Abstract
In this research, different contents of eugenol in the 2.5–25 wt.% range were first incorporated into ultrathin fibers of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) by electrospinning and then subjected to annealing to obtain antimicrobial monolayers. The most optimal concentration of eugenol in the PHBV monolayer was 15 wt.% since it showed high electrospinnability and thermal stability and also yielded the highest bacterial reduction against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). This eugenol-containing monolayer was then selected to be applied as an interlayer between a structural layer made of a cast-extruded poly(3-hydroxybutyrate) (PHB) sheet and a commercial PHBV film as the food contact layer. The whole system was, thereafter, annealed at 160°C for 10 s to develop a novel multilayer active packaging material. The resultant multilayer showed high hydrophobicity, strong adhesion and mechanical resistance, and improved barrier properties against water vapor and limonene vapors. The antimicrobial activity of the multilayer structure was also evaluated in both open and closed systems for up to 15 days, showing significant reductions (R ≥ 1 and < 3) for the two strains of food-borne bacteria. Higher inhibition values were particularly attained against S. aureus due to the higher activity of eugenol against the cell membrane of Gram positive (G+) bacteria. The multilayer also provided the highest antimicrobial activity for the closed system, which better resembles the actual packaging and it was related to the headspace accumulation of the volatile compounds. Hence, the here-developed multilayer fully based on polyhydroxyalkanoates (PHAs) shows a great deal of potential for antimicrobial packaging applications using biodegradable materials to increase both quality and safety of food products.
Highlights
Active antimicrobial packaging is one of the most relevant emerging technologies in the food industry
One can observe in the scanning electron microscopy (SEM) micrographs that higher concentrations of eugenol impaired the formation of PHBV fibers during electrospinning due to the potential agglomeration of the essential oil
Similar results were obtained during the electrospinning of PHBV with various essential oils, in which high contents of oil molecules prevented the formation of homogeneous fibers since the surface tension in the charged jet changed into droplets [59, 67]
Summary
Active antimicrobial packaging is one of the most relevant emerging technologies in the food industry. PHAs can be obtained from over 155 monomer subunits through fermentation by some bacteria from different renewable carbon sources that are accumulated as intracellular storage granules [8]. Bhatia et al [9] obtained high biomass (Yx/s, 0.31 g/g) and PHA (Yp/s, 0.14 g/g) yields using Ralstonia eutropha 5,119 bacteria and Miscanthus biomass hydrolysate (MBH) as carbon source. Bhatia et al [10] obtained a high PHB production (1.24 g/L) with 2% (w/v) starch as carbon source, using a Escherichia coli (E. coli) strain produced using different plasmids containing the amylase gene of Panibacillus sp. The structure and physical properties of PHAs vary depending on the bacteria specie, substrate (carbon source), and cultivation conditions [13].
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