Agar Films Research Articles

Tailoring the physico-chemical and antimicrobial properties of agar-based films by in situ formation of Cu-mineral phase

New agar-based composite films with increasing Cu-carbonate and Cu-phosphate mineral phase content were prepared by in situ mineralization and solvent casting method. SEM and optical analysis revealed that Cu-carbonate phase had better compatibility with agar matrix than Cu-phosphate phase. Incorporation of both mineral phases improved mechanical and water vapor barrier properties of the obtained mineralized films, in concentration dependent manner. When 5 mM of carbonate precursor was incorporated into agar matrix, mechanical resistance was enchanced for 44% and water vapor barrier property for 40%. The release of Cu (II) was higher in acidic conditions for both mineralized composites and remained in the range of specific release limits for this metal. In addition, both mineralized composite films exhibited distinctive antimicrobial activity against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. Overall, the Cu-carbonate and Cu-phosphate mineralized agar films showed potential to be used for food packaging materials, agriculture or medical purposes.

Cost-efficient bio-based food packaging films from unpurified agar-based extracts

• Unpurified agar-based extracts were used to produce bio-based food packaging films. • Unpurified agar-based films did not require the addition of plasticizer. • Polysaccharide/protein IPNs improved the mechanical and water barrier properties. • Unpurified agar-based films showed improved resistance to high humidity conditions. • The presence of impurities provided additional light blocking capacity. This work reports on the production of sustainable, cost-efficient food packaging films from unpurified agar-based extracts obtained from Gelidium sesquipedale seaweed. Unlike the purified agars, that required the addition of a plasticizer, the presence of impurities such as other polysaccharides (mainly floridean starch) and proteins exerted a plasticization effect in the unpurified agars, forming flexible films with no added plasticizer. The formation of interpenetrating polysaccharide/protein networks in the unpurified agar films had a positive impact in their mechanical and water barrier performance, which were similar or even better than those from the purified agars. Furthermore, these films showed improved resistance to high humidity conditions and retained their film structure, as opposed to the purified agar films, which gelled upon contact with moisture. Finally, the presence of impurities also provided the unpurified agar films greater light blocking capacity, inhibiting to a further extent the UV light-induced degradation of β-carotene.

Biocompatible agar/xanthan gum composite films: Thermal, mechanical, UV, and water barrier properties

Binary composite films were prepared from agar (A) and xanthan gum (X) with different weight percentages. The composite films were transparent, lightweight, eco‐friendly, and biodegradable. The structure and morphology of the prepared agar and AX composite films were confirmed by Fourier transform infrared spectroscopy (FT‐IR), X‐ray powder diffraction (XRD), and scanning electron microscopy (SEM) technique. The glass transition temperature (Tg) and melting temperature (Tm) of the AX composite films was slightly improved, when compared with the neat agar. Thermogravimetric analysis (TGA) analysis showed a considerable increment in the char yield and an improved thermal stability. The tensile strength was in the range of 25 to 40 MPa, and elongation at break was in the range of 28.9 to 39.4%. Though, the water vapor permeability (WVP) value reduced in the composite films the difference was not significant. From the mechanical studies, we can deduce that agar and xanthan gum are compatible and miscible with one another leading to prospects of their composite films being considered for different biomedical and packaging applications.

Agar-based antioxidant composite films incorporated with melanin nanoparticles

Melanin nanoparticles (MNP) were isolated from the sepia ink using a centrifugation method and used as a functional filler for the preparation of agar-based functional films. The MNP were spherical with an average diameter of 95.6 ± 21.2 nm. Field emission scanning electron microscopy (FE-SEM) and Fourier transform infrared spectroscopy (FTIR) test results indicated that the MNP were well dispersed in agar polymer to form free-standing composite films. The addition of MNP enhanced the UV-blocking, hydrophobicity, mechanical, and water vapor barrier properties of the agar film. Also, the agar/MNP composite films showed a high antioxidant activity comparable to ascorbic acid. The MNP separated from sepia ink can be used as a functional filler to develop antioxidant biopolymer films for food packaging and biomedical applications.

INVESTIGATION ON OXYGEN PERMEABILITY AND SURFACE PROPERTIES OF AGAR FILMS

In this work, agar (AG) films have been prepared using the cast method technique. Surface properties such as contact angle (CA) and morphology of the films were carried out. The water contact angle on the surface of agar film was recorded to be 74.91 ±0.35 indicating good hydrophilicity of the surface, which was confirmed by the scanning electron microscope (SEM) image of the AG film. The contact angle was also studied with the variation of time. After making the initial contact on the substrate, the water contact angle on the AG film was found to decrease gradually with time. Studying the oxygen permeability of the agar film showed that the film had an oxygen permeability of about 187 barer.

