| 1. |
- Akerberg, C, et al.
(författare)
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Modelling the influence of pH, temperature, glucose and lactic acid concentrations on the kinetics of lactic acid production by Lactococcus lactis ssp. lactis ATCC 19435 in whole-wheat flour
- 1998
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Ingår i: Applied Microbiology and Biotechnology. - : Springer Science and Business Media LLC. - 1432-0614 .- 0175-7598. ; 49:6, s. 682-690
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Tidskriftsartikel (refereegranskat)abstract
- A kinetic model of the fermentative production of lactic acid from glucose by Lactococcus lactis ssp. lactis ATCC 19435 in whole-wheat flour has been developed. The model consists of terms for substrate and product inhibition as well as for the influence of pH and temperature. Experimental data from fermentation experiments under different physical conditions were used to fit and verify the model. Temperatures above 30 degrees C and pH levels below 6 enhanced the formation of byproducts and D-lactic acid. By-products were formed in the presence of maltose only, whereas D-lactic acid was formed independently of the presence of maltose although the amount formed was greater when maltose was present. The lactic acid productivity was highest between 33 degrees C and 35 degrees C and at pH 6. In the concentration interval studied (up to 180 g l(-1) glucose and 89 g l(-1) lactic acid) simulations showed that both substances were inhibiting. Glucose inhibition was small compared with the inhibition due to lactic acid.
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| 2. |
- Andersson, M, et al.
(författare)
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Determination of the pore-size distribution in gels
- 1995
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Ingår i: Bioseparation. - 1573-8272. ; 5:2, s. 65-72
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Tidskriftsartikel (refereegranskat)abstract
- A method for determination of the accessible volume fraction in gels as function of the molecular weight of the solutes is presented. The pore-size distribution is determined by measuring the rate of diffusion of a mixture of solutes into a gel using gel filtration for separation. The solutes, of various sizes, are detected by refractive index measurements. Two marker molecules (blue dextran and glucose) were used to determine the gel void and the amount of liquid adhering to the surface. The technique is simple and can easily be adapted to other systems of a porous nature (membranes, catalyst pellets etc.). The method is applied to an N-isopropylacrylamide gel. This gel is sensitive to temperature changes. A considerable increase in volume is obtained when the temperature is decreased. This makes it suitable for use as a separation agent in gel extraction. In order to assess the performance of this unit operation the pore size distribution for the N-isopropylacrylamide gel was determined at 10 degrees C, 20 degrees C and 30 degrees C, using mixtures of different dextrans as well as different polyethylene glycols.
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| 3. |
- Andersson, M, et al.
(författare)
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Diffusion of glucose and insulin in a swelling N-isopropylacrylamide gel
- 1997
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Ingår i: International Journal of Pharmaceutics. - 1873-3476. ; 157:2, s. 199-208
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Tidskriftsartikel (refereegranskat)abstract
- The diffusional characteristics for poly(N-isopropylacrylamide) (NiPAAm) gel have been investigated. This gel is a critical gel which means that small changes in the environment influence the gel volume considerably. The effective diffusion coefficients for the solutes glucose and insulin were determined in batch experiments with the solutes diffusing out from small cylindrical gel bodies with diameters of 2.4-2.9 mm and at temperatures below the critical temperature: 10, 20 and 30 degrees C. The effective diffusion coefficients were obtained by fitting the experimental data to a mathematical model considering back-mixing and time delay in the experimental set-up, dilution due to the adsorbed liquid on the gel bodies and partition due to the exclusion effect. The effective diffusion coefficient for glucose increases from 2.7.10(-10) to 4.7.10(-10) m(2)/s when the temperature increases from 10 to 30 degrees C, following the Wilke-Chang relationship. This implies that the effect of the network is negligible compared with the effect of the temperature. However, for a solute with a molecular weight of about 6000 the network has a considerable effect. The effective diffusion coefficient for insulin increases from 4.4.10(-10) to 5.9.10(-10) m(2)/s when the temperature increases from 10 to 30 degrees C, which is less than expected from the Wilke-Chang relationship. This indicates an increased resistance for diffusion inside the gel due to shrinking. (C) 1997 Elsevier Science B.V.
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| 4. |
- Andersson, M, et al.
