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The melt viscosity of thermoplastic starch has been investigated as a function of temperature, moisture content, and processing conditions. The effects of various low molecular weight additives have also been studied. Starch melts exhibit power law behavior over the range of shear rates studied. Melt viscosity decreased with increasing temperature and moisture content (MC). The power law index m increased with increasing temperature. The consistency K decreased with increasing temperature and increasing moisture content. Moisture content during the pelletizing step influenced melt viscosities measured after equilibration to different MCs. All additives studied except glycerol monostearate (GMS) significantly lowered the melt viscosity of starch, some more effectively than water relative to starch with 15% MC. Starch with GMS had viscosities essentially the same as, or slightly higher than, starch/water. This behavior may be due to the presence of unmelted helical inclusion complexes of starch and GMS. Starch formulations at 160°C exhibited melt visocosities similar to an LDPE of melt index 1.8.

The elastic and loss moduli of fiue types of wheat flour doughs were measured in an eccentric rotating disc (ERD) rheometer. G’ and G”, determined from the linear portions of the response curve, were very sensitive to water content, decreasing as water content increased. Differentiation among samples was greatest at the highest water content. Results also depended on protein level, with higher moduli being observed at higher protein level. Samples mixed in a Farino‐graph to constant consistency showed differing values of both storage and loss moduli. The ERD geometry thus appears suited to characterization of doughs and to study of component interactions in such systems.

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Soy dough (defatted soy flakes), at 120° C and 30% moisture, was extruded through a series of dies of differing length (L) and radius (R). Rlots of pressure (P) were linear with the length to radius ratios (L/R) of the dies at constant output rate (Q) and R. The intercepts of the P versus (L/R) plots on the P = O axis give the end corrections (e) for capillary flow at each output rate. For soy dough, these end corrections were surprisingly large at low output rates and generally decreased rapidly with increasing output rate. This is contrary to the normal behavior of viscoelastic fluids, such as thermoplastics, where the end correction increases with increasing output rate. Both the end correction, which is a measure of the elastic properties of the soy dough, and the viscosity of the soy dough depended on the output rate as expected and also on the retention time of the material in the system.

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The use of starch as a thermoplastic material is a recent development. An understanding of the rheology of thermoplastic starch melts is needed in order to understand the effects of processing on structure/property relationships.This article discusses the effects of temperature, moisture content, molecular weight reduction (hydrolysis), and low molecular weight additives on the behavior of thermoplastic starch melts. Thermoplastic starch melts exhibit power law behavior. The melt viscosity decreases with increasing temperature, moisture content, and decreasing molecular weight. Low molecular weight additives also reduce the viscosity. Glycerol monostearate slightly increases the melt viscosity. This effect is attributed to the formation of helical inclusion complexes which are stable at the extrusion temperatures. The power law index and the rate of change in viscosity with temperature of thermoplastic starch melts are similar to those of synthetic polymers.

The orthogonal rheometer in principle provides a relatively simple means of applying a dynamic strain to polymers to determine the dynamic storage and loss moduli (G′, G″) as functions of frequency and strain. In applying this technique to soy doughs, surface evaporation of water from the samples was found to be a serious problem, but when evaporation was controlled, good reproducibility for G′ and G″, was obtained. The moduli were very sensitive to dough moisture level, with G’ changing by a factor of 10 over the moisture range 40 to 50%. Plate diameter and percent strain both affected the absolute values of G′ and G″, though the effects were relatively small. Log G′ and log G″ increased linearly with decreasing nitrogen solubility index. The two major advantages of the orthogonal rheometer for such food systems are the ease of operation and the fact that sample thickness can be chosen to obviate problems associated with sample heterogeneity. Nevertheless, the effects of plate diameter and percent strain need more extensive study.