Food rheology is the study of deformation and flow of foods under well-defined conditions. There are many areas where rheological data are required by the food industry including:

• plant design: pumps and pipe sizing and selection, heat and mass transfer calculations, filler designs and other process engineering calculations involving extruders, mixers, coaters and homogenisers
• quality control: both of raw material and the product at different stages of the process (including ingredient functionality determination in product development and also shelf life testing)
• evaluation of sensory attributes: quantitative measurement of consumer determined quality attributes by correlating rheology measurements with sensory data
• assessment of food structure and conformation of molecular constituents.

Food rheology is often confined to the behaviour of liquid foodstuffs. However, there is an increasing tendency to consider the response of both solid and liquid materials to applied stresses and strains as being two extremes of the same science. There are in fact some foods that will exhibit either behaviour depending on the stress applied; molten chocolate, fat-based spreads, mashed potato and some salad dressings will exhibit a solid-like behaviour at low stresses and a liquid-like behaviour at high stresses. This tendency is increasing as more food products are developed that would be classed by the consumer as being semisolid or semi-liquid. A more exact definition would therefore be the study of both the elastic and the plastic properties of foods. It is proposed, however, to place greater emphasis here on classical liquid rheology measurements, although elastic and viscoelastic properties will also be discussed in the context of semi-liquid foods.

There are many substantial reviews of basic rheology. However, before looking at rheological concepts, it is necessary briefly to consider here some of the fundamentals. It is also necessary to justify the need for measurement given the wealth of published data already available. The reason for this is as stated by Prins and Bloksma (1983): ‘Rheological measurements have to be made under the same conditions as those which exist in the system studied.’ In other words, there is little use in carrying out measurements on a product or extracting values from the literature, if the stresses used and their rates of application during the measurement differ from those in the process or calculation for which the measurement is required. (PRINS, A. and BLOKSMA, A.H. (1983), ‘Guidelines for the measurement of rheological properties and the use of existing data’, in Jowitt, R., Escher, F., Hallstrom, B., Meffert, H.F. Th. Spiess, W.E.L. and Vos, G., Physical Properties of Foods, arking, UK, Elsevier, 185–191.)

• Viscous flow: As has been stated, rheology is the study of deformation and flow of foods under well-defined conditions. These conditions could be defined in terms of their rate of deformation or in terms of the magnitude of the stress or the strain applied. Foods of differing internal structure and bonding will react in different manners to these applied conditions. We take as an example a system designed to apply a controlled rate of deformation to a fluid. In the simplest case the shear stress developed in the fluid is directly proportional to the rate of deformation or the rate of strain. In such cases, the liquid is said to be Newtonian

• Elastic deformation: Certain types of solids, known as hookean solids, display ideal elastic (or hookean) behaviour. This particular behaviour occurs when a force is applied to a solid material and the resultant response gives a straight line relationship between stress and strain. This relationship is known as Hooke’s law and occurs in an ideal elastic solid (also called Hooke’s body).

• Viscoelasticity:- Many complex structured foodstuffs display both viscous and elastic properties and are known as viscoelastic materials. The use of this term is often restricted to solids, with the term ‘elastico-viscous’ being used to describe liquids displaying similar characteristics. However, we will use the term viscoelastic to describe both, because it is often not possible to establish whether a material is behaving as a solid or as a liquid. Linear viscoelasticity is the simplest viscoelastic behaviour in which the ratio of stress to strain is a function of time alone and not of the strain or stress magnitude, while non-linear viscoelastic materials exhibit mechanical properties that are a function of time and the magnitude of stress used.

Ultimately the food product must be eaten, so sensory attributes become most important. However, en route from the farm to the mouth the product may have to be pumped, heated, stored or subjected to other processes, and must be amenable to flow when being placed in a container/package. Equally important is its ability to flow out of the container before consumption. Indeed, it is this ability (or the occasional lack of it) that first brings the consumer into a direct and sometimes frustrating contact with rheological principles. How often has the consumer experienced the dilemma of tomato ketchup refusing to flow from its bottle and found that the application of a sharp blow to the bottle base resulted in an excess amount being deposited on the plate? This provides an excellent example of a situation in which a product has a yield stress below which it will not flow, but flows perhaps too well once the consumer unknowingly provides the stimulus that exceeds it. Not only does this example illustrate yield stress, but it also shows the relationship between force and deformation and flow!

This simple example also gives emphasis to one of the basic rules of rheological measurements, namely that the product should be tested under a range of conditions of stress and shear rate that reflect those experienced during subsequent use, whether that use be tasting, pouring, shaking, stirring or any other action that requires movement of the material. Of course, rheological relevance does not stop when a food reaches the plate but influences the sensory perception or ‘mouthfeel’ of the product. Some authors have defined mouthfeel as the mingled experience deriving from the sensations of the skin of the mouth after ingestion of a food or beverage. It relates to density, viscosity, surface tension and other physical properties of the material being sampled. These relationships between rheology and mouthfeel have been the subject of extensive research, as reviewed in the author’s bibliography on food rheology (McKenna, B.M. 1990, The Liquid and Solid Properties of Foods – a Bibliography, London, Food Science Publishers).