A CONSTITUTIVE RELATIONSHIP FOR GELS UNDER LARGE COMPRESSIVE DEFORMATION

A three-parameter constitutive relationship was developed based on a strain energy function to describe the mechanical behaviour of gels subjected to large deformation under compressive load. The parameters in the relationship can be estimated from experimental data using regression techniques. Alternatively, these unknowns may be determined directly from three compression parameters σ_m, ɛ_m, β, which represent the strength, ductility and nonlinearity of the mechanical behaviour, respectively. The new constitutive relationship fits experimental data, for both brittle and ductile gels, up to the point of compressive failure.     

Contribution of Cellular Structure to the Large and Small Deformation Rheological Behavior of Kiwifruit

ABSTRACT: The relative contribution of turgor pressure, cell wall and middle lamellae to the rheology of kiwifruit was studied by performing large deformation assays and using an empirical model proposed by our group. Results were compared with those obtained previously through dynamic testing. Initial (σ₀) and residual relaxation (σ_∞) stresses determined under 14% constant deformation correlated significantly with complex moduli (G*) and they allowed to detect incipient plasmolysis but not to determine the individual contributions of cell wall and middle lamellae to tissue elasticity. Firmness (F_m) showed no correlation with G* because measurement of failure stress required tissue damage but it was affected by ripening allowing to determine the individual contributions of cell wall and middle lamellae to its value.     

RHEOLOGICAL BEHAVIOUR OF BUTTER AT SMALL DEFORMATIONS

Thirteen continuously churned commercial butter samples were evaluated for rheological properties at small deformations. The methods performed included dynamic experiments in shear and compression, stress relaxation in shear and static compression at 5–20C each. Linear viscoelastic behaviour was observed up to a critical strain of 0.001. The structure breakdown due to shear strains between 0.001 and 0.01 was found to be fully recoverable. No qualitative differences in the rheological behaviour were observed between the samples. Temperature-induced variations were mainly ascribed to changes in the solid fat content. Correlations between selected rheological measures in the linear viscoelastic region (i.e., complex modulus G* [Pa], relaxation modulus G(t) | _t ≃ 0[Pa], and modulus of deformability M_D[Pa]) were found to be causal and only slightly affected by the solid fat content.     

Vane Yield Stress of Starch Dispersions

ABSTRACT: Yield stresses of 5% (wt/wt) cross-linked waxy maize, tapioca, and Amioca starch dispersions (SDs) were measured with the vane method at different rotational speeds ( N ). The static (σ_0S) and dynamic (σ_0d) yield stress values of each SD were measured before and after breaking down its structure under continuous shear, respectively. The difference (σ_0S - σ_0d) was associated with the stress required to break the internal bonds (σ_b). Values of total yield stress (σ₀=σ_0S) and σ_b, and shear rate were related by power law relationships. The contribution of viscous and network stresses were estimated from an energy balance model. Textural characteristics of the SDs were analyzed in a texture map.     

VISCOELASTIC PROPERTIES OF ALGINATE GELS BY OSCILLATORY DYNAMIC TESTS

The response of several mannuronic and guluronic alginate gels to oscillatory tests was studied by varying the alginate concentration (c) from 1.0 to 1.75% w/w. Frequency dependence of the complex shear modulus G* discriminated between the alginate gels in terms of their different orders of the relaxation function α and "gel stiffness" A_α values using Friedrich and Heymann theory (1988). Guluronic alginate gels were approximately 4-5 times stronger than mannuronic ones, their networks exhibiting higher or smaller rates of stress relaxation, respectively. For both alginate types the gel stiffness parameter A_α was found to be a power function of alginate concentration and type (intrinsic viscosity [η], GG block diad frequency, F_GG, and average G-block length, N_G), its sensitivity with respect to c and N_G being greater than that to [η] and F_GG.     