Mild Synthesis of Copper Nanoparticles with Enhanced Oxidative Stability and Their Application in Antibacterial Films.

Copper nanoparticles possess unique physical and chemical properties; however, their application is often restricted, owing to their tendency to oxidize. In this work, we prepared copper nanoparticles with enhanced oxidative stability via a simple and low-cost method, where a modified starch was used as an environmentally friendly reducing agent and biocompatible polyethylenimine was used as a stabilizer. The prepared copper nanoparticles could be stored in air for at least 6 months without any oxidation in a dried state. Interestingly, our synthesis could even be performed at room temperature with a longer reaction time. We used various characterization methods to study the reaction mechanism. The prepared copper nanoparticles were further uniformly doped into an agar film, and this composite showed excellent bacterial killing efficiency, owing to the antibacterial properties of the copper nanoparticles. Our composite film shows potential for various clinical applications, such as wound dressing materials.

Isolation and characterization of cellulose nanocrystals from Saharan aloe vera cactus fibers

Saharan aloe vera cactus leaves fibers (SACLFs) are a biodegradable and renewable agro textile fiber enriched with cellulose polysaccharides. The isolation of cellulose nanocrystals (CNCs) from SACLFs is an alternative approach to reinforcement for bio-composites applications. In this study, we show that standard chemical methods for the extraction of cellulose microfibrils and sulfuric acid hydrolysis (64 wt%) will result in usable CNCs. The resulting CNCs were characterized by XRD, FTIR, TGA and TEM. Fourier Transform Infrared Spectroscopy (FT-IR) confirms that the maximum amount of noncellulosic materials is removed from SACLFs. High yield content (32%) of CNCs is achieved by sulfuric acid hydrolysis and sonication process. It also enhances the crystalline size and crystallinity index of CNCs, which are confirmed by XRD. TEM analysis of CNCs shows a rod-like structure with diameter (10–14 nm), length (200–255 nm) and high aspect ratio of 18–20, which confers good reinforcing properties. These were tested in biodegradable agar film, whose tensile strength is increased by 27% when 5 wt% of CNCs from SACLFs was incorporated into the matrix. The SACLFs represent an interesting source of cellulosic reinforcing materials.

High Sensitivity Humidity Fiber-Optic Sensor Based on All-Agar Fabry–Perot Interferometer

A high sensitivity humidity fiber sensor based on all-agar Fabry–Perot (FP) interferometer is proposed and experimentally demonstrated. The sensor is fabricated by filming a thin layer of moisture-sensitive agar to single mode fiber with end dipping method. Due to the all-agar FP cavity structure, both the size and the RI changes of agar film in the swelling effect are fully utilized to induce the frequency shift, the sensor exhibits a high sensitivity of 4.20 nm/%RH, with relative humidity (RH) ranging from 25%RH to 95%RH at 25 °C. Moreover, owing to the short FP cavity, $7.10~\mu \text{m}$ , the proposed sensor provides a short response time less than 340 ms with the humidity level changed from 64.0%RH to 74.2%RH. The proposed sensor also exhibits other superior performances in terms of good stability, low cost, and easy fabrication. Such good characteristics make the proposed sensor a good candidate for practical applications.

Quantitative SIMS Imaging of Agar-Based Microbial Communities

After several decades of widespread use for mapping elemental ions and small molecular fragments in surface science, secondary ion mass spectrometry (SIMS) has emerged as a powerful analytical tool for molecular imaging in biology. Biomolecular SIMS imaging has primarily been used as a qualitative technique; although the distribution of a single analyte can be accurately determined, it is difficult to map the absolute quantity of a compound or even to compare the relative abundance of one molecular species to that of another. We describe a method for quantitative SIMS imaging of small molecules in agar-based microbial communities. The microbes are cultivated on a thin film of agar, dried under nitrogen, and imaged directly with SIMS. By use of optical microscopy, we show that the area of the agar is reduced by 26 ± 2% (standard deviation) during dehydration, but the overall biofilm morphology and analyte distribution are largely retained. We detail a quantitative imaging methodology, in which the ion intensity of each analyte is (1) normalized to an external quadratic regression curve, (2) corrected for isomeric interference, and (3) filtered for sample-specific noise and lower and upper limits of quantitation. The end result is a two-dimensional surface density image for each analyte. The sample preparation and quantitation methods are validated by quantitatively imaging four alkyl-quinolone and alkyl-quinoline N-oxide signaling molecules (including Pseudomonas quinolone signal) in Pseudomonas aeruginosa colony biofilms. We show that the relative surface densities of the target biomolecules are substantially different from values inferred through direct intensity comparison and that the developed methodologies can be used to quantitatively compare as many ions as there are available standards.

