Effects of fluid shear and temperature on whey protein gels, pure or mixed with xanthan

The effects of shear on whey protein isolate (WPI) gels, pure or mixed with xanthan, have been investigated at pH 5.4 by dynamic oscillatory measurements and light microscopy (LM). The shear was performed on the suspensions under a constant stress of between 0.04 and 2.1 Pa. Various temperature conditions were chosen in order to describe the effects of shear at the different states of aggregation of the WPI. Shear-sensitive aggregation phenomena were already found around 40°C for the pure WPI samples. Continuous shearing during heating from 20 to 40°C, prior to heat treatment at 90°C, resulted in a gel with a storage modulus (G′) half that of the unsheared gel, independent of the shear stress. Continuous shearing during heating from 20 to 76°C resulted in a further decrease in G′. Inhomogeneities arose in networks formed from continuously sheared suspensions during heating from 20 to 50°C and above. Depending on the shear stress and on the heating range of the shear, the networks showed areas of varied compactness and different classes of pores, ranging from 10 to 200 μm. A higher G′, compared to that for the unsheared gel, was found for gels subjected to shear for short periods in the vicinity of the gel point. The presence of xanthan inhibited the aggregation and demixing of the WPI, described as a sterical phenomenon. Under static conditions, the presence of xanthan resulted in a more homogeneous WPI network. Exposing the mixed suspensions to shear generally increased the inhomogeneity of the network structure. Short periods of shearing in the vicinity of the gel point affected the kinetics of the gel formation and resulted in gels with higher G′ values than the unsheared gel. Continuous shearing under stresses below 0.09 Pa, during heating from 20 to 60°C and above, also resulted in gels with an increased G′. Continuous shear under stresses above 0.9 Pa resulted in gels with a decreased G′     

Heat-Induced Gelation of Individual Whey Proteins A Dynamic Rheological Study

Heat-induced gelation of the bovine whey proteins [serum albumin (BSA), β-lactoglobulin (β-Lg) and α-lactalbumin (α-La)] has been studied individually and in mixture at different conditions by a dynamic rheological method. Values in the shear stiffness modulus (/G*/) appeared on heating at low protein concentration for BSA (～2%) and at intermediate concentration for β-Lg (～ 5%). α-La did not form a heat-induced gel of concentrations up to 20% (w/v). The ratio of viscous to elastic properties (loss factor) at maximum possible measuring temperature was below 0.07 for the BSA gels and 0.1–0.3 for the β-Lg gels. The temperature of gelation was highly dependent on pH. In mixture one protein could not be exchange for another without changing the gelation behavior of the mixture.     

Modification of rheological properties of whey protein isolates by limited proteolysis

Whey protein isolate (WPI) was subjected to limited tryptic hydrolysis and the effect of the limited hydrolysis on the rheological properties of WPI was examined and compared with those of untreated WPI. At 10% concentration (w/v in 50 mM TES buffer, pH 7.0, containing 50 mM NaCl), both WPI and the enzyme-treated WPI (EWPI) formed heat-induced viscoelastic gels. However, EWPI formed weaker gels (lower storage modulus) than WPI gels. Moreover, a lower gelation point (77 °C) was obtained for EWPI as compared with that of WPI which gelled at 80 °C only after holding 1.4 min. Thermal analysis and aggregation studies indicated that limited proteolysis resulted in changes in the denaturation and aggregation properties. As a consequenece, EWPI formed particulated gels, while WPI formed fine-stranded gels. In keeping with the formation of a particulate gel, Texture Profile Analysis (TPA) of the heat-induced gels (at 80 °C for 30 min) revealed that EWPI gels possessed significantly higher (p < 0.05) cohesiveness, hardness, gumminess, and chewiness but did not fracture at 75% deformation. The results suggest that the domain peptides, especially β-lactoglobulin domains released by the limited proteolysis, were responsible for the altered gelation properties.     

Thermal gelation of globulin from Phaseolus angularis (red bean)

The heat-induced gelation properties of red bean globulin (RBG) were studied under the influence of salts and several protein structure perturbants. The viscoelastic properties of the gels were evaluated by small amplitude oscillatory tests and creep experiments. Gel structure developed progressively when a protein dispersion (≈10% w/v) was heated from 25 to 95 °C, and both the storage modulus (G′) and loss modulus (G″) increased rapidly during cooling. The addition of NaCl at lower concentrations led to increases in G′ and G″ values, with decreased creep compliance, suggesting higher viscoelasticity. At higher salt concentrations, viscoelasticity was decreased, and the optimum NaCl concentration to produce maximum gel rigidity was 0.2 M. Sodium dodecyl sulfate and urea caused more pronounced reduction of the gel moduli than dithiothreitol and N-ethylmaleimide. The data suggest that hydrophobic interactions and hydrogen bonding play an important role in heat-induced gelation, while electrostatic interactions and a balance of attractive–repulsive forces are also important. The gels formed at 95 °C exhibited a homogenous microstructure with extensive cross-links and sheet-like network structures.     

