Water vapour barrier and tensile properties of composite caseinate-pullulan films: Biopolymer composition effects and impact of beeswax lamination

The water sorption, water barrier properties and mechanical behaviour of pullulan (P) and sodium caseinate (SC), as well as their blend and bilayer films plasticized with sorbitol (25% dry basis), were investigated as a function of weight polymer ratio, water content and beeswax lamination. Very similar moisture sorption isotherms were obtained for blend and bilayer films with P/SC weight ratio of 1/3 and 3/1. Neither the type of film (blend or bilayer) nor the different P/SC ratio affected significantly (P > 0.05) the water vapour permeability (WVP) of the films. A mixture-process variable experimental design was applied to evaluate the effect of the proportion of the two polymers in relation with the relative humidity (RH, 53% and 75%) on the mechanical properties of the films. Increasing the P/SC ratio decreased the Young’s modulus (E), the tensile strength (σ_max) and increased the % elongation at break (% EB), suggesting that P imparts flexibility and SC stiffness to the composite films. With moisture content increase from 5% to 8% most of the films exhibited an increase in E and σ_max, whereas a sharp decline in both parameters and an increase in % EB were observed above this moisture level. The brittle to ductile transition of P coincided with its glass to rubber transition, whereas SC exhibited a ductile behaviour within the glassy state. The tensile characteristics of bilayer films at moisture content greater than 8% were dominated by the component present in higher proportion, while films made with the biopolymer blends showed mechanical behaviour closer to that of plain P films. Beeswax lamination of plain, bilayer and blend films resulted in a drastic decrease in water vapour permeance, whereas its effect on E and σ_max and in % EB was related to the mechanical properties of the hydrocolloid layers used and varied according to the moisture content of the films.