Electrospun fibers as food packaging material

Wrapping of fresh food with electrospun membrane containing nutrient preservation or antimicrobial substances.

The use of electrospun fibers as packaging material is mainly targeted at keeping the freshness of the food for a longer duration. The advantage of electrospinning for such applications lies in the relative ease of encapsulating active agents within the fiber and the process which forms a coating of nonwoven fibrous mat. High surface area of the fibers due to its small diameter also allows quick response to its surrounding conditions with timely release of active agents. Small pore size between the interconnected fibers also acts as a physical barrier against bacteria entry. Chaudhary et al (2014) used an electrospun polyacrylonitrile-silver composite filter media to cover a nutrient media in room condition and passes ambient air through the filter media. When compared to the negative control which is without the protective filter media, the nutrient media protected by the nanofibrous filter remains free of bacteria growth after two months while the unprotected nutrient media show microorganism growth.

Widespread use of food packaging provides an opportunity to reduce environmental impact through the use of abundant and renewable resources. One such material is starch. Zhu et al (2022) was able to electrospin pure starch by dissolving the starch in dimethyl sulfoxide (DMSO) at 70 °C before cooling to room temperature. Electrospinning was carried out at an ambient temperature of 60 °C to facilitate fast vaporization of the solvent to form distinct fibers. Post spinning cross-linking using glutaraldehyde (GTA) vapor was necessary to render the membrane insoluble in water and to increase its tensile strength from ~0.66 MPa to ~9.65 MPa.

Protection offered by electrospun membrane may be enhanced by the addition of anti-microbial agent into the fibers. To assure food safety, natural occurring anti-microbial agent has been used for loading into electrospun fibers. Plantaricin 423, produced by Lactobacillus plantarum, and bacteriocin ST4SA produced by Enterococcus mundtii has been blended with poly L-lactide and polyethylene oxide mixture and electrospun [Heunis et al 2011]. Despite the use of organic solvent, N, N-dimethyl formamide (DMF) for dissolving the polymers and the peptides and electrospinning at high voltage, the peptides were shown to retain their activity from their inhibition of Enterococcus faecium HKLHS. Dai (2013) demonstrated the release of allyl isothiocyanate vapor from mustard seed meal powder encapsulated in electrospun poly(lactic acid) (PLA)/poly(ethylene oxide) (PEO) fibers. Allyl isothiocyanate vapour is known to exhibit antimicrobial properties and the release of the it can be controlled by varying the ratio of PLA and PEO with maximum release rate recorded when the ratio was one is to one. Gallic acid, a naturally occurring phenolic acid which is known to exhibit anti-inflammatory and antimicrobial, has been encapsulated within electrospun zein fibers. Since zein is a naturally occurring protein, the electrospun packaging is fully made out of natural material. The electrospun mesh has been found to be effective against S. aureus and E. coli although it is only moderately effective against C. albicans with a log reduction of 1 to 2. The electrospun packaging material was found to be stable after 30 days of storage at 60 °C [Neo et al 2014]. Bugatti et al (2018) used electrospun bio-based polyamide 11 (PA11) loaded with halloysite nanotubes (HNTs) filled with lysozyme (50 wt % of lysozyme), a natural antimicrobial molecule, as bio-based pads for extending shelf-life of raw meat. With PA11/5.0 wt % HNTs-lysozyme, the release of lysozyme was more than a month. Anti-bacterial effectiveness of the membrane was tested using Pseudomonas spp., representing microbial dynamics during meat spoilage. Electrospun PA11 membrane without lysozyme was used as control. Over 13 days study period, the bacterial count on the PA11/5 wt % HNTs-lysozyme was one order of magnitude lower than the PA11 therefore demonstrating the potential of the lysozyme loaded membrane against food spoilage. Yue et al (2018) demonstrated the performance of electrospun carboxymethyl chitosan/polyoxyethylene oxide (CMCS) nanofibers for maintaining freshness of strawberries compared to commercial cling wrap and painted with the electrospinning solution to form a coating. Carboxymethyl chitosan was selected due to its antibacterial property and polyoxyethylene oxide was a binder to facilitate electrospinning. The electrospun composite fibers demonstrated good antibacterial property against Escherichia coli and Staphylococcus aureus. The electrospun CMCS membrane was able to reduce water loss while exhibiting adequate air permeability to maintain fruit freshness. After six days of storage at ambient temperature, only the strawberries covered with electrospun CMCS membrane maintained good outer appearance without any rot. Both unprotected control and strawberries wrapped with commercial cling wrap showed various degrees of rotting. Strawberries with CMCS coating on their surface suffered from browning and severe shrinkage but without any rotting. Zeinali et al (2021) demonstrated the potential use of jujube extract-loaded electrospun polyvinyl alcohol (PVA/JE) nanofiber for strawberry preservation. Both polyvinyl alcohol and jujube extracts are water soluble hence non-toxic solution can be prepared for electrospinning into a nanofibrous membrane. Jujube extracts were found to contain flavonoids which exhibited antioxidant properties. The same extracts also demonstrated antibacterial properties through their inhibition of Staphylococcus aureus and Pseudomonas aeruginosa. For their tests on maintaining the freshness of the strawberries, the strawberries are placed in polyethylene containers and its lid covered with the membrane. The PVA/JE membrane was shown to reduce the ripening process of the fruits with lower weight loss compared to the controls from day 6 to 12. Total soluble solids, firmness and antioxidant capacity of strawberries stored in the jar with PVA/JE membrane was also better than the control at the end of day 12. Sensory evaluation of the strawberries which includes color, texture, taste, appearance and general acceptance was also better in the PVA/JE membrane covered strawberries and no signs of decay was observed at the end of the 12th day.

