Influence of pH and NaCl on rheological properties of rennet-induced casein gels made from UF concentrated skim milk

Milk concentrates are used in the production of cast cheese. The effects of pH (5.19–6.21) and NaCl concentration (0, 1.75% and 3.50%, w/w) on the rheological and microstructural properties of rennet-induced casein gels made from ultra filtered skim milk (19.8%, w/w casein) were investigated. Low pH and high NaCl concentration reduced the development rate of the gel elasticity after coagulation of the ultra filtrated skim milk. Strain at fracture and stress at fracture from uniaxial compression of casein gels 48 h after coagulation showed maximum and minimum values at pH ∼5.8 and 5.29, respectively. Young's modulus from uniaxial compression of the same gels was almost constant between pH 5.52 and 6.21 but much lower at pH 5.28. Addition of NaCl resulted in increased Young's modulus in the interval pH 6.21–5.52. As pH decreased, the level of colloidal calcium phosphate decreased concomitantly, giving less cross-links in the casein network and partly explaining the changes in the rheological properties. Increased ionic strength by adjusting pH and addition of NaCl also influenced rheological results. The microstructure examined with confocal laser-scanning microscopy was unaffected by the changes in pH and concentrations of NaCl in the range studied as revealed by image analysis and calculations of two- and three-dimensional data from micrographs.     

Development of a restructured alginate food particle suitable for high temperature-short time process validation

Cylindrical particles of alginate and alginate/starch food purée were developed for use as carriers in a microbiological time temperature integrator (TTI). The mechanical properties of the restructured food particle, stress (σ_f) and strain at failure (ε_f), were studied as a function of different composition parameters (alginate and food concentration, pH, type offood added, and addition of starch). Addition of food purée produced gels that were weaker than purée alginate ones, although no differences in mechanical properties were obtained among the three levels of food purée concentration studied in this work. However, the type of food significantly affected these rheological parameters, producing the weakest gel when artichoke purée was added to the alginate. The pH also affected the mechanical properties: the lower the pH, the weaker the particle. When starch was added, the particles developed could be frozen without losing their mechanical resistance and handling. The results indicated that particles containing 2% alginate and 4% starch showed the best mechanical stability.     

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.     

Correlation of the Rheological Behavior of Egg Albumen to Temperature, pH, and NaCl Concentration

Rheological parameters of viscosity, gel strength, and elasticity were determined on heat-set egg albumen gels over various treatment combinations ranging from 65-90°C, pH 6.4-9.6, and NaCl concentrations of 0.0-0.1M added NaCl. Maximum viscosity was measured at a treatment combination of 77.5°C, pH 8.00, and 0.1M NaCl. Elasticity and gel strength were highest in gels with a treatment combination of 85.2°C, pH 9.0, and 0.08M NaCl. Temperature had the greatest effect on all three rheological parameters. Gels heated above 80°C were of unusual character, exhibiting syneresis and shrinkage.     

Storage of Shell Eggs Influences the Albumen Gelling Properties

Eggs from hens at 42 and 65 wk of age, respectively, were stored for time and temperature combinations of 0, 7, 10, and 14 d at 4, 12, 20, 28, and 37 °C in two experiments. The egg albumen was analysed for raw albumen quality parameters and dry matter content. Albumen gels prepared at either 85 °C or 90 °C were analysed for the textural properties corrected stress σ and Hencky strain ε at fracture point, σ_fandε_f , respectively, by uniaxial compression. The denaturation temperature, T_D, of ovalbumin in albumen samples was analysed by differential scanning calorimetry, DSC, and related to standards of N-ovalbumin and S-ovalbumin. Fresh egg albumen was characterised by the formation of gritty gels of poor water-holding capacity with lowσ_f and ε_fvalues. The stress and strain of gels at fracture were maximal after 14 d at 4 °C, and ε correlated significantly with ovalbumin T_Dfor gels prepared at 85 and 90 °C. The grouping of egg albumen samples with respect to the form of ovalbumin present (N-, intermediate, or S-) correlated with the egg mass loss during storage. Within the study, the storage temperature was more determining for the conversion of N-ovalbumin into S-ovalbumin than storage time.     

