Films made of sodium caseinate and gelatine were prepared by casting method from a water solution containing glycerol as a plasticizer to obtain environmentally friendly and fully biodegradable materials for agricultural and packaging applications. Additionally, enzymatic protein cross-linking by microbial transglutaminase was applied. Potassium nitrate (KNO3) was used as additive to investigate the influence of crystallization on the physical properties of protein films. Mechanical properties (tensile strength and elongation at break) of the protein films were determined versus ratio of protein to potassium nitrate in the presence and absence of microbial transglutaminase. Furthermore, optical properties (film formation and consistency, surface texture, crystal size, shape, and distribution of incorporated potassium nitrate) were examined. An increase of 122% and 177% in the elongation of the films was adopted due to the crystallization of KNO3 in enzymatic-modified sodium caseinate and gelatine films, respectively. Pure sodium caseinate films distinguished ultimate tensile strengths of 4.95 MPa, while MTG-treated gelatine films achieved ultimate tensile strengths of 13.52 MPa. Altogether, the most appropriate overall mechanical performance was obtained for KNO3/protein ratios of 1:6 in enzymatic-modified films. Furthermore, an increasing content of crystalline KNO3 results in increasing thickness, rough surfaces, decreased opacity, and whitish coloring of the films. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.