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.     

Torsion gelometry of cheese

Torsion gelometry, a fundamental rheological test in which specimens are twisted until they fracture, was applied to several different cheese varieties to determine its suitability for measuring their textural properties. Fresh and aged Brick, Cheddar, Colby, Gouda, Havarti, Mozzarella, and Romano cheeses were subjected to torsion analysis, and the results were compared with those from small amplitude oscillatory shear (SAOS) tests and texture profile analysis (TPA). Strong relationships (correlation coefficients > 0.8) were found between torsion shear stress and TPA hardness, and between torsion shear strain and TPA cohesiveness. SAOS, which measures rheological properties of intact samples, did not correlate well with torsion or TPA. A map showing trends during aging toward brittle, mushy, rubbery, and tough texture was drawn using the torsion data. The findings show that torsion gelometry provides fundamental rheological data on cheese at the fracture point. The information can be used to compare textural qualities of cheese samples as they are being cut.     

STRUCTURAL FAILURE IN SELECTED RAW FRUITS AND VEGETABLES1

A torsion test was developed for studying the structural failure of selected raw fruits and vegetables. Apple, melon and raw potato flesh were tested at a shear strain rate of approximately 0.26s^-1 in torsion and uniaxial compression. Low strain modulus values were determined in addition to shear stresses and normal strains at failure. Results corroborated the maximum normal strain failure criterion proposed by Segerlind and Dal Fabbro (1978) for apples and suggested its application to potatoes and melons if true strains are used rather than engineering strains. The maximum shear stress theory also seemed to be a possible failure criterion for potatoes. Results comparing compressible and incompressible materials suggest that bulk strain affects the shear stress at failure. The observed failure planes supported the quantitative results for stresses at failure. Scanning electron micrographs indicated that the cellular failure occurred in the cell wall, regardless of whether it was due to tension, Varying specimen lengths or diameters had negligible effects on the uniaxial compression modulus but did affect the shear stress at failure in a manner yet to be satisfactorily explained.     

COMPARISON OF TWO INSTRUMENTAL METHODS WITH SENSORY TEXTURE OF PROTEIN GELS2

The textural attributes of 8 different heat-induced protein gel preparations evaluated by torsion failure testing and Instron texture profile analysis (TPA) were compared to sensory ratings by a trained texture profile panel. The gels presented a wide range of textural properties as determined by the instrumental and sensory parameters. Among the instrumental parameters, true shear strain at failure was the most frequent and significant predictor of sensory notes. Initial shear modulus and 50% compression force had the poorest correlations with sensory notes. Comparison of the two instrumental tests produced high correlations between shear stress at failure and TPA hardness; true shear strain at failure and TPA cohesiveness; and, initial shear modulus and 50% compression force. High correlations were also observed among various panel notes. Canonical correlation analyses showed that sets of linear combinations of parameters from each one of the 3 tests (torsion, TPA or sensory) were highly correlated to sets from either of the other two. Regression equations relating each of the instrumental tests to sensory notes were developed. Of the torsion failure parameters, the logarithm of true shear strain most commonly appeared in the equations. Of the TPA parameters, cohesiveness and its logarithm were the terms that were most frequent. High R² values were obtained for regression equations developed for predicting torsion failure parameters based on TPA parameters.     

Texture Map of Cream Cheese

ABSTRACT: The controlled rate vane method was used to evaluate the yield stress and apparent yield strain of ten cream cheeses at refrigeration (5 °C) and room temperature (22 °C). Plotting yield stress versus apparent yield strain produced a "texture map", showing a trend where a decrease in yield stress corresponded to an increase in yield strain. The map can be used as a tool for evaluating product spreadability in quality control and product development applications. Yield stress values for all cream cheeses ranged from approximately 1.3 kPa to 6.6 kPa, while yield strain values from 0.2 rad to 0.6 rad were found.     

