Edible Coatings Containing Oregano Essential Oil Nanoemulsion for Improving Postharvest Quality and Shelf Life of Tomatoes

Edible coatings have attracted significant interest in maintaining quality and improving the shelf life of fresh fruit and vegetables. This study aimed to improve tomato storability by using edible coatings, based on alginate cross-linked with calcium chloride, and containing an oregano essential oil (OEO) nanoemulsion as a natural antimicrobial. The coating formulations were preliminary optimized in terms of alginate and calcium chloride concentrations, using response surface methodology, to obtain a thin (~5 µm) and uniform layer on the tomatoes surface. The optimized coating (prepared using sequential dipping in a 0.5% w/w sodium alginate solution and in a 2.0% w/w calcium chloride solution) was enriched by incorporating an OEO nanoemulsion, formulated with lecithin as a natural emulsifier, at an OEO concentration of 0.17% w/w in the alginate solution. The nanoemulsion did not significantly affect the coating thickness and uniformity but improved the wettability of the tomato skin. More specifically, the alginate-based edible coatings exhibited a strong interaction with the hydrophobic tomato skin surface (higher than water), promoting surface adhesion. The addition of OEO nanoemulsion in the coating, by providing more hydrophobic sites, further improved the wetting capability and adhesion of the coating solution on the tomato surface. The developed edible coatings successfully contributed to prolonging the tomato shelf life, by reducing the growth of the endogenous microbial flora (total microbial load, yeasts, and molds) over 14 days at room temperature in comparison with the control, with significantly better performances for the edible coating containing the OEO nanoemulsion.


Calibration curve of OEO
The nanoemulsions were tested in the Franz cells to calculate the effective diffusivity of OEO through a cellulose membrane. To evaluate the concentration of OEO in all analysed samples, the calibration curve ( Figure S1) was prepared by reading the absorbance of the Folin-Ciocoltaeu reagent, at a wavelength of 765 nm, when mixed with serial dilutions of the nanoemulsion. The dilutions were selected on the basis of the expected concentration in the receptor compartment of the Franz cells. The linear correlation between the UV-VIS absorbance and the OEO concentration was very high (R 2 = 0.9993), and could be expressed through equation S1. | 745 = 963.34 • [ ] (S1) Moreover, from calibration curve, according to Lambert Beer's law, it is possible to determine the molar absorptivity of OEO at 765 nm, which is about ε=A/(C•L)=963,34 mL/(g•cm).

Coating thickness optimization
The RSM was used to determine the number of experiments and to explore the effect of the three independent, i.e. dipping time (X1) and concentrations of the coating and cross-linking solutions (X2 and X3), on the resulting response variable, the thickness of the coating layers (Y1), listed in Table S1. * Non-uniform coating observed on tomato surface.
The figure S2 illustrates the tomato samples coated according to the combination of the three independent variables of Table S1. This picture confirm the results obtained for the thickness of the coating layers. In fact as it's noticeable the layer obtained form run at lower concentrations is less visible for lower concentrations then for those at higher concentrations. Details about regression analysis and ANOVA involved in the fitting model of film thickness response are shown in Table S2. The F-test value Freg of the coefficients of the polynomial model (β0) equal to 12.31 implies the model is significant. There is only a 0.01% chance that a Freg this large could occur due to noise. The F-test value for lack of fit of 2.58 implies the lack of fit is not significant relative to the pure error. There is a 15.55% chance that an F-test value for lack of fit this large could occur due to noise. Finally, the values obtained for R 2 and adj-R 2 (0.8503 and 0.7812, respectively) suggest that the regression model was suitable to accurately describe the observed experimental data. Figure S3 shows the contact angle of water and film solutions, as a function of time elapsed from droplet deposition, on tomato skin, using alginate coating solutions without and with OEO nanoemulsion, in comparison with pure water. In all cases, the contact angle instantly reached an asymptotic value and substantially constant values were recorded over the 120-s experiment, with a reduction of only 1.5%, 2.4% and 4.8% for water, sodium alginate and sodium alginate with OEO, respectively, between the beginning and the end of the experiment.

Color measurement
Tomato surface color was directly measured with a CR-400 Minolta chroma meter (Minolta, Inc., Tokyo, Japan). Color was measured using the CIE L*, a*, b* coordinates. The instrument was calibrated using a standard white reflector plate. Ten readings were made in each sample by changing the position of the tomato pieces.
The effect in the color parameters L*, a*, b* of tomatoes coated with calcium chloride crosslinked sodium alginate, with or without the addition of OEO nanoemulsion, has been evaluated (Table S3).