Irradiation dose and calcium effect on the mechanical properties of cross-linked caseinate films

The effect of irradiation dose on the formation of cross-links in calcium caseinate solutions was investigated. The formation of bityrosine was found to increase with the irradiation dose for the basic formulation containing calcium caseinate, carboxymethylcellulose, and glycerol. Addition of CaCl2 to the basic formulation increased the fluorescence signal of bityrosine for all irradiation doses suggesting that CaCl2 can have an indirect effect on the formation of bityrosine by shortening molecular distances between polypeptides, thus making easier the formation of bityrosine. Furthermore, gels were formed for solutions containing CaCl2 at irradiation doses greater than or equal to 16 kGy. The formation of gels was found to be accountable for the increased mechanical strength in the corresponding films. A maximum gel strength was obtained at an irradiation dose of 64 kGy for all formulations, suggesting a maximum cross-linking density. Such a feature was also noted when poly(ethylene glycol), mannitol, and sorbitol were added to the formulations. Although bityrosine cross-links increase with irradiation dose, some damage can occur in another part of the protein structure, accounting for the overall loss of puncture strength at doses >64 kGy.