Rheology, with Special Attention to Dispersions and Interfaces

Texture, as much as flavor, determines the oral experience of foods. The consumer has a conditioned expectation of the mouthfeel of a product and if he does not sense what is expected, the product is often considered unacceptable ice cream should be soft and creamy, pasta stretchy, potato chips crispy, and chocolate should melt slowly and smoothly in the mouth. Moreover, textural contrasts on a plate often add to the pleasure of eating. 

Most textured foods are built of a network of colloidal particles and molecules held together by a variety of forces. The structure, that is, consistency and texture, of such heterogeneous systems is strongly influenced by the strengths of these forces. The mechanical response of the food product during mastication, in particular its resilience against deformation, is a key factor for the in-mouth sensation. Therefore, rheological properties play a crucial role in the sensorial acceptance of food. 

Rheology deals with flow and deformation of materials. More precisely, rheology is the study of the deformation of matter in response to an applied force, as a function of time. The rheological properties of a material reflect the relation between the deformation and the force. The following extremes may be distinguished  

In general, if no bonds are broken during the time the force is applied, a material behaves fully elastic if all bonds are disrupted the material is viscous. Viscoelasticity is related to the ease with which bonds are broken or, more precisely, with which a material is able to relax the force that is needed to maintain a constant deformation. In this context, a characteristic time, the relaxation time, is defined as the time in which the required force/has decayed to e of its original value. In a purely elastic system, no bonds are broken and the force is constant in time. For a purely viscous 

FIGURE 17.1 Force required to maintain a constant deformation in time. 

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J Rheology, with Special Attention to Dispersions and Interfaces... 

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