Electrospinning edible fibers

Electrospun smooth fibers.

Electrospinning of edible food provides an opportunity to use the fibers as a safe delivery vehicle of vitamins, probiotics and nutrients. It may also be used to mimic meat using vegetable-based food source. Potential food material such as zein, soy protein [Shankar et al 2013] and whey protein [Sullivan et al 2014], has been electrospun. In an ambitious demonstration of the electrospinning ability to generate fibers, chocolate suspension comprising of salt, sugar, milk and cocoa were 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. Electrospinning has also been used to produce nanofibers from honey enriched with antibacterial herbal extracts such as the garlic, mint and edible mushroom [Shahbazi et al 2019]. To prepare the solution, honey was dissolved in ethanol and the other compounds are added to it before electrospinning. Smooth nanofiber with an average diameter of 240 nm were produced. The resultant honey nanofibrous mesh was immersed in acetate buffer solution with pH = 5.5 at 37 °C for 24 hours to stabilise the fibers. The stabilised mesh was removed from the solution and left to dry for 24 hrs in room temperature (25°C). Examination under SEM showed no apparent changes in the structure and morphology.

While protein is most commonly associated with animals, they may also be found in plants. Gelatin comes from animal collagen and zein is a plant protein from maize. 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 [Heuvel et al 2013]. Electrospun zein membranes are typically mechanically weak and may not be strong enough for certain applications. Cross linking agent such as glyoxal may be added to improve its mechanical properties and stability in water. Suwantong et al (2011) showed that with 10% v/v of glyoxal added to 40% w/v zein solution, the resultant membrane tensile strength more than doubled from 0.9 MPa to 2.2 MPa. However treatment of the cross-linked membrane with glycine is needed to reduce the toxicity of unreacted glyoxal. Alternative cross-linking agent such as citric acid has been used on electrospun zein membrane and this has shown to enhance mouse fibroblast cells adhesion and proliferation [Jiang et al 2010].

Numerous edible food are made from starch from different sources. They are able to dissolve in hot water and form a thick paste upon cooling. Researchers have found ways to electrospin fibers from starch either by modifying the setup or solution composition. 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.

Given the difficulty to electrospinning pure starch using conventional setup, alternative edible polymer may be added to improve fiber spinning. Guar gum from guar beans is often used in the food industry as a thickening and stabilizing agent. Pure guar gum (GG) has been electrospun to form fibers but the fibers were not uniform with solid aggregates between the fibers [Lubambo et al 2013]. Yang et al (2017) were able to successfully electrospin smooth fibers using a blend of GG and corn starch with amylose contents of 27.8% (CS28) and 50% (CS50). Defect free fibers were obtained with GG/CS ratio of 2:1 for CS28 and of 1:1 for CS50. For CS50, an additional ultrasonication step was necessary to yield defect free fibers.

Edible 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. To further improve the survivability of probiotics, Akkurt et al (2022) blended aqueous calcium caseinate (CaCAS) and sodium caseinate (NaCAS) with pullulan (PUL) aqueous solutions for electrospinning into fibers. Lactobacillus rhamnosus GG (LGG) was added into the solution just before electrospinning. The resultant electrospun fibers CAS-PUL-LGG had a mean diameter of 212 nm while NaCAS-PUL-LGG fibers had a mean diameter of 286 nm. LGG can be seen as elongated beads on part of the fibers. Viability of the LGG recovered from the nanofibrous mats after electrospinning showed no loss in the bacteria. The presence of milk milk proteins such as caseins may aid the survival of encapsulated LGG within the fibers and may potentially improve the survival of the probiotic if the CAS-PUL-LGG fiber mats were taken orally.

Scanning electron microscopy images of electrospun Lactobacillus rhamnosus GG (LGG) incorporated in fibrous mats from CaCAS-PUL-LGG [Akkurt et al 2022].