Study On The Structural Characteristics And Properties Of Bleached Pulverized Konjac Flour

The crude pulverized konjac flour generally presented strong fishy smell, high amount of residual SO₂ and other quality defects such as viscosity decrease and discoloration in a short storage course. So it must be refined for a wider utilization, such as fine chemicals, food additives or medical ingredients. The traditional refining process with stepwise ethanol washing cost a long time and the stability of refined products need further improvements, so it is necessary to develop a new method for the effective refining and bleaching of pulverized konjac flour.It has been reported that H₂O₂-citric acid can be used to bleach silk, wool without loss their qualities at slightly acid condition. H₂O₂ can be used to prepare oxidized konjac glucomannan at slightly alkaline condition in the presence of low concentration of isopropanol as well. However, there are no reports on bleaching of konjac flour with H₂O₂-citric acid treatments at slightly acid condition in the presence of low concentration of ethanol.Aiming at promoting the utilization of crude pulverized konjac flour, a bleaching process with H₂O₂-citric acid at slightly acid condition in the presence of low concentration of ethanol was developed and a bleached pulverized konjac flour (BPK) without much loss its viscosity was obtained, which allows potential effective refinement of the flour on an industrial scale. At slightly alkaline condition and strong alkaline condition, a bleached but oxidized konjac glucomannan (OKGM) and bleached but insoluble pulverized konjac flour (IPK) were obtained, respectively.The objectives of the research focus on investigation of the effects of H₂O_2-citric acid-low concentration of ethanol bleaching process on the quality properties, structural characteristics, hydration properties, pasting properties and rheological properties of BPK. The materials for the bleaching process was carefully chosen as a refined pulverized konjac flour (PK) which was obtained by a oscillatory ball mill pulverization of ethanol-refined konjac flour originally from the tubers of Amorphophallus albus P.Y.Liu et J.F.Chen, in order to simplify the bleaching reaction process.The mian results of the research as follows:(1) Compared with PK, BPK showed a higher degree of whiteness,slightly lower levels of viscosity, lower levels of undesirable odors and contents of SO₂ while the contents of KGM, the gel strength and the others physicochemical properties of BPK were not changed significantly. Furthermore, the results of a GC-MS analysis for the volatile components in SDE extracts from BPK confirmed the deodorization effects. It was thus demonstrated that the quality of konjac flour was improved using the newly developed process.(2) The particle size distributions, BET specific surface area and pore structure were measured by a Laser Granularity analyzer and a nitrogen adsorption technique (Tristar 3000, Micromeritics, U.S.A.), respectively.The result showed, compared with the PK, the mean size and particle size distribution of BPK was changed, The particle size of PK was distributed as a peak centered at 120μm with a shoulder at about 200μm, but the particle size of BPK showed larger and relatively narrower distribution and the specific surface area declined significantly, indicating the some of fine particle and amorphous substance on particle surface dissolved during the bleaching process.When observed of BPK, IPK under light microscopy, stereomicroscopy and scanning electron microscopy, compared with the PK, respectively, it is found that BPK and IPK exhibited a smoother surface structure with clear hollows and wrinkles that may be have resulted from the bleaching process.(3) The structures, molecular weight distributions and thermal properties were examined respectively.For the KGM from PK and BPK sample, The IR spectra were recorded using a Bio-Rad FTS-185 Fourier transform infrared (FTIR) spectrometer, the ¹H NMR spectra were recorded on Bruker DMX500MHz and the molecular weight distributions were examined by a CL-2B gel filtration chromatography, respectively. The results showed no remarkable changes were identified in IR spectra and the acetyl side groups from the backbones of KGM was still detected in FTIR and ¹H NMR, except of that the peak of 1728 cm^-1 from BPK was a slightly weaken in IR spectra Compared with the PK. The molecular weight of KGM from BPK was decreased slightly, but decreased significantly in the case of OKGM, which indicated a slight degradation of KGM molecules occur for BPK during the bleaching process.For the PK, BPK and IPK sample, CP/MAS ¹³C NMR spectra were carded out on a Bruker DSX300 NMR spectrometer, respectively. The result showed no remarkable changes were identified between the PK and BPK with CP/MAS ¹³C NMR. But for IPK, the peak of 21.2ppm and 173.6 ppm in CP/MAS ¹³C NMR spectra disappeared which confirmed the removal of the acetyl side groups from the backbones of KGM during bleaching process.For the PK, BPK and IPK sample, XRD patterns were analyzed using a Rigaku D/max-rB X-ray powder diffractometer. PK and BPK were found to exhibit almost a same broad peak with a hump-like diffuse peak over the 20 range from 14 to 24°. IPK was found to exhibit identical diffraction peaks at 20=19.02°, 2θ=22.26°and at 2θ= 10.76°, indicative of the existence of new crystalline domain with long-range order. However, the relatively low intensity of these diffraction peaks and the presence of a hump-like diffuse peak suggest that IPK still contained a high proportion of amorphousness.For the PK, BPK and IPK sample, The characteristic TG and DTG curves for PK, BPK and IPK were analyzed by a SDT Q600 Simultaneous DSC/TGA Analyzer and and the primary thermogram of PK and BPK were obtained at a scan rate of 10℃/min in an inert atmosphere. Both of PK and BPK involve only two steps of degradation, after the first stage loss of moisture at low temperature, the major weight loss occurred. In the secondstep, The maximum rate of weight loss for BPK occurs at 320.90℃, and loses 74.63% of its original weight, which could be attributed to a complex process including degradation of the saccharide rings and disintegration of macromolecule chains of KGM. However, in the case of PK, the temperature at the maximum rate of weight loss occurs is 310.74℃, and at this temperature, PK loses 66.13% of its original weight. These results indicated that the bleaching process slightly increased the thermal stability of PK.As can be seen from the TG and DTG plot of IPK samples, there is involve three steps of degradation, only about 35% weight loss of PK until a temperature of 268.19℃is reached. In addition, initial decomposition temperature and the temperature at the maximum rate of weight loss of IPK are much lower than that of BPK or PK, but the third degradation stage of IPK takes place at a higher temperature (361.15℃) than the corresponding stage of the BPK or PK. Namely, there existed both the lower and higher thermal stability of domain inside the IPK particles.