An Introduction to Electrospinning for food industry

A growing interest in the use of electrospun fibers in the food industries has seen electrospinning of biopolymers and the encapsulation of food ingredients, enzymes and other active compounds related to the food industry. Proposed specific applications of such composites are active packaging or preservation of nutrient activity for consumption. Another benefit of electrospinning food material is for the introduction of different textures and bite to the food.

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

To extend the shelf-life of food, anti-microbial agent may be added to electrospun fibers for use as packaging material. 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. Electrospun fibers have also been tested for meat preservation. 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.

Fluorescent images of labeled plantaricin 423 and polymers electrospun into nanofibers consisting of PEO 50:PDLLA 50. (A) Fluorescent image of PEO labeled with RBITC in the fibers; (B) fluorescent image of the bacteriocin labeled with FITC in the fibers; (C) overlay of the fluorescent images; (D) optical image of fibers with fluorescent signals; (E) co-localization of the signals; (F) fluorescent image of bacteriocin labeled with RBITC in the fibers; (G) fluorescent image of PDLLA labeled with FITC in the fibers; (H) overlay of the two fluorescent images; (I) optical image of fibers with fluorescent signals; (J) co-localization of the two signals. [Heunis et al. Int. J. Mol. Sci. 2011; 12: 2158. This work is licensed under a Creative Commons Attribution 4.0 International.]

The versatility of electrospinning for the fabrication of fibers out of different materials is well known. Prior to investigating electrospun fibers for use in the food industry, natural polymers such as collagen, chitosan, alginate and gelatin has already been electrospun and tested for medical applications. Later, potential food material such as zein, soy protein [Shankar et al 2013] and whey protein [Sullivan et al 2014], has been electrospun. However, due to difficulty in producing fibers out of these biopolymers only, other electrospinnable polymers are often added to the biopolymers prior to electrospinning. To eliminate the use of potentially harmful solvents, most companion polymers used are water soluble such as polyethylene oxide and polyvinyl alcohol. In an ambitious demonstration of the electrospinning ability to generate fibers, chocolate suspension comprising of salt, sugar, milk and cocoa was been used with the aim of creating food with varying microtexture and mouthfeel [Luo et al 2012]. Although evidence of fiber strands was shown, they were mostly short with elongated beads. Lepe et al (2014) showed that it is possible to electrospin "sugar syrup" fibers. Glucose syrup which contains a mixture of mono-, di- and tri-saccharides, was able to form fibers although they may not be stable at room condition due to absorption of atmospheric water vapor. Various combinations of mono-, di- and tri-saccharides dissolved in de-ionized water were tested for their electrospinnability. Their tests showed that only di-saccharides (sucrose and maltose) were able to form continuous fibers although beads can be found on them. The relative ease of fabricating highly soluble sugar syrup fibers provides an attractive option as fast dissolution carrier for vitamins, minerals or drugs.

The demonstration of electrospinning to use emulsion as the feed solution has widen the materials and encapsulation possibilities for transformation into fibers. Papadaki et al (2016) used coconut oil as solvent for recovery of multifunctional extracts from microalga Haematococcus pluvialis (HP) and the diatom Phaeodactylum tricornutum (PT). The coconut oil enriched with β-carotene and polyunsaturated fatty acids extracts was emulsified with an aqueous solution of ulvan and pullulan blend. The aqueous solution contains 15 mM H₃BO₃ and 7 mM CaCl₂ and Tween 20 surfactant was added to form the oil in water emulsion. Electrospinning of the emulsion was able to produce smooth nanofibers with diameters of about 65 nm. The resultant nanofibers provide a potential delivery vehicle as well as a protective barrier for the encapsulated extracts.

Preservation of active compounds through encapsulation in electrospun fibers is probably the most widely investigated field in the application of food technology. Folic acid without any coating is susceptible to degradation when exposed to light and acidic condition. However, when it is encapsulated within sodium alginate-pectin-poly(ethylene oxide) nanofibers, almost 100% of the folic acid is retained after 41 days of storage in the dark at pH 3. This contrasts against 8% recovery after one day of storage at pH 3 [Alborzi 2012]. Using pure zein, Fernandez et al (2009) was able to encapsulate β-carotene and increase its light (UV) stability up to 24 h compared with unprotected β-carotene and solution cast of fiber β-carotene powder.

