Resumen de Starch based coatings with thyme essential oil for fruit preservation
In this Thesis, different strategies have been analysed to tailor starch formulations for the purposes of obtaining useful coatings in postharvest fruit preservation.
Starch was partially substituted by microbial gums (xanthan, gellan, and pullulan) in order to improve film functional properties. Moisture sorption capacity and water vapour and oxygen permeability were reduced by the presence of gellan gum in the starch films. It also had a positive effect on the tensile properties, enhancing the films' strength and resistance to break and preventing retrogradation phenomena. Xanthan gum increased the tensile strength of the starch films, but did not reduce their water sorption capacity and water vapour permeability. Functional properties were not notably improved by the addition of pullulan. Then, 10 and 20 % starch could be substituted by gellan gum to obtain films with more adequate properties for food packaging/coating purposes.
Starch-gellan blend films containing thyme essential oil (EO) were also studied in order to provide antifungal activity to the formulations. This was incorporated either by direct emulsification or encapsulated in lecithin liposomes. These films exhibited antifungal effect in in vitro tests against A. alternata and B. cinerea. The encapsulation of the EO in lecithin liposomes allowed for greater EO retention in the films, enhancing their antifungal activity, which were more effective against B. cinerea than against A. alternata. The antifungal action was slightly affected by the polymer matrix composition. Thus, the greatest starch ratio enhanced the fungal growth when the content of the active compound was low. All the films exhibited high oxygen barrier capacity. The presence of lecithin enhanced their water vapour barrier capacity and reduced the film stiffness, resistance to break and extensibility. The films with lecithin-encapsulated EO, with a starch-gellan ratio of 8:2, were the most effective at controlling fungal growth.
In order to apply these starch-gellan formulations as fruit coatings, the surface properties of apple, tomato and persimmon, and the spreadability coefficient of these liquid formulations on the fruit surface, were analysed as a function of the concentration of Tween 85, as surfactant. The fruit skins behaved as low-energy surfaces. The values of the contact angles and surface tension of EO-free formulations were positively influenced by the addition of Tween 85. However, in the presence of emulsified or lecithin-encapsulated thyme EO, the surfactant exerted a negative effect, depending on its concentration. Coating-forming systems containing emulsified or encapsulated EO did not require surfactant to improve their already good spreadability, while Tween 85 at 5x104 mg/L notably improved this property in EO-free formulations.
Starch-gellan coatings, containing or not emulsified or lecithin-encapsulated EO, were applied on apples and persimmons. Coatings did not reduce the weight loss in apples, but they prevented water loss in persimmons. In contrast, no significant effect of the coatings was observed on respiration rates and respiration quotient of persimmons, whereas they increased the respiration rates and quotient in apples. Coatings did not affect the changes in fruit firmness either in apples or persimmons. In the in vivo assays, the coatings without lecithin reduced the incidence and severity of black spot caused by A. alternata in persimmons, and the severity of grey mould caused by B. cinerea in apples. The incorporation of EO did not exert an additional antifungal effect on the fruit and seemed to exert a negative effect on some other fruit quality attributes. This could be explained by the particular interactions of the coating components, fruit surface and pathogen. Starch-gellan coatings without lecithin or thyme EO could be used in persimmons to control weight loss and reduce the incidence of infections caused by A. alternata.
