Rheology is the science of flow and deformation of matter and describes the interrelation between force, deformation and time. The term comes from Greek rheos meaning to flow.Rheology is applicable to all materials, from gases to solids. The science of rheology is only about 70 years of age. It was founded by two scientists meeting in the late 1920s and finding out they have the same need for describing fluid flow properties. The scientists were Professor Marcus Reiner and Professor Eugene Bingham. The Greek philosopher Heraclitus described rheology as panta rei – everything flows. Translated into rheological terms by Marcus Reiner; this means everything will flow if you just wait long enough.

Fluid rheology is used to describe the consistency of different products, normally by the two components: viscosity and elasticity. By viscosity is usually meant resistance to flow or thickness and by elasticity usually stickiness or structure. Rheological relationships help us to understand the fluids we are working with so that we can either know how they are behaving or force them to behave according to our needs. Once a correlation has been developed between rheological data and product behaviour, the procedure can then be reversed and rheological data may be used to predict performance and behaviour.

Food rheology

Described as the study of the deformation and flow of the raw materials, the intermediate products and the final products of the food industry. The rheological property of a food system is dependent on the composition or the ingredients of the system. There are numerous areas where rheological data are needed in the food industry:

a. Process engineering calculations involving a wide range of equipment such as pipelines, pumps, extruders, mixers, coaters, heat exchangers, homogenizers, and on line viscometers;

b. Determining ingredient functionality in product development;

c. Intermediate or final product quality control;

d. Shelf life testing;

e. Characterizing ingredients and final products, as well as for predicting product performance and consumer acceptance.

f. Evaluation of food texture by correlation to sensory data;

g. Analysis of rheological equations of state or constituent equations.

h. They are also a way to predict and control a host of product properties, end use performance and material behaviour as well as sensory analysis and quality control of foods.

Why Make Rheological Measurements?

Anyone beginning the process of learning to think “Rheologically” must first ask the question, “Why should I make a rheological measurement?” The answer lies in the experiences of thousands of people who have made such measurements, showing that, much useful behavioural and predictive information for various products can be obtained, as well as knowledge of the effects of processing, formulation changes, aging phenomena, etc. A frequent reason for the measurement of rheological properties can be found in the area of quality control, where raw materials must be consistent from batch to batch. For this purpose, flow behaviour is an indirect measure of product consistency and quality. Another reason for making flow behaviour studies is that a direct assessment of process-ability can be obtained. For example, a high viscosity liquid requires more power to pump than a low viscosity one. Knowing its rheological behaviour, therefore, is useful when designing pumping and piping systems. It has been suggested that rheology is the most sensitive method for material characterization because flow behaviour is responsive to properties such as molecular weight and molecular weight distribution. Rheological measurements are also useful in following the course of a chemical reaction. Such measurements can be employed as a quality check during production or to monitor and/or control a process. Rheological measurements allow the study of chemical, mechanical, and thermal treatments, the effects of additives, or the course of a curing reaction.

Thinking Rheologically

To begin, consider the question, “Can some rheological parameter be employed to correlate with an aspect of the product or process?” To determine this, an instinct must be developed for the kinds of chemical and physical phenomena which affect the rheological response. For the moment, assume this information is known and several possibilities have been identified.

The next step is to gather preliminary rheological data to determine what type of flow behaviour is characteristic of the system under consideration. At the most basic level, this involves making measurements with whichever viscometer is available and drawing some conclusions based on the descriptions of flow behaviour types. Once the type of flow behaviour has been identified, more can be understood about the way components of the system interact.

THREE SCHOOLS OF THOUGHT ON RHEOLOGICAL MEASUREMENT

From experience, there are basically three schools of thought on the use of viscometers in rheological applications. They are presented here and you can then decide which you fall into, remembering that there is no “right” one and that each has its merits and demerits.

The Pragmatic School

This first school of thought is the most pragmatic. The person who adheres to this school cares only that the viscometer generates numbers that tell something useful about a product or process. This person has little or no concern about rheological theory and measurement parameters expressed in absolute terms. Quality control and plant production applications are typical of this category.

The Theoretical School

The second school of thought involves a more theoretical approach. Those adhering to this school know that some types of viscometers will not directly yield defined shear rates and absolute viscosities for non-Newtonian fluids. However, these people often find that they can develop correlations of “dial viscosity” with important product or process parameters. Many people follow this school of thought. The applications of rheology literature is replete with statements along the line of “I know the data isn’t academically defined, but I keep this fact in mind and treat the multi-point rheology information as if it were.” In many cases, this produces eminently satisfying results and eliminates the necessity of buying a highly sophisticated and very expensive piece of rheological equipment.

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