Physical stability, microstructure and antimicrobial properties of konjac glucomannan coatings enriched with Litsea cubeba essential oil nanoemulsion and its effect on citruses preservation.

This study purposed to develop konjac glucomannan (KGM) based antimicrobial coatings containing Litsea cubeba essential oil nanoemulsion (LNE) for citruses preservation. Physical stability, rheological, structural and antimicrobial properties of the coating solutions were investigated, along with the release characteristics of Litsea cubeba essential oil (LCO). Results showed that the coating solutions displayed shear thinning behavior. The oil droplets were distributed homogeneously in KGM phase with good stability. The coating structure became loose with increasing LNE content due to LNE interfering with molecular interactions and entanglement of KGM. The coating solutions showed stronger antibacterial activity against Escherichia coli than against Staphylococcus aureus and were effective in inhibiting the growth of Penicillium italicum on citrus surfaces. KGM-LNE 10 negatively affected citruses due to phytotoxicity caused by high levels of LCO. LCO was released slowly and continuously from the coatings, and its release was faster in deionized water than in an ethanol-water solution. KGM-LNE 2.5 coated citruses had the least weight loss, the greatest hardness, and kept the minimum changes in total soluble solids, total acid and vitamin C content, implying that KGM-LNE 2.5 best maintained the quality of citruses. The findings suggest that KGM-based coatings containing LNE have high potential for citruses preservation.

Modulating in vitro fecal fermentation behavior of sodium alginate by Ca2+ cross-linking.

Slow fermentable dietary fibers can be utilized by human gut microbiota in the distal region of the colon and thus exert a sufficient short-chain fatty acids (SCFAs) supplement in the distal region of the human colon. Alginate (Alg) based microgels are widely fabricated and used to control their digestion by digestive enzymes releasing active substances site-specifically. Herein, sodium alginate microgels with gradient calcium-ion (Ca2+) cross-linking densities were developed, restricting their degradation by gut microbiota. Alg microgels were prepared using high-speed shearing after Alg was cross-linked with 10, 40, and 60 mmol/L Ca2+, respectively (named 10-Alg, 40-Alg, and 60-Alg). The fluorescence and atomic force microscopic results showed that the 40-Alg particle has the densest structure among the three cross-linked Alg. In vitro human fecal fermentation results revealed that the Ca2+ cross-linking exerted more restricting effects than delaying effects on the fermentation of Alg, and the 40-Alg exhibited the slowest fermentation rate and the least fermentation extent, by characterizing the residual total carbohydrate content, residual monosaccharide content, pH, and total short-chain fatty acids. The 16S rRNA gene sequencing results indicated that cross-linking structures shaped a high specifical Bacteroides-type microbial community and that OTU205 (Bacteroides_xylanisolvens) highly correlated to the cross-linking density (R = 0.65, p = 0.047). In sum, Ca2+ cross-linking generated a dense and compact structure of sodium alginate that facilitated a more restricted fermentation property and specificity-targeting microbial community structure in comparison to the original sodium alginate.

Influence of interfacial properties/structure on oxygen diffusion in oil-in-water emulsions.

Oxygen diffusion played an important role in the lipid oxidation of food emulsions. In this study, a simple method was developed to quantitatively observe the oxygen diffusion in the oil-water biphasic system, and it was further applied to investigate the relationship between the oxygen diffusion and lipid oxidation in O/W emulsions. Various factors that related to the emulsion oxidation were considered, from their influence on the oxygen diffusion and lipid oxidation in the emulsions. Results showed that there was obvious correlation between the oxygen diffusion and lipid oxidation in O/W emulsions, which reveals the inhibition of oxygen diffusion could apparently slow down the lipid oxidation. Moreover, the changes of oil phase, water phase and interfacial layer of the emulsions, which were related to the oxygen diffusion, could improve the oxidative stability of the emulsions effectively. Our findings are helpful for deep understanding the mechanisms of the lipid oxidation in food emulsions.

Impact of heating and drying temperatures on the properties of konjac glucomannan/curdlan blend films.

The impact of preparation conditions including heating temperature (from 60 °C to 90 °C) and drying temperatures (from 25 °C to 90 °C) on the properties of pure curdlan film and konjac glucomannan (KGM) and curdlan blend films were analyzed. Microstructure analysis indicated the KGM addition could significantly improve the relatively poor film-forming property of curdlan. FTIR and X-ray analysis showed that at high heating temperature 90 °C, molecular interaction might be enhanced in the films due to the stretched structure of curdlan and dissociation of curdlan bundles or triple-helix structure. This was supported by the changes in the mechanical property, surface hydrophobicity, moisture barrier, and moisture tolerance property. The impacts of drying temperature were some different for the curdlan film and KGM/curdlan blend film, and were explained from the molecular hydrophilicity-hydrophobicity, compactness of the films, curdlan conformation, and molecular interaction. This work guided biodegradable film production especially with curdlan added.

Changes in microstructure and rheological properties of konjac glucomannan/zein blend film-forming solution during drying.

