Porosity of starch–proteins extrudates determined from nitrogen adsorption data

• Extrudates of potato starch with selected proteins determined from nitrogen adsorption data.
• Optimum extruder screw speed and temperature profile gives product with high porosity and expansion.
• Mild temperature profile and optimum screw speed gives the high and uniformity of SBET extrudates.

Extrudates of potato starch with selected proteins (concentrate of soybean proteins, whey proteins and acidic casein) were produced at 22% raw material moisture, and at various process temperatures and extruder screw speeds. Increase of screw speed and temperature had a significant effect on changes in the specific surface area and porosity of products which were tested with the use of low-temperature sorption of nitrogen. The nitrogen adsorption isotherms plotted are of types II and III according to IUPAC, both for the raw material and for the product. Equation BJH was used to determine the surface area (SBJH), volume (VBJH) and average pore diameter (DBJH). For diameters from 35 Å to 74 Å, the calculated pore volumes assumed values within the range of 23·10−5 cm3/g −55·10−5 cm3/g. Curves of pore size distribution based on average pore diameter within the range of 19.7–23.2 Å indicate a large cumulative volume of pores.Extrusion caused a decrease in the specific surface area of the product, from 0.19 m2/g to 0.32 m2/g. A high correlation was noted between increase in porosity and expansion, and the optimum screw speed and temperature profile.

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