The stimulus associated with oral evaluation of the viscosity of liquid and viscous foods appears to embrace a range of shear rates extending from 10-l to lo00 s-1. The value associated with a particular food depends on its flow characteristics. For liquid foods the stimulus appears to be the shear rate developed at an approximately constant shear stress (-100 dyn cm-2), whereas for viscous foods it appears to be the shear stress developed at an approximately constant shear rate (-10 s-1). Additional factors may be involved when evaluating highly viscous non-Newtonian foods or foods with an oil continuous medium. These are 'plug flow' or the spreading coefficient.respectively.

Abstract. Analysis of published chewing force patterns indicates that the ‘first bite’ involves an approximately linear application of force. The Instron Universal Testing Machine also offers a linear pattern of force application making it possible to obtain information about textural properties identified during the ‘first bite’ from force‐compression tests. However, the data obtained in such tests are influenced not only by the applied force but also by the rate at which it is applied. Both variables must be taken into account when analysing the data, since the applied force in chewing and also the rate of force application depend on the textural characteristics of the food being evaluated. Based on a correlation of sensory responses with instrumental force‐compression‐rate of force application data, a procedure is described for establishing the mechanical force conditions that should be used in Instron tests so that they simulate those associated with the sensory evaluation.

Creep compliance-time studies at low stresses give more information about the structural changes in work softened butter and margarine than the Cone Penetrometer. They show that margarine undergoes a greater structural change than butter. Margarine loses most of its instantaneous elasticity in an unrecoverable manner. However, most of the Newtonian viscosity loss is recovered. Butter loses much less of its instantaneous elasticity, and most of what is lost is recovered during aging, but less of the Newtonian viscosity which is lost is recovered. Retardation spectra derived from creep compliance-time curves for both butter and margarine before work softening show a peak at ∼10 sec. In contrast to margarine, butter does not lose this peak when work-softened. For margarine, the peak slowly redevelops during aging and occurs at ∼10 sec. Past interpretation of Cone Penetrometer data has been based on the simplified concept of primary irreversible bonds and secondary reversible bonds, and the changes in their relative proportions during work softening. The present approach avoids the allocation of bond strengths into these arbitrary categories, and is based on the realization that butter and margarine contain a wide range of bond strengths.

Flow properties of several liquid and semi-solid foods were characterized objectively with a coaxial cylinder viscometer. At the same time, a sensory evaluation panel compared the viscosities of these samples by tilting the containers and also by stirring the contents with a spoon. Correlation of the instrumental and sensory evaluation data established the shear stress-shear rate conditions prevailing during sensory evaluation of viscosity. The stimulus responsible for viscosity evaluation by tilting the container is the shear rate ( 0 . 1 4 sec-l) developed at a shear stress (60-600 dyne c m 9 related to the flow properties of the sample, whereas in stirring tests the stimulus is the shear stress (102-lOQdynecm-2) developed at a particular rate of shear (90-100 sec-l). In the latter type of test, the shear rate varies to some degree with the flow characteristics of the sample. Since different stimuli are involved in viscosity assessment by tilting the container or by stirring, it is possible for a series of samples to be given different ranking orders by the two methods of evaluation.

SUMMARY The creep behavior of frozen ice cream can be represented satisfactorily by a six‐element model. The parameters involved are the instantaneous elasticity (E0), two elastic moduli (E1 and E2), and two viscosity components (η1 and η2) associated with retarded elasticity, and a Newtonian viscosity (ηN). From the effect of fat, overrun, and temperature on the magnitude of these parameters it is suggested that E0 is affected primarily by the ice crystals, E1 and η2 by the weak stabilizer‐gel network, η1 by protein‐enveloped air cells, η1 by the fat crystals, and ηN by both fat and ice crystals. When ice cream mix is whipped and frozen, the fat globules undergo some coagulation from rupture of the protective protein‐emulsifier layer around the globules. Coagulation is hindered by the simultaneous conversion of water to ice.