Incorporation of Tannic Acid in Food-Grade Guar Gum Fibrous Mats by Electrospinning Technique.

The use of polysaccharides to produce functional micro- or nanoscale fibrous mats has attracted growing interest for their food-grade applications. In this study, the characterization and electro-spinnability of guar gum (GG) solutions loaded with tannic acid (TA) was demonstrated. Food-grade antioxidant materials were successfully produced by electrospinning while incorporating different loads of TA into GG fibers. Bead-free GG-TA fibers could be fabricated from GG solution (2 wt %) with 10 wt % TA. Increasing the amount of TA led to fibers with defects and larger diameter sizes. Fourier Transformed Infrared Spectroscopy and X-ray Diffraction of neat GG and TA loaded GG fibrous mats suggested that inclusion of TA interrupted the hydrogen bonding and that a higher density of the ordered junction zones formed with the increased TA. The high TA incorporation efficiency and retained antioxidant activity of the fibrous mats afford a potential application in active edible film or drug delivery system.

Electrospun ultra-fine cellulose acetate fibrous mats containing tannic acid-Fe3+ complexes.

Cellulose acetate (CA) fibrous mats with improved mechanical and antioxidant properties were produced by a simple, scalable and cost-effective electrospinning method. Fibers loaded with small amounts of TA-Fe3+ complexes showed an increase in tensile strength of ∼117% when compared to that of neat CA and were more resistant than those loaded with TA alone. The water uptake of the fibers increased upon TA or TA-Fe3+ incorporation while their thermal behavior was only slightly affected. Fibrous mats loaded with TA-Fe3+ showed comparable antioxidant activity with that of CA/TA mats, and a much slower TA release. These results suggest that TA-Fe3+ complexes can be incorporated into electrospun CA fibers to improve their mechanical properties and antioxidant activity which may be of interest for the development of active packaging that can extend the shelf life of perishable foods.

Electrospun Polymer Nanofibers Reinforced by Tannic Acid/Fe+++ Complexes.

We report the successful preparation of reinforced electrospun nanofibers and fibrous mats of polyvinyl alcohol (PVA) via a simple and inexpensive method using stable tannic acid (TA) and ferric ion (Fe+++) assemblies formed by solution mixing and pH adjustment. Changes in solution pH change the number of TA galloyl groups attached to the Fe+++ from one (pH < 2) to two (3 < pH < 6) to three (pH < 7.4) and affect the interactions between PVA and TA. At pH ~ 5.5, the morphology and fiber diameter size (FDS) examined by SEM are determinant for the mechanical properties of the fibrous mats and depend on the PVA content. At an optimal 8 wt % concentration, PVA becomes fully entangled and forms uniform nanofibers with smaller FDS (p < 0.05) and improved mechanical properties when compared to mats of PVA alone and of PVA with TA (p < 0.05). Changes in solution pH lead to beads formation, more irregular FDS and poorer mechanical properties (p < 0.05). The Fe+++ inclusion does not alter the oxidation activity of TA (p > 0.05) suggesting the potential of TA-Fe+++ assemblies to reinforce polymer nanofibers with high functionality for use in diverse applications including food, biomedical and pharmaceutical.

Strategies to improve the mechanical strength and water resistance of agar films for food packaging applications.

Agar films possess several properties adequate for food packaging applications. However, their high cost-production and quality variations caused by physiological and environmental factors affecting wild seaweeds make them less attractive for industries. In this work, native (NA) and alkali-modified (AA) agars obtained from sustainably grown seaweeds (integrated multi-trophic aquaculture) were mixed with locust bean gum (LBG) to make 'knife-coated' films with fixed final concentration (1 wt%) and variable agar/LBG ratios. Agar films were easier to process upon LBG addition (viscosity increase and gelling character decrease of the film-forming solutions observed by dynamic oscillatory and steady shear measurements). The mechanical properties and water resistance were optimal for films with 50 and/or 75% LBG contents and best in the case of NA (cheaper to extract). These findings can help reduce the cost-production of agar packaging films. Moreover, the controlled cultivation of seaweeds can provide continuous and reliable feedstock for transformation industries.

Alternative plasticizers for the production of thermo-compressed agar films.

Agar films were produced by thermo-compression using choline chloride (ChCl) as a plasticizer with urea. The three solid components were mixed together with the salt and urea (minor components) added to agar (main component) according to a fixed mass ratio of, respectively, 1.16:1:5. A central composite rotatable design (CCRD) with three parameters, 2(3), was used to evaluate the effects of temperature (X1; °C), time (X2; min) and applied load (X3; kN) of heat-pressing on the maximum tensile strength (TS) of the films (Y; MPa). Mixtures of urea and agar prepared at a mass ratio of 1:5 did not form homogeneous films suggesting the important plasticizing role of the salt. Heat-pressing the mixtures at more draconian conditions led to much darker and opaque films, with better mechanical resistance (higher values of TS). The most resistant film (∼ 15 MPa) was obtained at 140°C, 20 min and 176 kN. Selected films, including the optimal, showed similar water sorption profiles and close values of water vapor permeability (∼ 2.5-3.7 × 10(-9)gm(-1)s(-1)Pa(-1)). The fracture behavior and mechanical properties of the films were greatly affected by additional water plasticization when the films were stored at different conditions of relative humidity.

Electrospinning of agar/PVA aqueous solutions and its relation with rheological properties.

