RHEOLOGICAL PROPERTIES OF HOT PEPPER-SOYBEAN PASTE

Rheological properties of fermented hot pepper-soybean paste (HPSP) were evaluated at different total solid contents (TS, 43.6-54.7%) and temperatures (10-40C). HPSP samples at 20C are highly shear thinning fluids (n=0.25-0.33) with large magnitudes of Casson yield stresses (106-573 Pa). Consistency index (K) and apparent viscosity (η_a,20) increased with increase in TS. Apparent viscosity of HPSP obeyed the Arrhenius temperature relationship. The magnitudes of activation energy (7.6-20.4 kJ/mole) for HPSP increased with increase in TS. A single equation, combining the effects of temperature and concentration on η_a,20, was used to describe flow behavior of HPSP. The time-dependent model of Weltman was found to be most applicable (R²= 0.97-0.99) for HPSP. Storage (G') and loss (G") moduli increased with increase in TS, while complex viscosity (η*) decreased. Magnitudes of G'were higher than those of G" over the entire range of frequencies (ω).     

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.     

FLOW PROPERTIES OF BABACO (CARICA PENTAGONA) PUREES AND CONCENTRATES

A power law model describes adequately the rheological behaviour of Babaco purees and concentrates while a Herschel-Bulkley model describes concentrates only. A single equation η_a=α x cβ x exp (E_a/RT), combining the effects of temperature and concentration on apparent viscosity (η_a100), was used to describe rheological behaviour.     

RHEOLOGICAL BEHAVIOR OF PROCESSED MUSTARD. II. STORAGE EFFECTS

Samples of processed yellow mustard with three different particle size distributions (slightly-coarse, standard, and fine) were prepared according to a standard formulation and pilot-plant replication of the commercial methodology and stored for 3 months at 5°C, 25°C, and 45°C. After 3-month storage at 45°C, all samples showed visible liquid separation (syneresis) and aggregation of colloidal particles, as indicated by increase in d _pop and decrease in % colloids. The fine milled samples exhibited syneresis and an increase in d _pop after 3-month storage even at 25°C. The bimodal distribution did not show significant changes as a result of aggregation. In general, increase in d _pop increased apparent viscosity (η _ap ), flow behavior index (n) from the Herschel-Bulkley model, and exponent n" from the loss modulus (G") power law equation. As d _pop increased, Bingham yield stress _o), plastic apparent viscosity (η _p ), shear stress constant (A) from the Weltman stress decay model, yield stress (_o) and consistency index (m) from the Herschel-Bulkley model, exponent n' from the storage modulus (G') power law equation, and minimum values of loss tangent (tan δ) decreased. In heterodisperse semi-solid foods such as processed mustard, the particle size distribution influences the structure (i.e., packing) and the rheological behavior. Therefore, time and temperature dependent instabilities such as syneresis may be minimized by controlling particle size distribution during processing.     

Rheological behaviour of acorn starch dispersions: effects of concentration and temperature

Rheological properties of acorn starch dispersions at different concentrations (4%, 5%, 6% and 7%) were evaluated under steady and dynamic shear conditions. The flow behaviours of the acorn starch dispersions at different temperatures (25, 40, 55 and 70 °C) were determined from the rheological parameters provided by the power law model. The acorn starch dispersions at 25 °C exhibited high shear-thinning fluid characteristics ( n  = 0.23–0.36). Consistency index (K) and apparent viscosity (η_a,100) increased with an increase in starch concentration, and were also reduced with increasing temperature. Within the temperature range of 25–70 °C, the η_a,100 obeyed the Arrhenius temperature relationship with a high determination coefficient ( R ² = 0.97–0.99), with activation energies (E_a) ranging between 16.5 and 19.0 kJ mol⁻¹. Both the power law and exponential type models were employed in order to establish the relationship between concentration and apparent viscosity (η_a,100) in the temperature range of 25–70 °C. Magnitudes of storage ( G' ) and loss ( G ") moduli increased with an increase in the starch concentration and frequency (ω). The magnitudes of G ' were higher than those of G " over most of the frequency range (0.63–62.8 rad s⁻¹). The dynamic (η*) and steady shear (η_a) viscosities of acorn starch dispersion at 7% concentration follow the Cox–Merz superposition rule.     

