1.3 Two Kinds of Rheometry
Our modern way of life is unimaginable without rheometry. Rheometry is used to predict the avalanche menace in ski-fields, to minimize the risk of a heart attack by measuring and influencing the flow properties of blood, to estimate the weight-bearing capacity of shopping bags made from petropolymers or bio-polymers, and even to increase the service speed of tennis cracks by developing the right strings for their rackets. Rheology has a multitude of uses and is all around us. In food production it is used to assess the quality of raw materials and end products, and it has therefore become an important and powerful aid to food technologists. It also aims to determine the texture of foods and thus replace sensory testing in the judgement of quality. But the latter is hardly likely to succeed, partly because there are no suitable measuring techniques so far, and partly because experienced technologists of long standing are unlikely to allow themselves to be ousted or challenged by measuring instruments.

Rheology has a sub-discipline, rheometry, whose task is to make and explain measurements. We speak of empirical (also known as descriptive or imitative) rheometry and fundamental (absolute) rheometry, depending on the measuring principle and the possibilities offered by the instruments used. Empirical rheometry may also be termed conventional rheometry. From the point of view of rheologists and practical users, both groups of instruments and measuring techniques have advantages and disadvantages, most of which result from the design of the instruments and the measuring principle. The users and advocates of the two different rheometries regard each other with suspicion. One rheologist has described this situation aptly:
• With the empirical methods we don't know what we are measuring, but it works;
• with the fundamental methods we know exactly what we are measuring, but it doesn't work.

"It works" means that the measurements reflect the behaviour of a raw material during processing and also the quality of the end product. How the measurements are interpreted doubtless depends on the experience of the people who carry them out. In practice the measurements are often performed under conditions different from those of the process. Most of the instruments used in empirical rheometry are relics of the early days of dough rheology and have scarcely been modified to this day. They are very common, easy to use, and their results have found a permanent place in the terminology of cereal technologists.

The comment "but it doesn't work", said of fundamental rheometry years ago, has now ceased to apply. In the final decades of the last century, rheology in general experienced a considerable upswing with the development of new, versatile, sensitive, precise and efficient measuring instruments. These instruments were used chiefly in the plastics industry to demonstrate the structures of "noble" (and therefore expensive) thermoplastic polymer melts. Systematic work with measuring instruments of this kind yielded structural models for wheat and rye doughs too, and the biopolymer "dough" was found to conform to the same laws as the organic polymer melts. Moreover, it also undergoes phase transitions as it passes through the various temperature ranges. This discovery is not only of academic interest; it has also had practical value since the technique of freezing dough and heating pre-baked frozen dough portions became common practice at bakeries. But in spite of all the advantages of rheometers and fundamental rheometry, the high price of such measuring instruments and the more intricate work they require explain why they are not yet in general use in the laboratories of cereal processors. It is a sad fact that bread rolls are less lucrative than plastic polymer melts!

Being a branch of physics, rheology cannot quite do without mathematics; in some areas, in fact, it uses a great deal of mathematics and calculation. But that should not frighten us and prevent us from using rheometry. Rheology can manage without higher mathematics, especially when the results are considered in categories of "too much" or "too little" in relation to the desired optimum or used for drawing curves or diagrams. In the meantime simpler measuring instruments suitable for the routine work of cereal laboratories have come onto the market. Some combine the advantages of the two rheometries, overcome the old, strict divisions between empirical and fundamental rheometry, and are affordable into the bargain. So in future we may expect them to find their way into the research and development laboratories more often than in the past.

The use of rheometry in dough rheology has already been described in detail in the relevant standard methods (e.g. ICC, AACC, AOAC) and in textbooks on cereal technology. In our discussion of dough rheology and dough rheometry, their application in laboratories and interpretation of the results we do not intend to repeat this information. Our aim is to give "the practical man" an insight into what is going on backstage in nature and in the measuring instruments and dispel his fear of advanced science in an ivory tower.

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