Gilles Fayard

Glass transition of extruded wheat, corn and rice starch

Commercial wheat, corn and rice starch were extrusion cooked under a specific mechanical energy input (SME) ranging from 81 to 365 Wh/kg (288 to 1314 kJ/kg). Extrusion cooking at low and high SME resulted in products having significant differences in molecular weight distribution and having crystalline structures of the V- and E-type, as determined by gel permeation chromatography and X-ray diffraction analysis. Differential scanning calorimetry revealed that the glass transition temperature (T g ) of the extruded starches was independent of the botanical source, the degree of extrusion-induced molecular fragmentation and the formation of the V- and E-type crystalline structures. The obtained master curve, defined by the relationship between water content and T g of the amorphous starch, may be used as a predictive tool in modelling the extrusion process of starch or starch containing blends, especially with regard to the formation of the morphological structure and texture attributes of directly expanded products.

Description of Extrudate Characteristics in Relation to the Shear Stress of Plasticised Starches Determined In-line

The behaviour of wheat, corn and rice starch as well as of mixtures thereof was investigated with the aid of the shear stress of material plasticised under extrusion cooking conditions. The shear stress was determined in a laboratory extruder using an in-line viscometer. The extrusion experiments were performed using a combination of a fractionated factorial design and mixture experiments. It was shown for each starch and mixture of starches that a functional dependency existed between the extrusion process parameters and the shear stress of the plasticised materials. A second relationship was found between the shear stress of the plasticised materials and the product characteristics of the extrudates. With respect to the extrusion cooking of various types of starch, it could thus been demonstrated that it is possible to predict the development of product characteristics by determining the shear stress of plasticised materials in-line. Furthermore, regression analysis showed that the levels of the test ranges for the relationship between the shear stress and the process parameters are dependent on the starch or starch mixtures used. Molecular degradation of the starch polymers of the different types of starch and their mixtures resulted in extrudates with different functional properties. This also applied to extrudates with the same mean molecular weight. Extrudates made of wheat starch had a much higher cold paste viscosity than extruded rice or corn starches. Extrudates made of corn starch and those made of mixtures of rice, corn and wheat starch had a higher cold water solubility and a lower sediment volume than those made of wheat or rice starch.

Influence of starch, gluten proteins and extraction rate on bread and pasta quality

Abstract Wholemeal products and white products of bread and pasta were made of the same material consisting of a set of bread wheat varieties. The protein content varied little, whereas the protein quality varied significantly among the varieties. The composition of the high molecular weight glutenin subunits and a set of gliadins could explain most of the variations in Zeleny sedimentation volume, farinogram data and bread quality. Total amylose content in white flour varied significantly among the wheat varieties. All varieties with good protein quality gave good bread quality. Among those with poor protein quality, one had higher and one had lower total amylose content than average, and both gave poor baking results. Small differences in gelatinization temperature and gelatinization enthalpy were observed among the wheat varieties. The gelatinization temperature was slightly higher in wholemeal flour than in white flour, whereas the gelatinization enthalpy was lower in the former. White flour from bread wheat gave pasta of unacceptable quality, whereas the quality of pasta made from wholemeal flour was close to that of commercial pasta made from wholemeal durum wheat. The poor pasta quality of the white bread flours may be attributed to the fine granulometry of the flours. Variations in pasta quality and bread quality between different varieties were not correlated. T-8020 and Bastian with the same protein quality characteristics and the same bread quality gave pasta of different qualities.

Application of an In-line Viscometer to Determine the Shear Stress of Plasticised Wheat Starch

An in-line viscometer was designed and constructed to enable determination of the shear stress of plasticised wheat starch during extrusion cooking. The viscometer was installed between the end of the barrel section and the extruder die plate so that the shear stress could be determined for the plasticised material, irrespective of the geometrical shape into which it was subsequently moulded by the extruder die. The extrusion conditions were described in terms of the process parameters, i. e. water content, barrel temperature, screw speed and screw configuration; and of the system parameters, which were the specific mechanical energy input (SME), product temperature (PT) and mean residence time (MRT). The parameters were measured and the results evaluated using statistical methods. Regression equations were used to describe functional relationships between the shear stress and the extrusion conditions on the one hand, and between the shear stress and the product characteristics of the extrudates on the other. The shear stress of plasticised wheat starch determined in-line can be used to predict the morphological structure (volumetric expansion) and the functional properties (cold paste viscosity and cold water solubility) of extruded, directly expanded starch with a high degree of accuracy. The measurement technique used and the results of the extrusion tests undertaken for this project will therefore enable the shear stress of plasticised material in an extruder to be used as the principle parameter for controlling extruders on-line.