STRUCTURAL FAILURE AND NONDESTRUCTIVE RHEOLOGICAL ANALYSES OF FRANKFURTER BATTERS: EFFECTS OF HEATING RATES AND SUGARS1, 2

The effects of heating rate and sugar type and concentration on the values of frankfurter textural parameters at structural failure were evaluated by a torsion test and a two cycle uniaxial compression test. Structural failure tests indicated that heating rate and sugar concentration had major effects on shear stress; in addition to minor effects on true shear strain. In general, heating rate had a greater effect on stresses and strains than sugar. Torsion and compression tests yielded different values, however, trends due to treatment were similar. The method used for interpretation of texture profile analysis parameters from Instron force-deformation curves had an effect on the statistical analysis results. A thermal scanning rheology monitor nondestructively measured rheological changes during thermal processing. During thermal processing, the major modulus of rigidity increase started at 58°C and the major decrease in energy loss occurred in the 40–60°C range. Unlike structural failure rheological properties, rigidity at 70°C was not affected by heating rate.     

Fracture Analysis of Alginate Gels

The fracture properties of alginate gels were investigated using torsion and compression. The gel fracture stress correlated with Ca²⁺ and alginate concentration, whereas the fracture strain was insensitive to composition. Considering the relationship of fracture stress with gel network crosslink density and the energy to break covalent and noncovalent bonds, the fracture of alginate gels is hypothesized to result from the disruption of junction zones. Consequently, the fracture stress was the stress required to overcome electrostatic forces that formed junction zones. The fracture stress‐strain relationship for alginate gels can be described by the Blatz, Sharda, adn Tschoegl (BST) equation, suggesting that for a given gel, the fracture strain can be predicted based on fracture stress, small‐strain shear modulus, and a fitted parameter describing nonlinearity of the gel. In addition, the fracture properties were affected by deformation rate. The influence of deformation rate on fracture was ascribed to structural changes among the alginate junction zones.     

VISCOSITY CHARACTERIZATION OF A COMMERCIAL YOGURT AT 5C USING A CUP IN BOB AND A VANE GEOMETRY OVER A WIDE SHEAR RATE RANGE (10−5S−1-103s−1)

Equilibrium viscosity data at 5C, over a wide shear rate range (10^?5 s^?1 -10³ s^?1), are presented for a typical commercially available stirred yogurt. A bob in cup and a vane in cup geometry were used in the work. The vane was manufactured to the same outer dimensions as the bob. A series of constant shear stress creep tests was performed, ranging from 2 Pa to 60 Pa. Equilibrium shear rate values were found for each shear stress. Equilibrium viscosity data obtained with both geometries were of the same order of magnitude. This indicated that slip (wall depletion) effects were not significant with the bob in cup geometry used in the study. Application of existing rheological models showed that the equilibrium viscosity behavior of yogurt may be described using either a power-law of the form or the Cross equation The Cross model better predicted the almost constant viscosities observed at low shear stresses.     

Measurement techniques for steady shear viscosity of Mozzarella-type cheeses at high shear rates and high temperature

While measuring steady shear viscosity of Mozzarella-type cheeses in a rotational rheometer at 70 °C, three main difficulties were encountered; wall slip, structural failure during measurement and viscoelastic time dependent effects. Serrated plates were the most successful surface modification at eliminating wall slip. However, even with serrated plates shear banding occurred at higher shear rates. Because of the viscoelastic nature of the cheeses, a time dependent viscous response occurred at shear rates <1 s⁻¹, requiring longer times to attain steady shear conditions. Prolonged continuous shearing altered the structure of the molten cheeses. The effects of structural change were greatly reduced by minimising the total accumulated strain exerted on the sample during flow curve determination. These techniques enabled successful measurement of steady shear viscosity of molten Mozzarella-type cheeses at 70 °C at shear rates up to 250 s⁻¹.     

TEXTURAL CHARACTERIZATION OF SOY-BASED YOGURT BY THE VANE METHOD

The vane method was applied to evaluate failure characteristics of soy-based yogurts prepared from five soybean varieties at Brix values of 6, 8, and 10°. Yield stress, yield strain, and water-holding capacity were compared. Yield stress values ranging from 133 to 420 Pa at 2.5% protein and 498 to 1171 Pa at 4.0% protein were dependent on soybean variety and increased with increasing protein concentration. The average yield strain of samples was not affected by protein or variety. Compared to commercial dairy yogurt, soy yogurt had 132 to 445% higher yield stress at similar protein content, and was less deformable based on yield strain measurements. Water-holding capacity of soy yogurts was variety dependent, although this dependence was less pronounced at higher protein concentrations. The vane method may be effectively used as a rapid and inexpensive technique for detecting textural differences of soy-based yogurts.     

