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 and microwaved 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.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1541-4337.2003.tb00020.x

Nucleation and Expansion During Extrusion and Microwave Heating of Cereal Foods

Relations between rheological properties of molten starches and their expansion behaviour in extrusion

Shear viscosity (η), first normal stress difference (N1), and extensional viscosity (ηE) of polymer melts measured under processing conditions are important in process modeling, quality control, and process control. A slit rheometer that could simultaneously measure η, N1, and the planar extensional viscosity (ηp) was designed and tested by attaching it in-line to a laboratory model single-screw extruder. A tube (circular cross-section) rheometer to measure η and the uniaxial extensional viscosity (ηu) simultaneously was also designed and tested. Two commercial grades of LDPE (low density polyethylene) with melt index values of 6 and 12 were used as test materials for the study. Exit and hole pressure methods were used to estimate N1, and the entrance pressure drop method using the analyses of Cogswell, Binding, and Gibson (the last analysis used with the axisymmetric case only) was used to estimate ηE.

In-line measurement of rheological properties of polymer melts

Results on some physical properties and on melt processing of a starch-based polymer under steady-state shearing are presented. A peculiar microstructure involving a strong pseudoplastic behavior at high shear rates as well as yield stress at lower ones is discussed. A model is proposed to explain the characteristic viscoelastic behavior of this material based on hydrophylic and hydrophobic interactions between starch and vinyl-alcohol copolymers.

Microstructure and melt flow behavior of a starch-based polymer

The effect of surfaces on the rheology and processing of polymers is a newly developing area of research. In this work, a stainless steel slit die was used with a variety of inserts of other metals, at the exit. The viscosity measured with the slit die was well predicted by the generalized Maxwell model containing a damping function with parameters determined from separate shear experiments. The viscosity was not affected by different metal type at the die exit. However, there was a marked increase in exit pressures for flow over brass inserts at the die exit. This effect coincided with the production of a porous, copper‐rich brass surface and it was shown that this was the result of surface dezincification. This was postulated to cause increased adhesion between the LLDPE and the brass via physical interlocking at the porous surface which subsequently increased the exit pressure.

The effect of metals on the processing of LLDPE through a slit die

A slit die apparatus is used to measure exit pressures for five different polymer melts. Viscosity data obtained from the same apparatus agree well with values obtained from a cone-and-plate rheometer or a capillary rheometer. Except for a PVC sample where thermal degradation was found to occur, the exit pressures obtained by linear extrapolation of the measured pressure profiles are all positive, and increase with increasing shear stress. The values of the first normal stress difference calculated according to the exit pressure theory are of the right order of magnitude and in some cases correlate satisfactorily with values measured in a cone-and-plate rheometer. However, the high sensitivity of the exit pressure values to the method of extrapolation and the wild scatter of exit pressure data for some materials make it difficult to use the exit pressure method as a routine procedure for accurate determination of the first normal stress difference.

T. W. Chan

Papers

An experimental study of exit pressures for polymer melts