Engineering Properties of Edible Transglutaminase Cross-Linked Caseinate-Based Films

In this work, the effect of the weight ratio (Y _E/C) between transglutaminase (TGase) and sodium caseinate on the main engineering properties and microstructure of maltodextrin/caseinate/glycerol-based films, as well as cross-linking kinetics of the corresponding film-forming mixtures, was assessed. In the absence of TGase, the film forming solution did not jelly at all even after a 20-h gel cure experiment at 30 °C. As Y _E/C was increased from 0.5 to 8.3 mg/g, the gel point time exhibited quite a hyperbolic dependence on Y _E/C, whereas the pseudo-first order reversible kinetic constant rate of casein reticulation linearly increased with the enzyme concentration. Despite a certain data scattering, the initial and equilibrium complex shear moduli appeared to be practically constant and independent of Y _E/C. This finding was also extended to the mechanical and water barrier properties of the films under study. This was also indirectly confirmed by scanning electron microscopy, the film structure being quite compact, dense, and about free of air bubbles. In particular, their complex tensile modulus (E*) resulted to be not only independent of the cross-linking agent dosage but also interconvertible with the elastic modulus (E) determined in the quasi-static mode at an initial strain rate equal to the angular frequency. The water vapor permeability (WVP) for all the films examined was found to be extremely dependent on the water, desiccant, or Permatran-W standard methods used as well as on the difference in water activity (a _W) at the inside (a _Wi = 1.0 or 0) and outside (a _Woi = 0 or ∼0.5) of the test cup. By maintaining a _Wi = 0 and a _Woi = 0.5, the Permatran-W method yielded practically constant WVP values (30 ± 4 pg m⁻¹ s⁻¹ Pa⁻¹) whatever the enzyme/caseinate ratio used. Nevertheless, TGase appeared to decrease the water vapor permeability of not cross-linked maltodextrin–caseinate films.