Staff Publications

Staff Publications

  • external user (warningwarning)
  • Log in as
  • language uk
  • About

    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

    We have a manual that explains all the features 

    Records 1 - 10 / 10

    • help
    • print

      Print search results

    • export

      Export search results

    Check title to add to marked list
    Engagement als antwoord op krimp, de lessen van de Ruimtevolk studiereis "Leren krimpen in Duitsland"
    Breman, B.C. - \ 2012
    In: Ruimtevolk Jaarboek 2012, nieuwe eigenaarschap in de ruimtelijke ordening / Lekkerkerker, J., de Vries, S., Arnhem : RUIMTEVOLK - ISBN 9789081961202 - p. 121 - 125.
    Landschappen te kust en te keur
    Roncken, P.A. ; Woestenburg, M. - \ 2012
    In: Ruimtevolk Jaarboek 2012 : Nieuw eigenaarschap in de ruimtelijke ordening / Lekkerkerker, J., de Vries, S., Arnhem : RUIMTEVOLK - ISBN 9789081961202 - p. 146 - 148.
    De discussie over de Hedwigepolder is een aanfluiting in het licht van de rijke Nederlandse traditie in het vormgeven van landschappen. Integrale gebiedsontwikkeling biedt ongekende mogelijkheden.
    Vrijloopstallen in winterse omstandigheden
    Galama, P.J. ; Hoekstra, N. ; Lekkerkerker, A. - \ 2011
    Veeteelt 28 (2011)6. - ISSN 0168-7565 - p. 58 - 60.
    melkveehouderij - loopstallen - huisvesting van koeien - stallen - dairy farming - loose housing - cow housing - stalls
    Het succes van vrijloopstallen in Nederland hangt erg af van het klimaat. Vooral in koude en vochtige winters is het lastig om de bodem droog en schoon te houden. Studenten van Van Hall Larenstein beschrijven de ervaringen van vier pioniers in Nederland met een vrijloopstal.
    Phase diagram for a mixture of colloids and polymers with equal size
    Tuinier, R. ; Smith, P.A. ; Poon, W.C.K. ; Egelhaaf, S.U. ; Aarts, D.G.A.L. ; Lekkerkerker, H.N.W. ; Fleer, G.J. - \ 2008
    Europhysics Letters 82 (2008). - ISSN 0295-5075
    volume-restriction - behavior - spheres - dispersions - suspensions - cyclohexane - separations - mechanism - chains - latex
    We present the phase diagram of a colloid-polymer mixture in which the radius a of the colloidal spheres is approximately the same as the radius R of a polymer coil (q=R/a1). A three-phase coexistence region is experimentally observed, previously only reported for colloid-polymer mixtures with smaller polymer chains (q0.6). A recently developed generalized free-volume theory (GFVT) for mixtures of hard spheres and non-adsorbing excluded-volume polymer chains gives a quantitative description of the phase diagram. Monte Carlo simulations also agree well with experiment
    Analytical phase diagrams for colloids and non-adsorbing polymer
    Fleer, G.J. ; Tuinier, R. - \ 2008
    Advances in Colloid and Interface Science 143 (2008)1-2. - ISSN 0001-8686 - p. 1 - 47.
    thermodynamic perturbation-theory - critical end-point - hard-spheres - monte-carlo - protein limit - interfacial-tension - depletion thickness - metastable liquids - pair interaction - rigid spheres
    We review the free-volume theory (FVT) of Lekkerkerker et al. [Europhys. Lett. 20 (1992) 5591 for the phase behavior of colloids in the presence of non-adsorbing polymer and we extend this theory in several aspects: (i) We take the solvent into account as a separate component and show that the natural thermodynamic parameter for the polymer properties is the insertion work Pi v. where Pi is the osmotic pressure of the (external) polymer solution and v the volume of a colloid particle. (ii) Curvature effects are included along the lines of Aarts et al. [J. Phys.: Condens. Matt. 14 (2002) 755] but we find accurate simple power laws which simplify the mathematical procedure considerably. (iii) We find analytical forms for the first, second. and third derivatives of the grand potential, needed for the calculation of the colloid chemical potential, the