Measurement of extensional viscosity of viscoelastic liquid foods

Abstract 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).