Extrusion behaviour of different soya isolates and the effect of particle size

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Abstract
Eleven different commerically produced soya isolates, and one defatted soya flour, were extruded at temperatures of 120 and 180 °C at a moisture content of 35% on a dry solids basis (dsb), using a Brabender laboratory extruder. The pressure, torque and flow rate during extrusion, and product expansion and texture of the final extrudate following rehydration varied considerably between different soya isolate preparations. Analysis by multiple regression yielded about twenty significant (P < 0.05) correlations between the three extrusion variables (pressure, torque and flow rate), the four feed variables (bulk density, solubility in water, particle size distribution and angle of slope) and the three extrudate characteristics (resistance to shear, solubility in water and product diameter) measured. It was concluded that the best extrusion behaviour was obtained with feed containing insoluble but native protein. Fine powders containing insoluble protein were less prone to agglomeration, and, hence, were easier to feed than powders containing high levels of soluble protein. In addition, higher shear force values after retorting were obtained with extruded products prepared from feed containing insoluble, but native protein. This was interpreted in terms of the formation during extrusion of covalent disulphide linkage between initially insoluble regions of protein. The wide variation in extrusion behaviour of chemically similar materials prepared in different ways suggests that extrusion studies on single materials should be interpreted with caution.

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