Extrusion cooking of wheat starch

PhD Student: Gena Nashed
Supervisors: Dr. Rulande Rutgers and Dr. Peter Sopade

Starch is one of the most abundant constituents of the human diet as it is widely used in food processing. Gelatinised starches have numerous industrial, non food uses such as in drilling oil wells, making paper and in water-base paints (Chiang and Johnson 1977) . In food, gelatinised starches are typically used as thickening agents. The gelatinisation of starch has a significant effect on the characteristics and quality of food such as texture, elasticity and softness of paste products, digestibility and stability of bread and cakes (Chiang and Johnson 1977) . There are many food processes that can gelatinise starch and one notable one is extrusion cooking.

Extrusion cooking has become one of the most popular new processes developed by the food and feed industries. Its basic principle is to convert a solid material to a fluid state by applying moisture and shear, then to extrude the material through a die to form a product of predetermined geometric and physical characteristics. Depending on the process parameters, directly expanded or non-expanded products can be obtained. This project investigates the effect of extrusion parameters on the gelatinisation of wheat starch during extrusion. Effects of moisture content, pressure, temperature, shear rate and residence time will be investigated. The effect of these parmeters on the structural and rheological properties of wheat starch extrudate will help understand the various physical transitions that occur.
Experiments will be carried out on a single screw and twin screw extruder. The properties of the final extrudates will be compared to help determine the effect of different screw configurations on starch extrusion.

Onset temperature for starch:water:glycerol mixtures as a function of effective moisture content of the starch phase in the mixture, corrected for the higher rate of moisture uptake by glycerol, compared to To for starch:water mixtures published by Hoseney [27].(From Nashed et al., Submitted to Starch/Stärke, 2002).