| 1. |
- Lundqvist, Jon, et al.
(författare)
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Characterization of galactoglucomannan extracted from spruce (Picea abies) by heat-fractionation at different conditions
- 2003
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Ingår i: Carbohydrate Polymers. - 0144-8617 .- 1879-1344. ; 51:2, s. 203-211
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Tidskriftsartikel (refereegranskat)abstract
- Water-soluble hemicelluloses were extracted from spruce chips by microwave heat-fractionation. The galactoglucomannan (GGM) extraction was evaluated on the basis of weight-average molecular weight (MW), yield and carbohydrate composition of the GGM. The MW was determined by size-exclusion chromatography with column calibration using off-line MALDI-MS analysis, and determination of mannan content in the fractions collected. Water impregnated spruce chips were heat-fractionated at three different temperatures (180, 190, and 200oC). The spruce chips were also impregnated in NaOH solutions of different concentrations, and then heat-fractionated at 190oC for 5min. The highest mannan yield (78% based on the amount in the raw material) was obtained from water impregnated spruce chips heat-fractionated at 190oC for 5min (MW of 3800). The highest MW (14,000) was obtained from impregnation with 2% NaOH (190oC, 5min), but the yield of mannan was very low (3%). Impregnation with 0.025% NaOH and heat-fractionation at 190oC for 5min resulted in extraction of GGM with MW of 9500 and a mannan yield of 31%. When the spruce chips were impregnated with =<0.05% NaOH an O-acetyl-galactoglucomannan was extracted, whereas when higher NaOH charges were used in the impregnation, the extracted GGM lacked acetyl groups.
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| 2. |
- Anthonsen, M.W., et al.
(författare)
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Aggregates in acidic solutions of chitosans detected by static laser light scattering
- 1994
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Ingår i: Carbohydrate Polymers. - : Elsevier BV. - 0144-8617 .- 1879-1344. ; 25:1, s. 13-23
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Tidskriftsartikel (refereegranskat)abstract
- Chitosans having degrees of N-acetylation, FA, ranging from 0 to 0·6, were randomly degraded to different molecular weights and studied by multi angle static laser light scattering (LLS). Under the given experimental conditions, negative second virial coefficients of the solutions, A?2, revealed the presence of concentration dependent aggregates. Attempts to remove the aggregates, or to influence the aggregation behavior, were made by ultracentrifugation and extensive filtering of the solutions. Modification of the solvent conditions such as pH, ionic strength and temperature were carried out, and chitosan solutions were digested with an acidic proteinase. Non-degraded samples and chitosans prepared by both heterogeneous and homogeneous N-deacetylation of chitin were also studied. In all cases, the negative A?2 remained. However, it was observed that ultracentrifugation and filtering of the solutions decreased the measured molecular weights and radii of gyration, indicating that some of the material of high molecular weight and size could be removed by ultracentrifugation and filtration. The chemical nature of the physical basis of the molecular association was not revealed. Nevertheless, by the use of gel permeation chromatography coupled to an on-line low angle laser light scattering instrument and a differential refractive index concentration detector (HPSEC-LALLS-RI), a bimodal molecular weight distribution was observed in which about 5% of the sample had a very high molecular weight. These results coupled with the positive virial coefficients obtained earlier from osmotic pressure measurements suggest that a small fraction of the chitosan is aggregated to high molecular weight material, probably following a closed association model. Electron microscopy revealed the presence of some supramolecular structures. The positive second virial coefficients obtained earlier from osmometry are in harmony with these findings. The results demonstrate the occurrence of reversible aggregation in chitosan solutions. Static laser light scattering therefore cannot readily be used to determine molecular weights and sizes of chitosans under these conditions. It was not possible to correlate the extent of aggregation with the chemical composition of the chitosans. © 1994.
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| 3. |
- Berggren, R., et al.
(författare)
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Alkaline degradation of birch and spruce : influence of degradation conditions on molecular mass distributions and fibre strength
- 2003
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Ingår i: Carbohydrate Polymers. - 0144-8617 .- 1879-1344. ; 51:3, s. 255-264
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Tidskriftsartikel (refereegranskat)abstract
- The alkaline degradation of birch and Norway spruce during kraft pulping was studied on a laboratory scale by two degradation strategies, by varying the pulping time using the same initial alkali level and by varying the initial alkali concentration using a constant time. The degradation at the molecular level was monitored by determining the carbohydrate composition, intrinsic viscosity and-molecular mass distribution (MMD). The influence of the degradation on fibre strength was studied as zero-span tensile index. The alkaline degradation was to a large extent homogeneous on a molecular level. However, some significant differences in degradation patterns were found. In the case of birch, the two different degradation strategies (increased alkali level and increased pulping time) caused differences in MMD and fibre strength (comparisons made at a given intrinsic viscosity or M-w). For spruce pulps, the decrease in fibre strength and the shift in MMD were the same in both series, regardless of degradation strategy.
