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Rheological properties of fine-stranded whey protein isolate gels
Food Hydrocolloids, 2003
The rheological and fracture properties of fine stranded WPI gels were determined over a range of shear strain rates (0.014-0.69 s 21 ). All gels had the highest fracture strain when deformed at a strain rate of 0.014 s 21 . Fracture stress was relatively constant over all strain rates. These effects were observed at all protein concentrations and suggest a link with molecular or network relaxations. The apparent modulus (stress/strain at any level of strain) exhibited decreasing or linear behavior at low to moderate strains, followed by non-linear strain-hardening behavior at higher strains. As strain rate increased, the gels displayed increased non-linear behavior. The strain where the modulus switched from linear to non-linear behavior decreased as protein concentration or strain rate was increased. Therefore, both strain rate and protein concentration affect the large-strain viscoelastic behavior of WPI gels during deformation and fracture. This observation suggests a link with the molecular relaxation processes occurring in the gel network. q
A comparative rheological study of heat and high pressure induced whey protein gels
Food Chemistry, 1995
The rheological properties of whey protein concentrate (WPC) gels induced by high pressure (4000 bar/30 min), were compared to those induced by heat (80°C 30 min) at protein concentrations ranging from 110 up to 183 g/liter. Oscillation measurements at 1 Hz and 0.001 strain showed the highest storage and loss moduli for heat set gels, while creep experiments at a stress level of 40 Pa gave larger sample deformations for high pressure induced gels. Relaxation experiments performed at 17 and 33% deformation were characterized by a higher force decay as a function of time for the high pressure gels, while during compression the compression modulus was always higher in the case of heat set gels. Electron microscopy showed a higher level of cross links in the heat induced gels; high pressure generated a more porous network with a lower amount of intermolecular cross links.
The mechanism behind microstructure formation in mixed whey protein–polysaccharide cold-set gels
Food Hydrocolloids, 2009
Microstructure formation in acidified mixed protein/polysaccharide gels is an important feature to control the mechanical properties of these gels. Previous work showed the relation between the charge density of the polysaccharide and the type of microstructure obtained. This manuscript deals with the mechanism behind the microstructure formation. This was studied with a combination of techniques, including light scattering, confocal microscopy (CLSM) and small deformation rheology. The microstructure obtained is a result of the competition between gelation of the protein aggregates and the phase separation between protein aggregates and polysaccharide molecules. The onset of gelation and microstructure formation occurs at a constant pH independent of the type and amount of polysaccharide added. The gelation of the protein aggregates induces the phase separation process that leads to the microstructure formation. The rate of gelation is controlled by the acidification rate, which can be used to modulate the microstructure of the mixed gels. A slower acidification rate results in a coarser microstructure because there is a longer time for the phase separation to occur.
Dynamic viscoelastic properties of thermally induced whey protein isolate gels with added lecithin
Food Hydrocolloid, 1999
The effect of crude egg yolk lecithin on the formation of heat-induced whey protein isolate (WPI) gels was investigated using dynamic small-strain rheometry. Three different types of gel networks, fine-stranded, mixed, and particulate network, were formed by varying NaCl concentration. In all conditions, storage modulus (G′) increased during holding at 80°C, development of which was well-described by first-order reaction kinetics. There was an additional increase in G′ during cooling to 25°C. The final values of G′ and the fracture modulus showed similar trends in the effect of lecithin addition: a substantial reinforcing effect on fine-stranded and mixed gels and slightly negative effect on particulate gels. For fine-stranded and mixed networks, lecithin addition decreased the gelation time and increased the gelation rate constant at 80°C. In contrast, lecithin addition to particulate gels did not affect these rate parameters. The positive effect of lecithin on rheological properties of WPI gels with fine-stranded or mixed networks was not only due to the acceleration of gelation rates during the heating process but also due to the enhancement of the elastic nature of the networks during cooling.
