Search

Extensional rheology of foods can be important and helpful in various stages of food manufacture, starting from the initial stage of formulation to the final stage of sensory perception of the product by the consumer. Several new techniques have been developed over the last two decades to measure the extensional flow properties of polymeric liquids. Some of these techniques can be employed to measure the extensional viscosity of foods. Three such techniques are reviewed in this paper: the fiber wind-up and the entrance pressure drop technique for high viscosity liquids, and the opposed jets device for low viscosity liquids. The fiber wind-up technique requires a special set of fixtures which is attached to a conventional rotational rheometer. The entrance pressure drop technique requires a capillary rheometer. A large-diameter barrel capillary rheometer is useful to obtain data for wheat doughs. A commercial rheometer is available with the opposed jets configuration for low viscosity liquids (RFX). Extensional viscosity data from all these techniques are examined and their application to foods discussed. The transient extensional viscosity of a high viscosity polyisobutylene (a polymer melt at room temperature) obtained using the fiber wind-up technique over extension rates of 0.5–10 s−1, and up to strains of three is examined. The extensional viscosity of two different wheat flour-water mixtures estimated using the entrance pressure drop technique is presented. Both doughs showed extension-thinning behavior over the range of extension rates examined. The use of the opposed jets device to characterize polysaccharide solutions is discussed based on a study conducted by R. C. Clark in 1991 (Extensional viscosity of some food hydrocolloids. Paper presented at the Sixth International Conference on Gums and Stabilizers in the food industry, Wrexham, Wales, 15–19 July 1991).

Rheological properties of food dough melt, such as steady shear viscosity (η) and first normal stress difference (N1), were measured using an in-line slit die rheometer attached to a laboratory model single-screw extruder. The flow rate through the rheometer was varied by altering the screw speed. Alternatively, a sidestream valve, to vary the flow rate through the rheometer at fixed screw speed, was also used to obtain rheological data. The slit die rheometer was tested with corn meal at three different experimental conditions: 25% and 35% moisture contents at 180C barrel temperature and 35% moisture content at 150C barrel temperature. N1 was measured using the hole-pressure and exit-pressure method. Exit pressure measurements were found to be erratic and unreliable. Hole pressure increased monotonically with increasing flow rate and was found to be affected by the processing history. The magnitude of the hole pressure ranged between 2% and 21% of pressure of the flush mounted transducer. For the experimental conditions used in this study, N1 ranged from 3 × 104 to 6 × 105 Pa for shear rates between 30 to 400 s−1. Possible sources of errors in the hole pressure measurements are discussed.

Uniaxial extensional viscosity of cornmeal at moisture contents of 30 and 35% (dry weight basis) and temperature of 160 and 180 o C was estimated from entrance pressure drop measurements during extrusion cooking. The value of the sheat component of the entrance pressure drop was an order of magnitude below that of the extensional component. Food doughs exhibited sheat thinning and extension thinning behavior during processing conditions used. Increasing moisture and temperature decreased extensional viscosity at highet extension rates. The Trouton ratio ranged from 25 to 50 and was affected by product moisture content, barrel temperature, and deformation rate

Slit and capillary dies attached in-line to an extruder are a popular configuration for obtaining the flow curves of food doughs during extrusion-cooking. However, it is demonstrated in this paper that the use of screw speed, especially for a single-screw extruder, to vary the shear rate through the die can result in erroneous flow curves for single-screw extruders. Furthermore, the use of a sidestream valve to vary the flow rate for a given screw speed is shown to result in flow curves that are different and better behaved. Such a procedure will be helpful in single-screw extruder systems, not only in obtaining the true flow curves of the food dough, but also in rheological measurements with in-line rheometers. Data for hole pressure (a measure of the first normal stress difference) are also presented to demonstrate the validity of using the sidestream valve and the error in using the screw speed to obtain rheological measurements during extrusion of starchy foods.

The application of the entrance pressure drop method to evaluate the extensional viscosity of corn meal during extrusion-cooking was investigated. An approximate planar extensional flow was generated by forcing the dough from a circular channel barrel into a slit die. The pressure drop along the contraction was calculated to obtain the entrance pressure drop Δ P en . Both Cogswell's and Binding's analyses were applied to estimate the apparent planar extensional viscosity η pa from Δ P en . The magnitude of η pa from Cogswell's analysis was found to be much larger than that predicted by Binding's analysis. In addition, the severe extension-thinning behavior exhibited by the fluid introduced problems in the calculations of Binding's analysis for some cases. Calculation and interpretation of the Trouton ratios have to be done carefully when extrapolation of the viscosities is required. Moderate to large entrance corrections were obtained, with lower values obtained for the lower moisture contents and temperatures of extrusion. In all cases the entrance correction decreased with increasing flow rate.

The use of slit rheometers for on-line viscosity measurements of fluid foodstuffs has been gaining increasing popularity in recent years. However, the viscoelastic nature of fluid foodstuffs necessitates the measurement of their elastic properties, which will provide a better overall understanding of the behavior of the foodstuffs during processing. This review discusses the exit-pressure and hole-pressure methods for the on-line measurement of fluid elasticity, and the application of these methods to the control of the extrusion-cooking process. The potential application of the methods to the rheological characterization of viscoelastic fluid foodstuffs in general is also discussed.

Volumetric flowrate was studied in the die of a single screw extruder. Variables included the ratio of die length to die diameter, barrel to die diameter, barrel and die temperature, screw speed and product moisture. Die conductance was determined graphically and analytically. Results indicated that for a constant pressure drop across the die, increasing temperature, die diameter or moisture increased flowrate. Dimensional analysis was used to study the effect of each parameter on the flow through the extruder die. An expression for temperature rise due to viscous dissipation for isothermal flow in the die was developed and nomographs are presented.

Entrance pressure drop of extruder dies increased with a decrease in the ratio of barrel-to-die diameter during the extrusion of corn. For the low Reynolds number flow studied (1.77 x 10−3≤ Re ≤ 9 x 10−5) the viscous component, obtained from Weissberg's analysis, contributed 2% to 40% of the entrance pressure drop and increased as the barrel-to-die diameter ratio decreased. The friction factor for laminar fully developed flow through the die followed the expression f = 16/Re'. Fully developed flow within the die was achieved at a die length-to-diameter ratio of 9.