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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.

The rheological properties of heat whey protein concentrate gels were studied by dynamic oscillation rheometry. A whey protein concentrate of 75% protein was used to make solutions of 10.3, 12.5 and 14.5% protein (w/w), which were heated to 90C for gel formation. Specific attention was focused on the temperature dependence of the mechanical properties of the gels during cooling and reheating. In all cases the magnitude of the complex modulus |G*| was found to increase with decreasing temperatures from 90 to 30C. The tan δ, which is related to the relative viscoelasticity of the gels, increased with decreasing temperatures from 90 to 60C. At temperatures between 60 and 30° C, tan δ remained constant. The dependence of |G*| and tan δ on temperature was found to remain constant during heating and cooling between 30 and 70C, indicating that rheological changes were reversible within this temperature range.

Creep compliance was used to determine the effects of the addition of α-, β-, and κ-casein, and Na-caseinate on the viscoelastic properties of skim milk curd. The results of all measurements can be represented by a six-element mechanical model. Addition of α- and β-casein, and Na-caseinate (1.80g/L) to raw skim milk reduced the instantaneous modulus of rigidity and final viscosity of the curd, while κ-casein addition at the same level increased both viscoelastic parameters. Shielding of κ-casein and depletion of serum Ca++ ions by α- and β-casein is thought to have caused the reduction of curd rigidity and viscosity. Subsequent experiments indicated that the addition of β-casein before and after rennet hydrolysis produced different curd strength with the latter producing a stronger curd.

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.

Time dependent flow properties of commercial mayonnaise have been measured at shear rates of 0.530, 0.169, 0.052, and 0.0169 s−1. The flow curves were fitted with a series of two first order rate functions. It appears that the time dependent flow properties of mayonnaise are better characterized by this model than with only the initial and final stress values of the stress decay curves or with a single first order rate function.

Description de la methodologie d'un systeme de collecte et protocole de modelisation des proprietes d'ecoulement, avec application a la mayonnaise

Advances in the characterization of the complex time-dependent flow properties of liquids and semisolids are reviewed, with special emphasis placed on recent developments in elucidating the behavior of food systems when sheared. Time dependency is discussed in terms of viscoelasticity and structural changes. Systems as diverse as mineral oils, lotions and creams, egg custards, and mayonnaise are shown to exhibit a similar stress decay over time when sheared at a constant rate. Various mathematical descriptions, both theoretical and empirical, are used to characterize the stress decay, which is due to structural changes (e.g. thixotropy) that occur within the system as it is sheared. Evidence is provided that the stress decay varies with product formulation and experimental conditions, such as temperature, shear rate, and viscometer gap width. A discussion of the uses and disadvantages of steady shear measurements is included.