Gel Characteristics—Structure as Related to Texture and Waterbinding of Blood Plasma Gels

Heat induced denaturation and aggregation of plasma protein solutions were studied by low shear viscometry and turbidity measurements. The microstructure of blood plasma gels was evaluated by scanning electron microscopy (SEM). Relationships between gel structure, texture, and waterbinding properties of blood plasma gels prepared under various conditions such as different heating temperatures, pH and protein and salt concentrations were investigated. Generally, it was found that the degree of elasticity and waterbinding properties decreased with an increasing degree of random aggregation of the protein gel network. The degree of aggregation increased with increasing protein and salt concentration and decreasing pH from 9 to 6. With increasing heating temperature from 77° C to 92° C, a partial disruption of the gel structure due to local aggregation phenomena was demonstrated by SEM micrographs.     

Functional Properties of Myofibrillar Proteins from Cold-Shortened and Thaw-Rigor Bovine Muscles

Prerigor bovine sternomandibularis muscles were stored at 15, 0 and ?29°C to examine cold-shortening and thaw-rigor effects on myofibrillar protein extractability and gelation properties of myofibrils and salt-soluble protein (SSP). Frozen muscle that underwent severe contraction at thawing showed greater protein extractability (35%) than muscles stored at 0 and 15°C (27% extractability). Of the three tempered muscles, thaw-rigor muscle produced the strongest myofibril gel and cold-shortened muscle formed the most elastic SSP gel as determined by dynamic shear and penetration measurements. However, thermally induced changes in gel viscoelastic moduli for all protein samples followed similar patterns. Results indicated that physicochemical changes accompanying muscle contraction affected protein network formation during gelation.     

On the gelation of mungbean starch and its composites

Light microscopy and the concentration dependence of the elastic modulus of mungbean starch (MB) gel indicate that the gelation of MB is mainly governed by a close‐packing mechanism. The mechanical property of the entire network is influenced by the strength of swollen amylose‐rich discrete gel particles rather than the surrounding matrix. However, when the physicochemical properties of the surrounding matrix is altered by the presence of flour fillers possessing lower gelatinisation temperature and enthalpy, both the stress‐related and strain‐related parameters of MB‐based mixed flour gels were reduced (P < 0.05). Addition of iota‐carrageenan (1.0% w/v), in both the absence and presence of calcium lactate (0–20 mm ), further reduced both the stress‐related and strain‐related parameters of the composite gels. This study shows the detrimental effect of the anionic hydrocolloids and divalent cation addition on the mechanical properties of a composite structure stabilised by close‐packing mechanism.     

Influence of inulin/oligofructose on the acid‐induced cold aggregation and gelation of preheated soy proteins

BACKGROUND: In recent years inulin‐type prebiotics have attracted much attention due to consumers' awareness of the health benefits of functional foods. Currently no information is available about the possible texture‐modifying effect of these non‐ionizable polar carbohydrates in different soy‐based food systems. In this study, the effect of inulin/oligofructose on the cold aggregation and gelation of preheated soy protein isolate (SPI) and its fractions (7S, 11S, and their mixture), induced by glucono‐δ‐lactone (GDL), were evaluated by turbidity (A₆₀₀) and dynamic rheological measurements. RESULTS: Oligofructose significantly delayed the aggregation of all protein samples and decreased the end‐point optical density of 11S fraction and SPI. Inulin, a long‐chain fructan, only delayed the aggregation of 7S globulin and reduced the capacity of aggregation (A₆₀₀) of SPI. While oligofructose showed no significant effect, the addition of 5% (w/v) inulin enhanced the gelation of SPI and the 7S/11S mixture, which was demonstrated by the increase in gel storage modulus up to 13.6% and 10.1% (P < 0.05), respectively. CONCLUSION: Inulin was found to enhance the viscoelastic properties of GDL‐induced cold‐set soy protein gels. It is expected that ‘functional’ cold‐set gel products with improved texture can be prepared from preheated soy proteins and inulin. Copyright © 2009 Society of Chemical Industry     

Thermal aggregation and gelation of kidney bean (Phaseolus vulgaris L.) protein isolate at pH 2.0: Influence of ionic strength