In this work, we report the successful fabrication of agar-based nanofibers by electrospinning technique, using water as solvent media. A tubeless spinneret was attached inside the electrospinning chamber, operating at 50°C, to avoid agar gelation. Agar pure solution (1 wt%) showed inadequate spinnability regardless of the used electrospinning conditions. The addition of a co-blending polymer such as PVA (10 wt% starting solution) improved the solutions viscoelasticity and hence, the solutions spinnability. Agar/PVA solutions were prepared with different mass ratios (100/0, 50/50, 40/60, 30/70, 20/80 and 0/100) and electrospun at various sets of electrospinning conditions. Best nanofibers were obtained with 30/70 and 20/80 agar/PVA blends while samples with higher agar contents (50/50 and 40/60 agar/PVA) were harder to process and led to discontinuous fibrous mats. This first set of encouraging results can open a new window of opportunities for agar-based biomaterials in the form of nanofibers.

Improving agar electrospinnability with choline-based deep eutectic solvents.

Very recently our group has produced novel agar-based fibers by an electrospinning technique using water as solvent and polyvinyl alcohol (PVA) as co-blending polymer. Here, we tested the deep eutectic solvent (DES), (2-hydroxyethyl)trimethylammonium chloride/urea prepared at 1:2 molar ratio, as an alternative solvent medium for agar electrospinning. The electrospun materials were collected with an ethanol bath adapted to a previous electrospinning set-up. One weight percent agar-in-DES showed improved viscoelasticity and hence, spinnability, when compared to 1 wt% agar-in-water and pure agar nanofibers were successfully electrospun if working above the temperature of sol-gel transition (∼80 °C). By changing the solvent medium we decreased the PVA concentration (5 wt% starting solution) and successfully produced composite fibers with high agar contents (50/50 agar/PVA). Best composite fibers were formed with the 50/50 and 30/70 agar/PVA solutions. These fibers were mechanically resistant, showed tailorable surface roughness and diverse size distributions, with most of the diameters falling in the sub-micron range. Both nano and micro forms of agar fibers (used separately or combined) may have potential for the design of new and highly functional agar-based materials.

Choline chloride based ionic liquid analogues as tool for the fabrication of agar films with improved mechanical properties.

In the present paper, we test the suitability of ChCl/urea (DES-U) and ChCl/glycerol (DES-G) eutectic mixtures, each one prepared at 1:2 molar ratio, for the production of agar films. A three-step process is proposed: pre-solubilization of polymer in DES followed by compression-molding and subsequent drying. The mechanical properties, water resistance and microstructure of the films were evaluated at different polymer concentrations (i.e. 2-6%, w/w). DES-U showed by far, the best film forming ability. Agreeing with the diffusion and SEM data, films with the best mechanical properties were found at the lowest and highest agar concentrations (tensile strengths of 24.2-42 MPa and elongations of 15.4-38.9%). The water sorption and contact angle studies suggested increased hydrophilicity for the film containing the lowest concentration of agar. The use of choline chloride based ionic liquid analogues as solvent and plasticizer might be a promising tool for the development of new non-aqueous materials based on seaweed polysaccharides.

Influence of the extraction process on the rheological and structural properties of agars.

Agars obtained by traditional hot-water (TWE) and microwave-assisted (MAE) extractions were compared in terms of their rheological and physicochemical properties and molecular self-association in solutions of low (0.05%, w/w) and high (1.5%, w/w) polymer concentrations. At low concentration, thin gelled layers were imaged by AFM. Slow or rapid cooling of the solutions influenced structure formation. In each case, TWE and MAE agar structures were different and apparently larger for MAE. At high concentration, progressive structural reinforcement was seen; while TWE agar showed a more open and irregular 3D network, MAE agar gel imaged by cryoSEM was denser and fairly uniform. The rheological (higher thermal stability and consistency) and mechanical (higher gel strength) behaviors of MAE agar seemed consistent with a positive effect of molecular mass and 3,6-anhydro-α-l-galactose content. MAE produced non-degraded agar comparable with commercial ones and if properly monitored, could be a promising alternative to TWE.

Complexation of WPI and microwave-assisted extracted agars with different physicochemical properties.

The complex formation between whey protein isolate (WPI) and agar has been investigated and their interactions were monitored as a function of the physicochemical properties of agar, the pH and the ionic strength of the medium. Agars from Gracilaria vermiculophylla were extracted under different MAE conditions and characterized according to their physicochemical properties. By using microwave irradiation a wide variety of agars was obtained, as different MAE conditions results in polyelectrolytes with distinct properties. UV-vis (in optical dispersion (O.D.) model) spectroscopy and isothermal titration calorimetry (ITC) have been used to study the formation of insoluble (coacervate) complexes. MAE agars revealed excellent properties for complex formation with WPI. The binding of WPI to MAE agar samples has been shown to be the result of different contributions. O.D. and ITC results showed that the molecular mass and the sulfate content of different agars had a determinant effect on coacervate formation.

Structural, physical, and chemical modifications induced by microwave heating on native agar-like galactans.

Native agars from Gracilaria vermiculophylla produced in sustainable aquaculture systems (IMTA) were extracted under conventional (TWE) and microwave (MAE) heating. The optimal extracts from both processes were compared in terms of their properties. The agars' structure was further investigated through Fourier transform infrared and NMR spectroscopy. Both samples showed a regular structure with an identical backbone, ß-d-galactose (G) and 3,6-anhydro-α-l-galactose (LA) units; a considerable degree of methylation was found at C6 of the G units and, to a lesser extent, at C2 of the LA residues. The methylation degree in the G units was lower for MAE(opt) agar; the sulfate content was also reduced. MAE led to higher agar recoveries with drastic extraction time and solvent volume reductions. Two times lower values of [η] and M(v) obtained for the MAE(opt) sample indicate substantial depolymerization of the polysaccharide backbone; this was reflected in its gelling properties; yet it was clearly appropriate for commercial application in soft-texture food products.