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.     

RHEOLOGICAL PROPERTIES OF NONCOHESIVE APPLE DISPERSION WITH HELICAL AND VANE IMPELLERS: EFFECT OF CONCENTRATION AND PARTICLE SIZE

The proportionality constant, k_s, between shear rate, γ, and agitation velocity, N, for a helical ribbon‐screw (HRS) agitator was 17.8. Using the HRS agitator, values of consistency index K and the flow behavior index n of 14 apple pulp suspensions at seven different solids concentrations and two average particle diameters 0.71 mm and 1.21 mm were determined; in addition, values of the Casson viscosity η_c and yield stress σ_OC were also calculated. The magnitudes of K increased and of n decreased with increase in pulp concentration. Experimental values of the vane yield stress, σ_Oν, measured with a six‐blade vane increased with increase in pulp content. The values of σ_OC obtained using the Casson model were close to the experimental values σ_Oν. The effect of particle size on the relative viscosity, η_r, was correlated with Peclet number.     

ORAL PERCEPTION OF VISCOSITY IN FLUID FOODS AND MODEL SYSTEMS

The perceived in-mouth thickness ( T ) of a range of fluid foods and model systems was assessed by a trained sensory panel, using a ratio scaling technique, and correlated with objective measurements of viscosity (η_N). For Newtonian samples a simple linear correlation (r²= 0.98) is observed between log T and log η with exponent η= 0.22. The linear relationship between log T and log η for Newtonian materials was used to calculate the equivalent Newtonian viscosity (η_N) from subjective panel scores for the non-Newtonian samples studied. The shear rate at which observed viscosity is equal to η_N decreases with η_N, as reported previously by Shama and Sherman, but also decreases with increasing shear-rate dependence of viscosity. This implies oral perception of viscosity over a range of shear rates. Comparison of flow curves for samples assigned similar thickness scores, but showing very different shear thinning behaviour, suggests that the perceived thickness of extremely shear-thinning materials is dominated by their high viscosity at low rates of shear. As a simple practical index, viscosity at 10 s⁻¹ shows a better correlation (r²= 0.95) with the perceived thickness of the samples studied than values calculated by the methods suggested by Wood, or by Shama and Sherman (r²= 0.90 in both cases). The close agreement between panel scores for perceived thickness and perceived stickiness previously observed for concentrated solutions of random coil polysaccharides does not apply for several of the food systems studied, or for very concentrated solutions of xanthan.     

Flow Characterization of Peach Products During Extrusion

Suitability of Bingham, Herschel-Bulkley, Casson, and Mizrahi-Berk models, to characterize the flow behavior of peach products during extrusion was investigated. The Casson equation sufficiently described the flow of peach extrudates within the 49 to 125 s^-1 shear rate range. As concentration increased, yield stress and consistency coefficients increased. A rheological model was proposed to describe the viscosity of peach extrudates. The model incorporates the effect of shear rate by the Casson equation and the effect of concentration by a linear expression. The model provided good fit to the experimental data for peach extrudates reconstituted from drum-dried peach purees.     

FLOW PROPERTIES OF ORANGE DIETARY FIBER SUSPENSIONS

The flow behavior of 7–20% orange dietary fiber (DF) suspensions at 15–65C was most adequately described by the simple power law model. The effect of the temperature on the apparent coefficient of viscosity (η_app) followed an Arrhenius-type relationship. The flow activation energy (E_a) was in the range 3.0–14. 7kJ/mol, depending on the concentration. The η_app increased exponentially with concentration within the temperature range studied. The combined effect of temperature and concentration on the η_app could be represented by an exponential equation. In all the cases, the effect of concentration on η_app was more pronounced than that of the temperature.     

PREDICTING STEADY AND OSCILLATORY SHEAR RHEOLOGICAL PROPERTIES OF CMC/GUAR BLENDS USING THE BIRD-CARREAU CONSTITUTIVE MODEL

The Bird-Carreau constitutive model, a five parameter semi-empirical set of equations, was used to predict the steady shear viscosity η, the dynamic viscosity η', and the out-of-phase component of the complex viscosity divided by frequency η"/ω, for aqueous blends of sodium carboxymethylcellulose (CMC) and guar gum. The five Bird-Carreau constants were derived from equations which correlate the constants to concentration and molecular weight of the CMC/guar blends. Blends of ratios 3:1, 2:1, 1:1, 1:2, and 1:3 by weight were studied in concentrations of 0.5, 0.75, 1.0, and 1.5% by weight. For most blends the model accurately predicted the experimental η and η' data in the frequency/shear rate range of 0.1–100 s^-1 and the experimental η"/ω data in the frequency/shear rate range of 1.0–100 s⁻¹.     

