4.7. Physical Characterization of the Extracts

The extraction yield was expressed as the dry matter content of the selected A. vera extracts (the samples obtained under the optimal extraction conditions in every employed extraction technique for achieving the highest polyphenol content) and calculated using the following equation:

where a represents the weight of the vessel containing the extract before drying (g), b represents the weight of the vessel containing the extract after drying at 105 °C to constant mass, and m represents the weight of the extract. Dry matter content is expressed as %.

The zeta potential and conductivity of the selected A. vera extracts (the same samples as in the case of the extraction yield) were determined using photon correlation spectroscopy in Zetasizer Nano Series, Nano ZS (Malvern Instruments Ltd., Malvern, UK). Each extract was measured three times at 25 °C.

The density and surface tension of the selected A. vera extracts (the same samples as in the case of the extraction yield) were determined using silicon crystal as the immersion body and Wilhelmy plate, respectively, in Force Tensiometer K20 (Kruss, Hamburg, Germany). Each extract (20 mL) was examined three times at 25 °C.

The viscosity of the selected A. vera extracts (the same samples as in the case of the extraction yield) was examined using Rotavisc lo-vi device equipment with VOL-C-RTD chamber, VOLS-1 adapter, and spindle (IKA, Königswinter, Germany). Each extract (6.7 mL) was examined three times at 25 °C.