# Chitosan Films with Caffeine and Propolis as Promising and Ecofriendly Packaging Materials
## 3 Results and Discussion
### Sorption Experiments  
The experimental EMC data and fitted adsorption isotherms of the examined film samples are presented in Figure 2. The adsorption isotherm for these chitosan-based films demonstrated a minor increase in EMC below RH 0.50, followed by a notable surge in EMC in the range of RH exceeding 0.50. This occurrence has been documented in prior studies [67; 68; 69; 70]. The results showed a positive effect of caffeine and caffeine-propolis on reducing the hygroscopicity of chitosan films. The characteristic feature of tested chitosan-based created using the studied additives is a negligibly low EMC (less than 0.01 kg/kg) in the RH range below 0.35. A similar effect of lowering EMC in the described RH range was observed in earlier studies of films created using chitosan and kombucha solutions derived from coffee as well as three different types of tea [71]. The EMC of pure chitosan film (sample A) produced with acetic acid is clearly higher in the whole RH range. In turn, the application of citric acid as a solvent in the production process of crab chitosan-based films (pure chitosan) results in a greater reduction in their hygroscopic properties than in the case of acetic acid. When exposed to air at a temperature of 25 \({}^{\circ}\)C RH below 0.80, the maximum value of equilibrium moisture content (EMC) of the tested samples of the C-type film did not exceed 0.292 kg/kg. Under analogous conditions, the observed EMC for samples of A film was almost 2-fold greater (0.546 kg/kg). The results of the sorption experiment shown in Figure 2 confirmed that the hygroscopicity of the tested film samples based on crab chitosan may be significantly reduced by the addition of CAF and CAF with EP. The proposed modification variants for chitosan-based films, i.e., an addition of CAF and CAF with EP, cause a further decrease in EMC within the RH range below 0.80. An advantageous effect of EP on the reduction in hygroscopicity of film based on crab chitosan confirmed the results of earlier studies [26]. Figure 2: Adsorption isotherms of chitosan-based film samples including caffeine and caffeine-propolis solution (chitosan dissolved in acetic acid—left plot; chitosan dissolved in citric acid—right plot; dots represent experimental data; solid lines—fitting with the GDW sorption model, temperature of 25 \({}^{\circ}\)C). The results of the conducted sorption experiments confirmed that a greater range of EMC reduction is found for film samples produced using acetic acid (A-type samples). The application of an addition of caffeine or caffeine-propolis in the case of samples obtained using citric acid (C type samples) causes only a slight reduction in the hygroscopicity of the produced chitosan films. The estimated coefficients of the GDW model for the investigated chitosan-based films, taking into account the caffeine and caffeine with propolis extract used, are presented in Table 4.  
Generally, the estimated parameter \(m\) (monolayer water content) is lower than 1 for all the investigated chitosan-based films since the water molecules absorbed on primary sites do not undergo a complete transformation into secondary sorption sites. Moreover, the use of caffeine and caffeine with propolis extract as additives to produce a chitosan film affected the monolayer water content. Nevertheless, while in samples of film A and C (pure chitosan) samples, the value of the described parameter varies slightly (i.e., 0.5609 vs. 0.5419, respectively), in the case of chitosan-based films produced with the addition of acetic acid it was significantly reduced. As a result of the use of caffeine and caffeine with propolis extract, the accessibility of the primary sorption sites is limited. The value of \(m\) for the caffeine (AC) and caffeine-propolis (ACP) variants, i.e., 0.1216 and 0.0984, represents, respectively, 21.7 and 17.5% of the corresponding value determined for the chitosan film produced with acetic acid (0.5609).