Beyond encapsulation of vitamins and minerals, electrospun fibers may also be used for delivery of pro-biotic bacteria. Liu et al (2016) used an aqueous solution containing two edible polysaccharides, pectin (PEC) and pullulan (PUL) for encapsulation of probiotic bacteria Lactobacillus rhamnosus GG (LGG). The electrospun PEC/PUL fibers containing the bacteria were cross-linked by soaking in 5% CaCl₂ solution. 90% of the bacteria were found to remain viable after electrospinning and cross-linking which demonstrates the potential use of edible polysaccharides as bacteria carriers.

Encapsulation of active compounds into fibers is often achieved by mixing the active compounds into the polymer solution and electrospinning. To avoid the use of toxic solvents for the generation of food-related products, most polymers used for electrospinning of food-based application are dissolved in water or ethanol. This presents a challenge when the active compounds to be encapsulated are hydrophobic. Kayaci et al (2012) demonstrated the potential use of cyclodextrin to incorporate organic compound, vanillin, into an aqueous polymer solution for electrospinning. Three different types of cyclodextrin (CD), α-CD, β-CD and γ-CD were tested for the stability of vanillin in polyvinyl alcohol electrospun fibers. Storing at about 30% relative humidity and 24 °C, their tests showed that without the use of CD in the polymer fibers, 81% of vanillin was loss after 8 days while α-CD and β-CD showed 60% and 66% loss respectively. γ-CD was the most stable with 20% loss at 8 days and 42% loss after 50 days while the others showed negligible vanillin remaining. Since PVA is a man-made polymer and may not be suitable for food products, Fuenmayor et al (2013) used pullulan, a polysaccharide, as the carrier for β-cyclodextrin and limonene. Mixing the particles into pullulan solution formed water-in-water emulsion (cylcodextrin rich aqueous microdroplets dispersed within aqueous pullulan rich phase). The stability of the volatile limonene was confirmed when there was no further loss up to 45 days after the initial release of excess limonene. Similar method (using β-cyclodextrin) was used to encapsulate other aroma compound like perillaldehyde [Mascheroni et al 2013]. The encapsulated perillaldehyde was relatively stable with initial loss of 15% after 7 days but little loss thereon to 45 days when stored at 55% relative humidity and 23 °C. At elevated humidity of more than 92% relative humidity which is the environment where packed fresh food is stored, 90% of perillaldehyde was released in a day. Given the antimicrobial properties of perillaldehyde, the electrospun fibrous composite may potentially be used in packaging of food products where high humidity is experienced. Another chemical that forms complex with α-CD and loaded into electrospun fiber is 1-methylcyclopropene (1-MCP). 1-methylcyclopropene (1-MCP) is used to delay ethylene-induced color change, ripening, and senescence in various produce and its release rate from electrospun polystyrene is low at low relative humidity of less than 30% [Neoh et al 2011]. Apart from use as packaging material, electrospun products have been tested as potential edible food substitute.

The development of electrospun product as edible food substitute may sound bizarre at first. However, its use in tissue engineering due to its resemblence to natural extracellular matrix has inspired research in the use of electrospun products for food substitute. As a carrier for proteins or active compounds, the selected polymer for electrospinning should be natural, edible, does not require the use of toxic solvents and can be electrospun to give fibers without the need to introduce man-made polymer in the mixture. Heuvel et al (2013) seek to address this issue by testing zein and gelatin against the requirements. Zein is soluble in water/ethanol mixture while gelatin is water soluble. These polymers were mixed with protein (sodium caseinate, whey, ovalbumin, soy or BSA) to test their ability as carriers to these proteins for electrospinning under food-grade conditions. The results showed that zein is a poor carrier of proteins for electrospinning while gelatin was able to form fibers with up to 17.5% whey protein content. This demonstrates an opportunity to fabricate meat substitutes that better mimic the texture and bite of meat. Another potential material for this purpose is natural starch. Sutjarittangtham et al (2014) has successfully electrospun natural tapioca starch solution (using deionized water) in a modified electrospinning setup. In their setup, the metal target is placed in a -20 °C cooled ethanol solution to accelerate the dehydration process. The optimum concentration for producing fibers was found to be 4.5 wt%. Concentration below that produces mat with areas of completely fused fibers while higher concentration resulted in clogged spinneret due to high viscosity. Cardenas et al (2016) reported electrospinning of potato starch microfibers from natural potatoes instead of industrial starch. Dimethyl sulfoxide (DMSO) was used to prepare the starch solution for electrospinning. Similar to Sutjarittangtham et al (2014) and other reported fabrication of fibers from electrospinning starch, a liquid collector was used comprising of ethanol-water 70:30 for the collection of the fibers. This solution composition is able to extract DMSO so that starch microfibers can be formed upon deposition.