RHEOLOGICAL BEHAVIOR OF PROCESSED MUSTARD. I: EFFECT OF MILLING TREATMENT

The influence of milling level on the rheological behavior of processed mustard was investigated using three different particle sizes (slightly-coarse, standard, and fine) prepared according to a standard formulation and pilot-plant replication of the commercial methodology. All samples showed unimodal population-based particle size distributions which were described by the population-mode (d_pop). On a volume basis, all milling levels had well-defined bimodal distributions consisting of a smaller- and a larger-size group of particles described by their modes (d and D). The most important particle size index in relating milling level to rheological behavior was not the population mode (d_pop) but the larger-size volume mode (D) from the bimodal distribution. The coarser the milling, the higher the larger-size volume mode (D), and higher the values of: a) Apparent viscosity of nonmixed samples (η_p), b) Bingham yield stress (o⁻_o) and plastic apparent viscosity (σ_p), c) Shear stress constant (A) and coefficient of thixotropic breakdown (B) from the Weltman stress decay model, d) Yield stress (o⁻_o) and consistency index (K) from the Herschel-Bulkley model, and e) Storage (G¹) and loss (G^") moduli. As indicators of structure-texture relationship, the above rheological parameters can be profitably utilized to study the effect of manufacturing practices on long term storage stability of the product.     

INFLUENCE OF UNIAXIAL COMPRESSION RATE ON RHEOLOGICAL PARAMETERS AND SENSORY TEXTURE PREDICTION OF COOKED POTATOES

The effect of uniaxial compression rate (20–1000 mm/min) on the parameters: Stress (σ_ftrue), strain (ε_fHencky) and work to fracture (W_f), modulus of deformability (E_d), maximum slope before fracture (E_max) and work during 75% compression (W_total) was investigated for ten potato varieties. Multivariate data analysis was used to study the correlation between and within the sensory and nonsensory measurements by Principal Component Analysis (PCA) which showed σ_ftrue, E_max, W_f, and W_total to explain the same type of information in the data, and ε_fHencky versus E_d another type of information in the data. The deformation rate had a large effect on ε_fHencky. Nine sensory texture attributes covering the mechanical, geometrical and moistness attributes were evaluated. Relationships between uniaxial compression data at various deformation rates and the sensory texture attributes were studied by Partial Least Squares Regression (PLSR). A minor effect of deformation rate on the correlation with the sensory texture properties was obtained. Mechanical properties were predicted to a higher extent than the geometrical attributes and moistness. The prediction of the mechanical, geometrical and moistness attributes increased largely by using uniaxial compression supplemented by chemical measures such as dry matter and pectin methylesterase, but here no relevant effect of deformation rate was obtained.     

Isostrength Comparison of Large-Strain (Fracture) Rheological Properties of Egg White and Whey Protein Gels

The effects of protein concentration and heating conditions on the physical properties of whey protein isolate (WPI) (in 50 mM NaCl, pH 7) and egg white (pH 9) gels were examined. Egg white and WPI gels had similar values for shear stress at fracture (i.e., isostrength), while trends for shear strain at fracture were protein-type specific. The rigidity ratio (R_0.3), ratio of the rigidity at fracture (G_f) to the rigidity at 30% of fracture strain, measured departure from the stress-strain relationship of an ideal Hookean solid. All gels fit master curves of G_f vs R_0.3, which were described by a power law model of R_0.3=A(G_f)-°19, where "A" showed protein type-specific characteristics.     

Changes in rheological and viscoelastic properties and protein breakdown during the ripening of 'Port Salut Argentino' cheese

Changes in the rheological behaviour and viscoelastic properties during ripening at 10°C of a soft cheese (Port Salut Argentino) packaged in a plastic film (EVA-EVA) were analysed. Casein degradation was measured by electrophoresis slab gels; α_s1 casein degradation was rapid and striking compared with that of β casein. Rheological parameters obtained from uniaxial compression tests changed during ripening: hardness decreased, adhesiveness and cohesiveness increased.
An exponential decay equation with two maxwellian elements and one elastic in parallel was fitted to characterize stress relaxation curves. Viscoelastic parameters (elastic moduli and relaxation times) were obtained using non-linear regression analysis. The elastic equilibrium modulus decreased 80% during the ripening period and the viscosity of the element with the highest relaxation time decreased 20%. These parameters represented the changes observed in cheese hardness and elasticity during ageing time and were related to the extent of casein breakdown.     