Vane Rheometry for Textural Characterization of Cheddar Cheeses: Correlation with Other Instrumental and Sensory Measurements

The relationship between instrumental (vane method, texture profile analysis (TPA), uniaxial compression) and sensory texture measurements of Cheddar cheeses was investigated. A Haake VT 550 viscotester equipped with a four-bladed vane rotor was used for the vane test. Instrumental TPA was performed with a TA.XT2 Texture Analyser, and compression variables were calculated from TPA data. Vane parameters were significantly correlated with respective variables of compression and TPA (r=0.56-0.91), and sensory tests (r=0.54-0.88). Multivariate analysis indicated that seven sensory attributes of ten commercial Cheddar cheeses were satisfactorily predicted (calibration regression coefficient,R_cal >0.62) by variables of the vane, uniaxial compression and TPA tests. In particular, cheese firmness and cohesiveness evaluated by sensory panel were well described by vane stress and apparent strain. The results validate the vane method as an alternative to the existing cheese testing methods for rapid evaluation of cheese texture.     

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.     

Strain hardening and anisotropy in tensile fracture properties of sheared model Mozzarella cheeses

We studied the tensile fracture properties of model Mozzarella cheeses with varying amounts of shear work input (3.3–73.7 kJ/kg). After manufacture, cheeses were elongated by manual rolling at 65°C followed by tensile testing at 21°C on dumbbell-shaped samples cut both parallel and perpendicular to the rolling direction. Strain hardening parameters were estimated from stress–strain curves using 3 different methods. Fracture stress and strain for longitudinal samples did not vary significantly with shear work input up to 26.3 kJ/kg and then decreased dramatically at 58.2 kJ/kg. Longitudinal samples with shear work input <30 kJ/kg demonstrated significant strain hardening by all 3 estimation methods. At shear work inputs <30 kJ/kg, strong anisotropy was observed in both fracture stress and strain. After a shear work input of 58.2 kJ/kg, anisotropy and strain hardening were absent. Perpendicular samples did not show strain hardening at any level of shear work input. Although the distortion of the fat drops in the cheese structure associated with the elongation could account for some of the anisotropy observed, the presence of anisotropy in the elongated nonfat samples reflected that shear work and rolling also aligned the protein structure.     

AN ATTEMPT TO IDENTIFY AND MODEL TRANSIENT VISCOELASTIC FLOW IN FOODS2

Shear stress development data for commercial ketchup, mustard, mayonnaise, apple butter, butter, margarine and canned frosting were obtained using the cone and plate geometry of the Rheometrics Mechanical Spectrometer. At sudden imposition of four shear rates, .1, 1, 10, and 100 s^?1, shear stresses displayed different degrees of overshoots with margarine exhibiting a maximum overshoot of 320% that of the steady-state value at a shear rate of 100 s^?1. The actual extent of overshoot depended on the particular food and the particular shear rate applied. An attempt was made to explain observed transient shear stresses with the Bird-Leider equation, a four parameter empirical model which incorporates both steady viscous and elastic properties of food materials. This model assumes both the steady viscosity function, η, and the steady primary normal stress coefficient Ψ to have a prominant power-law region with increasing shear rate. This assumption is shown to be justified for all foods studied between shear rates of .1 s^?1 and 100 s^?1. When this assumption is justified a time constant Λ can be constructed. This time constant is found to be an approximate indicator of relative stress overshoot for the foods studied. It is found that the Bird-Leider equation provides a moderately good prediction of peak shear stresses and peak times and only a crude prediction of shear stress decay. Nevertheless, the model is a definite improvement over time independent models such as the power-law model.     