Table 1. List of electropsun carrer material and the corresponding active compounds.

Active compound	Electrospun Carrier	Reference
Folic acid 0.01% (w/w)	Sodium alginate-pectin-poly(ethylene oxide)	Alborzi 2012
β-carotene 1.65 wt%	Zein	Fernandez et al 2009
Limonene 3.1 wt% to dry membrane	Pullulan and β-cyclodextrin	Fuenmayor et al 2013
Perillaldehyde 1.85 wt% to dry membrane	Pullulan and β-cyclodextrin	Mascheroni et al 2013
Vanillin 5 wt% to dry membrane	Polyvinyl alcohol and α-cyclodextrin	Kayaci et al 2012
Vanillin 5 wt% to dry membrane	Polyvinyl alcohol and β-cyclodextrin	Kayaci et al 2012
Vanillin 5 wt% to dry membrane	Polyvinyl alcohol and γ-cyclodextrin	Kayaci et al 2012
1-Methylcyclopropene	Polystyrene and α-cyclodextrin	Neoh et al 2011

For water soluble polymer, high surface area of electrospun nanofibers allows it to quickly dissolve upon contact with water and releases the load it is carrying. Kyzy et al (2014) used this property to introduce nutritional iodine based on electrospun fast dissolving oral mat. With diameter ranging from 120 nm to 400 nm, the polyethylene oxide nanofibers carrying KIO₃ achieves complete dissolution within 5 mins in water. Porous and thin electrospun mats containing other ingredients such as mint or medications may offer advantages over off-the-shelf oral dissolving film strips.

Development in electrospinning applications in other areas may also be used in the food industry. Air and liquid filtration membranes made from electrospinning process have been in development before its application in the food industry. Veleirinho et al (2009) demonstrated the potential use of electrospun poly(ethylene terephthalate) nonwoven nanofiber mat as apple juice clarification membrane. Comparing the average processing time between ultrafiltration and electrospun PET membrane, the former requires 35 min with pressure of 50.8 psi while the later take just 6 min with pressure of 0.7 psi. Physico-chemical properties such as total solids, soluble solids and acidity remains unchanged. Composition in phenolic, sugars and organic acids also remains unchanged compared to unclarified control although the protein content is reduced for the electrospun membrane.

Easy detection of bacteria or bacteria activities are very useful in many applications from food preparation to bacteria contamination in an area. Colorimetric means are particularly attractive due to the convenience of visual cues. Kinyua et al (2022) constructed a colorimetric sensor made of electrospun chitosan/polyethylene oxide nanofibers (CS/PEO NFs) grafted with chromogenic substrate 5-bromo-4-chloro-3-indolyl-β-D-glucuronide (X-Gluc) for the purpose of detecting bacteria, Escherichia coli, activities. In the presence of the enzyme β-glucuronidase (β-GUS) from Escherichia coli (E. coli), hydrolytic cleavage of the chromogenic substrate X-Gluc leading to the release of an indoxyl derivative which forms a blue colored dichlorodibromo indigo dye in air. Comparing the reaction rate of CS/PEO NFs and CS/PEO hydrogel of the same mass and enzyme concentration, the higher surface area of CS/PEO NFs gave a much higher reaction rate and greater amount of dye released. The limit of detection for β-glucuronidase by the nanofibers was 13 nM compared to 25 nM of CS/PEO hydrogel. The lower limit of detection for the CS/PEO NFs was 13 nM compared to 25 nM CS/PEO hydrogel sensors. For the lower limit of quantification for the enzyme, CS/PEO NFs was 45 nM while CS/PEO hydrogel was 60 nM.

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