Bioactive Andean sweet potato starch-based foam incorporated with oregano or thyme essential oil

https://doi.org/10.1016/j.fpsl.2019.100457Get rights and content

Highlights

  • Bioactive foams were made from sweet potato starch and essential oil.

  • Essential oil droplets were encapsulated into the first layer of the foams.

  • Oregano essential oil showed the best antimicrobial activity.

  • Salmonella was totally inhibited in foams with 10 % oregano essential oil.

  • Solubility decreased significantly in foams with essential oil.

Abstract

In this research, sweet potato starch and oregano (OEO) or thyme (TEO) essential oil at two concentrations (7.5 and 10 %) were used to produce bioactive foams by thermopressing. The foams were characterized according to microstructure, mechanical properties, antimicrobial properties, and structural properties by X-ray diffraction, scanning electron microscopy, and Fourier-transform-infrared spectroscopy (FT-IR). In all cases, essential oil addition affected the foam color, yielding reddish/yellowish foams, but not the foam thickness. FT-IR spectrum and X-ray diffraction revealed starch-lipid interactions. According to the micrographs, the lipids were localized in the first layer. Thus, formation of amylose-essential oil complexes in the foam may have prevented the essential oil from degrading under the thermoforming temperature. Essential oil addition yielded starch foams with low water solubility and mechanical resistance, especially for 10 % OEO. Meanwhile, these foams were more effective against Salmonella (Gram-negative bacteria) and L. monocytogenes (Gram-positive bacteria). The antimicrobial activity of the foams containing essential oil makes them beneficial for application as bioactive materials. Therefore, bioactive sweet potato starch-based foams can be prepared by thermopressing and be applied as food container.

Introduction

There has been growing interest in obtaining biopolymeric packaging consisting of biodegradable and renewable material (Gilfillan, Moghaddam, Bartley, & Doherty, 2016; Kaisangsri, Kerdchoechuen, & Laohakunjit, 2014) as opposed to petroleum-based packaging, whose accumulation is a real and worrisome problem for the environment. In addition, biopolymeric packaging with bioactive properties can be designed to offer protective effects against microbes.

Active packaging is defined as a system where product, packaging, and free space interact, to result in improved product quality and safety (Suppakul, Miltz, Sonneveld, & Bigger, 2003; Vermeiren, Devlieghere, & Debevere, 2002). However, incorporating bioactive components into food products could alter the food product flavor and make the food product susceptible to fast degradation, not to mention that the bioactive component could interact negatively or positively with the other food matrix elements (Quirós-Sauceda, Ayala-Zavala, Olivas, & González-Aguilar, 2014). In contrast, when appropriate concentrations of these components are added in a polymeric matrix (e.g., starch foam), they can generate active packaging that impact the food organoleptic properties to a lesser extent because the active components are gradually released.

Biomaterials like starch allow incorporating of antimicrobial and antioxidant agents (Shen & Kamdem, 2015), such as essential oils (Atarés & Chiralt, 2016). Essential oils are classified as Generally Recognized as Safe (GRAS). They could be used to produce "bioactive packaging" with antimicrobial and antioxidant properties (Burt, 2004) and to help reduce the hydrophilic behavior. Several works have evaluated how diverse essential oils including thyme or oregano essential oil (Romani, Prentice-Hernández, & Martins, 2017; Yahyaoui, Gordobil, Herrera Díaz, Abderrabba, & Labidi, 2016), rosemary or clove (Mulla et al., 2017; Qin, Li, Liu, Yuan, & Li, 2017) essential oil, Thai essential oil (Klangmuang & Sothornvit, 2016), and cinnamon or ginger essential oil (Atarés, Bonilla, & Chiralt, 2010) affect film properties. Debiagi, Kobayashi, Nakazato, Panagio and Mali (2014) incorporated essential oil as an additive in baked foams made from cassava bagasse and polyvinyl alcohol. The authors reported that the thermoforming temperatures volatilized the essential oil, and that the foams exerted no antimicrobial effect. Ketkaew et al. (2018) recently evaluated how oregano essential oil affected cassava starch foams and pointed out that bioactive foam foams could be obtained by thermopressing. Nevertheless, information on how starch and essential oils interact under high temperature (> 140 °C) and pressure to generate a bioactive foam is scarce. In fact, whether the essential oil is preserved in the foam layers after the thermoforming process has not been well established, so new information about this topic is necessary. In addition, the study of new starch sources as a basis for bioactive foams is relevant for industrial applications.

In our previous work, we showed that native sweet potato starch can be a very useful source to generate resistant foams made by thermopressing, but it is necessary to improve its hydrophilicity and active property (Cruz-Tirado, Vejarano, Tapia-Blácido, Barraza-Jáuregui, & Siche, 2019). Therefore, here we hypothesize that addition of an essential oil can decrease the sweet potato starch hydrophilicity and improve the antimicrobial properties of sweet potato foams, yielding bioactive packaging. During the thermoforming process, microdrops of an essential oil can be encapsulated inside the gelatinized starch, preventing the oil from evaporating completely during foam drying. In this context, this work aims to evaluate how the addition and the concentration of thyme or oregano essential oil affect the physical and antimicrobial properties of foams based on native sweet potato starch.

Section snippets

Materials

Sweet potato (Ipomoea batatas) starch (32.65 % amylose, 9.27 % moisture, and 0.30 protein) was provided by the Agroindustrial Processes Engineering Laboratory of the National University of Trujillo (Trujillo, Peru). Glycerol and magnesium stearate were acquired from Su Man (Pflücker e Hijos S.A., Lima, Peru).

Thyme essential oil (Thymus vulgaris) and oregano essential oil (Origanum vulgaris) were purchased from Runcato (Runcato E.I.R.L, Lima, Peru). Oregano essential oil contained carvacrol

Foam color and appearance

Fig. 1 shows the appearance of the sweet potato starch foams without and with added essential oil (OEO or TEO). The thermoforming conditions provided complete foams. Foams with 7.5 % or 10 % OEO or TEO had small cracks, holes, and deformations because water vapor interacted with OEO or TEO during starch expansion. The foam color changed slightly to reddish/yellowish, which was consistent with the color results (Table 2).

Table 2 presents the color parameters luminosity (L*), redness (a*),

Conclusion

Bioactive sweet potato starch-based foams with oregano or thyme essential oil were prepared by thermopressing. The foams with OEO showed a greater antimicrobial effect than the foams with TEO. Besides, the foams with 10 % OEO completely inhibited Salmonella (Gram-negative bacteria). The results suggested that small essential oil drops were trapped in the first layers of the foams, which caused an effective antimicrobial effect. However, the drops altered the polymer matrix structure and

Funding

This work was supported by Universidad Nacional Trujillo Contract CanonPIC 2-2013.

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgement

J. P. Cruz-Tirado acknowledges scholarship funding from FAPESP, grant n° 2018/02500- 4. Ramon Sousa Barros Ferreira acknowledges scholarship funding from CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnologico), grant n° 132428/2018-0. The authors acknowledge the Innovation Program for Competitiveness and Productivity (INNÓVATE PERÚ) Contract 407-PNICP-PIAP-2014.

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