Effect of Partial Substitutes of NaCl on the Cold-Set Gelation of Grass Carp Myofibrillar Protein Mediated by Microbial Transglutaminase

The objectives of this study were to investigate the changes and the relationship between structure and physiochemical properties of low sodium salt substitutes (NaCl partially replaced by KCl, CaCl₂, and MgCl₂) on grass carp myofibrillar protein gels mediated by microbial transglutaminase during cold-set gels and to provide more information about the gel characteristics. The gel strength, water holding capacity, whiteness, rheological characteristics, differential scanning calorimeter (DSC), and Raman spectra of cold-set gels were determined. The Raman spectra data were fitted to four secondary structures (α-helix, β-sheet, β-turn, and random coil). The gel properties of cold-set gels varied both with the low sodium salt types and incubation time. Myofibrillar protein (MP) gels added with NaCl and KCl had significantly higher water holding capacity than the MgCl₂, CaCl₂, and control groups. Additionally, the results showed that the gel strength and G’ value increased with the incubating time. No significant difference was detected in whiteness between the NaCl group and partial substituted groups. Cold-set gels added with the same molar amount of NaCl and KCl had fairly similar gel properties. There is a strong correlation between structural properties and gel properties of MP gels determined by DSC during the cold-set gelation process.     

Factors Affecting the Gelation Properties of Hydrolyzed Sunflower Proteins

The effects of temperature and several chemicals on gelation time and strength of gels formed by heating (pH 8) 5% solutions of trypsin hydrolyzed sunflower proteins were studied by dynamic rheological methods. The storage modulus reached a maximum at 80°C. Ca₂Cl (and NaCl at > 0.2M) accelerated gelation and weakened the gel. NaCOCH₃Na₂SO₄ and NaSCN decreased the storage modulus. Urea decreased gelstrength and at high concentrations slowed gelation. Time for gelation diminished and gel strength increased with increasing mercaptoethanol concentration up to 0.1M. Propylene glycol at 5–20% concentrations accelerated gelation and at 5% also increased gel strength. Trypsin hydrolyzed sunflower proteins could be useful in products requiring strong gels at high temperatures.     

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.     

Combined microscopic and dynamic rheological methods for studying the structural breakdown properties of whey protein gels and emulsion filled gels

The microstructural and large deformation rheological properties of model food gels were studied by performing notch propagation tensile testing on the gels using a tensile stage and observing changes in the microstructure of the gels during tensile testing using confocal laser scanning microscopy (CLSM). Heat-set whey protein (WP) gels containing either added sodium caseinate (NaCN) or sunflower oil droplets emulsified with WP or NaCN as the emulsifier protein were prepared in 0 or 50 mM NaCl. The WP gel structure strengthened in the presence of added NaCl and NaCN. The rheological properties of WP gels containing sunflower oil droplets emulsified with WP or NaCN were influenced by the NaCl concentration, oil concentration and extent of oil droplet aggregation in the gel or by the type of emulsifier protein used. During tensile testing, the notch length in all gels increased above a certain critical stress, leading to fracture of the gels through the notch. Also, the microstructural changes in the oil phase of emulsion filled gels subjected to tensile testing were influenced by the structural properties of the WP gel matrix and the proximity of the oil droplet to the fracture path.     

Xanthan Enhances Water Binding and Gel Formation of Transglutaminase-Treated Porcine Myofibrillar Proteins

ABSTRACT: In this study, the effect of xanthan on dynamic rheological properties, textural profile, and water binding of transglutaminase (TG)-treated myofibrillar protein (MP) gels was investigated. In experiment 1, MP suspensions (40 mg/mL protein, 0.6 M NaCl) at pH 6.45 with or without 0.05% xanthan were treated with 0%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% TG; in experiment 2, MP suspensions (40 mg/mL protein, 0.6 M NaCl) at pHs 6.13, 6.30, 6.45, 6.69 with or without 0.05% xanthan were treated with 0.3% TG. Treated samples were analyzed with differential scanning calorimetry for thermal stability and oscillatory rheometry and Instron penetration tests for gelation properties. The TG treatments lowered the transition temperature (T_m) of MP by as much as 6 °C (P < 0.05) but increased apparent enthalpy of denaturation. However, there was no detectable thermal stability difference between MP samples with or without xanthan. The shear storage modulus (G′) of MP gels increased markedly upon treatments with ≥0.3% TG, and the presence of xanthan further enhanced the gel strength (P < 0.05). The addition of 0.05% xanthan decreased cooking loss of TG-treated MP gels by 17% to 23% when compared with gels without xanthan at all pH levels evaluated (6.13 to 6.67). Thus, the combination of TG and xanthan offered a feasible means to promote cross-linking and gelation of MP while reducing cooking losses.     