(författare)
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Swelling kinetics of poly(N-isopropylacrylamide) gel
- 1998
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Ingår i: Journal of Controlled Release. - 1873-4995. ; 50:1-3, s. 273-281
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Tidskriftsartikel (refereegranskat)abstract
- In many gel applications the swelling and shrinking kinetics are very important, e.g. in controlled/slow release, where the kinetics determine the rate of out-diffusion of the active component, and in gel extraction where the gel is swollen and shrunk several times. In this study swelling kinetics of poly(N-isopropylacrylamide) gel (NiPAAm gel) was determined by monitoring the swelling process using a stereo microscope and a video camera. The swelling of spherical gel bodies could conveniently be studied after a temperature change. The results obtained were treated according to the approach of Tanaka and Fillmore, in which the swelling and shrinking of a gel is described as a motion of the gel network according to the diffusion equation. This was shown to be valid when the temperature changes are kept below the critical point of the gel. However, the model fails to describe the shrinking process when passing from below to above the critical temperature. The collective diffusion coefficient (D), defined as the osmotic bulk modulus divided by the friction factor, was determined by fitting to the experimental data. D was found to increase with temperature according to the Wilke-Chang relation D=2.0.10(-11)+7.6.10(-17).T/mu. The results were used to simulate the swelling/shrinking process. The simulations show the importance of having sufficiently small gel bodies to achieve a short swelling time. (C) 1998 Elsevier Science B.V.
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| 5. |
- Ehlde, M, et al.
(författare)
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A general formalism for metabolic control analysis
- 1997
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Ingår i: Chemical Engineering Science. - 0009-2509. ; 52:15, s. 2599-2606
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Tidskriftsartikel (refereegranskat)abstract
- A general formalism for Metabolic Control Analysis is derived using general sensitivity analysis and structural information of the metabolic pathway inherent in the stoichiometry matrix. The equations derived provide a general procedure for calculating the control coefficients from the elasticity coefficients using matrix algebra, and is valid for any pathway stoichiometry. The procedure diminishes the risk of deriving erroneous relations and is, due to its generality, well suited for computer handling. The formalism is mathematically stringent and is a complement to the original theorems of Metabolic Control Analysis, which were derived using ad hoc reasoning. (C) 1997 Elsevier Science Ltd.
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| 6. |
- Ehlde, M, et al.
(författare)
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Influence of experimental errors on the determination of flux control coefficients from transient metabolic concentrations
- 1996
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Ingår i: Biochemical Journal. - 0264-6021. ; 313, s. 721-727
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Tidskriftsartikel (refereegranskat)abstract
- The influence of experimental errors on the determination of flux control coefficients from transient metabolite concentrations with the method proposed by Delgado and Liao [(1992) Biochem. J. 282, 919-927] has been investigated by using Monte Carlo simulations. The method requires least-squares fitting of the transient metabolite concentrations. Three different fitting methods have been evaluated. Simulated metabolite concentrations of a fictive metabolic pathway were scattered randomly, emulating experimental errors, before performing the fits. This was repeated a large number of times; the mean values and standard deviations of the resulting control coefficients are reported. The results show that the proposed method for determining control coefficients is too sensitive to experimental errors to be practicable, with theoretically justified fitting methods. This is in particular due to the high degree of correlation between the concentrations. An alternative ad hoc fitting method produced biased mean values of the estimates of the control coefficients, but with remarkably low standard deviations.
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| 7. |
- Ehlde, M, et al.
(författare)
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MIST - A user-friendly metabolic simulator
- 1995
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Ingår i: Computer Applications in the Biosciences. - 0266-7061. ; 11:2, s. 201-207
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Tidskriftsartikel (refereegranskat)abstract
- The Metabolic Interactive Simulation Tool, MIST, is a software package, running under Microsoft Windows 3.1, which can be used for dynamic simulations, stoichiometric calculations and control analysis of metabolic pathways. The pathways can be of any complexity and are defined by the user in a simple, interactive way. The user-defined enzymatic rate equations can be compiled either by an external or an internal compiler. Simulations of pathways compiled by an external compiler run significantly faster, but since these compilers are commercial software, they are not distributed together with MIST. The simulations are performed by numerical integration of a set of ordinary differential equations. The integration can be done by either an explicit fourth-order Runge-Kutta algorithm or a semiimplicit third-order Runge-Kutta algorithm, both with adjustable step size. The second algorithm can be used if the set of differential equations is stiff Vector-based drawing facilities are included in the program, with which results can be presented in graphs. Results of simulations, including graphics, can be stored in files. MIST is a very user-friendly, flexible and yet powerful program, with the mathematical details regarding models, simulations and calculations hidden from the user. This makes it suitable for scientists and students with limited computer experience.
