| Refined konjac flour is widely used in food, medicine, chemicals and materials fields due to its thickening, stabilizing and other functional characteristics. Although A.guripingensis konjac flour(ARF) was the dominant product in the world-wide konjac market. Recently, refined konjac flour extracted from a wild variety from Southeast Asian(AGF) attracts much attention in the konjac community and enters the domestic market due to its disease resistance. However, there are fewer understandings about the characteristics of this variety and it is urgent to study its properties systematically in order to provide the database for processing and utilizing in the manufacture.In this paper, to give useful information on konjac industry, refined konjac flours from a typical commercial species(ARF) and a wild new species(A.rivirei konjac flour, AGF) were studied and compared in different aspects, including physical and chemical properties, swelling properties, rheological properties, gel properties and emulsifying properties. Consequently, these results might lay the solid theoretical foundation for the production and application in the future. The main contents and results of this paper are listed as follows:1. Physical and chemical properties of konjac flour from two species. Konjac flour chromaticity, particle size distribution as well as apparent morphology were investigated with the colorimeter, particle analyzer and scanning electron microscopy respectively. The compositions of these two refined konjac flour were studied analysized. Then the structures and molecular parameters of the main composition, a kind of neutral polysaccharide called konjac glucomnannan(KGM), were also investigated. It was found that ARF had better whiteness, smoother surface and broader particle distribution. Content of moisture, ash, protein and soluble sugar of ARF was slightly higher than that of AGF, while insoluble polysaccharide content was lower. KGM extracted from ARF with high yield(72.80±1.945%) possessed the high ratio of acetyl groups and its molecular weight was 1.669×106. However, AGF has rough particle surface, yellow color and relatively narrow particle size distribution. Compared with ARF, KGM obtained from AGF had low yield(61.35 ± 2.293%), less acetyl groups and small molecular weight(approximately 1.236×106 Da). Whereas the insoluble polysaccharides contents in AGF were higher, such as cellulose. In addition, there were no significant differences between structures of KGM from two species.2. Swelling properties of konjac flour from two species. The influences of konjac concentration, pH as well as temperature on the swelling behavior were studied using a dynamic method. Then the transparency of konjac solutions and influences of various factors on solution stability were studied further. The results showed that konjac flour particles swelled rapidly and then dissolved gradually in the distilled water. Nevertheless, konjac flour from ARF dissolved faster than AGF for the sake of the differences between cell wall structures. Moreover, with an increase of konjac concentration, the dissolving speed was accelerated and the viscosity increased. The swelling behaviors of ARF were almost not affected by pH values only with the viscosity slightly increased at strong acid condition. However, the swelling behaviors of ARF were sensitive to pH values and the viscosity of AGF solution showed a significant increase to 9.551 Pa.s at the pH=2. With an increasing temperature, the swelling rate was quicker. As for AGF, the viscosities at every temperatures were remained the same at the beginning 1.5 h, but the viscosities at different temperature exhibited significant differences after 1.5 h. With continues stirring after fully swollen, the viscosity of ARF solution remained stable, but the viscosity of AGF solution reduced gradually. Transparency was influenced by the concentration obviously while pH and temperature almost not affected the transparency at all. Additionally, storage time, outside temperature and microorganism will result in the solution instability, leading to the reduced viscosity.3. Rheological properties of konjac flour from two species. The effect of konjac concentration, pH as well as temperature on fluid properties and viscoelasticity of flour solutions were investigated. Models of steady flow, frequency sweep and temperature sweeps were performed. The obtained results were fit into the models. It was found that under various conditions, konjac flour solution was exact pseudoplastic fluid, showing the typical shear-thinning behavior. Besides, increased concentration resulted in the increased viscosity and enhanced elasticity. When the concentrations was lower than 4.5wt%, AGF solution viscosity was also lower than ARF. Once the concentrations reached up to 4.5wt%(or higher than 4.5wt%), the viscosities of two species were similar. Viscosity and viscoelasticity of ARF solution exhibited pH stability while that of AGF solution showed pH sensitivity. As the temperature increased, both the viscosity and elasticity of konjac solutions decreased. Furthermore, AGF gel network was formed at 85 oC, showing relative low gel-forming temperature.4. Gelation properties of konjac flour from two species. The influences of several factors on the textural properties of konjac gel were explored and the sol-gel transition was monitored. The gelation mechanisms of the two kinds of konjac flour were studied further. The results indicated that concentrations, alkaline contents as well as temperatures played an important role in characteristics of the two kinds of konjac gels. Under the same circumstance, AGF gel breaking force was remarkably higher than that of ARF. Increased concentrations gave rise to the enhanced breaking force. With an increasing amount of alkaline within the limited concentration, breaking force of AGF gel increased. Whereas the breaking force was declined slightly with the continue addition of alkaline. However, with an increasing addition of alkaline within the tested range(4wt%- 12wt% of konjac flour), breaking force of ARF gel increased continually. Rising temperature was helpful to increase the konjac gel breaking force. AGF gel deformation was greater than ARF at low concentrations, low alkaline contents or low temperatures. Once the external conditions were strengthened, ARF gel deformation was greater than AGF. Therefore, combined with the rheological data, it could be conclude that AGF gel with high gel strength could be formed at relative low concentration, low alkali content or low temperatures when compared with ARF. The water-holding capacities of the obtained gels from two species were more than 90%. The results of FT-IR and chemical titration demonstrated that deacetylation was an important step for the konjac gelation. Rising temperature promoted the deacetylation process. Finally, the degree of acetylation reached to a constant value and the final degree deacetylation from AGF gel was higher than that of ARF. Moreover, AGF gel possessed of the high crystallinity, ordered molecular structure and dense gel network. But the thermal stabilities of the konjac gel formed from ARF and AGF were the same.5. Emulsifying properties of konjac flour from two species. Emulsifying capacity and emulsion stability of ARF and AGF were studied. Their emulsifying properties were investigated and compared, which were based on parameters of particle size, microscopic pictures, rheological properties as well as interfacial tension. It was found that AGF possessed of better emulsifying capacity. Its emulsion with low viscosity and poor stability showed concentrated droplet distribution. Besides, oil phase was also affected the emulsion properties. However, ARF possessed of good emulsion stability. Its emulsion with high viscosity and great stability exhibited wide droplet size distribution. Moreover, its emulsion stability was independent on the choice of oil phase. Since KGM with large molecular weight was not easy to adsorb at the oil/water interface, two kinds of konjac flour showed poor emulsion capacities in comparison with the commercial emulsifiers. Nevertheless, after mixed the konjac flour with WPI, the mixtures exhibited better emulsifying properties. And emulsification mechanism was studied further. The obtained results showed that there are low interfacial tensions in the mixture and there are also significant interactions between WPI and the composition in the konjac flour. |
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