1. Rheology
1.1. Introduction
Bread in all its diversity has existed for over 6,000 years; rheology as a branch of physics is a great deal younger, though scarcely less diverse. What, then, is the relationship between bread and the rheology of dough? Descriptions of the first attempts to measure the physical properties of food in general and of bread doughs in particular date back to the 18th century, when Beccari assessed the quality and structure of wheat doughs sensorily in 1728 and Bolland and Kunis later carried out tests with the Aleurometer in 1836 and 1885 respectively. Dough rheology as we know it today did not originate until the early 20th century and was born of necessity. Hungary was considered the granary of the Austro- Hungarian Empire and exported wheat to the rest of Europe. The Hungarian wheat varieties were popular in North America too – some were in fact related to the American varieties, having common parents. Hungarian wheat was much in demand for its quality. So the breeders made every effort to cultivate and grow higher-yielding varieties. But the quality of these new wheat varieties no longer met the requirements of the market. In order to assess the baking properties of wheat varieties or wheat lots without performing expensive and time-consuming baking trials, the first recording mixers (Hankoczy) and dough stretching instruments (Hankoczy, Rejtö, Gruzl) were constructed in the early 20th century; they may be regarded as the forerunners of the Swanson Working Mixograph in America and also the Brabender Farinograph and Extensograph and the Chopin Alveograph in Europe. After World War II the situation changed in that in Germany the Brabender Farinograph and Extensograph were used to determine the suitability of the quality wheat lots imported from North America (USA and Canada) and South America (Argentina). The Alveograph was used from the start to compare and check the quality of flour batches. An interesting description of this development is given by Muller and Wassermann (Muller, 1964 and 1966; Wassermann, 1993).
1.1. Introduction
Bread in all its diversity has existed for over 6,000 years; rheology as a branch of physics is a great deal younger, though scarcely less diverse. What, then, is the relationship between bread and the rheology of dough? Descriptions of the first attempts to measure the physical properties of food in general and of bread doughs in particular date back to the 18th century, when Beccari assessed the quality and structure of wheat doughs sensorily in 1728 and Bolland and Kunis later carried out tests with the Aleurometer in 1836 and 1885 respectively. Dough rheology as we know it today did not originate until the early 20th century and was born of necessity. Hungary was considered the granary of the Austro- Hungarian Empire and exported wheat to the rest of Europe. The Hungarian wheat varieties were popular in North America too – some were in fact related to the American varieties, having common parents. Hungarian wheat was much in demand for its quality. So the breeders made every effort to cultivate and grow higher-yielding varieties. But the quality of these new wheat varieties no longer met the requirements of the market. In order to assess the baking properties of wheat varieties or wheat lots without performing expensive and time-consuming baking trials, the first recording mixers (Hankoczy) and dough stretching instruments (Hankoczy, Rejtö, Gruzl) were constructed in the early 20th century; they may be regarded as the forerunners of the Swanson Working Mixograph in America and also the Brabender Farinograph and Extensograph and the Chopin Alveograph in Europe. After World War II the situation changed in that in Germany the Brabender Farinograph and Extensograph were used to determine the suitability of the quality wheat lots imported from North America (USA and Canada) and South America (Argentina). The Alveograph was used from the start to compare and check the quality of flour batches. An interesting description of this development is given by Muller and Wassermann (Muller, 1964 and 1966; Wassermann, 1993).
The last four measuring instruments – the Farinograph, Mixograph,
Extensograph and Alveograph – are used virtually unchanged to this day in the
service of the science and practical task of cereal processing. However, they use
relatively strong deformation forces as a measuring principle, and they only
describe the properties of the dough in the cold phase of the bread-making
process, during mixing and after fermentation. These two points may be regarded
as disadvantages. The pasting properties of the cereal starch as a function of alpha-amylase
activity at high temperatures similar to those of an oven have been determined with
the Brabender Amylograph and the Hagberg-Perten Falling Number instrument since
the 1920s and 1960s respectively, and more recently with the Newport Scientific
Rapid Visco Analyzer. For technical reasons these measurements can only be made
in flour/water slurries of various concentrations.
In the early 1930 s, Scott Blair laid the foundations of fundamental
rheometry and rheology of food by measuring and describing the viscoelastic
properties of wheat dough (Schofield and Blair, 1932). During the 1970s and
1980s, fundamental rheology experienced a major upswing with the construction
of new, precise and sensitive instruments whose measurements permitted an
insight into the structures and behaviour of foods. These new rheometers apply
the dynamic, oscillating mode as a measuring principle, with which they can
simultaneously record complex viscosity and its components elasticity and plasticity
("pure viscosity"). Moreover, the deformation forces they apply are
extremely small; since these do not interfere with the structure of the
specimens, they permit continuous monitoring of changes in the viscoelastic
properties of a dough as a function of time and temperature. This is really the
basis of the simulating "recording baking trial" which makes it
possible to monitor the changes in the viscoelastic properties of the dough in
the baker's oven (Weipert, 1987 and 1992).
Bread baking is ultimately a process of the uptake and binding of
water by the swelling substances (such as proteins and pentosans) in the dough
and the rebinding of water by the pasted starch in the heated, baked dough or the
crumb of the bread which results in changes in the viscosity or consistency of
the dough or bread crumb and can therefore be demonstrated well by rheometry.
In other words: rheology in general and rheometry in particular are good tools
for studying, interpreting, predicting and checking baking properties.
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