YIELD STRESS OF FOOD DISPERSIONS WITH THE VANE METHOD AT CONTROLLED SHEAR RATE AND SHEAR STRESS

The vane method was used to measure yield stresses of 15 commercial food dispersions under controlled shear stress (Cσ) and controlled shear rate (C     ) operating conditions. Magnitudes of yield stress (σ_σ) at Cσ were higher than those of yield stress (σ     ) at C     on tomato products and baby foods. There were good linear correlations ( R ²= 0.96, 0.86) between γ     and σ^σ for food dispersions with undisturbed structure (UDS) and with broken down structure (BDS). Data obtained under Cσ were also used to calculate shear moduli of foods.     

STRESS RELAXATION AND TENSILE STRENGTH TESTING OF A PROCESSED FISH PRODUCT

Minced silvercarp was prepared as cooked loaves containing only various added salts. The loaves were then subjected to stress-relaxation and tensile strength tests. The various treatments gave different compression forces, relaxation T_50% values and tensile strengths. The compression force measurements were closely correlated with tensile strength values. Both compression force and tensile strength might be related to the binding quality (cohesiveness) of the loaves. The relaxation T_50% values were not closely correlated with the compression forces nor the tensile strengths, and might be related to the product's viscoelastic properties. Different percent deformation did not have a significant effect on the relative order of the compression force values, but did affect the relaxation T_50% values, with larger differences between samples at 25% deformation.     

NEW PARAMETERS FOR INSTRUMENTAL TEXTURE PROFILE ANALYSIS: INSTANTANEOUS AND RETARDED RECOVERABLE SPRINGINESS

Cylindrical samples of gelatin, K-carrageenan/locust bean gum and gellan gum gels were subjected to two successive cycles of compression at three predetermined deformation levels corresponding to 25, 50 and 75% of their respective degrees of compression at failure. Values of "recoverable instantaneous springiness" (S_ins) and "recoverable retarded springiness" (S_ret) were measured. Values of S_ins and S_ret for the gelatin samples were similar and close to 1, indicating a very fast and almost complete recovery of their initial height. By contrast, for K-carrageenan/locust bean gum and gellan gels the values of S_ins and S_ret were quite different, indicating that recovery was retarded. These parameters seem to be a good index of the relative magnitude of elastic and viscous components of foods. This was confirmed by relaxation tests.     

RHEOLOGICAL INDICES OF FRUIT CONTENT IN JAMS: EFFECT OF FORMULATION ON FLOW PLASTICITY OF SHEARED STRAWBERRY AND PEACH JAMS

The effect of formulation factors on Casson yield values measured at low shear rates (0.08 ≤γ≤ 1.01s^-1) (σ₀₁) and at medium shear rates (2.58 ≤γ≤ 387.30s^-1) (σ₀₂) was analyzed in previously sheared strawberry and peach jams. Twenty three samples of each fruit jam were prepared according to a second order composite rotatable design. Composition ranges were: fruit content, 25–55%; soluble solids content, 60–70° Brix; added pectin in strawberry jams, 0.3–0.7% and in peach jams, 0.1–0.5%. Variation of σ₀₁ values in strawberry jams depended mainly on the interactions between fruit and soluble solids and between fruit and pectin, while in peach jams, it depended on fruit-soluble solids and soluble solids-pectin interactions. Patterns of change of σ₀₂ values with composition were similar to those observed for σ₀₁ in both strawberry and peach jams. Predictive power of σ₀₁ and σ₀₂ values for estimation of fruit content was low, but taken in conjunction with soluble solids content and total pectin values, 79.5% of the variability of fruit content in strawberry jams and 91.1% of same in peach jams could be explained.     

TEXTURAL PARAMETERS IN COMPRESSION TESTING OF A GEL MADE FROM FRESH AND FROZEN FISH MINCE

The relation between deformability modulus and yield stress is evaluated in compression testing of a model product made from fresh and frozen stored mince of cod. The experiments were performed with a compression rate of 120 mm/min. Preliminary experiments showed decreasing values of yield stress when compression rates were below 50 mm/min. Two moduli were calculated from the slope of the compression curve; α₁, initially (small deformations) and α₂ in a linear part of the curve shortly before rupture of the gel. α₂ was linearly related to yield stress in gels of different water content, and in gels made from fresh and frozen raw material. α₁ showed different relations to yield stress depending on quality of the raw material.     