DETERMINATION OF CHOCOLATE VISCOSITY

A critical appraisal of the Casson model for measuring chocolate versus a new proposed method is performed. The new method is validated with both research grade rheometers and factory grade viscometers. This includes elements of measurement uncertainty. In this way, a new reference method is validated for the viscosity of chocolate and related cocoa products using factory grade viscometers. It involves recording: (1) the value of the stress at a shear rate of 5 s ^-1 to represent the yield stress of chocolate, (2) the value of the viscosity at a shear rate of 40 s ^-1 to represent the high shear viscosity, and (3) the difference between the viscosity measured at a shear rate of 40 s ^-1 during the ramp up and down in shear rate to represent thixotropy. Results relate to the solid to liquid glass transition of dispersions under applied stress, and it is shown that they reflect existing theories of stress induced formation and rupture of fractal aggregates, for particle crowded colloidal suspensions with low interaction energy.     

COMPARISON OF STEADY SHEAR RHEOLOGICAL PROPERTIES AND SMALL AMPLITUDE DYNAMIC VISCOELASTIC PROPERTIES OF FLUID FOOD MATERIALS

Small amplitude dynamic viscoelastic properties of apple butter, mustard, tub margarine, and mayonnaise are compared with their respective properties in steady shear flow in the range of shear rates and frequencies of 0.1-100 sec⁻¹ using a Rheometrics Mechanical Spectrometer. Comparison of dynamic and steady viscosities shows that dynamic viscosities (η*) are much larger than steady viscosities (η). Consequently the Cox-Merz rule is not obeyed. Moreover the obvious nonlinearity suggests that shift-factor type relationships are not obeyed either. The in-phase (η') component of the dynamic viscosity shows similar behavior when compared to the steady viscosity (η).
Comparison of G'/ω² and Ψ₁ also suggests that any of the results predicted by classical theories of viscoelasticity are not obeyed. G'/ω² is always much larger than Ψ₁ and the relationship between these two material functions is nonlinear.     

RHEOLOGICAL BEHAVIORS OF GINSENG EXTRACTS IN STEADY AND DYNAMIC SHEAR

The steady and dynamic rheological properties of ginseng extracts were evaluated at 60.6–69.8% total solid contents. Ginseng extracts at 20C showed a very low shear-thinning flow behavior (n = 0.94 - 0.98) with low consistency index (K = 0.39 - 5.02 Pa.sⁿ). Over the temperature range of 5 - 50C, the magnitudes of K and apparent viscosity (η_a,100) decreased with increase in temperature. The apparent viscosity obeyed the Arrhenius temperature relationship; the activation energies for flow of ginseng extracts were in the range of 56.9 - 85.9 kJ/mol. The exponential type relationship between total solids and apparent viscosity was found to be more suitable than a power type relationship. Viscosity data at various temperatures could be reduced to a composite viscosity curve using a reduced apparent viscosity (η_aR). Magnitudes of storage (G') and loss (G") moduli increased with increase in total solids. Magnitudes of G" were much higher than those of G' over the entire range of frequencies (ω) with the high dependence on ω.     

The rheological properties of ketchup as a function of different hydrocolloids and temperature

The flow properties of ketchup were assessed upon addition of commonly used food thickeners: guar, xanthan and CMC gum at three different concentrations (0.5%, 0.75% and 1%) and four temperatures (25, 35, 45 and 55 °C). The ketchup without supplementation served as a control. All ketchup formulations exhibited non-Newtonian, pseudoplastic behaviour at all temperatures and hydrocolloid levels. The Power-law and Herschel-Buckley model were successfully applied to fit the shear stress versus shear rate data. The flow behaviour indices, n and n' , varied in the range of 0.189–0.228 and 0.216–0.263, respectively. The consistency coefficients, k and k' , were in the range of 8.42–27.22 and 6.56–20.10 Pa s ⁿ , respectively. The addition of hydrocolloids increased the yield point (τ₀) and apparent viscosity of the ketchup in comparison to that of the control. The Arrhenius equation was successfully used to describe the effects of temperature on the apparent viscosity of the prepared formulations. The E _a value appeared in the range between 5492.6 and 21475.8 J mol⁻¹.     