Thermal aggregation and gelation of kidney bean protein isolate (KPI) at pH 2.0 and varying ionic strengths (0–300 mM) were investigated using dynamic light scattering (DLS), atomic force microscopy (AFM), and turbidity and dynamic oscillatory measurements. DLS and AFM analyses showed that the extent of thermal aggregation at pH 2.0, or contour length of the worm-like and fine-stranded aggregates, progressively increased with increasing ionic strength. Turbidity and dynamic rheological analyses indicated that, the turbidity and mechanical moduli of the formed gels also increased with the increase in both ionic strength and protein concentration (c). The c dependence of the elastic modulus G′ could be well described using both fractal and percolation models, though in the case of fractal model applied, two distinct scaling regimes were observed. These results suggest that at pH 2.0, the thermal aggregation and gelation behaviors of the proteins in KPI could be remarkably affected by a change in electrostatic repulsion, and homogenous fine-stranded gels formed at ionic strengths in the 0–300 mM range.     

大豆球蛋白自组装聚集及凝胶特性

研究了在低pH值、低离子强度下,分别加热诱导不同浓度11S(大豆球蛋白)和7S(大豆伴球蛋白)自组装纤维化聚集体的形成。通过平均粒径和Thioflavin T(硫磺素T)荧光光谱,对自组装纤维化聚集体的性质进行表征,并对其热致凝胶的流变学,硬度和微结构特性进行考察。结果表明:在低pH值、低离子强度的诱导条件下,蛋白浓度对自组装聚集的形成起着关键作用,随着诱导浓度的增大,蛋白的纤维化聚集越明显,7S比11S更容易形成纤维化聚集。在酸性环境下,大豆球蛋白的纤维化聚集程度越高,越有利于热致凝胶网络结构的形成。在相同的预处理条件下,11S自组装凝胶硬度强于7S。扫描电镜结果显示7S自组装凝胶的网络结构较11S致密,但有序性较11S低。     

Effect of CMC Molecular Weight on Acid‐Induced Gelation of Heated WPI‐CMC Soluble Complex

Acid‐induced gelation properties of heated whey protein isolate (WPI) and carboxymethylcellulose (CMC) soluble complex were investigated as a function of CMC molecular weight (270, 680, and 750 kDa) and concentrations (0% to 0.125%). Heated WPI‐CMC soluble complex with 6% protein was made by heating biopolymers together at pH 7.0 and 85 °C for 30 min and diluted to 5% protein before acid‐induced gelation. Acid‐induced gel formed from heated WPI‐CMC complexes exhibited increased hardness and decreased water holding capacity with increasing CMC concentrations but gel strength decreased at higher CMC content. The highest gel strength was observed with CMC 750 k at 0.05%. Gels with low CMC concentration showed homogenous microstructure which was independent of CMC molecular weight, while increasing CMC concentration led to microphase separation with higher CMC molecular weight showing more extensive phase separation. When heated WPI‐CMC complexes were prepared at 9% protein the acid gels showed improved gel hardness and water holding capacity, which was supported by the more interconnected protein network with less porosity when compared to complexes heated at 6% protein. It is concluded that protein concentration and biopolymer ratio during complex formation are the major factors affecting gel properties while the effect of CMC molecular weight was less significant.     

魔芋葡甘聚糖/表面脱乙酰甲壳素纳米纤维复合凝胶的制备及流变性能

研究魔芋葡甘聚糖（konjac glucomannan,KGM）与不同质量分数的表面脱乙酰甲壳素纳米纤维（surface deacetylated chitin nanofiber,S-ChNF）制成的复合凝胶的微观结构及性能变化。结果表明,KGM/S-ChNF复合凝胶均呈现剪切稀化现象,符合幂定律模型,是一种假塑性流体;且随着S-ChNF添加量的增加,凝胶的黏度增加,剪切应力降低,稠度系数由23.174 Pa·sn增大至29.950 Pa·sn,而流动指数则由0.436 63降低至0.413 08,表明其假塑性能提高。动态黏弹性分析表明,储能模量和损耗模量均表现出对角频率的依赖性,且随着S-ChNF添加量的增加,两者呈现上升趋势。此外,交叉点由6.77 s－1向低角频率方向移动至3.77 s－1,表明分子间氢键作用力增强,增大了KGM分子链移动的阻力,松弛时间变长,凝胶倾向于呈现弹性特征。傅里叶变换红外光谱、扫描电子显微镜及热重分析结果显示,随着S-ChNF含量的增加复合体系内分子间相互作用增强,形成具有稳定网络结构的体系,从而改善复合凝胶的流变性特并且提高了复合凝胶的热稳定性。     

