Chitosan card paper system (CH-Sys) preparation
Film suspensions were prepared by dispersing chitosan (CH) in aqueous acetic acid according to Yoshida et al. (2009). The stoichiometric amount of acetic acid was calculated from sample weight, taking into account the value of DA to achieve the protonation of all the NH2 sites (Notin et al., 2006). Anthocyanin (ATH) was added to the filmogenic suspensions and homogenized by magnetic stirring at room temperature (25 ± 1 °C) for 45 min. Different concentrations of CH and ATH were prepared to apply as coating on card paper (Table 1). The suspensions were spread on the card paper surface (0.045 m2) using an 80 μm wire bar coater (TKB Erichsen, Brazil) forming the CH-Sys. The coated paper sheets were dried in an oven at T = 150 °C for 90 s. In this drying process the purple colour characteristics of ATH was maintained, which at higher ATH concentration was not possible note the colour variation.