pressure, gas-liquid critical points and the critical endpoint (cep), where the (stable) critical line ends and then coincides with the triple point. This cep determines the boundary condition for a stable liquid. We first apply these modifications to the so-called colloid limit, where the size ratio q(R)=R/a between the radius of gyration R of the polymer and the particle radius a is small. In this limit the binodal polymer concentrations are below overlap: the depletion thickness delta is nearly equal to R, and H can be approximated by the ideal (van 't Hoff) law Pi=Pi(0)=phi/N, where phi is the polymer volume fraction and N the number of segments per chain. The results are close to those of the original Lekkerkerker theory. However, our analysis enables very simple analytical expressions for the polymer and colloid concentrations in the critical and triple points and along the binodals as a function of q(R). Also the position of the cep is found analytically. In order to make the model applicable to higher size ratio's q(R) (including the so-called protein limit where q(R)>1) further extensions are needed. We introduce the size ratio q = delta/a, where the depletion thickness delta is no longer of order R. In the protein limit the binodal concentrations are above overlap. In such semidilute solutions delta approximate to xi, where the De Gennes blob size (correlation length) xi scales as xi similar to phi(-gamma), with gamma = 0.77 for good solvents and I for a theta solvent. In this limit Pi = Pi(sd) similar to phi(3 gamma). We now apply the following additional modifications: iv) Pi = Pi(0) + Pi(sd), where Pi(sd) = A phi(3 gamma); the prefactor A is known from renormalization group theory. This simple additivity describes the crossover for the osmotic pressure from the dilute limit to the semidilute limit excellently. (v) delta(-2) = delta(-2)(0) + xi(-2), where delta(0)approximate to R is the dilute limit for the depletion thickness delta. This equation describes the crossover in length scales from delta(0) (dilute) to xi(semidilute). With these latter two modifications we obtain again a fully analytical model with simple equations for critical and triple points as a function of q(R), In the protein limit the binodal polymer concentrations scale as q(R)(1/gamma), and phase R diagrams phi q(R)(-1/gamma) versus the colloid concentration eta become universal (i.e., independent of the size ratio q(R)). The predictions of this generalized free-volume theory (GFVT) are in excellent agreement with experiment and with computer simulations. not only for the colloid limit but also for the protein limit (and the crossover between these limits). The q(R)(1/gamma) scaling is accurately reproduced by both simulations and other theoretical models. The liquid window is the region between phi(c) (critical point) and phi(t) (triple point). In terms of the ratio phi(t)/phi(c) the liquid window extends from 1 in the cep (here phi(t)-phi(c) = 0) to 2.2 in the protein limit. Hence. the liquid window is narrow: it covers at most a factor 2.2 in (external) polymer concentration. (C) 2008 Elsevier B.V. All rights reserved.
    Lamellar phase-Structured liquid detergents and mesoscopic physics
    Linden, E. van der; Pas, J.C. van de; Hogervorst, W.T. ; Droge, H.A. ; Lekkerkerker, H.N.W. - \ 1999
    In: Abstracts of the 23rd World Congress and Exhibition of the International Society of Far Research , The Brighton Centre, 3-7 october 1999, Brighton (England) Brighton : - p. 43 - 44.
    An exocellular polysaccharide and its interactions with proteins
    Tuinier, R. - \ 1999
    Agricultural University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer; C.G. de Kruif; P. Zoon. - S.l. : S.n. - ISBN 9789058080493 - 184
    polysacchariden - lactococcus lactis subsp. cremoris - fysische eigenschappen - polysaccharides - lactococcus lactis subsp. cremoris - physical properties