Rheological behaviour of processed avocado pulp emulsions

Shear rate vs. shear stress data were obtained on avocado pulp in water emulsions using a concentric cylinder rheometer and fitted to a power law model. Dilution, as volume fraction of water, had a pronounced effect on the apparent viscosity of the pulp emulsions and the Richardson equation, (η_R= exp.( a φ) for the emulsion viscosity fitted the data well. A mean slope coefficient, a , of 4.57 can be used as a first approximation. Enzymatic treatment (40°C, 1 h), is slightly more effective than thermal treatment (65°C, 1 h), in reducing the initial apparent viscosity of the pulp-water emulsions     

Effect of processing on rheological,structural and sensory properties of apple puree

The relation between rheological, structural and sensory properties of apple purees was studied taking into account the effect of processing. For this reason, a grinding - separation strategy was established in order to vary pulp content and particle size. By grinding, three different particle size distributions were obtained. A second heat treatment was applied to purees to see the impact on its rheological and structural properties. An experimental design was constructed, with two factors (pulp content and particle size) and 4 levels (25, 31, 42, 60%) for pulp content and 3 levels (200, 500, 1100 μm) for particle size. The rheological properties of purees were characterized using a controlled stress rheometer by the flow curves obtained from 2.14 to 214 ^s−1 shear rate range; frequency sweeps measurements were performed within the linear viscoelastic region, in the range of 0.1-40 rad/s. Purees behaved as shear-thinning fluids presenting a yield stress. Apparent viscosity and yield stress increased as pulp content increased, and they decreased as particle size decreased. The least shear thinning behaviour was observed in purees with low pulp content and small particles. A second heat treatment affected cell wall structure inducing a decrease of the rheological properties of the puree. The most important attributes to explain the texture of apple purees are consistency and graininess, parameters that can be manipulated by controlling processing conditions.     

YIELD STRESS OF STRUCTURED AND UNSTRUCTURED FOOD SUSPENSIONS

Yield stresses of 40 model suspensions of apple pulp particles with unimodal and bimodal particle size distribution in water, of 13 commercial food suspensions, and of 11 prepared apple sauce samples were determined by the vane method (_0V), and by application of the Casson (_0C) model. For the unstructured (US) apple pulp suspensions magnitudes of _0V and _0C were nearly equal, but for the structured commercial and prepared apple sauce samples, magnitudes of _0V were much higher than _0C. The ratio of _0V/_0C can be used to determine the shear diameter and the degree of structure of a suspension. Energy dissipation due to viscous drag was also higher for the structured suspensions than for the US suspensions. The results suggest that some trends observed with rheological properties of nonfood suspensions may not be applicable to structured food suspensions.     

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.     

YIELD STRESSES IN MOLTEN CHOCOLATES

Yield stress and flow properties of four molten chocolate samples at 40C were measured using coaxial cylinder and vane fixtures. The possibility for slip to occur during measurement was demonstrated by significant differences (p < 0.05) in Gasson parameters when measured between coaxial cylinders of different radius ratios. Significant differences in the Casson flow parameters (p ≤ 0.05) were also found when samples were sheared for 12 min as compared to 30 min. A piecewise regression technique, which fitted the Casson model to the low shear rate ranges and the Bingham model to higher shear rate ranges, could be used to best describe the flow behaviour of chocolate melts. Stress relaxation produced very low o_y values, which were believed to be an artifact of the measuring fixture and instrument. In the case of the single vane method, use of different start-up speeds had a marginal but significant effect (p < 0.05) on o_y values. When the multiple vane method was employed, start-up speeds below 0.23 rpm had no significant effect on o_y. The multiple vane method proved to be simple and more rapid than obtaining the steady shear data and then fitting the Casson flow model, and thus may be more suitable for routine yield stress measurements of molten chocolate for quality assurance purposes.