Results of renin- and ACE-inhibitory activities were mixed and did not correspond to the overall polyphenolic content and antioxidant test results, probably indicating the importance of components specific to individual extracts.     

Effect of soy protein substitution for sodium caseinate on the transglutaminate-induced cold and thermal gelation of myofibrillar protein

The influence of soy protein isolate (SPI) substitution for sodium caseinate (SC) on the properties of cold-set (4 °C) and heat-induced gels of pork myofibrillar protein (MP) incubated with microbial transglutaminase (TG) was investigated. The strength of cold-set MP–SC gels (formed in 0.45 M, NaCl, 50 mM phosphate buffer, pH 6.25) increased with time of TG incubation, but those gels with more than 66% SPI substituted for SC had a >26% reduced strength (P < 0.05). Upon cooking, both incubated and non-incubated protein sols were quickly transformed into highly elastic gels, showing up to 6000 Pa in storage modulus (G′) at the final temperature (72 °C). However, no differences (P < 0.05) in G′ were observed between heated samples with SPI and SC. Myosin heavy chain, casein and soy proteins gradually disappeared with TG incubation, contributing to MP gel network formation. Both cold-set and heat-induced gels had a compact protein matrix, attributable to protein cross-linking by TG.     

Effect of preheating temperature and calcium ions on the properties of cold-set soybean protein gel

To elucidate the effect of preheating temperature and calcium ions on the properties of cold-set soybean protein gel, Ca²⁺ induced gelation of soybean protein were investigated by rheological approaches, electrophoresis analysis, confocal scanning laser microscopy (CSLM) and surface hydrophobicity (S₀). The results showed that, both CaCl₂ concentration (20–40 mM) and preheating temperature (80–120 °C) took significant influence on the dynamic viscoelasticity of the gel samples. The bands distribution of samples preheated at 120 °C were different from the bands distribution of samples preheated below 100 °C in denature and native electrophoresis. The CSLM analysis showed that the gel became coarser as the Ca²⁺ increased. On the other hand, two different kinds of gel were shown under the same Ca²⁺ concentration: a bead-like structure (below 100 °C) and a filamentous structure (above 100 °C). According to fractal theory, weak-linked gel (α was >0.8) was formed when preheating below 100 °C, while a transition gel (α = 0.52–0.62) was obtained when preheating over 100 °C. All these results suggested that the preheating temperature influence the type of cold-set gel.     

Dynamic oscillatory rheological measurement and thermal properties of pea protein extracted by salt method: Effect of pH and NaCl

The effects of two important factors, pH (3.0-10.0) and NaCl (0-2.0 M), on pea protein gelation properties were studied using dynamic oscillatory rheometer and differential scanning calorimeter (DSC). The strongest gel stiffness was achieved at 0.3 M NaCl; higher or lower salt concentrations lead to weakening of the gel. The gelation temperature was also influenced by ionic strength; salt had a stabilization effect which inhibited pea protein denaturation at higher salt concentrations resulting in higher gelling points (p < 0.05). At a NaCl concentration 2.0 M, pea protein gelation was completely suppressed at temperatures ?100 °C. The pH also played an important role in gel formation by pea protein isolates since acid and base cause partial or even total protein denaturation. In this paper the maximum gel stiffness occurred at pH 4.0 in 0.3 M NaCl; higher or lower pH values resulted in reduced gel stiffness (p < 0.05). pH also altered the denaturation temperature of the pea protein; higher pH values resulted in higher denaturation temperatures and higher enthalpies of denaturation (p < 0.05). At pH 3 pea proteins seem like completely denatured by acid as the DSC curve showed a straight line. The gelation temperature (gelling point) peaked at pH ∼6.0 (89.1 °C). Careful adjustment of pH and NaCl concentration would enable the food industry to effectively utilize the salt-extracted pea protein isolate as a gelling agent.     

The influence of bovine serum albumin on β-lactoglobulin denaturation,aggregation and gelation

The effect of bovine serum albumin (BSA) on the heat-induced denaturation, aggregation and subsequent acid-induced gelation of β-lactoglobulin (β-lg) was investigated in this work. Changes in the denaturation kinetics of β-lg during heating at 78 °C were determined by monitoring the disappearance of the native protein by reverse-phase chromatography. Replacing β-lg with increasing amounts of BSA, while keeping the total protein concentration constant at 5% (w/w), significantly increased the denaturation rate of β-lg from 2.57±0.30×10⁻³(g L⁻¹)⁽¹⁻ⁿ⁾s⁻¹ to 5.07±0.72×10⁻³(g L⁻¹)⁽¹⁻ⁿ⁾s⁻¹ (β-lg: BSA ratio of 3:1 w/w). The reaction order for β-lg was 1.40±0.09. Partial replacement of β-lg with BSA (β-lg: BSA ratio of 3:1 w/w) significantly increased the reaction order to 1.67±0.13. Heat-induced aggregates between β-lg and BSA were studied by dynamic light scattering, two-dimensional electrophoresis and size exclusion chromatography. The partial replacement of β-lg with BSA significantly changed the gelling properties of the acid-induced gels. A rapid rate of acidification resulted in a significant decrease, while a slow acidification rate resulted in a significant increase in gel strength. Size exclusion chromatography demonstrated that intermolecular disulphide bond formation occurred during both heat-induced denaturation/aggregation and subsequent acid-induced gelation. Results clearly indicate that BSA contributed to the formation of these disulphide bonds.