    In the food industry polysaccharides are used as thickening or gelling agents. Polysaccharides are usually extracted from plants. Micro-organisms are also capable of excreting polysaccharides: exocellular polysaccharides (EPSs). In some cases EPSs are produced in-situ in food products, notably in acidified milk products. These EPSs function effectively as food thickeners but do not need to be declared in the food label.

    Systematic physical analysis of an exocellular polysaccharide produced by a lactic acid bacterium has hardly been performed until now. In order to obtain a better understanding of the role of EPS in (acidified) milk products the physical properties of an EPS from the lactic acid bacterium strain Lactococcus lactis subsp. cremoris B40 were studied (Chapters 2-4) as well as its interactions with milk proteins (Chapters 5-8). The ionic strength of the EPS solutions was always set at 0.10 M, about the ionic strength in milk.

    In Chapter 2 the isolation, purification and analysis of the molecular properties of EPS from L. lactis B40, our 'model' EPS, are investigated. The polysaccharide was separated from most low molar mass compounds in the culture broth by filtration processes. Gel permeation chromatography (GPC) was used to size-fractionate the polysaccharide. Fractions were analyzed by multi-angle static light scattering in aqueous solutions from which a number- (M n ) and weight-averaged (M w ) molar mass of (1.47 ± 0.06)·10 3and (1.62 ± 0.07)·10 3kg/mol, respectively, were calculated so that M w /M n1.13. The number-averaged radius of gyration was found to be 86 ± 2 nm. The hydrodynamic radius as determined from dynamic light scattering was consistent with the radius of gyration.

    The viscosity of the EPS solutions was studied in simple shear flow as described in Chapter 3. Firstly, the zero-shear viscosity was determined as a function of the concentration. The intrinsic viscosity was determined from the data in the low concentration range. The intrinsic viscosity and the concentration dependence of the (zero-shear) viscosity of the B40 EPS could be predicted from the molar mass and the hydrodynamic radius. In addition the shear-thinning behavior was measured at several concentrations. The shear rate at which the viscosity starts to decrease scales with polymer concentration in accordance with the Rouse theory. By combining existing theories (Rouse and Bueche) it is possible to predict the intrinsic viscosity, concentration dependence of the viscosity, and shear-thinning behavior in terms of the molar mass and the hydrodynamic radius.

    The measurements and theoretical description of the dynamic rheological properties of the EPS are presented in Chapter 4. Dynamic rheological measurements were performed as a function of frequency and EPS concentration. The dynamic properties could be described by the bead-spring model of Rouse. Concentrated EPS solutions have a significant elasticity at high concentrations and high frequencies, which is indicative of the presence of significant normal stress differences. It is suggested that these normal stresses may explain the contribution of the EPSs to the ropy behavior of yogurts.

    Having characterized the EPS in aqueous solution, its interaction with the most relevant colloidal (protein) particles present in milk products was studied. As the polysaccharide studied in this thesis occurs in dairy products our focus was on the interactions and phase behavior of EPS with the colloidal components in milk. There are three distinctly different types of particles in the colloidal size range in milk: fat globules, casein micelles and whey proteins. Smaller molecular species (over 100,000 in milk) are considered as part of the continuous phase.

    In Chapter 5 the interactions with whey proteins are described. Native whey proteins and EPS were co-soluble; they could be mixed in all proportions. However, an effective attraction (a depletion interaction) is induced between aggregated-whey-protein colloid (AWC) particles when they are mixed with the EPS. This depletion interaction originates from a loss of conformational entropy of the EPSs near the surface of neighboring AWC particles and leads to a phase separation at high enough EPS and/or AWC concentrations. The effect of the depletion interaction on the properties of the mixtures of EPS and AWC particles was first studied in the stable, i.e. one-phase region. The strength of attractions was characterized by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The SANS results could be described quantitatively by the Vrij theory and integral theory (Ornstein-Zernike with HNC closure) in combination with the Schaink-Smit theory and allowed a determination of the position of the spinodal. The DLS results could be described reasonably well by using a theory of Dhont and Kawasaki.

    Furthermore, the experimental phase boundary was determined and compared with the Schaink-Smit theory, a mean-field theory which evaluates the free energy of a mixture of colloids and large non-adsorbing polymers. The spinodal so calculated was found to be consistent with the experimentally determined position of the phase boundary.

    Spinodal phase separation kinetics was investigated by small-angle light scattering (SALS). At low Q a scattering peak was detected which shifted to lower Q's with time, in agreement with other experimental data and theoretical predictions for spinodal decomposition. Both the scaling of the scattered intensity with Q and the scaling of the Q-position of the peak with time agree with theoretical predictions of Furukawa and Siggia.

    The interactions between EPS B40 and casein micelles are treated in in Chapter 6. Casein micelles become mutually attractive when the EPS is added to skim milk. The attraction can be explained as a depletion interaction between the casein micelles induced by the non-adsorbing EPS. We used three scattering techniques (SANS, turbidity measurements and DLS) to measure the attraction. The Vrij theory in combination with integral theory and all the experiments showed that casein micelles became more attractive upon increasing the EPS concentration.

