Active LDPE films loaded with biopesticides by supercritical CO2-assisted impregnation for stored grain protection
Introduction
Hermetic storage systems such as silo bags are considered an effective tool to prevent damaging from pest infestations during food storage. However, many biological interactions, such as grain-insect-fungi, can arise when good agricultural practices are not taken into account, causing grain deterioration and losses up to 10% (Casini, Rodriguez, & Bartosik, 2009). The weevil Sitophilus zeamais Motschulsky (Coleoptera: Curculionoidae) and the fungal pathogen Fusarium verticilliodes are well known among the main pests with economic relevance, being involved in significant annual losses of stored grains such as wheat, rice and maize. (Casini & Santajulia, 2015). In Argentina, two synthetic pesticides, methyl bromide and phosphine, are generally used to manage grain protection (Bartosik, Cardoso, & Manetti, 2016), even though these substances have been listed as “prohibited” and “restricted”, respectively, by the Argentinean National Ministry of Health (Digón, 2014), due to their extensive use is related to resistance development, human diseases, as well as food and environment contamination (Cordeiro, Corrêa, Rosi-Denadai, Tomé, & Guedes, 2017; Who, 2002). In an attempt to minimize these environment and health issues, an alternative approach for controlling stored grain pests could be based upon the use of biopesticides, active compounds which are considered as minimum risk agents. Therefore, biopesticides appear as novel eco-friendly tools with an increasing demand (EPA, 2017). Nevertheless, considering the generally high volatility of these natural compounds, an adequate implementation in the application sites should seek mechanisms for their retention and protection against environmental agents (Stella, Quintana, Olivero-verbel, & Stashenko, 2009). Thus, the development of active packaging incorporating biopesticides into polymer materials is an innovative technology for food preservation during storage (Rojas et al., 2015). 1-octen-3-ol (Fig. 1) is a fungal volatile organic compound (VOC) which mediates interspecies interaction, being a semiochemical for insects (Usseglio et al., 2017). Numerous studies have shown its antifungal, insecticidal, repellent and herbicidal activities (Zhao, Yang, Lix, Mu, & Liu, 2011). In particular, the biological activity of 1-octen-3-ol against the insect S. zeamais and the fungus F. verticillioides and its mycotoxin, fumonisin B1, have been already reported by our research group (Herrera, Pizzolitto, Zunino, Dambolena, & Zygadlo, 2015). This compound presents moderate toxicity in mammals (DL50: 340 mg/kg) (EPA, 2017) and it is currently approved for its use as food additive (US FDAs Center for Food Safety & Applied Nutrition, 2008).
Supercritical CO2-assisted impregnation is an environmentally friendly process that can be used for the incorporation of natural active compounds into polymeric materials (Kikic & Vecchione, 2003). Currently, this technique is used in the preparation of polymeric systems for controlled delivery of drugs and volatile bioactive compounds (Kiran, 2016; Rojas et al., 2015; Torres, Romero, Macan, Guarda, & Galotto, 2014). CO2 is chemically inert, non-toxic and in supercritical conditions (P>Pc = 7.38 MPa and T>Tc = 31 °C) can dissolve a wide range of organic compounds and swell polymeric matrices, appearing as a suitable solvent for incorporating bioactive compounds into polymers. In addition, its low critical temperature ensures the preservation of thermolabile compounds during processing (Cocero, Martín, Mattea, & Varona, 2009).
Low-density polyethylene (LDPE) is a commercial and economic material commonly used for food packaging due to their excellent thermal stability and mechanical resistance (Goñi, Gañán, Strumia, & Martini, 2016; Teymouri, 2011). This material is also used for the production of hermetic bags for postharvest grain preservation (Casini et al., 2009).
Based on these facts, the aim of this contribution was to obtain a bioactive material for stored grain protection, using an ecofriendly technology such as supercritical CO2-assisted impregnation. The bipoesticide 1-octen-3-ol was incorporated into LDPE film, evaluating the influence of the process conditions on impregnation yield and the release behavior of the obtained materials. Finally, the insecticidal and antifungal activities of the impregnated films against S. zeamais and F. verticillioides were tested, in order to evaluate their potential use as active materials for stored grain protection.
Section snippets
Materials
1-octen-3-ol (purity 98%, MW: 128.21 g mol-1, bp: 84 °C) was purchased from Sigma-Aldrich (Steinheim, Germany).
Commercial low density polyethylene film (LDPE, Mw: 229300, Mn: 22500, MFI: 0.6 g/10 min at 190 °C/2.16 kg, density: 921 kg·m-3, thickness: 130 ± 20 μm, Dow-Polisur, Argentina) was used as polymeric matrix in the impregnation tests.
Industrial extra-dry carbon dioxide (water content ≤ 10 ppm v/v) used as impregnation solvent was purchased from Linde (Argentina).
Insects
The S. zeamais adults
Supercritical CO2–assisted impregnation of LDPE films with 1-octen-3-ol
As previously mentioned, impregnation of LDPE films with 1-octen-3-ol was performed at four CO2 pressure values, corresponding to a wide range of fluid densities, and two depressurization rates, maintaining constant the other process conditions (i.e., temperature, solute mole fraction, stirring rate and contact time). CO2 density, which is controlled by setting the pressure, is a key parameter in the impregnation process as it influences the solubility of the active compound, the polymer
Conclusions
The incorporation of agents with biological activity, such as biopesticides, in a polymeric matrix is a new technology to develop active packaging materials for food preservation. In this work, bioactive films were prepared loading 1-octen-3-ol into LDPE by supercritical CO2-assisted impregnation, evaluating the effect of different process conditions on the total amount of bioactive compound incorporated into the polymer matrix and its release behavior. Results indicated that the maximum
Acknowledgements
This investigation was financed by CONICET PIP11220090100114 (Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina), FONCYT 2012-2146 (Agencia Nacional de Promoción de Ciencia y Tecnología, Argentina), and UNC (Universidad Nacional de Córdoba, Argentina). NAG, JAZ and REM are Career Members of CONICET and JMH and MLG hold fellowships from CONICET.
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