During film formation at 60 °C, the microstructure and rheological properties of konjac glucomannan (KGM) film-forming solution and KGM/zein blend film-forming solution were investigated. The drying process of film-forming solutions was divided into two stages according to the drying curves. Scanning electron microscopy showed that KGM chains in the blend solution aggregated into thicker chains and formed a molecular network with larger pores. Zein particles grew larger but were homogeneously distributed during drying as observed by confocal laser scanning microscopy. The addition of zein improved the thermal stability of the film-forming solution. As the drying proceeded (up to 8 h), KGM solution exhibited a typical concentrated solution behavior due to molecular entanglement; whereas the blend solution gradually formed a weak gel after 2 h. Complex viscosity data for the film-forming solutions were well-fitted by the power-law model. The information obtained from the study is important for understanding the film-forming mechanism.

Fabrication and characterization of a novel konjac glucomannan-based air filtration aerogels strengthened by wheat straw and okara.

The konjac glucomannan (KGM)-based aerogel as an air filtration material was fabricated through sol-gel and freeze-drying methods. Results showed that gelatin and starch addition could increase the filtration efficiency and compressive strength of aerogel significantly, due to the appearance of more microporous structure and the formation of dense structure in aerogel. The addition of wheat straw could decrease the filtration resistance and increase the breathability of KGM-based aerogel, which was attributed to the multi-cavities of wheat straw. The aerogel with wheat straw had a filtration efficiency of 93.54% for particle matters ≥ 0.3 µm, a filtration resistance 29 Pa, and an air permeability 271.42 L/s·m2. Okara addition could increase the hydrophobicity of KGM-based aerogel by increasing the water contact angle and decreasing the equilibrium water content. The water contact angle of the aerogel containing okara reached 105.4°, and the equilibrium water content was decreased by 17.03%-81.10% compared with that without okara, with relative humidity 0%-80%. The results demonstrated that the KGM-based aerogel had good performance on filtration, mechanical and hydrophobic properties, indicating high potential application as an air filtration material.

Effect of drying temperature on structural and thermomechanical properties of konjac glucomannan-zein blend films.

Konjac glucomannan (KGM)/zein blend films were successfully prepared by solution casting at different drying temperatures (40, 50, 60, 70 and 80°C). The effects of drying temperature on the films' structural, thermomechanical, mechanical and water barrier properties were investigated. Microstructural observations indicated that zein particles were homogeneously dispersed in KGM continuous matrix, and the blend film dried at 60°C showed the most compact and smooth surface. Dynamic mechanical thermal analysis curves showed that with increasing drying temperature from 40 to 60°C, glass transition temperature (Tg) of films increased; however, with further increase in temperature, the Tg decreased, indicating the compatibility of film components was the highest when dried at 60°C. The hydrophobicity of blend film dried at 60°C was significantly stronger than that of other blend films, supported by the highest water contact angle, and the lowest swelling ratio and solubility. Moreover, the film dried at 60°C showed the highest tensile strength, elongation at break, and the lowest water vapor permeability. Therefore 60°C was preferred for KGM/zein blend film preparation. This study indicated that intermolecular interactions among film components were greatly influenced by the drying temperature, and should be carefully noticed for film preparation.

Stability, microstructure and rheological behavior of konjac glucomannan-zein mixed systems.

This study aims to investigate the stability, microstructure and rheological properties of konjac glucomannan (KGM)/zein mixed systems in different mixing ratios. A phase diagram was established by centrifugation and visual observation. KGM/zein could form a stable homogeneous dispersion with appropriate mixing formula, and the particle size in mixed systems increased with increasing zein content. During storage, zein particles increased in size but were homogeneously distributed in the continuous phase of KGM without flocculation as observed by confocal light scanning microscopy. The rate of particle size change slowed down with increasing concentration of KGM. Transmission electron microscopy and atomic force microscopy images showed that zein particles were distributed in the KGM molecular network. The mixed systems showed shear-thinning behavior, and the temperature dependence of the viscosity was well-fitted by the Arrhenius equation. Based on dynamic viscoelasticity analysis, the mixed systems showed typical behaviors for entangled polymer solutions. The shift of cross-over frequency of storage (G') and loss (G″) moduli to higher frequencies with increasing concentration of zein implied the shortening of the lifetime of the temporary entangled junction in the mixed systems.

Structural characterization and properties of konjac glucomannan and zein blend films.

A series of konjac glucomannan (KGM)/zein blend films were successfully prepared with zein in proportions 0-30%. The hydrophobicity of blend films were significantly stronger than pure KGM film, indicated by increased contact angle, swelling and solubility properties, and moisture absorption. Moreover, other properties including mechanical, thermal, water vapor and oxygen barrier were also found to be increased. FTIR indicated that hydrogen bond interactions and Maillard reaction occurred between KGM and zein molecules, and microstructural observations indicated that the aggregated zein was homogeneously dispersed in the KGM continuous matrix. However, these zein aggregations were larger with increased proportion of zein, leading to weakened molecular interactions with zein proportion >10%. A mixing ratio of KGM:zein=9:1 was suggested to provide best film properties. This research offers an alternative improvement for KGM-based biodegradable films.

pH-Sensitive drug delivery system based on hydrophobic modified konjac glucomannan.