    The phase separation arising from depletion interaction in mixtures of casein micelles and EPS is described in Chapter 7. We have determined a phase diagram that describes the separation of skim milk with EPS into a casein-micelle-rich phase and an EPS-rich phase. We compared the phase diagrams with those calculated from theories developed by Vrij, and by Lekkerkerker and co-workers, showing that the experimental phase boundary can be predicted quite well. From measurements of the self-diffusion of the casein micelles in the presence of EPS the spinodal was calculated, which corresponds to the visual observations.

    The effect of adding the EPS to an oil-in-water emulsion, stabilized with whey proteins, is reported in Chapter 8. Even at low EPS concentrations the emulsion phase separates. The phase line could be described by depletion interaction theory of Vrij. At high EPS concentrations and dispersed phase volume fractions above 10% we found a stable 'gel'-like region in the phase diagram. In that region the oil droplets attract one another so strongly that a space-filling network is formed at sufficient oil volume fractions.

    A kinetic study showed that the rate of creaming/demixing decreases with volume fraction of oil of the system (hydrodynamics) and strongly depends on the concentration of EPS (strength of depletion interaction and continuous-phase viscosity). At low EPS concentration the creaming rate strongly increased with EPS concentration since attractions enhance creaming. At higher EPS concentrations creaming was slowed down by the viscosity increase of the continuous phase and the particle network which was created. This network became so strong at high EPS concentrations that creaming was absent in the 'gel' region. The rheological behavior of the 'gel' was studied by measuring flow curves which could be interpreted by the Potanin model, which describes the rheology of a dispersion of weakly aggregating particles.

    In Chapter 9 the practical implications of this work are described. In order to understand the thickening effect of EPSs the molar mass, radius of gyration, and their interrelation are very important. It is indicated how the effectivity of a polysaccharide can be analyzed on the basis of the molar mass and the radius of gyration. The relation between the radius of gyration and the molar mass depends on the kind of monosaccharide residues, the linkage type, and the solvent. Further it is addressed how a fundamental understanding of the interactions between polysaccharides and proteins leads to predictions of the phase line and interpretation of the measured phase behavior. The unwanted effect of phase separation can then be suppressed by using only biopolymer concentrations at which the system is still stable. An understanding of the biopolymer interactions may thus make it possible to adjust the properties of food dispersions. Finally, some suggestions for further research are given.

    Identification of distinct functional domains of the cowpea mosaic virus movement protein by mutational analysis.
    Bertens, P. ; Lekkerkerker, A. ; Verver, J. ; Lent, J. van; Wellink, J. ; Goldbach, R. ; Kammen, A. van - \ 1996
    In: Abstract 10th Int. Congr. of Virology. Jerusalem, Israel (1996) 118, pw05-24
    Distinct functional domains in the cowpea mosaic virus movement protein.
    Lekkerkerker, A. ; Wellink, J. ; Yuan, P. ; Lent, J. van; Goldbach, R. ; Kammen, A. van - \ 1996
    Journal of Virology 70 (1996). - ISSN 0022-538X - p. 5658 - 5661.
    Molecular characterization of the beet cyst nematode (Heterodera schachtii) resistance locus Hs1
    Salentijn, E.M.J. - \ 1995
    Agricultural University. Promotor(en): A. van Kammen; W.J. Stiekema. - S.l. : Salentijn - ISBN 9789054853930 - 119
    beta vulgaris - suikerbieten - plantenveredeling - ziekteresistentie - plaagresistentie - moleculaire biologie - plantenziektekunde - plantenplagen - pratylenchus - heteroderidae - tylenchidae - moleculaire genetica - beta vulgaris - sugarbeet - plant breeding - disease resistance - pest resistance - molecular biology - plant pathology - plant pests - pratylenchus - heteroderidae - tylenchidae - molecular genetics

