Structure characterization by means of rheological and NMR experiments as a first necessary approach to study the l-(+)-ascorbic acid diffusion from pectin and pectin/alginate films to agar hydrogels that mimic food materials

Pectin (P) and pectin-alginate (PAL) edible films, developed for antioxidant preservation, were placed on agar cylinders, mimicking food materials, in order to understand the release of l-(+)-ascorbic acid (AA) from the films. To improve the release properties of polymeric systems, it is crucial to describe and understand the macro- and microscopic properties of the matrices. Rheological studies performed within linear and non linear frames permitted to select, among different polymer concentrations (0.50-2.00% w/w), a 2.00% w/w agar gel as food model as this system shows the higher pure elastic contribution. Rheological and Low Field NMR (LFNMR) tests performed on 0.50-2.00% w/w agar gels as well as on P- and PAL-films after exposure (up to 6 h) to 2.00%-agar gels, showed that in spite of the higher glycerol (plasticizer) content, P-network is characterized by more numerous calcium-junction zones than PAL-matrix. The determined average network mesh size (ξ¯) for both of P- and PAL-films did not significantly change during 6 h of contact with 2.00%-agar gel. However, due to a higher swelling degree, PAL-film leads to higher ξ¯ value and water mobility inside the polymeric network. These results are of paramount importance as “ξ¯” is the main parameter affecting the release kinetics of AA from film networks to agar gels and also the diffusion of AA into the agar gel or food.