Effects of agar films incorporated with fish protein hydrolysate or clove essential oil on flounder (Paralichthys orbignyanus) fillets shelf-life

This study evaluated the effect of protein hydrolysate (PH) and clove essential oil (CEO) on agar film properties and the shelf-life improvement of flounder fillets. Firstly, Argentine croaker muscle protein was alkali-solubilised and recovered to obtain a stable substrate. This substrate was hydrolysed to 20% degree of hydrolysis, by Alcalase (A20) and Protamex (P20). The molecular weight (MW) distribution, amino acid and in vitro antimicrobial activity of these hydrolysates were tested. Agar films were prepared with and without (control) incorporation of PH (A20 hydrolysate) or CEO and characterised by mechanical properties, water vapour permeability, solubility and optical properties. The microbiological, physical and chemical properties of flounder fillets coated with these films were monitored during storage at 5 °C for 15 days. Compared to P20, A20 hydrolysate possessed lower MW, higher hydrophobic amino acid content and the most effective antimicrobial activity against selected pathogens, such as Staphylococcus aureus, Yersinia enterocolitica, Aeromonas hydrophila, Debaryomyces hansenii and Listeria innocua. The presence of PH increased the water solubility, water vapour permeability, elongation at break and yellowness of the films. The CEO films possessed less transparency than PH films and control films, due to the presence of oil in the formulations. Fillets covered with PH and CEO films, especially the latter, presented lower total volatile bases and pH values, and delayed considerably the growth of H2S-producing microorganisms compared to the control films. The CEO might be used as a natural biopreservative to extend the flounder shelf-life. However, further studies are necessary, to assess the antimicrobial effect of PH when used as food packaging.

Enhanced antimicrobial activity of the food-protecting nisin peptide by bioconjugation with silver nanoparticles

Nisin is an antimicrobial peptide widely used in the food industry. The efficacy of nisin has decreased due to the development of resistant bacteria. For instance, bacteria such as Staphylococcus aureus have resistance by digesting nisin using the nisinase enzyme. The efficacy of nisin could be improved using bioconjugation with metal nanoparticles. Here we synthesized silver nanoparticles using the extract of Cymbopogon citratus; then, we bioconjugated those silver nanoparticles with nisin to form a nanosilver bioconjugate. Silver nanoparticles and silver bioconjugate were characterized by UV–Vis spectroscopy, nanoparticle tracking analysis, zeta potential measurement and transmission electron microscopy. In vitro antimicrobial efficacy of both silver nanoparticles and silver bioconjugate was evaluated against selected food spoilage microorganisms such as Listeria monocytogenes, S. aureus, Pseudomonas fluorescens, Aspergillus niger and Fusarium moniliforme. Results show that the antimicrobial potential of nisin increased after bioconjugation with silver nanoparticles. Further, we developed agar film containing nanosilver bioconjugate and also evaluated its antimicrobial activity against selected food spoilage microorganisms. The agar film demonstrated maximum activity against P. fluorescens, of 19 mm, and the minimum against F. moniliforme, of 12 mm. Overall, agar film containing nisin and silver nanoparticles can be used against food spoilage microorganisms.

Chemical characterization of wash water biomass from shrimp surimi processing and its application to develop functional edible films.

The aim of this work was to recover and study the composition of the biomass obtained from the surimi processing industry, and to explore the feasibility of its valorization by developing functional edible films. A concentrate from wash water of minced shrimp muscle of low commercial value [shrimp concentrate (SC)] was obtained by alkaline solubilization and isoelectric precipitation, which had protein content of 61.8% and fat 23.6%. The protein fraction of low molecular weight (≤37kDa), with maximum solubility at basic pH 9-11, and was rich in essential amino acids (296 essential amino acids/1000 residues), whereas the fat fraction showed a high degree of unsaturation (26.8% MUFA, 28.3% PUFA). SC, showed antioxidant activity measured by FRAP and ABTS assays and, was successfully incorporated into an agar film matrix (agar-to-protein ratio of 3:1). The films were translucent and brownish and maintained integrity upon immersion in water, but swelled considerably. Films were able to release protein and exert antioxidant activity in water and 95% ethanol (aqueous and fatty food simulants, respectively), the release being faster and greater in the former. In conclusion, the biomass obtained from shrimp mince/surimi processing was for developing edible films.