    The white beet cyst nematode (BCN), Heterodera schachtii Schm. is a serious pest in sugar beet ( B. vulgaris L.) cultivation and is widely distributed throughout most of the beet-growing areas in the world (Cooke 1987). The economical losses due to infestation with the nematode are considerable (approximately 1200 dutch guilders or $ 600 per ha. at a rate of 25 % -30 % loss) and can mainly be ascribed to the intensive growing of sugar beet and other crops like oilseed rape which allow the nematode to multiply. The damage consists of wilting and a loss in root yield and sugar content (Mesken & Lekkerkerker 1988). Due to the lack of paying non-host crops to widen the rotation scheme, control of the beet cyst nematode population relies heavily on the use of nematicides. An alternative way, in which control might be achieved, is the use of resistant varieties. However, breeding for nematode resistance in sugar beet is extremely difficult and time consuming and did not yet result in stable nematode resistant material. Nevertheless, from crosses between sugar beet and BCN resistant wild beets three types of BCN resistant plants were obtained: monosomic additions, monosomic fragment additions and diploid introgressions. The BCN resistance of the monosomic additions (2n = 18 + 1) and the monosomic fragment additions (2n = 18 + f) is highly unstable whereas the resistance of the diploids appears to be more stable but also does not reach an acceptable level of stability (Lange et al. 1990; Van Geyt et al. 1990). Because of the difficulty to obtain stable resistant sugar beet varieties by traditional breeding, a program was started in april 1988 which aims at the isolation of BCN resistance gene(s) from wild beets of the section Procumbentes. The ultimate goals of this project are the transfer of the isolated resistance gene(s) to sugar beet to obtain stable resistant varieties and to elucidate the mode of action of the BCN resistance gene. Although, several groups are working on the isolation of genes conferring resistance to plant-parasitic nematodes, no such gene is isolated yet.

    This thesis describes work aimed at the isolation of the BCN-resistance genes Hs1 pat-1 and Hs1 pro-1 via 'positional cloning' (Wicking and Williamsom 1991). 'Positional cloning' is a strategy for isolating genes which are only defined by their phenotype, a condition that holds for the BCN-resistance genes. For positional cloning the gene of interest is localized on the genome with respect to molecular markers. Next, flanking markers can be identified and used for the onset and termination of a chromosomal walk, which is the identification of a continuous set (contig) of overlapping DNA clones that connect the two flanking markers. A Yeast Artificial Chromosome (YAC) library (Burke et al. 1987, Ward & Jen 1990) that contains large cloned DNA-fragments of several hundred kilobases can aid the spanning of large chromosomal distances between the markers. Furthermore, the separation and manipulation of large chromosome fragments by Pulsed Field Gel Electrophoresis can be employed for the construction of a long-range physical map of the region. Finally, the essential chromosomal region, cloned in one or several contiguous YACs and subcloned in cosmids, is analyzed for the presence of candidate genes which are then screened for a functional BCN resistance gene.

    Chapter 1 of this thesis describes morphological and genetic features of the plant-BCN interaction. This information is important for the ultimate development of nematode resistant plants. Furthermore, the positional cloning strategy for isolating genes is described in detail and the state of art for the identification and cloning of various nematodes resistance genes is given.

    Chapter 2 describes the isolation of molecular markers linked to the BCN resistance locus, Hs1, which is the first prerequisite for positional cloning of the gene(s). The plant material which was used consisted of sugar beets with an introgressed wild beet chromosome fragment containing the resistance gene(s). Since the resistance in this material segregates in a nonMendelian way, a deletion mapping strategy was employed to order the markers with respect to the resistance locus.

    Chapter 3 describes the characterization of a marker which is highly repetitive in wild beets and closely linked to Hs1 pat-1 and Hs1 pro-1 . The long-range physical organization of the repeat is studied by employing the PFGE-technology.

    Chapter 4 describes the construction of a YAC-library from BCN-resistant sugar beet (AN5-203b) containing an additional fragment of a wild beet chromosome. This library was screened with a marker localized near the Hs1 pat-1 gene to provide a starting point for the assembly of a YAC- contig spanning the resistance locus.

    Chapter 5 describes the isolation, characterization and deletion mapping of additional PCR-based RAPD markers.

    In Chapter 6 physical distances around three markers linked to Hs1 pat-1 are determined which has resulted in a first generation physical map of the resistance locus.

    Chapter 7 is a general discussion of the research which will be necessary for the ultimate positional cloning of the resistance genes. Furthermore, different strategies for the engineering of nematode resistant plants are discussed.

    Check title to add to marked list

    Show 20 50 100 records per page

    Please log in to use this service. Login as Wageningen University & Research user or guest user in upper right hand corner of this page.