Viscoelastic Characterization of Sage Seed Gum

Wild sage seed is a small, rounded, and mucilaginous seed, which comes from Salvia macrosiphon. The viscoelastic behavior of sage seed gum, at different concentrations (0.5–2%, w/w), was examined by measuring the transient (in-shear structural recovery and creep/recovery tests) and dynamic (stress and frequency sweeps) rheological properties. The mechanical spectra showed typical weak gel behavior at all concentrations, with storage modulus higher than loss modulus, and little variation with frequency. Both moduli greatly increased with increasing the concentration, and the concentration dependency was well described by the power-law model. The loss tangent was increased slightly with increasing the frequency in the range of 0.25–0.67, although it was not affected by an increase in gum concentration. Moreover, the complex viscosity was found to increase with the increase of sage seed gum concentration and to decrease linearly with the increase of frequency. All samples showed typical viscoelastic response to stress in creep/recovery tests, with recoverable strain increasing in direct proportion to sage seed gum concentration. Creep curves were adequately fitted with a Burger model of four parameters. The elastic and viscous contributions to the general viscoelastic behavior were analyzed through the obtained parameters. The concentration had no specific effect on the in-shear recovery properties of sage seed gum gels, and the gel structure was highly recovered after applying shear. The results of this article indicated that sage seed gum may offer an excellent alternative for commercial gums as a thickening/gelling agent.     

Factors Influencing Whey Protein Gel Rheology: Dialysis and Calcium Chelation

Influence of dialyzable compounds on the Theological properties (shear stress and shear strain at failure) of heat-induced whey protein concentrate (WPC) and whey protein isolate (WPI) gels was examined. Dialyzing WPC and WPI suspensions prior to gelation increased the stress of two of three WPC gels and a WPI gel. Dialysis also significantly increased the strain of the same two WPC gels, normalizing all strain values. Replacement of calcium lost through dialysis did not significantly change gel rheology. However, chelating calcium caused a significant decrease in the stress of all gels: a minimum amount of calcium and/or a calcium complex appears to have a major role in whey protein gelation.     

Rheology of whey protein isolate-xanthan mixed solutions and gels. Effect of pH and xanthan concentration

Flow properties at pH 5.5-7.5 of whey protein isolate (WPI)-xanthan solutions containing 0-0.5 w/w% xanthan were studied by viscosimetry, although rigidity and fracture properties of the corresponding heat-set gels (90°C, 30 min) were determined by uniaxial compression. All the studied solutions displayed generalized shearthinning flow behaviour. Synergistic WPI-xanthan interactions has been revealed by observing that rheological parameters [σ_msf, K, n, η (γ)] characterizing blends were larger than those calculated from the two separated solutions. Such a behaviour was attributed to segregative phase separation of whey proteins and xanthan. Effects of xanthan on WPI-xanthan gel properties both depended on pH and xanthan concentration. Simultaneous increased xanthan concentration and decreased pH inhibited gelation of WPI-xanthan blends. Regarding gel strength, synergistic WPI-xanthan interactions were observed at pH >7.0 and low xanthan concentration (0.05 or 0.1 w/w%). Antagonism between the two macromolecules occurred at low xanthan concentration and pH ≤6.5, and high xanthan concentration (0.2 or 0.5 w/w%) at all pH tested. Low xanthan concentration rendered mixed gels more brittle than protein gels, and high xanthan concentration decreased pH effects on gel stress-strain relationships. The balance between strong thermal aggregation of concentrated whey proteins - in presence of incompatible xanthan -, high viscosity of blends and repulsive surface forces of protein molecules was thought to be at the origin of WPI-xanthan gel mechanical properties.     

How micro-phase separation alters the heating rate effects on globular protein gelation

This study was conducted to determine how the combination of heating rate and pH can be used to alter viscoelastic properties and microstructure of egg white protein and whey protein isolate gels. Protein solutions (1% to 7% w/v protein, pH 3.0 to 8.5) were heated using a range of heating rates (0.2 to 60 °C/min) to achieve a final temperature of 80 °C. The gelation process and viscoelastic properties of formed gels were evaluated using small strain rheology. Single phase or micro-phase separated solution conditions were determined by confocal laser scanning microscopy. In the single phase region, gels prepared by the faster heating rates had the lowest rigidity at 80 °C; however, a common G' was achieved after holding for 4 h at 80 °C . On the other hand, under micro-phase separation conditions, faster heating rates allowed phase separated particles to be frozen in the network prior to precipitation. Thus, gels produced by slower heating rates had lower rigidities than gels produced by faster heating rates. The effect of heating rate appears to depend on if the solution is under single phase or micro-phase separated conditions. PRACTICAL APPLICATION: The effect of heating rate and/or time on protein gel firmness can be explained based on protein charge. When proteins have a high net negative charge and form soluble aggregates, there is no heating rate effect and gels with equal firmness will be formed if given enough time. In contrast, when electrostatic repulsion is low, there is a competition between protein precipitation and gel formation; thus, a faster heating rate produces a firmer gel.     