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| 4. |
- Hermansson, Ann-Marie
(författare)
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Rheological and microstructural evidence for transient states during gelation of kappa-carrageenan in the presence of potassium
- 1989
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Ingår i: Carbohydrate Polymers. - 0144-8617 .- 1879-1344. ; 10:3, s. 163-181
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Tidskriftsartikel (refereegranskat)abstract
- Kappa-carrageenan gelation was studied by a combination of electron microscopy and dynamic viscoelastic measurements. There was an initial maximum in the storage modulus during cooling and gel formation of 1% kappa-carrageenan in the presence of potassium. The structure of the transient state was found to consist of a fine network structure, where the junction zones were believed to be double helices. The structure had the characteristics of a true gel but was unstable. When the temperature was lowered aggregation took place and the fine network structure was partly broken down. Ordered superstrands formed which aligned themselves in parallel or were densely packed together. The degree of aggregation depended on the potassium ion concentration. In the weakest gel formed in 0·01 m KCl, the superstrands did not form a network but were rather dispersed in the fine network structure. In 0·1 and 0·2 m KCl the superstrands formed a three-dimensional network, where aligned superstrands formed the junction zones and branching occurred when superstrands deviated from each other and aligned with new superstrands. In 0·1 m KCl the supermolecular network dominated but in 0·2 m KCl a mixed gel was formed of the fine network and the coarse supermolecular network. The mixed gel gave rise to the firmest gel. © 1989.
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| 5. |
- Hermansson, Ann-Marie
(författare)
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Shear induced changes in the viscoelastic behaviour of heat treated potato starch dispersions
- 1990
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Ingår i: Carbohydrate Polymers. - 0144-8617 .- 1879-1344. ; 13:1, s. 29-45
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Tidskriftsartikel (refereegranskat)abstract
- The viscoelastic behaviour of native potato starch dispersions was studied using dynamic viscoelastic measurements. The influence of various shear and heat treatments of gelatinized starch pastes were compared in the concentration range 4-10%. Heating had a minor effect compared to the effect of shear treatment. The complex modulus (G*) decreased to about 5% of its maximum value when the starch pastes were sheared during the heating cycle, whereas heating alone resulted in a decrease to about 60% of the maximum G* value. Shear also caused an increase in the phase angle and characteristic changes were observed with regard to frequency and strain dependencies. When samples were prepared in a Brabender viscograph, the shear induced changes took place early in the pasting cycle. The characteristics of the concentration dependence differed due to the shear history of the sample. Peculiar concentration dependencies, some even negative, could be obtained from samples with different shear histories. The results imply that even moderate shear treatment gives rise to structure changes which are reflected by a transition in the type of the viscoelastic behaviour. © 1990.
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| 6. |
- Lundin, Leif, et al.
(författare)
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Influence of locust bean gum on the rheological behaviour and microstructure of K-?-carrageenan
- 1995
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Ingår i: Carbohydrate Polymers. - 0144-8617 .- 1879-1344. ; 28:2, s. 91-99
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Tidskriftsartikel (refereegranskat)abstract
- Mixtures of ?-carrageenan-locust bean gum (LBG) were analysed by transmission electron microscopy and dynamic viscoelastic measurements. The effect of two different mannose:galactose (M:G) ratios of LBG on the rheology and microstructure of ?-carrageenan in KCl was studied. The rheological differences observed in the mixed gel as LBGs were added were found to be dependent on M:G ratio, salt concentration and LBG content. Monolayers of dilute samples of ?-carrageenan mixed with LBGs in varying ratios in 0.10 M KCl were visualised by low-angle rotary metal shadowing for transmission electron microscopy. The results obtained showed that LBG hindered the self-association of ?-carrageenan, and that the effect was enhanced at a higher M:G ratio and as the LBG contents were increased. Small deformation viscoelastic measurements were performed on ?-carrageenan-LBG mixtures in 0.050 and 0.10 M KCl at constant temperatures of 38 and 48 °C, respectively. Increased rheological stability was observed for mixtures containing a relatively high LBG content. Mixtures with a relatively low LBG content showed a maximum in storage modulus and an increase in phase angle. The stabilising effect of LBG on the rheology of the blends was more effective for the LBG with high M:G and as the LBG content was increased. © 1996.
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| 7. |
- Lundin, Leif, et al.
(författare)
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Supermolecular aspects of xanthan - locust bean gum gels based on rheology and electron microscopy
- 1995
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Ingår i: Carbohydrates polymer. - 0346-718X .- 0144-8617 .- 1879-1344. ; 26:2, s. 129-40
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Tidskriftsartikel (refereegranskat)abstract
- The viscoelastic properties and supermolecular structure of synergistic gels, formed by xanthan and locust bean gum (LBG) of two different mannose:galactose ratios (M:G), have been investigated by small deformation viscoelastic measurements and by low angle rotary-shadowing for transmission electron microscopy. The rheological properties at 20°C for mixtures subjected to heating and cooling cycles in the temperature range 30-80°C were found to be dependent on the M:G ratio. Mixtures of xanthan and LBG mixed at temperatures ?40°C were found to form true gels with low phase angles. Blends of xanthan and LBG with a low M:G ratio did not show any increase in synergistic effects as the temperature was increased, whilst the mixture of xanthan and LBG with a high M:G ratio showed a strong increase in synergistic effects as the temperature was raised above 60°C. A difference in gelation temperature (Tg) of approx.13°C was observed between the mixtures of xanthan and the two LBG fractions. The Tg for xanthan with a high M:G ratio was approx. 53°C, whilst the Tg for mixtures of xanthan and LBG with a low M:G ratio was approx. 40°C. Results obtained using electron microscopy showed that the xanthan-LBG network was formed from xanthan supermolecular strands, and addition of LBG did not influence the xanthan structure. The observed structural features of the gels were independent of heat treatment and LBG fraction. The structural similarities and rheological differences observed between xanthan and the LBG fractions are discussed in comparison with existing interaction models at the molecular level. Based on these results, a speculative network model at the supermolecular level is presented.