Structure–fracture relationships in gas-filled gelatin gels

Food aeration has become one of the fastest growing unit operations practiced in the food industry. Dispersed air (or other gases) provides an additional phase within the gel that may accommodate new textural and functional demands. This paper addresses the relationships between structural characteristics and fracture properties of gas-filled gelatin gels (GGG), and compare these properties with those of control gelatin gels (CGG). Three gases were used in the fabrication of GGG: air, nitrogen and helium. Experimental methods to determine density, gas hold-up, bubble sizes and bubble size distributions as well as fracture properties of GGG are presented. Increasing protein concentration produced higher density, lower gas hold-up and decreased polydispersity of bubbles due to its effect on increased solution viscosity. Type of gas affected density and gas hold-up due to the different diffusivities of gases and structures (bubble size, size distribution and number of bubbles per area) formed in GGG. Fracture values increased for both GGG and CGG with increasing protein concentration for the three gases used. GGG were weaker and less ductile than CGG, the decrease in stress and strain at fracture being between 70 and 80%, and 40 and 65%, respectively. A power law relationship (σ_f = 2.73 × 10⁻¹²ρ_G^4.76) was found between the fracture stress and gel density for the three gases studied. This study shows that the presence of bubbles in gel-based food products results in unique textural properties conferred by the additional gaseous phase.     

Deformation and fracture of emulsion-filled gels: Effect of oil content and deformation speed

The large deformation properties of gelatine, κ-carrageenan and whey protein isolate (WPI) gels filled with bound and unbound oil droplets were studied as a function of compression speed. The rheological properties of the gel matrices controlled the compression speed-dependency of the gels containing oil droplets. Polymer gels (gelatine and κ-carrageenan gels) showed a predominantly elastic behaviour. Their Young's modulus was not affected by the compression speed. The increase of fracture stress and strain observed with increasing compression speed was related to friction between the structural elements of the gels and, for gelatine, to the unzipping of physical bonds. Particle gels (WPI gels) showed a more viscoelastic behavior. Their Young's modulus and fracture stress increased with compression speed. This was attributed to the viscous flow of the matrix and friction phenomena between structural elements of the gel. The effect of an increase in the oil volume fraction (φ) on the Young's modulus was for all gels according to the Van der Poel theory. In addition, oil droplets embedded in the gel matrix acted as stress concentration nuclei and increased friction. The relative impact of these two effects was related to the viscoelastic properties of the gels and to droplet–matrix interaction. For polymer gels and gels with bound droplets, stress concentration phenomena played a relatively larger role. For particle gels and gels with unbound droplets, friction phenomena were relatively more important, increasing the viscoelastic character of the gels. As a result, an increase in φ resulted in a decrease of both fracture stress and fracture strain for polymer gels and in an increase of the fracture stress and a decrease of fracture strain for particle gels.     

Hydroxyl Radical and Ferryl-Generating Systems Promote Gel Network Formation of Myofibrillar Protein

ABSTRACT: The objective of the study was to examine how oxidatively induced protein cross-linking would influence the gelation properties of myofibrillar protein (MP) under meat processing conditions. MP suspensions in 0.6 M NaCl at pH 6 were treated with an iron-catalyzed oxidizing system (IOS: 10 μM FeCl₃, 0.1 mM ascorbic acid, 0.05 to 5 mM H₂O₂) or a H₂O₂-activated metmyoglobin oxidizing system (MOS: 0.01 to 0.1 mM metmyoglobin/H₂O₂) that produced hydroxyl radical and ferryl species, respectively. Both oxidizing systems promoted MP thermal gelation, which was evidenced by rapid protein–protein interaction and the enhancement in storage modulus (elasticity) of the gel network as revealed by dynamic rheological testing in the 20 to 74 °C temperature range. This gelation-enhancing effect was attributed to the shift of myosin aggregation in the early stage of heating from predominantly head–head association (nonoxidized control samples) to prevalently tail–tail cross-linking through disulfide bonds. However, both hardness and water-holding capacity of chilled gels tended to decline when MP was exposed to ≥1 mM H₂O₂ in IOS and to all concentrations of metmyoglobin in MOS. Microscopic examination confirmed a more porous structure in oxidized gels when compared with nonoxidized protein gels. The results demonstrated that mild oxidation altered the mode of myosin aggregation in favor of an elastic gel network formation, but it did not improve or had a negative effect on water-binding properties of MP gels. Practical Application: Mild oxidation promotes protein network formation and enhances gelation of myofibrillar protein under normal salt and pH conditions used in meat processing. This oxidative effect, which involves disulfide linkages, is somewhat similar to that in bakery product processing where oxidants are used to improve dough performance through gluten protein interaction.     

