Rheology is the science of deformation and flow of matter. The rheological prop- erties of a material provide relevant information on its structure, its behavior during processing, and its end-use properties. These properties influence the flow process and are themselves influenced by the structural changes generated during the process. Initially a branch of mechanics, this science has developed consider- ably and, for example, in the field of synthetic polymers, aims to compare results from continuum mechanics, molecular theories, and computer simulations. Un- fortunately, application of these approaches to biopolymers and cereals is more difficult, for the following reasons:
In contrast to synthetic polymers, cereal grains are variable products, whose suitability for a given process/product goal is affected by the genotype, the environmental conditions encountered during grain development and finally the milling process; these difficulties have been reinforced by progress in plant breeding and by the large offers created by the interna- tional cereals trade.
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Cereal products very often have a complex formulation, with several com- ponents (starch, proteins, water, sugars, lipids) that can interact and lead to more or less organized structures; starch itself is made of two types of macromolecules, the linear amylose and the branched amylopectin. It can thus be considered as a multiphasic, rheologically complex material.
Doughs and melts from cereal products are highly non-Newtonian, with a high level of elasticity, and are very sensitive to the temperature, the water content, and, more generally, the composition (starch origin, pres- ence of lipids).
Some components, even in small quantities, like lipids, may induce slip at the wall and thus totally modify the flow conditions.
Biopolymers are not as thermally stable as casual synthetic polymers. They start to decompose at 200–220°C. But before this threshold, chemical interactions can occur, leading to intra- and intermolecular covalent cross-linking. The kinetics of these reactions are highly determined by temperature and also by water content: Material processing itself induces important changes to the structure.
All of these reasons imply that any investigation should be based upon samples whose biological origin and conditions of preparation are not only known but also reproducible. The variability induced by these various factors leads to a broad range of behavior during the three basic operations in cereal technology:
mixing, dough processing (including shaping), and baking or cooking. These op- erations commonly lead to the following phenomenological changes:
Obtaining a macroscopic homogeneous phase: from a powder (flour) to a dough or a melt
Flow and dough forming (laminating, sheeting)
Foam creation by bubble nucleation and growth during fermentation and heating, vaporization: from a dough or a melt to a solid foam
Various states or types of organization (suspension, network, melt) may thus be encountered under many different flow conditions, as expressed inTable 1.To face the complexity of these changes, empirical rheological tools have first been used in the laboratories, to predict cereal and flour performances. Their goal is to provide qualitative information related to process adequacy. Section II.A is devoted to these tests, which are still widely employed, not only in industry but also in research laboratories. However, no device has ever been developed to follow the on-line rheological properties of dough during baking: This would explain, for instance, why all final testing procedures for bread making include a small baking test. This underlines that this last step of bread technology can reveal unusual behaviors, unexpected on the basis of all empirical tests devoted to only the two first steps, mixing and dough processing.
Table 1 Examples of Products, Behaviors, and Processing Conditions Encountered During Overall Processing
Application and/or end Bread baking Biscuit dough forming Dough development Breakfast cereals and snacks product
Process and/or phenomenon Fermentation and Laminating, sheeting Mixing and kneading Extrusion and die flow bubble growth
Typical rheological behavior Viscoelastic solid Concentrated suspension Transient network Polymer melt, viscous liquid
Range of moisture content and 10–40% 10–30% 30–50% 10–30%
temperature 50–200°C 10–60°C 10–60°C 100–200°C
Range of shear rate (s⫺1) 10⫺3–10⫺2 1–102 10–102 10–103
More objective experimental approaches are now available. They are de- scribed in Section II.B. The adaptations that have to be made to classical methods to study biopolymers are also emphasized in Section II.C. Since the pioneering work of Schofield and Scott-Blair (1), many applications of these approaches to cereal dough characterization can be found in the literature. The variety of prod- ucts, processes, and methods encountered explain why no global literature synthe- sis is available on this topic. Thus, rather than trying to be exhaustive, we have chosen, in Section III, to present some recent examples of the use of such methods for different types of products encountered during different processing steps, such as those mentioned inTable 1. This chapter also aims to show the point up to which rheological methods can actually be useful for cereal processing and which improvements they eventually require.