Spoilage of food is commonly caused by fungi and inhibition of fungi growth is important to maintain the freshness of food. Zhang et al (2022) examined the effectiveness of electrospun polyvinyl alcohol/β-Cyclodextrin (PVA/β-CD, 6:1) with Zanthoxylum bungeanum essential oil (ZBEO, 10%) as packaging material for maintaining freshness of strawberries and cherries. The antifungal activity of PVA/β-CD loaded with ZBEO was tested against Penicillium, Aspergillus flavus or Botrytis cinerea. Inhibition of close to 90% of the fungi exposed to the vapors of ZBEO emitted from the electrospun PVA/β-CD membrane was observed at day 7. The ZBEO loaded PVA/β-CD membrane was also found to be superior in maintaining the freshness of strawberries and cherries compared to untreated fruits, blank film and free ZBEO enclosed in a jar with the fruits.

In the development of smart packaging for food preservation during transport, the release or increased release rate of antimicrobial agents can be triggered by specific stimulus. Shi et al (2021) encapsulated salicylaldehyde precursors (SP) and hexanal precursors (HP) into electrospun ethyl acetate (EA)/polyethylene oxide (PEO) fibers. Both hexanal and salicylaldehyde are known to have antimicrobial properties. However, as these aldehydes are volatile and susceptible to oxidative degradation in the environment, they need a polymer carrier for protection until it is released. Shi et al (2021) showed that the release of SP and HP was higher with exposure to higher concentration of citric acid vapor. When used as packaging material, increasing concentration of citric acid vapor within the headspace would induce faster release of SP and HP which would offer protection to the fruits and vegetables. However, more investigations need to be carried out as aldehydes self-react or react with other molecules through acid-catalyzed aldol condensation which would reduce the amount of SP and HP in the head-space. Despite this, the amount of SP and HP concentration is higher than in the absence of citric acid vapor.

Food spoilage generally requires the presence of oxygen and moisture. Packaging with oxygen scavenging ability is able to control headspace oxygen content and improve product quality and shelf-life. Cherpinski et al (2019) constructed an oxygen-scavenging multilayered biopaper containing palladium nanoparticles as a potential oxygen scavenging packaging material. The multilayered biopaper comprised of a cellulose base paper, coated with electrospun poly(3-hydroxybutyrate) (PHB) and polycaprolactone (PCL) polymer-containing palladium nanoparticles (PdNPs). The layer of electrospun PHB function as a water barrier and electrospun PCL is the carrier for oxygen scavenging palladium nanoparticles. Such configuration is necessary as electrospun PHB loaded with PdNPs was found to exhibit poor oxygen scavenging capacity after annealing to improve water barrier performance. PHB is a better water barrier material compared to PCL but electrospun PCL loaded with PdNPs was found to exhibit much better oxygen scavenging ability. To take advantage of the better water barrier performance of electrospun PHB and oxygen scavenging ability of electrospun PdNPs/PCL fibers, both fibers were incorporated onto a cellulose base layer to form a multilayered biopaper.