LWT - Food Science and Technology, 2000
High strength concrete has been used in situations where it may be exposed to elevated temperatures. Numerous authors have shown the significant contribution of polypropylene fibre to the spalling resistance of high strength concrete. This investigation develops some important data on the mechanical properties and microstructure of high strength concrete incorporating polypropylene fibre exposed to elevated temperature up to 200 -C. When polypropylene fibre high strength concrete is heated up to 170 -C, fibres readily melt and volatilise, creating additional porosity and small channels in the concrete. DSC and TG analysis showed the temperature ranges of the decomposition reactions in the high strength concrete. SEM analysis showed supplementary pores and small channels created in the concrete due to fibre melting. Mechanical tests showed small changes in compressive strength, modulus of elasticity and splitting tensile strength that could be due to polypropylene fibre melting. D
Food Hydrocolloids, 2015
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Strain hardening and fracture of heat-set fractal globular protein gels
Journal of Colloid and Interface Science, 2006
Non-linear mechanical behavior at large shear deformation was been investigated for heat-set β-lactoglobulin gels at pH 7 and 0.1 M NaCl using both oscillatory shear and shear flow. These gels have a self-similar structure at length scales smaller than the correlation length of the gel with fractal dimension d f = 2. Strain hardening is observed that can be well described using the model proposed by Gisler et al. [T.C. Gisler, R.C. Ball, D.A. Weitz, Phys. Rev. Let. 82 (1999) 1064] for fractal colloidal gels. The increase of the shear modulus normalized by the low strain value (G 0 ) is independent of G 0 . For weak gels the elasticity increases up to a factor of ten, while for strong gels the increase is very small. At higher deformation irreversible fracture occurs, which leads eventually to macroscopic failure of the gel. For weak gels formed at low concentrations the deformation at failure is about 2, independent of the shear modulus. For strong gels fracture occurs at approximately constant stress (2 × 10 3 Pa).
Food Hydrocolloids, 2008
We have studied the effect of a combined heat and shear treatment on the formation and rheological properties of fibrillar whey protein aggregates. The amount and length distribution of whey protein fibrils were characterized using flow-induced birefringence and transmission electron microscopy (TEM). Fibril solutions were characterized macroscopically using crossed polarizers and the flow behaviour was measured with steady-shear viscosity measurements. Fibril growth was dependent on protein concentration. The use of shear flow influenced the amount of fibrils formed when the protein concentration was sufficiently high (above 3 wt%). A shear rate was found for which the amount of fibrils was maximal. The increase in the amount of fibrils as a function of shear rate was explained by enhanced supply of protein monomers towards the fibril tips in the flow field, while the following decrease at higher shear rates could be caused by the breakage of non-matured bonds inside the fibril. Viscosity measurements of the fibril solutions showed that above a critical fibril concentration, the viscosity became independent of the fibril concentration.
Journal of the Science of Food and Agriculture, 2001
The viscoelastic properties of corn starch (CS) gels were more dependent on heating temperature, while the properties of whey protein isolate (WPI) gels were more dependent on pH. Thus heating temperature (75, 85, 95°C) and pH were varied to obtain a series of mixed gels with interesting viscoelastic properties. WPI gels showed extensive stress relaxation (SR) indicative of a highly transient network structure, while CS gels relaxed very little in 2000 s. Based on SR results, it appeared that CS/WPI mixed gels with 25 and 50% CS formed compatible network structures at 15% total solids only at pH 9. This supposition was supported by SEM microstructures obtained for dehydrated gels and a synergistic increase in the large-strain fracture stress for these gels. Some synergy was also found for mixed gels at 30% total solids at pH 9, while at pH 7 the mixed gels seemed to contain separate additive WPI and CS networks unlike the case for pH 7 at 15% total solids. In both cases (15 and 30% total solids) the degree of elasticity of the mixed gels decreased as the WPI content increased. Mixed gels (CS:WPI = 0.5) at pH 9 showed increased fracture stress and fracture strain relative to the same gels at pH 7. This suggests that a unique chemical compatibility exists at pH 9 and results in gels that combine the elasticity of CS and the internal stress dissipation of WPI.
Journal of Food Science, 2012
The effect of heating rate and pH on fracture properties and held water (HW) of globular protein gels was investigated. The study was divided into 2 experiments. In the 1st experiment, whey protein isolate (WPI) and egg white protein (EWP) gels were formed at pH 4.5 and 7.0 using heating rates ranging from 0.1 to 35 • C/min and holding times at 80 • C up to 240 min. The 2nd experiment used one heating condition (80 • C for 60 min) and probed in detail the pH range of 4.5 to 7.0 for EWP gels. Fracture properties of gels were measured by torsional deformation and HW was measured as the amount of fluid retained after a mild centrifugation. Single or micro-phase separated conditions were determined by confocal laser scanning microscopy. The effect of heating rate on fracture properties and HW of globular protein gels can be explained by phase stability of the protein dispersion and total thermal input. Minimal difference in fracture properties and HW of EWP gels at pH 4.5 compared with pH 7.0 were observed while WPI gels were stronger and had higher HW at pH 7.0 as compared to 4.5. This was due to a mild degree of micro-phase separation of EWP gels across the pH range whereas WPI gels only showed an extreme micro-phase separation in a narrow pH range. In summary, gel formation and physical properties of globular protein gels can be explained by micro-phase separation.