RHEOLOGICAL CHARACTERIZATION OF SALAD DRESSINGS. 1. STEADY SHEAR, THIXOTROPY AND EFFECT OF TEMPERATURE

Shear stress-shear rate data of five commercial salad dressings at 2.0, 10.0, 18.3, 26.7, 32.2 and 43.3°C were obtained using a concentric cylinder viscometer. The power law and Casson equations were applicable for all products (r ≫ 0.98). The Arrhenius equation was applicable for the effect of temperature on the apparent viscosity at 100 s^−l (r ≫ 0.97). Rheological data were obtained at a constant shear rate of 275s^−l at 2.0 and 10.0° C. The results show that the Weltman equation was better than the Hahn equation in describing the thixotropic behavior of the five salad dressings.     

THE RHEOLOGICAL CONDITIONS ASSOCIATED WITH JUDGEMENT OF POURABILITY AND SPREADABILITY OF SALAD DRESSINGS

A panel of 25 students compared the pourability and spreadability of two commercial brand salad dressings (non-Newtonian) and a CMC-sugar solution (Newtonian). Correlation of their responses with visco-metric data for the 3 samples indicated that the shear stress and shear rate associated with the sensory evaluation of these two properties are 190–430 dyne cm⁻² and 0.9–6.3 s⁻¹, and < 190 dyne cm⁻² and < 0.9⁻¹, respectively.     

SALAD DRESSINGS—PRELIMINARY RHEOLOGICAL CHARACTERIZATION1

The Weissenberg rheogoniometer was used to characterize a number of commercial mayonnaises and salad dressings. Steady shear measurements were used to determine flow properties and to estimate the degree of structure breakdown with shear. Dynamic measurements, at strain amplitudes greater than the linear uiscoelastic limit, were made to establish the properties of the essentially undisturbed samples. The results were analyzed in terms of the parameters of a modified Bingham body. Ranges for the various rheological properties of commercial salad dressings were established.     

DYNAMIC VISCOELASTIC PROPERTIES OF SOME COMMERCIAL SALAD DRESSINGS

A controlled stress rheometer (Carrimed CS) was used to carry out oscillatory viscoelastic experiments within the linear viscoelastic region for some commercial salad dressings: mayonnaise, reduced calorie mayonnaise and salad creams. All the mayonnaise samples exhibited higher values of maximum stress amplitude which guarantee linear viscoelasticity than either the reduced calorie mayonnaise samples or salad creams.
The frequency dependencies of the storage modulus and the phase angle revealed that all the standard mayonnaise samples exhibited similar viscoelastic properties, showing a higher elasticity than the reduced calorie mayonnaise samples and these, in turn had a higher elasticity than salad creams.
The absolute magnitude of the complex viscosity, |η*|, followed the power law equation with respect to frequency except for the salad creams and was compared to the steady viscosity, η. |η*|/η was related to the degree of shear destruction in the structure, which was minimum for one of the reduced calorie mayonnaise samples.
The viscoelastic properties of the salad dressings studied are related to their different structures.     

RHEOLOGICAL CHARACTERIZATION OF TOFU2

The rheological behavior of four types of tofu was evaluated through compressive stress-strain relationships, relaxation parameters, and percent recoverable work at different strains. It is shown that tofu's deformability mode, like that of sponges, is regulated by both the solid matrix properties and internal hydrostatic pressure. Because the latter is strongly affected by the specimen geometry and the deformation rate, the dependency of mechanical parameters on these factors should be considered as an inherent characteristic of tofu's texture.     

EFFECT OF STORAGE ON THE RHEOLOGICAL BEHAVIOUR OF BABY FOODS

The effect of storage temperature on rheological behaviour of four varieties of baby foods was studied. After 24-month storage at 15C and 25C, the constant A in Weltman model increased significantly for the vegetables, meat and fish samples, and decreased significantly for the fruit samples. After 24-month storage at 5C and 15C there were no significant changes in flow behavior index, consistency index and yield stress of the Herschel-Bulkley model for the vegetables, meat, fish and fruit samples. Increase in storage temperature decreased the consistency index and yield stress of the fruit samples.     

RHEOLOGICAL CHARACTERIZATION OF KETCHUP

Ketchup flow properties were investigated with a concentric cylinder viscometer in the temperature range from 30°C to 80°C. A partial characterization of the product viscoelastic behavior at 30°C was also made by “Strain test” and “Frequency test.” The Harper and El Sahrigi equation (η_a= acβ e^Ea/RT), as well as the equation that links the apparent viscosity of ketchups to the serum viscosity and to the pulp content (η_a=η_serum+A(%P)^B), do not describe adequately the ketchup flow behavior that we found to be highly affected by the production technology and by the raw materials used.     

RHEOLOGICAL CHARACTERIZATION OF APPLE CORTEX

The incremental creep and recovery theory was used to develop a constitutive relationship for apple cortex without assuming the nature and the characteristics of the material. Successive loading and unloading cycles of increasing duration were applied on the cylindrically shaped specimens of apple cortex. The elastic strain component was linear with stress. The viscoelastic and permanent strain components were time dependent and nonlinear functions of stress.
Three-step creep tests and stress-strain tests at various deformation rates were also conducted on cylindrically shaped specimens of apple cortex. Prediction of strains under these loading histories by using the constitutive equation showed a close agreement between the theoretical and experimental strains. An equivalence between the critical creep stress (found from the incremental creep and recovery tests) and the ultimate stress, obtained from the stress-strain tests at various deformation rates, were also established.     

