*2.1. Materials*

240 bloom type A fish gelatin (FG), with an average molar mass of 125–250 kDa and a lysine content of 2.62%, was obtained from Healan Ingredients, North Newbald, UK, 280 Bloom type A gelatin from porcine skin (PG) was obtained from Italgelatine SpA, Via Statale, Italy. Anhydrous citric acid was provided by Panreac, Barcelona, Spain, sodium hydroxide, hydrochloric acid, 2,4,6-trinitrobenzenesulfonic acid (TNBS) and diethyl ether were purchased from Sigma-Aldrich, Milano, Italy. All chemicals were used as received without further purification.