Amphiphilic aliphatic amines grafted konjac glucomannan (KGM-g-AH8, KGM-g-AH12 and KGM-g-AH18) micelles were prepared via a simple two-step synthesis with Schiff's base as the "switch" to achieve intracellular acid-triggered curcumin release. The KGM-g-AH8 self-assembled into spherical nano-micelles (107.6±11.6nm) in an aqueous medium, and presented high curcumin loading capacity as well as good physical stability in 28 days. The in vitro curcumin release behaviors proved the controlled release property and the endosomal/lysosomal pH response of KGM-g-AH8 micelles. The cytotoxicity and cellular uptake studies were also investigated to exhibit the intracellular pH-sensitivity, safety and biocompatibility of KGM-g-AH8 micelles. This research focuses on the feasibility of KGM-based micelles to be extrapolated as promising strategies for cancer therapy and offers new potential options for intracellular drug delivery.

The control of ice crystal growth and effect on porous structure of konjac glucomannan-based aerogels.

Konjac glucomannan (KGM)-based aerogels were prepared using a combination of sol-gel and freeze-drying methods. Preparation conditions were chosen to control ice crystal growth and aerogel structure formation. The ice crystals formed during pre-freezing were observed by low temperature polarizing microscopy, and images of aerogel pores were obtained by scanning electron microscopy. The size of ice crystals were calculated and size distribution maps were drawn, and similarly for aerogel pores. Results showed that ice crystal growth and aerogel pore sizes may be controlled by varying pre-freezing temperatures, KGM concentration and glyceryl monostearate concentration. The impact of pre-freezing temperatures on ice crystal growth was explained as combining ice crystal growth rate with nucleation rate, while the impacts of KGM and glyceryl monostearate concentration on ice crystal growth were interpreted based on their influences on sol network structure.

Physical stability and rheological properties of konjac glucomannan-ethyl cellulose mixed emulsions.

Konjac glucomannan (KGM)/ethyl cellulose (EC) could form a stable homogeneous emulsion with appropriate mixing formula. This study aimed to investigate the stable mechanism of KGM/EC emulsion at different mixing ratio through its rheological properties, and droplet size and morphology changes with up to 6days storage time. Though emulsions samples with high KGM content had larger droplet size and worse uniformity at the fresh stage, they appeared to be more stable and droplets growth rate were slow during storage. Aggregation and morphology of droplets could be observed under confocal microscopy. Shear-thinning behavior were found in mixed emulsions, and mainly accredited to the KGM component. The effect of temperature on viscosities of the emulsions was well-described by Arrhenius equation. Increased KGM content led to larger storage modulus (G') and loss modulus (G″), and the difference in cross-over point of G' and G″ implied changes in microstructures of the emulsions.

Characterization of konjac glucomannan-ethyl cellulose film formation via microscopy.

Konjac glucomannan-ethyl cellulose (KGM-EC, 7:3, w/w) blended film shows good mechanical and moisture resistance properties. To better understand the basis for the KGM-EC film formation, optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) were used to observe the formation of the film from emulsion. Optical microscopy images showed that EC oil droplets were homogeneously dispersed in KGM water phase without obviously coalescence throughout the entire drying process. SEM images showed the surface and cross-sectional structures of samples maintained continuous and homogeneous appearance from the emulsion to dried film. AFM images indicated that KGM molecules entangled EC molecules in the emulsion. Interactions between KGM and EC improved the stability of KGM-EC emulsion, and contributed to uniformed structures of film formation. Based on these output information, a schematic model was built to elucidate KGM-EC film-forming process.

Carboxymethyl modification of konjac glucomannan affects water binding properties.

The water binding properties of konjac glucomannan (KGM) and carboxymethyl konjac glucomannan (CMKGM) are important for their application in food, pharmaceutical, and chemical engineering fields. The equilibrium moisture content of CMKGM was lower than that of KGM at the relative humidity in the range 30-95% at 25°C. The water absorption and solubility of CMKGM in water solution were lower than that of KGM at 25°C. Carboxymethyl modification of KGM reduces the water adsorption, absorption, and solubility. Both carboxymethylation and deacetylation could confer hydrophobicity for CMKGM. These data provide the basis for expanding CMKGM application.

Interactions between carboxymethyl konjac glucomannan and soy protein isolate in blended films.

To elucidate biopolymer interactions between carboxymethyl konjac glucomannan (CMKGM) and soy protein isolate (SPI) in different ratios on physicochemical properties of the blended films, biodegradable CMKGM/SPI films were prepared and characterized. The results showed that CMKGM and SPI are highly compatible in blended film formation, and that Maillard reactions and hydrogen bonds interactions between CMKGM and SPI occurred. The water adsorption of the CMKGM/SPI films progressively decreased with increasing CMKGM level, the surface wettability of the blended films was improved with increasing CMKGM content; the CMKGM/SPI blend films had enhanced tensile strength (TS) and elongation at break (EAB) compared to pure CMKGM and SPI films; the oxygen permeability of blend films was decreased; the roughness was decreased with increasing CMKGM content. Moreover, the CMKGM/SPI film was biocompatible and biodegradable.