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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