Variations of Hausdorff Dimension and Selected Textural Indices of Cheddar and Gouda Cheeses During Storage

Two imported commercial cheeses (Cheddar and Gouda) were analyzed to characterize their textural changes during a storage period from 176 to 362 days at 4°C. Fractal dimension analysis was used to examine the structure of the cheeses, and a scaling model based on stress values was used to calculate fractal dimension (D_f) by the Hausdorff dimension count method. It was found that the variation of D_f of two cheeses showed a similar trend. When two cheeses were stored from 176 to 300 days, their D_f ranged from 2.37 to 2.43 for Cheddar or from 2.06 to 2.23 for Gouda cheeses. After that, the final values of D_f increased obviously to 2.87 (Cheddar cheese) or 2.63 (Gouda cheese) at the end of the storage period. The experiments carried out showed that the variation of D_f had a poor relationship with the changes of pH 4.6 soluble nitrogen and two instrumental textural indices (hardness and springiness) of the cheeses, especially in the late storage period, but the inhomogeneous cavities existing in the cheeses could be reflected by their D_f.     

QUANTITATIVE MEASUREMENT OF FOOD SPREADABILITY USING THE VANE METHOD

This study was performed to describe an application of the vane method for the rapid, quantitative measurement of spreadability, a subjective, textural property. Yield points of thirteen commercial food items were determined and related to spreadability using models HBTDV-I and 5X HBTDV-I Brookfield viscometers equipped with three different vanes. Spreadability strongly impacts consumer approval of a food. This textural property has been linked to the yield stress of a material, but observations support that strain at yielding may also be important. A textural map, showing yield stress and yield strain of the selected foods was created to chart regions of spreadability.     

Stress-Strain Curve Analysis of Cheddar Cheese under Uniaxial Compression

Samples were subjected to uniaxial compression at six deformation rates until fracture occurred. Fracture strain, stress and work, deform-ability modulus, and biaxial extensional viscosity were determined. No significant effect of surface lubrication was observed on magnitudes of the selected mechanical properties. Irrespective of deformation rate Cheddar cheese fractured at a strain of 55.5% when aspect ratio was 0.65, and at a strain of 59.7% when aspect ratio was 1.0. The fracture stress ranged from 34 to 107 kPa and fracture work from 22 to 63 kJ/m³. Mean deformability modulus of Cheddar cheese was 240 kPa. Biaxial extensional viscosity was a decreasing function of strain rate.     

RHEOLOGICAL BEHAVIOUR OF BUTTER AT LARGE DEFORMATIONS

Eleven traditionally produced butter samples were evaluated for rheological properties at large deformations by lubricated and nonlubricated uniaxial compression with varying sample geometries and compression rates, by stress relaxation in shear and creep experiments in compression at 5–20C each. Compression forces required to deform cylindrical samples in nonlubricated conditions were higher than in lubricated cases. Correction of shape changes in nonlubricated stress-strain curves resulted in good agreement with lubricated experiments at low compression rates up to stress maxima or yielding. As frictional forces were relatively smaller at high deformation rates, lower stresses were obtained in nonlubricated conditions. Samples with higher height/diameter ratios showed more pronounced stress maxima. A biaxial extensional viscosity, η be , was computed for nonbrittle, soft samples. Typical slopes in logarithmic plots of viscosity against strain rate were approximately -1. At strains of about 0.01 creep compliance depended on compression stress. The average ratios of recovered to initial instantaneous compliance were between 0.31 and 0.59, and increased with sample temperature. A calculated ηBE from creep compression showed fair agreement with ηBE from static compression experiments. At comparable strains, a conversion of creep compliance to the stress relaxation modulus by an approximate method showed a good fit of experimental results.     

MEASUREMENT OF YIELD STRESS IN DARK CHOCOLATE USING CONTROLLED STRESS VANE METHOD

The controlled stress vane method was used to measure the yield stress in dark chocolate. Three vanes with different L/D ratios and five different stress rates were used in three different chocolate formulations. The vane displacement data did not indicate a definite yield phenomenon. When the vane displacement data were transformed into a ratio of vane velocity to elapsed time, the square root of vane velocity (SV) or the cube root of displacement, a definite yield phenomenon was observed. The SV was preferred on the basis of physical considerations and the linear regression of its derived yield stress on vane dimensions, stress rate and chocolate particle size. The value of the yield stress was the least when determined for a vane height to diameter ratio of 2 and at a stress rate of 3 Pa/min using the SV transformation.