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| 8. |
- Stading, Mats, et al.
(författare)
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Humidity-induced structural transitions in amylose and amylopectin films
- 2001
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Ingår i: Carbohydrate Polymers. - 0144-8617 .- 1879-1344. ; 45:3, s. 209-217
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Tidskriftsartikel (refereegranskat)abstract
- The effect of humidity on the dynamic mechanical and barrier properties and on the microstructure of amylose and amylopectin films was studied. The films were solution-gel-cast, with and without glycerol as added plasticizer, and dried to equilibrium weight under various relative humidities (RH). The network microstructure of glycerol-plasticized amylose films was shown to change from dense and homogeneous to a more open structure with fluctuations in the pore size when the surrounding RH was increased. The structural change was attributed to plasticization of the amorphous areas by increased water content, leading to higher mobility in the network and subsequent inhomogeneous swelling. This structural change had a direct influence on the oxygen permeability, which drastically increased. The onset of the increase occurred at lower surrounding RH for the films containing glycerol, and these films also had higher oxygen permeability. Glycerol-plasticized amylopectin films with gradually increased crystallinity were prepared by varying the RH during film formation. Dynamic mechanical analysis at both varied temperature and RH showed that the increased crystallinity led to higher glass transition temperature, T g, and a smaller effect of the surrounding humidity on mechanical and barrier properties. The RH during the formation of glycerol-plasticized amylose films did not affect crystallinity or T g. These films formed at 70% RH were shown to have a marked heterogeneous network. © 2001 Elsevier Science Ltd.
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| 9. |
- Stading, Mats, et al.
(författare)
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Rheological behaviour of mixed gels of kappa-carrageenan-locust bean gum
- 1993
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Ingår i: Carbohydrate Polymers. - 0144-8617 .- 1879-1344. ; 22:1, s. 49-56
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Tidskriftsartikel (refereegranskat)abstract
- Mixed gels of the K, Na and Ca-forms of ?-carrageenan and locust bean gum at a total polysaccharide concentration of 1% (w/w) were studied by dynamic viscoelastic measurements. Synergistic effects were only observed for mixed gels with K-?-carrageenan in ?0·1 m KCl. The composition favouring maximum synergy shifted from 90%-?-carrageenan in distilled water down to 35% K-?-carrageenan in 0·1 m KCl. Gels of K-?-carrageenan/locust bean gum did not show synergistic effects at high potassium concentration (0·2 m KCl). The typical behaviour of K-?-carrageenan at ?50 mm KCl dominated the rheological behaviour of the mixed systems even when the locust bean gum content was high. The synergistic effects probably originate from different mechanisms at low, 0-10 mm KCl, and high, 50-100 mm KCl, concentrations because of differences in the potassium-induced microstructure. The mixed gels with the Na or Ca-form of ?-carrageenan showed no synergy. Addition of locust bean gum caused the storage modulus to decrease and the phase angle to increase. The gels were weak, with G? < 170 Pa for the Na form and G? < 350 Pa for the Ca form. The mixed gels with both the Na and Ca forms had a broken frequency dependence of G?(f). © 1993.
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| 10. |
- Svegmark, Karin, et al.
(författare)
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Changes induced by shear and gel formation in the viscoelastic behaviour of potato, wheat and maize starch dispersions
- 1991
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Ingår i: Carbohydrate Polymers. - 0144-8617 .- 1879-1344. ; 15:2, s. 151-169
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Tidskriftsartikel (refereegranskat)abstract
- The effect of shear and gel formation on the behaviour of heat-treated wheat, maize and potato starch dispersions was studied using dynamic viscoelastic measurements. The starch types were compared both at a concentration of 10% and at concentrations where the same level of shear forces was applied during the paste preparation procedure (11% wheat, 10% maize, and 4% potato). The cereal and the potato starch pastes initially showed similar viscoelastic properties after gelatinization, but the cereal starch pastes underwent a transition in their viscoelastic behaviour when cooled. This transition was believed to be due to the gelation of amylose. The main changes observed were a sharp increase in the shear modulus and a drop in the phase angle to below 2 degrees. No such transition occurred on cooling the potato starch pastes, at 25°C for 1 h, in the concentration range studied (4-10%).
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