Edible film incorporated with chitosan and Artemisia annua oil nanoliposomes for inactivation of Escherichia coli O157:H7 on cherry tomato

SummaryAs a natural antibacterial agent, Artemisia annua oil (AAO) exhibited significant antibacterial effect on Escherichia coli O157:H7. However, AAO was volatile and not suitable for surface coatings of fresh produce. Therefore, the edible agar films containing chitosan and AAO liposomes were engineered to overcome the defect. Chitosan was added in the film to enhance the antibacterial activity. Finally, the results showed the mean size of AAO liposomes was about 191.8 nm with a polydispersity index (PDI) of 0.463. The absolute zeta potential value was greater than 30 mV, and the loading efficiency (LE) of 5.0 mg mL−1 AAO liposome was 67.45%. As a proof of concept, the antibacterial activity of the films against E. coli O157:H7 on cherry tomatoes was evaluated. The results indicated that the incorporation of AAO liposomes as a natural antibacterial agent possessed bacteriostatic effect, which had potential for using the developed film as an active packaging.

Effect of hydrocarbon surfactants on dexamethasone solubilization into silicone surfactant micelles in aqueous media and its release from agar films as carriers

Silicone surfactant (SS) micelles are one of the unexplored systems especially for drug solubilization. SS are increasingly becoming important as pharmaceutical additives because, the polydimethylsiloxane part of it is very permeable to the diffusion of active drugs. In the present work, the effect of hydrocarbon surfactants (HS) namely sodium dodecyl sulfate (SDS), or dodecyltrimethylammonium bromide (DTAB), or polyoxyethylene octyl phenyl ether (TX-100), as additives on the solubilization of dexamethasone (dex), a poorly water soluble drug into the micelles of a nonionic trisiloxane silicone surfactant in water was measured. The dex loaded micellar solutions were uniformly dispersed in the agar hydrogels as carriers. The release of dex (by simple dilution) from the dry cast gel films was monitored at different time intervals and the release data were analyzed mathematically in terms of diffusion models. The drug diffusion coefficients, Ddrug were calculated and it was found that Ddrug values linearly decreased with the concentration of HS or SS and while the addition of HS increased the Ddrug in SS micellar solutions indicating that HS facilitate not only release of dex but also enhance its rate of release. The enhancement of dex release from SS micelles are closely related to the dissolution of SS micelles.

Preparation and characterization of agar-based nanocomposite films reinforced with bimetallic (Ag-Cu) alloy nanoparticles

Agar-based active nanocomposite films were prepared by incorporating silver-copper (Ag-Cu) alloy nanoparticles (NPs) (0.5–4wt%) into glycerol plasticized agar solution. Thermo-mechanical, morphological, structural, and optical properties of the nanocomposite films were characterized by texture analyzer, differential scanning calorimetry (DSC), scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transforms infrared (FTIR) spectroscopy, and surface color measurement. Tensile strength and the melting temperature of the film increased linearly with NPs loading concentration. Color, transparency and UV barrier properties of agar films were influenced by the reinforcement of Ag-Cu NPs. XRD analysis confirmed the crystalline structure of the Agar/Ag-Cu nanocomposite films, whereas the smoothness and the homogeneity of film surface strongly reduced as observed through the SEM. The nanocomposite films exhibited a profound antibacterial activity against both Gram-positive (Listeria monocytogenes) and Gram-negative (Salmonella enterica sv typhimurium) bacteria. Overall, the agar nanocomposite films could be used as packaging material for food preservation by controlling foodborne pathogens and spoilage bacteria.

Ambient Mass Spectrometry Imaging with Picosecond Infrared Laser Ablation Electrospray Ionization (PIR-LAESI).

A picosecond infrared laser (PIRL) is capable of cutting through biological tissues in the absence of significant thermal damage. As such, PIRL is a standalone surgical scalpel with the added bonus of minimal postoperative scar tissue formation. In this work, a tandem of PIRL ablation with electrospray ionization (PIR-LAESI) mass spectrometry is demonstrated and characterized for tissue molecular imaging, with a limit of detection in the range of 100 nM for reserpine or better than 5 nM for verapamil in aqueous solution. We characterized PIRL crater size using agar films containing Rhodamine. PIR-LAESI offers a 20-30 μm vertical resolution (∼3 μm removal per pulse) and a lateral resolution of ∼100 μm. We were able to detect 25 fmol of Rhodamine in agar ablation experiments. PIR-LAESI was used to map the distribution of endogenous methoxykaempferol glucoronide in zebra plant (Aphelandra squarrosa) leaves producing a localization map that is corroborated by the literature. PIR-LAESI was further used to image the distribution inside mouse kidneys of gadoteridol, an exogenous magnetic resonance contrast agent intravenously injected. Parallel mass spectrometry imaging (MSI) using desorption electrospray ionization (DESI) and matrix assisted laser desorption ionization (MALDI) were performed to corroborate PIR-LAESI images of the exogenous agent. We further show that PIR-LAESI is capable of desorption ionization of proteins as well as phospholipids. This comparative study illustrates that PIR-LAESI is an ion source for ambient mass spectrometry applications. As such, a future PIRL scalpel combined with secondary ionization such as ESI and mass spectrometry has the potential to provide molecular feedback to guide PIRL surgery.

