Film-forming Mechnism Of Polysaccharide-based Edible Films And Effects Of Water Molecules On Their Properties

In this study, pullulan and sodium alginate were used as film-forming materials. The film-forming mechanism of polysaccharide-based edible films and interaction between water and polysaccharide were investigated.Rheological properties of pullulan, sodium alginate and blend solutions were studied at 20°C, using steady shear and dynamic oscillatory measurements. The intrinsic viscosity of pure sodium alginate solution was 7.340 dl/g, which was much higher than that of pure pullulan. Pure pullulan solution showed Newtonian behavior between 0.1 to 100 s^-1 shear rate range. However, increasing sodium alginate concentration in pullulan-alginate blend solution led to a shear-thinning behavior. The effect of temperature on viscosities of all solutions was well-described by Arrhenius equation. Results from dynamical frequency sweep showed that pure sodium alginate and blend solutions at 4% (w/w) polymer concentration were viscoelastic liquid, whereas the pure pullulan exhibited Newtonian behavior. The mechanical properties of pure sodium alginate and pullulan-alginate mixture were analyzed using the generalized Maxwell model and their relaxation spectra were determined. Correlation between dynamic and steady-shear viscosity was analyzed with the empirical Cox-Merz rule.The change of conformation and chains mobility for pullulan, sodium alginate and blend film forming solutions during drying process were determined by rheometer. With the water content continuously decreasing, the entanglement network was formed by pullulan chains. However, the sodium alginate chains interacted to form physical crosslinking network structure. Moreover, the film-forming mechanism of pullulan, sodium alginate and blend films was studied by ATR-FTIR and 2D correlation spectroscopy.Pullulan-sodium alginate blend films were prepared and characterized as a function of water activity. At low a_w, the incorporation of sodium alginate into pullulan film increased the tensile strength and elastic modulus, but decreased the elongation at break of the composite films; the opposite trends were observed at elevated a_w. Above 0.43 a_w, water exerted a typical plasticization effect upon the biopolymer blends. As a_w increased from 0.23 to 0.43, an anti-plasticization effect was observed as tensile strength and elastic modulus increased. Water vapor permeability (WVP) of all samples increased with the increase of a_w from 0.23 to 0.84,the lowest WVP was detected in pure pullulan films. Moreover, the glass transition temperature (T_g) of all samples decreased substantially with the increase of a_w from 0.23 to 0.84. Within this a_w range, one transition temperature for all the specimens was observed.The stretching vibration band of O-H was investigated using attenuated total reflection Fourier transform infrared spectroscopy, and different water-polysaccharide interactions were identified from 0.23 to 0.84 a_w. At high a_w (0.84), the hydrogen bonding strength for all samples could be qualitatively assigned as very weak, weak, medium, and strong, respectively. However, three type of hydrogen bonding strength were observed below 0.84 a_w.The moisture sorption isotherms of pullulan, sodium alginate and blend films were determined using a standard gravimetric method at 25, 35 and 45°C. The thermodynamic properties were calculated through direct use of moisture isotherm by applying the Clausisus-Clapeyron equation. Net isosteric heat value for all films decreased with the increase of moisture content to approximately 0.1g/ g H₂O dry basis and then was plateau on axis of moisture content. The plots of differential enthalpy in contrast to entropy provided the isokinetic temperatures for all samples, indicating an enthalpy-controlled sorption process. Net integral entropy for all samples decreased with moisture content to a minimum value but increased thereafter.