Gelation of Turkey Breast and Thigh Myofibrils: Effects of pH, Salt and Temperature

Salt concentration >0.1M and pH were important to the development of gel fracture properties. Howcvcr, meat type and salt concentration (0.5–1.0M) did not influence gel shear stress or shear strain. Isothermal heating temperature (55°C or 70°C) affected only gel shear strain. Rheological properties at fracture and nonfracture did not respond alike to changes in gclation conditions. General similarities between breast and thigh myofibril gels implied that protein isoforms were not the main factor influencing gel structure formation.     

Small strain oscillatory shear and microstructural analyses of a model processed cheese.

The rheological and microstructural properties of a model processed cheese were investigated. The cheese contained 20% protein (rennet casein), 27% anhydrous milk fat, 1.5% NaCl and 1 to 3% Na2HPO4 (DSP). Processing time (10, 20, or 30 min), DSP (1, 2, or 3%) and pH of the DSP-NaCl solution (5.4, 5.6, or 5.8) were adjusted to alter rheological properties. Small-strain oscillatory shear rheological parameters that correlated with transitions previously observed by large-strain (fracture) properties were moduli at 8 degrees C and changes in moduli with cooling from 80 to 25 degrees C. Mechanical spectra at 80 degrees C indicated cheeses containing less than 2.0% DSP were entangled macromolecular dispersions, and those containing more than 2.0% DSP were gels. A compilation of small strain results suggested samples could be divided into three basic rheological states. A model describing the gel network is proposed based on the results of this investigation and large-strain (fracture) rheological properties. The model was based on increased breakdown of para-casein aggregates into amphiphilic molecules and rheological properties being related to the combination of para-casein aggregates and amphiphilic molecules.     

RHEOLOGICAL PROPERTIES OF RENNET-INDUCED GELS DURING THE COAGULATION AND CUTTING PROCESS: IMPACT OF PROCESSING CONDITIONS

The effects of gelation temperature (GT), pH, milk solids nonfat (MSNF) content and aging time on the small and large deformation rheological properties of rennet‐induced skim milk gels were studied. Small amplitude oscillatory rheometry (SAOR) was used to study gel formation. A constant shear rate was applied to gels of various ages to try to simulate the cutting process used in cheese vats. Second‐order polynomial models successfully predicted (R² ≥ 0.83) the relationship between processing parameters and rheological properties of gels. The processing parameters – gelation pH, GT and MSNF – had a significant effect on the rheological properties of rennet‐induced gels. The type and the nature of bonds in these networks and the time scale of applied deformation affected the rheological properties of rennet gels. As time after rennet addition increased, storage modulus, loss modulus and yield stress values increased. This resulted from an increase in the number and strength of bonds with time. The yield strain decreased with time probably because of rearrangements in the network making the gel shorter/brittle in texture. When the impact of the time scale of the applied deformation was compared between the small (storage modulus as a function of frequency) and large (yield stress as a function of constant shear rate) deformation properties of rennet‐induced gels, similar power law exponents were obtained. This similarity presumably reflects the type and relaxation behavior of bonds in this casein network. These results identify the impact of several important processing variables on both the small and the large deformation rheological properties of rennet‐induced gels, which could be useful in identifying gelation properties that improve cheese yield.     

Mechanical Properties of Ovalbumin Gels Formed at Different Conditions of Concentration,Ionic Strength,pH, and Aging Time

Ovalbumin gels were prepared by heat treatment at constant pH and ionic forces. Ovalbumins are widely utilized as emulsifying or binding agents. However, due to their protein origin, mechanical properties of ovalbumins are enclosed in a wide range of rheological responses depending on concentration, ionic strength, pH, and aging time. The objective of this work was to study the effect of processing conditions (pH, ionic strength, and protein content) on the textural attributes of an ovalbumin protein system by means of uniaxial compression. Gels were prepared by dispersing proteins (purity 98%) (8.3–12.5% w/w) until complete dissolution in deionized water at 90°C by 45 min, pH (6.3–9.1) was adjusted using citric acid, and the ionic strength (0–100 mM of NaCl) was adjusted using NaCl. The storage of gels was done at 63°C (24–168 h). The rheological tests of gels were done by uniaxial compression. A rupture force peak was observed at high protein content together with an increase in the Young’s modulus. At fixed conditions of ion content (NaCl 50 mM) and pH of 7, the gels presented a maximum in fracture force and Young's modulus after 7 days of storage. The addition of minimum amount of citric acid increases the stability of ovalbumin gels. This information is useful to ensure that the final product will remain stable during storage time at longer shelf lives.