Specific surface area and porosity of starch extrudates determined from nitrogen adsorption data

Extrudate structure produced from potato starch at various operational parameters was examined using low-temperature adsorption of nitrogen. Adsorption isotherms, capacity of the monolayer, and specific surface area were calculated based on the BET equation, while the BJH equation was used to determine the surface, volume and average diameter of the pores. Sorption isotherm patterns of the source and extrudates were identical and could be categorized as type II in the IUPAC classification. Native potato starch adsorbed more nitrogen in the range of relative pressure, p/p0 = 0–0.40 than the extrudates. Extrusion decreased specific area, SBET, of the products independently of the applied extruder type and extrusion regime. SBET varied between 0.243 and 0.265 m2/g. Only in two samples, SBET was higher by 0.020 m2/g slightly exceeding that of the source (0.281 m2/g). Similarly, specific SBJH pore area varied. The average diameter of the pores in extrudates was more uniform at a lower velocity of the extruder screw. Based on the BJH equation, index of the radial expansion correlated against SBET and cumulative surface of pores (SBJH), volume mesopores (VBJH) and average diameter of mesopores (DBJH). Highest expansion providing best development of the specific surface area, volume, and average diameter of the pores, max ƒ(EX)BJH, reached 4.41. In this paper, for the first time, the structure of extrudates was identified using low-temperature nitrogen adsorption from the gas phase.