A concern of using electrospun fibers in packaging is whether loose fiber strands may get into the food. This may be mitigated by using proper adhesion methods to fuse the fibers or fuse the fibers to a base substrate. Taking advantage of the good dispersion of Ag nanoparticles in electrospun fibers, Castro-Mayorga et al (2017) coated a commercial polyhydroxyalkanoate (PHA) substrate with electrospun PHA loaded with Ag nanoparticles to introduce antibacterial properties to the material. To fuse the electrospun fibers with the base substrate, hot compression was used. With compression under heat, the electrospun fibers completely fused with the base substrate such that no fibers were observed following the heat treatment. The nanoparticles were being transferred to the combined substrate and its distributions indicates where the fibers once were. The multilayered substrate retains good transparency and are active against Salmonella enterica (gram negative) but did not show any antibacterial effect against Listeria monocytogenes (gram positive). Pardo-Figuerez et al (2018) used a combination of electrospinning and electrospraying to create a superhydrophobic coating on polyethylene terephthalate (PET) film as transparent food packaging. Electrospinning of polylactide (PLA) solution was first carried to deposit a layer of fibers on the PET film. This increases the hydrophobicity of the film from 82° to 96° using water contact angle test. Unfortunately, annealing of the bilayer film at temperatures ranging from 90°C to 170°C reduces the water contact angle to 73 °C at 170°C annealing temperature. This may be attributed to the partial loss of fiber structure especially at temperatures above 120°C. To increase hydrophobicity of the film, SiO₂ microparticles were electrosprayed on top of electrospun PLA fibers. An optimum annealing temperature of 160°C was found to create good adhesion between the fibers, SiO₂ microparticles and the film. A water contact angle of 171° and sliding angle of 6° were obtained thus making the film superhydrophobic. The film also exhibited good optical transparency following heat treatment.

The release of active agents may also be triggered by environmental stimuli. Liu (2016) used the moisture released by fresh tomatoes to trigger the release of thymol encapsulated within electrospun mats. The polymer used for electrospinning was polyvinyl alcohol which is swells when comes into contact with water or moisture. It is important to note that for encapsulation of volatile compound like thymol, the compound gets vaporized during electrospinning and the remaining concentration needs to be determined post-electrospinning. Liu (2016) was able to demonstrate the increased release of thymol when the mat was placed in higher humidity environment. Their study successfully showed that the shelf life of fresh tomatoes can be extended by at least 5 days.

For roots and tubers, keeping them fresh is about preventing sprouting. Some chemicals such as peppermint essential oil have been shown to be able to prevent or delay sprouting of tubers. Ramachandran et al (2017) encapsulated peppermint essential oil in electrospun polyurethane (PU)/ polymethylmethacrylate (PMMA) fibers to use as anti-sprouting agent in stored potatoes. The test subject with the loaded electrospun fibers showed no sprouting for 30 days while the controlled samples (electrospun mat without peppermint essential oil) sprouted in 10 days. This shows that the loaded peppermint essential oil is effective in delaying sprouting. However, it is important to note that the test was carried out in an airtight container. Dumitriu et al (2017) recommended using electrospinning to create a coating for existing high performance barrier films. The electrospun fibers will be loaded with anti-oxidant compound to prevent oxidative degradation reactions of fats, proteins and pigments which will lead to degradation of the meat. In their experiment, polycaprolactone (PCL) was used as the carrier matrix and vitamin E (α-tocopherol), selected as plant-based phenolic antioxidant. The vitamin E loaded into the electrospun PCL fibers were shown to be accessible and effective as an antioxidant. However, more studies will be required to demonstrate its effectiveness in delaying food product degradation and the increment in shelf-life.

Investigation of electrospun membrane to retain freshness of food products are not restricted to just fruits and vegetables. With the proper selection of additives, fish has also been shown to benefit from a protective layer of electrospun membrane. Li et al (2021) investigated the use of core-shell electrospun fibers with antibacterial methyl ferulate as the active ingredient encapsulated within the core and zein as the shell layer for preserving sea bass. The electrospun core-shell methyl ferulate/zein fiber membrane showed an initial release of 30% of the load in the first 8 h. The release rate slowed to 77.5% of the load at 84 h and stabilized at 83% from 84 to 132 h. Inhibition of bacteria was demonstrated by soaking the membrane in the culture medium of Shigella putrefaciens. Through determining the pH value, lipid oxidation content and total volatile basic nitrogen (TVB-N) content, the electrospun core-shell methyl ferulate/zein fiber membrane was able to keep the sea bass fresher compared to uncovered and zein only fiber membrane covered sea bass.

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