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| 8. |
- Eklund, R, et al.
(författare)
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Simultaneous saccharification and fermentation of steam-pretreated willow
- 1995
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Ingår i: Enzyme and Microbial Technology. - 0141-0229. ; 17:3, s. 255-259
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Tidskriftsartikel (refereegranskat)abstract
- Ethanol was produced from SO2-impregnated steam-pretreated willow by simultaneous saccharification and fermentation (SSF). Experiments with SSF were performed in stirred 300-ml vessels and in a 3-1 fermenter. Commercial cellulases were used in combination with either Saccharomyces cerevisiae or Zymomonas mobilis. The amounts of substrate, enzymes, yeast, bacteria, and nutrients were varied. The temperature was maintained at 37 degrees C, and the pH at 5.0. The results indicate that it is possible to reach over 85% of the theoretical ethanol yield, based on the glucan available in the raw material, in 3 days. This was achieved with a substrate concentration of 10% (wt/wt) and a yeast concentration of 10 g l(-1); but even with 1 and 3 g l(-1) yeast, yields of 84.7 and 80%, respectively, were reached. A cellulase concentration corresponding to 18 filter paper U g(-1) substrate was sufficient to complete SSF in 3 days. Zymomonas mobilis gave slightly reduced ethanol yields; however, a reduction in the by-product formation was also obtained compared with the yeast. The nutrient concentration could be considerably reduced compared with literature data without any decrease in the ethanol yield. An increase of substrate concentration from 10 to 15% (wt/wt) reduced the ethanol yield to below 80% as a result of insufficient mass transfer. It was shown that SSF could be performed successfully under nonsterile conditions; however, the substrate and SSF medium must be sterilized prior to SSF.
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| 9. |
- Eklund, R, et al.
(författare)
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The Influence of SO2 and H2SO4 Impregnation of Willow Prior to Steam Pretreatment
- 1995
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Ingår i: Bioresource Technology. - 1873-2976. ; 52:3, s. 225-229
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Tidskriftsartikel (refereegranskat)abstract
- In this investigation steam pretreatment of willow with the addition of SO2 or H2SO4 was studied, the aim being to recover both the cellulose and the hemicellulose fr actions. The willow was steamed (1 bar saturated steam) prior to impregnation with 0-3% H2SO4 or 1% SO2 (w/w substrate). The pretreatment experiments were performed at temperatures in the range 160-230 degrees C for 10 min. The highest xylose recovery with H2SO4 impregnation was obtained using 3% H2SO4 at 190 degrees C, where 80% of the available xylose could be recovered by washing. At these pretreatment conditions, a glucose yield after enzymatic hydrolysis of 67%, based on the glucan available in the raw material, was achieved. The maximum glucose yield, 95%, was obtained when the willow was impregnated with 1% SO2 and pretreated at 200 degrees C. At these conditions, 62% of the original xylose was solubilized. As glucan is the main constituent of willow, a high yield of glucose is of more importance than a high yield of xylose, leading to the conclusion that impregnation with SO2 is better than impregnation with H2SO4.
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| 10. |
- Hofvendahl, Karin, et al.
(författare)
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Simultaneous enzymatic wheat starch saccharification and fermentation to lactic acid by Lactococcus lactis
- 1999
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Ingår i: Applied Microbiology and Biotechnology. - : Springer Science and Business Media LLC. - 1432-0614 .- 0175-7598. ; 52:2, s. 163-169
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Tidskriftsartikel (refereegranskat)abstract
- Simultaneous saccharification of starch from whole-wheat flour and fermentation to lactic acid (SSF) was investigated. For saccharification the commercial enzyme mixture SAN Super 240 L, having alpha-amylase, amyloglucosidase and protease activity, was used, and Lactococcus lactis ssp. lactis ATCC 19435 was used for the fermentation. SSF was studied at flour concentrations corresponding to starch concentrations of 90 g/l and 180 g/l and SAN Super concentrations between 3 mu l/g and 8 mu l/g starch. Kinetic models, developed for the saccharification and fermentation, respectively, were used for simulation and data from SSF experiments were used for model verification. The model simulated SSF when sufficient amounts of nutrients were available during fermentation. This was achieved with high wheat hour concentrations or with addition of yeast extract or amino acids. Nutrient release was dependent on the level of enzyme activity.
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