2.4.4. Water Absorption Index, Water Solubility Index, and Swelling Index

The water solubility index (WSI) and water absorption index (WAI) were determined by the method of Singh and Smith [20]. The extrudates were first milled to a mean particle size of 180–250 µm. A 2.5 g sample was dispersed in 25 g of distilled water, using a rod to manually break up any lumps. After stirring for 30 min using a magnetic stirrer, the dispersions were rinsed into tared 50 mL centrifuge tubes, made up to 32.5 g and centrifuged at 3000× g for 10 min. The supernatant was decanted for determination of its dissolved solids content, and the sediment was weighed. WAI and WSI were calculated according to Uribe-Wandurraga et al. [21] and Equations (2) and (3).

The swelling index (SWE) was measured using the bed volume technique. Samples were accurately weighed (1 g) and transferred to a calibrated cylinder and 10 mL of distilled water was added. Cylinders were left to stand undisturbed for 18 h at ambient temperature. The bed volume was recorded and expressed as mm of swollen sample per g of dry initial sample [22].