Physicochemical changes in surimi with salt substitute

Protein endothermic transitions (thermal denaturation), rheological properties (protein gelation), and fundamental texture properties (shear stress and strain at mechanical fracture) of Alaska pollock surimi gels made with 0 (control), 1, 2, and 3 g/100 g of salt (NaCl) were determined and compared with equal molar concentration of salt substitute. Salt and salt substitute shifted the onset of myosin transition to higher temperature and resulted in larger myosin peaks (i.e., transition enthalpy). Endothermic transitions showed similar trends to rheological properties. The elastic modulus (G′) increased when salt or salt substitute was added to surimi, except at the highest concentration of salt and salt substitute. Salt and salt substitute also induced the onset of protein gelation (i.e., as measured by significant increase of G′) at lower temperature. Surimi gels with salt substitute and salt at equal molar concentrations had similar texture properties (shear stress and strain). Based on the present study, salt substitute can be used in the development of low-sodium surimi seafood products without significant change in gelation and texture.     

Rheological properties and gel characteristics of polysaccharides from fruit-bodies of Sparassis crispa

Rheological properties and gel characteristics of Sparassis crispa polysaccharides (SCPs) were investigated under various concentrations, temperature, pH, salt concentrations, and sucrose concentrations. SCP solutions behaved as shear thinning pseudoplastic fluids; apparent viscosity increased with concentrations but decreased with extreme conditions and was highest for 1% SCPs at 80 ℃ under neutral conditions; 5% SCPs solutions formed a hysteresis loop and exhibited thixotropic properties. By oscillatory measurements, SCPs were viscoelastic materials. 0.5% and 1% SCPs solutions exhibited viscous behavior at low frequency and enhanced elastic property with the oscillation frequency increased. With the concentration increased to 3% and 5%, the elastic property was predominant in solutions and exhibited gel-like behavior. SCPs gel textural properties and water holding capacity increased with concentration (to 20%) and decreased with salinity, extreme sucrose, and pH. 10% SCPs gels were optimized at 10% sucrose in neutral conditions. Thus, these results implied SCPs had the potential utilization as a new hydrocolloid source in food industries.     

蛋白氧化对肌原纤维蛋白凝胶构效关系的影响

研究不同浓度H2O2氧化体系中肌原纤维蛋白的氧化情况及氧化程度对其凝胶特性的影响,并测定了肌原纤 维蛋白凝胶二级结构和空间结构的变化。结果表明：H2O2形成的羟自由基可以促进肌原纤维蛋白的氧化,蛋白质的 氧化程度随着H2O2浓度的升高而增加;肌原纤维蛋白的凝胶特性随着蛋白氧化程度的增加而发生规律性的变化,蛋 白凝胶强度、凝胶保水性、储能模量和损失模量随着H2O2浓度的升高而降低,表面疏水性呈现出增加的趋势;随着 H2O2浓度的升高,结构稳定的α-螺旋和β-折叠部分遭到破坏,蛋白质由稳定的结构向不稳定转变;通过扫描电子显 微镜观察蛋白凝胶的网状结构,表明蛋白氧化阻碍了蛋白质稳定凝胶空间结构的形成。     

Use of Konjac glucomannan as additive to reinforce the gels from low-quality squid surimi

This paper reports a study of the influence of a Konjac glucomannan aqueous dispersion (KAD) as ingredient, at different alkalinity levels, on the thermal stability of low-quality squid surimi (Dosidicus gigas) and the viscoelastic properties of its gel. An increase in elastic (G^′) and viscous (G^″) moduli at T < 50 °C, reflecting protein aggregation, and a strong decrease at T > 50 °C, reflecting structural damage, were observed in thermal gelation profiles of low-quality squid surimi. The contribution of 1% KAD (10%) to enhancement of gelation ability was assessed by evaluating the viscoelastic properties of the gels, with and without KAD, at increasing alkalinity levels. Gels with KAD at high-pH had the best rheological properties. Small amplitude oscillatory shear (SAOS) tests showed a significant decrease in rigidity, an increase in strain amplitude and a decrease in the frequency-dependence of G^′ and G^″. These results were in agreement with instrumental texture analyses, meaning that KAD may be used to overcome the negative effect of the poor protein functionality of low-quality squid surimi and achieve better gels from them.