The porcine skins were soaked in 0.2% (w/v) sodium hydroxide (NaOH) for 40 minutes. After washing out sodium hydroxide (NaOH), two acid incubations were performed, each for 40 min, first in a sulphuric acid (H2SO4 0.2%, v/v) and then in a citric acid solution (1.0%, w/v). The acid solutions were drained and then samples were washed with cold water once.
The final extraction of gelatin was performed in distilled water at 45 C for 18 hours. Solubilized gelatin was separated from residual skin fragments by filtration through a Whatman No. 4 filter paper, collected and kept at -20oC for at least 24 hours. Residual water in the gelatin extract was removed by a freeze dryer.