Development of starch/gelatin blends added with turmeric extract and cross-linked with citric acid

Gelatin and starch are two polymers widely applied in the development of biodegradable films that can act as food packaging. Turmeric (Curcuma longa L.) is a rhizome that has curcuminoids, the main one being curcumin, an aromatic polyphenolic compound with antioxidant and antimicrobial properties. In this work, it were produced gelatin/cassava starch blends G/A (100/0, 70/30, 50/50, 30/70, 0/100) added with turmeric extract (EC) and cross-linked with citric acid were produced and characterized in order to evaluate their effects on the mechanical, functional, antioxidant and antimicrobial properties of the films. The effect of the G/A ratio in the blends (100/0, 70/30, 50/50, 30/70, 0/100) on the properties of the films was also evaluated. The films were prepared using the casting method using glycerol as plasticizer. The concentration of curcuminoids was performed by HPLC-DAD in reverse phase with mobile phase of 45% acetonitrile (ACN): 55% aqueous solution 1% acetic acid gradient: 0 – 12 min 45% ACN; 32 min 45% - 100% ACN; 32 – 40 min 100% ACN. The mechanical properties were measured by perforation test using a TA Instrument texturometer (TX Plus model). Thickness were measured with an electronic digital micrometer (ZAAS Precision) and the mass of a predetermined area. Moisture was determined by a gravimetric method (105°C, 24 hours), surface wettability by contact angle (OCA- 20 Dataphysiscs equipament) and microstructure by Scanning Electron Microscope (ZEISS, EVO-50). The antioxidant activity by DPPH• were also performed. Antimicrobial activity was measured by the halo test for E. Coli, P. Aeruginosa, S. Aureus, S. Uberis and S. Agalactiae for G/A 70/30 blends: control, crosslinked (R), non-crosslinked 10% EC (10EC) and crosslinked 10% EC (R EC10). Inoculation of the media at 37ºC for 24h and then addition of film discs for incubation under the same conditions. Films with a higher proportion of gelatin presented higher values of tensile strength and elongation at break, while a high proportion of starch provided lower solubility and permeability to water vapor. The addition of EC and its concentration affected the mechanical and functional properties of the blends. Curcumin inhibits the FtsZ protein and thereby inhibits cell
division, stopping cell growth. The blend G/A controls did not show antimicrobial activity, while the blend R EC10 acted against S. agalactiae, S. uberise and S. aureus. Addition of 10% EC improved the antioxidant activity by 2.5% (G/W films) to values around 86.97% for all analyzed G/W ratios. Crosslinking improved the interaction of gelatin and starch in the film and the dispersion of high EC concentration (10%) within the blend matrix, including those with higher starch concentration. This allowed to obtain films that are less permeable to water vapor, less wettable in water, but more elongable than the non-crosslinked films without addition of EC. Since the bioactive properties of films were evidenced, we suppose that there is a synergistic effect between citric acid and curcumin in the film.