COMPARATIVE STUDY OF LARGE STRAIN METHODS FOR ASSESSING FAILURE CHARACTERISTICS OF SELECTED FOOD GELS

Vane rheometry was compared with uniaxial compression and torsion in evaluating the effects of strain rate on failure shear stress and deformation of soybean protein (tofu) and gellan gum gels. A Haake VT 550 viscotester was used for torsion and vane tests, and compression was performed with an Instron/MTS universal testing machine. Strain or angular deformation at failure was independent of strain rate in the three testing modes. In vane rheometry, failure shear stress increased with increasing low shear rates (< 0.100 s ⁻¹) and was rate independent at higher rates. This strain rate dependency was also evident in compression, varying with the material. For torsion, fracture stress appeared to be rate independent. Shear fracture stresses measured in torsion and compression were in good agreement at strain rates above 0.025 s ⁻¹ and 0.100 s ⁻¹ for tofu and gellan gels, respectively. Shear stresses from the vane method were lower than shear stresses of torsion and compression. Similar texture maps of the food gels studied were generated by plotting stress and strain or angular deformation values of the three testing methods. The findings validate the vane technique as an alternative to torsion and compression for rapid textural characterization of viscoelastic foods.     

DYNAMIC VISCOELASTICITY AND STRESS RELAXATION IN STARCH PASTES

The dynamic viscoelasticity and stress relaxation of ten types of starch pastes were measured. The dynamic moduli increased with increasing frequency, and were different for each starch paste, changes in tan δ for cassava and potato starch pastes being about 0.5–0.6. The stress relaxation behaviour of all the samples tested could be represented by a four-element mechanical model. The relationship between spinnability and stress relaxation was determined, the spinning distance increasing with increased relaxation time (τ₂). No satisfactory linear relationship between spinning distance and relaxation time (τ₂) was apparent with the 10 starch paste samples.     

Gelation mechanism of surimi studied by 1h NMR relaxation measurements

ABSTRACT:  In order to elucidate the gelation mechanism of surimi, the temperature dependence of water proton spin–spin relaxation time (¹H T ₂) has been described by a theoretical approach, in which the exposed protein surface is taken into account. Water ¹H T ₂ measured for horse mackerel surimi in the presence of 2.5% NaCl was analyzed on the basis of the consideration for the denaturation and the aggregation of protein in order to explain the macroscopic structural change during the heating and the cooling processes. The temperature dependence of water ¹H T ₂ and the fraction of rigid component gave a clear explanation for the gelation mechanism of surimi. Differential scanning calorimetry thermogram and dynamic viscoelastic measurements supported the results of nuclear magnetic resonance (NMR) measurements. It has been demonstrated that the measurement of NMR relaxation times is useful to describe the gelation mechanism of surimi.     

CORRELATION BETWEEN TRANSIENT ROTATIONAL VISCOMETRY AND A DYNAMIC OSCILLATORY TEST FOR VISCOELASTIC STARCH BASED SYSTEMS

Gelatinized starch formulations including lipids and xanthan gum, typical of viscoelastic foodstuffs, were analyzed using a rotational viscometer (transient conditions) and an oscillatory rheometer. Ultra-rapid and slow freezing processing conditions, which can modify product stability, were also tested. Transient shear stress curves showed a typical overshoot and a structural breakdown; the Bird-Leider model satisfactorily fitted these curves. Model parameters were correlated with characteristic peak stress (α_max), peak time (t_max) and relative overshoot corresponding to the α vs time curve. Relationships between transient shear stress data and dynamic measurements (G', G*, δ) for viscoelastic ranges were found for the starch-based systems and were used to assign a physical meaning to Bird-Leider parameters. Viscoelastic starch systems were grouped in different zones of the correlation diagrams based on structure properties like rigidity, stability to the applied strain and fluid characteristics after breakdown.     

Effect of heating temperature,pH, concentration and starch/whey protein ratio on the viscoelastic properties of corn starch/whey protein mixed gels

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 (5, 7, 9) 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. © 2001 Society of Chemical Industry