Abstract
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.
Highlights
Edible coatings represent a method traditionally used to extend the shelf life and maintain the physical and gustatory characteristics of fresh produce [1]
Numerous studies have been conducted to date to assess the effect of edible coatings on preserving the quality and increasing the shelf life of fresh/fresh-cut fruit [4]
The oregano essential oil (OEO) nanoemulsion was characterized in terms of droplet size distribution and ζ-potential, with the values reported in Table 2, together with the nanoemulsion formulation
Summary
Edible coatings represent a method traditionally used to extend the shelf life and maintain the physical and gustatory characteristics of fresh produce [1]. The use of edible coatings has been further promoted by the coating functionalization with bioactive compounds, such as natural antimicrobial compounds, antioxidants, minerals, and vitamins, which contribute to improving safety and preserving the quality of food and to delivering health benefits to the consumer [5,6]. Because of their high efficiency in reducing the occurrence of deteriorative processes, Foods 2020, 9, 1605; doi:10.3390/foods9111605 www.mdpi.com/journal/foods. The coating efficiency in protecting food products was reported to depend on (i) the method of application; (ii) the nature of the coating ingredients and their concentrations [8], (iii) the uniformity of wetting and spreading on the surface and on the adhesion, cohesion, and durability [9]; and (iv) the capability to act as barriers against water or oils permeation, and gas or vapor transmission [9,10]
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