Seasonal Preservation Success of the Marine Dinoflagellate Coral Symbiont, Symbiodinium sp.

Coral reefs are some of the most diverse and productive ecosystems on the planet, but are threatened by global and local stressors, mandating the need for incorporating ex situ conservation practices. One approach that is highly protective is the development of genome resource banks that preserve the species and its genetic diversity. A critical component of the reef are the endosymbiotic algae, Symbiodinium sp., living within most coral that transfer energy-rich sugars to their hosts. Although Symbiodinium are maintained alive in culture collections around the world, the cryopreservation of these algae to prevent loss and genetic drift is not well-defined. This study examined the quantum yield physiology and freezing protocols that resulted in survival of Symbiodinium at 24 h post-thawing. Only the ultra-rapid procedure called vitrification resulted in success whereas conventional slow freezing protocols did not. We determined that success also depended on using a thin film of agar with embedded Symbiodinium on Cryotops, a process that yielded a post-thaw viability of >50% in extracted and vitrified Symbiodinium from Fungia scutaria, Pocillopora damicornis and Porites compressa. Additionally, there also was a seasonal influence on vitrification success as the best post-thaw survival of F. scutaria occurred in winter and spring compared to summer and fall (P < 0.05). These findings lay the foundation for developing a viable genome resource bank for the world’s Symbiodinium that, in turn, will not only protect this critical element of coral functionality but serve as a resource for understanding the complexities of symbiosis, support selective breeding experiments to develop more thermally resilient strains of coral, and provide a ‘gold-standard’ genomics collection, allowing for full genomic sequencing of unique Symbiodinium strains.

Effect of lignin on water vapor barrier, mechanical, and structural properties of agar/lignin composite films

Biodegradable composite films were prepared using two renewable resources based biopolymers, agar and lignin alkali. The lignin was used as a reinforcing material and agar as a biopolymer matrix. The effect of lignin concentration (1, 3, 5, and 10wt%) on the performance of the composite films was studied. In addition, the mechanical, water vapor barrier, UV light barrier properties, FE-SEM, and TGA of the films were analyzed. The agar/lignin films exhibited higher mechanical and UV barrier properties along with lower water vapor permeability compared to the neat agar film. The FTIR and SEM results showed the compatibility of lignin with agar polymer. The swelling ratio and moisture content of agar/lignin composite films were decreased with increase in lignin content. The thermostability and char content of agar/lignin composite films increased with increased lignin content. The results suggested that agar/lignin films have a potential to be used as a UV barrier food packaging material for maintaining food safety and extending the shelf-life of the packaged food.

Effect of post-treatments and concentration of cotton linter cellulose nanocrystals on the properties of agar-based nanocomposite films

Cellulose nanocrystals (CNCs) were prepared by acid hydrolysis of cotton linter pulp fibers and three different purification methods, i.e., without post purification (CNC1), dialyzed against distilled water (CNC2), and neutralized with NaOH (CNC3), and their effect on film properties was evaluated by preparation of agar/CNCs composite films. All the CNCs were rod in shape with diameter of 15–50nm and length of 210–480nm. FTIR result indicated that there was no distinctive differences in the chemical structure between CNCs and cotton linter cellulose fiber. No significant relationship was observed between the sulfate content and crystallinity index of CNCs. The CNC3 showed higher thermal stability than the other type of CNCs due to the less adverse effect on the thermal stability of sulfate groups induced by the neutralization with NaOH. The tensile strength (TS) of agar film increased by 15% with incorporation of 5wt% of CNC3, on the contrary, it decreased by 10% and 15% with incorporation of CNC1 and CNC2, respectively. Other performance properties of agar/CNCs composite films such as optical and water vapor barrier properties showed that the CNC3 was more effective filler than the other CNCs. In the range of concentration of CNC3 tested (1–10wt%), inclusion of 5wt% of CNC3 was the maximum concentration for improving or maintaining film properties of the composite films. The neutralization of acid hydrolyzed cellulose using NaOH was simple and convenient for the preparation of CNC and bionanocomposite films.

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 (1wt%) 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.