Material ParametersOver the years, numerous studies have investigated extrusion expansion, particularly of cereal foods. Chinnaswamy (1993) reviewed most of the work on extrusion expansion of starches and cereal flours up to that date. In the last decade, many other researchers continued to study expansion and the factors that affect it, aiming to create the basis for new or improved extruded foods. ABSTRACT: Expansion of biopolymer matrices is the basis for the production of a wide variety of cereal foods. A limited number of manufacturing processes provide practical solutions for the development of an impressive variety of expanded products, just by changing process variables. It is therefore essential that the mechanisms involved in expansion are well known and controlled. This paper summarizes the knowledge of nucleation and expansion in extruded andmicrowaved products available to date. The effect of processing conditions and properties of the biopolymeric matrix on nucleation and expansion are discussed. Moisture content enables the glassy polymeric matrix to turn into rubbery state at process temperatures, which allows superheated steam bubbles to form at nuclei and then expand, expansion being governed by the biaxial extensional viscosity of the matrix. Nucleation and expansion theories are presented along with quantitative data that support them.

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Among all flour components, starches play a key role during extrusion. Starch undergoes several significant structural changes, which include gelatinization, melting, and fragmentation. This review focuses on transformations in starch during extrusion and their effect on rheological properties. The effect of extrusion operating parameters, such as residence time distribution, thermal and mechanical energy input and pressure inside the die, and their effect on rheological properties are reviewed. The relationships between rheological properties and extrudate expansion and final extrudate texture are discussed. The dependence of starch viscosity on temperature, moisture content, time‐temperature history, shear rate, gelatinization, and fragmentation kinetics during extrusion are reviewed.

Statistical analysis of subjective assessments obtained by ratio scaling suggested that a ten attribute vocabulary describing liquid texture in the mouth could be reduced to a smaller number without great loss of predictive power, Three attributes, “thickness,”“smoothness,” and “slipperiness,” which could best predict the remaining seven were selected for further study. Each attribute was found to be closely related to a specific force in the mouth. “Thickness” was shown to be proportional to the viscous force between the tongue and the roof of the mouth. “Smoothness” seemed to be inversely proportional to the frictional force caused by the contact between the tongue and the mouth. “Slipperiness” was shown to be inversely proportional to a known average of viscous and frictional forces.

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