Comparison of Different Methods of Crosslinking Sodium Caseinate Films

MARIANA PEREDA; NORMA MARCOVICH; MIRTA I. ARANGUREN

Siegen, Germany

Congreso; POLYCHAR 18 World Forum for Advanced Materials; 2010

The goal of this work was to investigate different methods to crosslink sodium caseinate (NaCas) films, by analyzing and comparing the final properties of the films (potentially useful as food packaging): mechanical, water vapor sorption and permeability. The crosslinking chosen methods were: a) use of glutaraldehyde (GTA) as chemical crosslinker, b) thermal treatment, c) complexation with a polysaccharide (chitosan, Chit). The films were obtained by a casting/solvent evaporation method from dilute aqueous solutions, using glycerol (28 wt%) as plasticizer. The first method (a) was followed by measuring the total soluble material (TSM), which indicated that the level of crosslinking increases as glutaraldehyde content increases. Visual inspection of the films showed a more intense yellow coloration as glutaraldehyde content was increased, due to the crosslinking reaction, which proceeded via Schiff-base formation.1 Tensile and thermal degradation (TGA) properties, as well as water vapor permeability, were measured to characterize the films. Thermal gravimetric analysis showed an increase of about 15 ºC in the initial degradation temperature due to GTA crosslinking, indicating improved thermal stability of the films. Final properties showed not much improvement at concentrations below 10 wt.% with respect to those of the uncrosslinked film. This was explained as the result of the disruption of the strong H-bonding interactions present in neat caseinate films, which was barely compensated by the introduction of low amounts of covalent SC-GTA bonds. The thermal treatment (b) was much more successful with the added advantage of not incorporating potentially toxic crosslinkers. Samples were prepared by the usual casting method and thermally treated at constant different temperatures (95-105°C) for different amount of time (0-24 hours), to investigate the effect of heating the caseinate films. Disulfide bridges formed between the protein chains were responsible of crosslinking. The properties were largely improved; by heating at 95 °C during 6 hours: total soluble mass was reduced by ~ 40wt.%, tensile modulus more than double the value for the unheated sample (251 to 525 MPa), with a slight decrease in the elongation at break.  Although there was just a small decrease in the equilibrium moisture content, the permeability dropped to 7.79± 1.25 1010 g m/(Pa s m2), a value almost 30% lower than the corresponding to the untreated film (measured at 100:64.5% relative humidity, 24°C). The use of chitosan to form a polyelectrolyte complex (method c) was also quite successful.  Chitosan is edible and also has biocide properties, which make it an interesting crosslinker, although the pH conditions had to be carefully selected to allow for the polysaccharide dissolution and to avoid protein coagulation. The co-dissolution was done at pH =5.  Tensile and impact testing showed that the complex films presented a synergistic effect, so that tensile modulus and impact energy increased with respect to the control caseinate films. Moreover, the equilibrium moisture content was lower that any of the one-component films.