Selection of process conditions for the integrated process of supercritical CO2 extraction from green tea leaves and impregnation onto polymer films

Green tea is considered to be a plant with a variety of health benefits due to its antioxidative, antimicrobial, anti-inflammatory, and anticancer activity. Besides the most important bioactive constituents, including polyphenols, especially catechins, and flavonoids, green tea is rich in caffeine, and such complex composition makes its extract suitable as a component of active food packaging or skin care systems [1,2]. The technologies commonly proposed to obtain such materials involve some limitations, mainly related to the use of liquid organic solvents that lead to low penetration of the active compound into the polymeric platform and, consequently, limited loading efficiencies, non-unique dispersion, and solvent residue in the final material. An excellent technique for direct loading of natural bioactive compounds from plant materials into polymeric carriers that can overcome the aforementioned challenges is an integrated process accomplished by merging supercritical fluid extraction (SFE) and supercritical solvent impregnation (SSI). Therefore, in this study, the preparation of two types of polymer films and their functionalization with green tea leaf extract using the SFESSI process was tested. Starch/chitosan (SC) films were prepared by mixing the 1% chitosan solution with the 1% starch solution in a mass ratio of 1:1, while starch/chitosan/β-cyclodextrin (SCCD) films were prepared by addition of 1% β- cyclodextrin to starch and chitosan to obtain the ratio of 1:1:1. After solvent casting and drying at room temperature, the films were additionally dried in an oven overnight at 35 °C. Initially, supercritical CO2 (scCO2) extraction was investigated as a method for isolating the extract from green tea at temperatures of 35 and 70 °C and pressure of 30 MPa, with and without the ethanol as a co-solvent. The obtained extraction yields ranged from 0.91 to 4.22%, being the highest at a higher temperature. The chemical composition of the obtained extracts was determined, as well as the total phenolic content (TPC), total flavonoid content (TFC), total chlorophyll, and total carotenoid content. HPLC analysis revealed that caffeine was the major constituent of all extracts with the amount ranging from 24.1 to 202.9 mg/g. The IC50 values determined using the DPPH assay showed that the extract obtained at 35 °C with the addition of ethanol exhibited the strongest antioxidant activity. After determining the SFE conditions that gave the green tea extract with high content of bioactive compounds, prepared films were impregnated using the integrated SFE-SSI process for 4 h at the same conditions as scCO2 extraction. The loading of green tea extracts onto films was up to 7.7% showing that lower temperature (35 °C) and the addition of ethanol had a favourable influence. The presence of the extract on the surface of impregnated samples was confirmed by FTIR analysis