Resumen de Incorporation of active components in biopolymer-based films for food use
In the present Doctoral Thesis, different strategies have been used to incorporate non-volatile (polyphenols from thyme extract) and a volatile active compound (eugenol) into biodegradable films prepared with starch (S) of different sources or chitosan (CH) or with mixtures of CH:S. The functional properties and release kinetics of casted films prepared with pea starch or CH or CH:S blends were evaluated as affected by the incorporation of polyphenols from an aqueous thyme extract (TE) and tannic acid (TA), a polyphenol which was used as a cross-linking agent. Moreover, the functional properties and release kinetics of corn starch films, obtained by casting or by thermo-compression moulding, were evaluated as a function of the addition of eugenol (E) in free form or pre-encapsulated with different wall materials. In order to encapsulate E, whey protein isolate (WP) or soy lecithin (LE), as wall material, and maltodextrin (MD), as drying coadjuvant, were selected. Microencapsulate powders were obtained by spray-drying. The effect of the addition of oleic acid (OA), as eugenol carrier, and CH, as a potential capsule stabilizer, was also analysed. The different formulations were characterized before and after drying, in terms of encapsulation efficiency, thermal stability, release kinetics and antioxidant capacity and antimicrobial activity. Results showed that TE provided pea starch and chitosan films with antioxidant activity. Polyphenols (TE and TA) interacted with CH chains and acted as cross-linkers, thus improving the tensile behaviour of films and reducing the release rate and the amount of polyphenol released from the films in water and ethanol aqueous solutions. The opposite effect was observed when TE was incorporated into the pure S matrix. All the films became darker, more reddish and less transparent when TE was incorporated, and this effect was more marked in pure S matrices, which suggests that the TE compounds were poorly encapsulated. Thus, S:TE films showed the fastest delivery rate and the highest delivery ratio of TE. A good encapsulation efficiency (EE) of E was observed in the CH-free powders (87-98%). However, the use of CH provoked a marked EE decrease in both WP and LE powders (22% and 46%, respectively). The formulations exhibited similar E release behaviour in food simulants of different polarity, where practically the total E content was delivered at a similar rate. The antibacterial effect of CH-free powders against E. coli was also coherent with their eugenol content. An additional positive effect of OA was detected in terms of the antilisterial action of this powder. The process of thermo-compression moulding of the films led to important E losses (80-65%), which were minimised when using OA in the microcapsules (EOA-WP or EOA-LE). In the films containing non-encapsulated E, the retention of E was promoted due to the formation of E-starch complexes. The presence of microencapsulated eugenol increased the presence of discontinuities in the polymeric matrix, thus yielding films that were mechanically less resistant and stretchable, except for E-LE-S films, which became more resistant to break. The barrier properties were improved when adding microencapsulates to the films, except for OA-based microcapsules. Eugenol release from the films was affected by microcapsule, their composition and the food simulant. In starch film obtained by casting, the addition of microencapsulated eugenol powder in starch film-forming dispersions modified film microstructure, yielding less resistant and less elastic films with reduced moisture content, transparency and oxygen permeability as compared to casted films formulated with non-encapsulated eugenol. The addition of eugenol microencapsulated with OA promoted the preservation of the antioxidant activity of the films, especially in less polar food simulants. S-EOA-LE films were effective in preventing sunflower oil oxidation during 53 days of storage at 30¿C.