(4) A novel rapid techniques were set up to determination of hydration and pasting properties of PK, BPK or OKGM by using RVA hydration curve and RVA pasting curve.A novel method for measuring hydration and dissolution kinetics of pulverized konjac flour (PK) using RVA-3D⁺ Rapid Visco Analyzer (RVA; Newport Scientific Pty Ltd., Australia). The results showed that RVA was a rapid and reproducible method for measuring the dissolution kinetics of the PK flour. This study has also identified the optimum conditions in order to rapidly determine the hydration rate of the PK. These conditions include the concentration of PK flour with 1.0%, test temperature at 30±1℃, stirring speed at 160 RPM (Revolution Per Minute), and test time of 16 Minutes. An empirical logistic model has been established to describe the dissolution kinetics of PK flour, which was tested to be reliable for predictive purposes:η=η_max/(1+EXP(a-bÏ„))Where b is he rate constant andÏ„p is the time to reach to the half of maximum viscosity. These two can be used as indicating parameters for the hydration process of PK flour. Based on this model, the different hydration rates of 1.0% PK flour with associated these two parameters were defined when the system was in the presence of sodium chloride, sucrose, or maltodextrin. The results indicated that RVA is a reliable fast technique for determining the hydration dissolution kinetics of PK flour.As for the hydration dissolution kinetics of the tested pulerized konjac flour, it was shown that the hydration propertise of of pulverized konjac flour related to the degree of oxidation. As for BPK sample, there was no remarkble change for the hydration rate.A modified Weibull model has also been established to describe the dissolution kinetics of BPK flour, which has been applied to several problems in various fields as a general statistical distribution function, When applied to the hydration process, the function could be represented as:η=η_max·EXP(-b/Ï„) Where b is a constant andÏ„p is also defined as the time to reach to the half of maximum viscosity(η_max), butÏ„p=1.4427 b in the modified Weibull model.Comparing the RVA pasting curves of BPK, OKGM with PK or WPK (as same treatment as BPK except for without hydrogen peroxide addition), it was shown that the pasting propertise of pulverized konjac flour related to the degree of oxidation. As for the pasting propertise of BPK, it was typically formed a high peak viscosity which slightly breaks down under shear and with a higher final vicosity. there was no remarkble changes between. BPK and WPK, but the peak viscosity, final viscosity of BPK was slightly lower and the peak time of RVA pasting curve was slightly prolonged compared to that of PK. Conversely, the OKGM forms a relatively low peak viscosity and final viscosity but was modified to resist breakdown under shear.(5). The rheological properties of the pastes for PK, BPK and OKGM were anylyzed by ARES and RVAin this studies.Steady shear properties of the pastes for PK, BPK and OKGM were analyzed by ARES and RVA, respectivety. A typical non-Newtonian pseudoplastic behavior was exhibited. All samples showed marked shear-thinning behavior, i. e. decreasing viscosity with increasing shear rate under steady shear conditions. But the zero shear Viscosity of BPK was decreased compared with PK. The phenomena are compatible with the observed slightly decrease in molecular weight for BPK as mentioned earlier. Due to the scission of polymer chains and decline of molecular weight, the number of effective entanglement couplings will be reduced and lead to easy flow and low apparent viscosity.For OKGM sample, non-Newtonian pseudoplastic behavior was not obvious, even showed close to Newtonian fluid behavior. The data presented from steady shear test by RVA in high shear rate also in good agreement these findings.Dynamic shear properties of the pastes for PK, BPK and OKGM were analyzed in storage (G') and loss modulus (G") as a function of frequency at 18℃, respectively. We note that, for 3 types of sample, the magnitudes of G' and G" increased with increasing frequency, this tendency is in good agreement with those found for other macromolecular dispersions. For PK and BPK, there existed a cross over point between the two curves, and the intersected point of G' and G" is 4, 7 rad s^-1 for the 2.0% (w/w) pastes of PK and BPK, respectively. In the case of OKGM sample solutions, G" was always higher than G' throughout the frequency domain studied; the expected cross-over point can hardly be seen due to instrumental limitations. This observable fact showed that PK, BPK behaved like entanglement network system, while OKGM displayed liquid-like behavior. In addition, the values of the dynamic modulus are slightly lower for BPK than for PK. This effect is associated with the relatively lower molecular weight of the latter, and coincides with the steady shear properties discussed above. As described previously the decreasing molecular weight will induce shorter molecular chains connecting junction zones, which can decrease the number of elastically active chains and weaken the dynamic modulus of polymers. This is also true in the case of wheakening effetcts for Na₂SO₃ added to the BPK paste even without change of the molecular weight, which indicated the important influence of intramolecular interaction on the rheological properties of BPK.