RHEOLOGICAL CHARACTERIZATION OF STIRRED YOGHURT: VISCOMETRY

Rheological characteristics of stirred yoghurt, varying in dry matter content, fermentation temperature and composition of bacteria cultures, were evaluated using a Bohlin VOR Rheometer. Flow curves, covering the shear rate range 29–920 s⁻¹, generally indicated the presence of a yield point and pseudoplastic behaviour, but also Bingham plastic behaviour was observed for some of the specimens. Systematic variations in the profile of the flow curves were observed as a result of the applied manufacturing conditions. Flow curves exhibiting pseudoplastic behaviour with a yield stress could not, over this broad shear rate interval, be described satisfactorily by the often applied Casson or Herschel-Bulkley models, and consequently a new one, the QRS-model was introduced. This model contains three parameters and gave a significantly better fit, with R² values generally above 0.99.     

SMALL DEFORMATION PROPERTIES OF MODEL SALAD DRESSINGS PREPARED WITH REDUCED CHOLESTEROL EGG YOLK

Low-fat, low-cholesterol materials were extracted from spray-dried egg yolk using as solvents petroleum ether, petroleum ether/ethanol, ethanol/water, and supercritical CO₂. The four extracts and their source (spray-dried yolk) were incorporated into traditional recipes for oil in water salad dressings. The linear viscoelasticity of these systems was characterised by dynamic oscillation and creep/relaxation curves on a stress-controlled Carri-Med rheometer. The high protein content of the extracts generates stronger and more elastic structures than those of commercial salad dressings but they are comparable to the viscoelastic properties of commercial mayonnaises which contain high cholesterol egg yolk. Measurements of the size of oil droplets over a period of eleven days demonstrate that these emulsions maintain acceptable consistency. The methodology adopted and the conclusions derived might assist in the purposeful formulation of low-cholesterol salad dressings.     

RHEOLOGICAL CHARACTERIZATION OF ASEPTICALLY PACKAGED PUDDING

The flow behavior of commercially available, aseptically packaged vanilla pudding was measured using a Thermo Haake RheoStress rheometer (RS1, Thermo Electron Corporation, Waltham, MA) with a concentric cylinder system. Six different shear rates, ranging from 1 to 100/s, and three reasonable serving temperatures were investigated using constant shear rate tests. A time‐dependent, Herschel–Bulkley model characterized the thixotropic nature of pudding. The model's five parameters were related to temperature. The model was validated at three constant shear rates and at two different temperatures, and was found to predict viscosity well as a function of time, shear rate and temperature.     

INTERACTION BETWEEN POTASSIUM SORBATE, OIL AND TWEEN 20: ITS EFFECT ON THE GROWTH AND INHIBITION OF Z BAILII IN MODEL SALAD DRESSINGS

The effect of the interaction between potassium sorbate (KS), oil and Tween® 20 on the growth and inhibition of Z bailii in model salad dressings was studied. Z. bailii grew in emulsions containing 110 and 230 g/kg of oil and the addition of 0.5 g/kg of KS inhibited growth. When the oil content was raised to 460 g/kg, a steep death rate curve was observed and the effect of KS addition was hidden by the inhibitory action of the high oil level. Tween® 20 action depended on oil level: for emulsions containing 110 or 230 g/kg, a depression in the activity of KS was observed. This behavior was attributed to the decrease in the free form of the preservative due to the partition between surfactant micelles and the water. In contrast, when the oil content was 460 g/kg, KS activity was enhanced in spite of the decrease in the amount of its free form. These results highlight the importance of considering ingredient interactions when evaluating shelf-life.     

COMPLETE RHEOLOGICAL CHARACTERIZATION OF TIME-DEPENDENT FOOD PRODUCTS

Rheological data are presented for three mayonnaise samples. The material shows pseudoplastic behaviour, and also exhibits a yield stress and time-dependent characteristics. Mayonnaise data are used to demonstrate how the time-dependent behaviour of a food product can be completely characterized using a simplified kinetic and rheological model.     

RHEOLOGICAL CHARACTERIZATION OF COMMERCIALLY PROCESSED FLUID MILKS

Flow profiles of commercially processed whole, two-percent, one-percent and nonfat milks with added solids as well as skim milk without added solids were measured using a rotational steady shear viscometer. Also tested were mixtures of concentrated skim milk added to skim milk over a range of 9.7% to 20.2% total solids. A shear rate range of 121 to 486 s^-1 was used with a cone and plate geometry. In all cases, linear plots of shear stress versus shear rate with small nonzero intercepts were obtained.     

NONLINEAR RHEOLOGICAL BEHAVIOR CHARACTERIZATION OF SOLID FOODS

Nutting's general law of deformation was used to characterize the nonlinear rheological behaviour of bread. A simple experimental technique submits a sample of regular geometry to a sequence of compression runs in which the ultimate strain, or the ultimate load, was kept constant. In order to avoid sample failure as a consequence of mechanical fatigue an ultimate strain as low as 5% and an ultimate load of 150 g were chosen. An extremely good agreement (correlation coefficient 0.9997) between the model prediction of bread stress-strain behaviour and the experimental results was found up to 80% compression. This result appears quite promising since it provides a simple way to predict and compare material performances under very large deformations.