The role of casein micelles and their aggregates in foam stabilization
Chen, Min - \ 2016
Wageningen University. Promotor(en): Erik van der Linden; Toon van Hooijdonk, co-promotor(en): Marcel Meinders; Guido Sala. - Wageningen : Wageningen University - ISBN 9789462579842 - 124
foams - foaming - milk - casein - micelles - physical properties - stabilization - schuim - schuimen - melk - caseïne - micellen - fysische eigenschappen - stabilisatie
Many foam products derived from milk or specific dairy ingredients suffer from drainage, coalescence and/or disproportionation. Previous studies indicated that foam properties of milk are strongly influenced by the composition of the milk as well as by the processing conditions during foam production. The aim of this research was to get a better understanding of these two factors. Interestingly, the presence of aggregates of casein micelles was found to result in very stable foams. The interfacial properties (adsorption speed, adsorption energy, dynamical interfacial tension, interfacial dilatational moduli), thin film stability (rupture time) and foam properties (foamability, drainage, coalescence) of casein micelle dispersions were determined. Based on these data, the very stable foams were concluded to result from properties of the thin films in the foam, which were affected drastically by the presence of the large aggregates of casein micelles.
Core-shell particles at fluid interfaces : performance as interfacial stabilizers
Buchcic, C. - \ 2016
Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): R.H. Tromp; Marcel Meinders. - Wageningen : Wageningen University - ISBN 9789462578968 - 140
stabilization - stabilizers - particles - colloidal properties - adsorption - interface - fluids - stabilisatie - stabiliseermiddelen - deeltjes - colloïdale eigenschappen - adsorptie - grensvlak - vloeistoffen (fluids)
There is a growing interest in the use of particles as stabilizers for foams and emulsions. Applying hard particles for stabilization of fluid interface is referred to as Pickering stabilization. By using hard particles instead of surfactants and polymers, fluid interfaces can be effectively stabilized against Ostwald ripening and coalescence. A drawback of the use of hard particles as interfacial stabilizers is that they often experience a pronounced energy barrier for interfacial adsorption and that hard particles are very specific with regard to the type of fluid interface they can adsorb to. Soft particles, on the other hand, are known as good stabilizers against coalescence and they spontaneously adsorb to a variety of different fluid interfaces.
The aim of this thesis was to investigate core-shell particles comprising a hard core and soft shell with regard to their interfacial behaviour and their ability to act as sole stabilizers for foams and emulsions. We hypothesised that the presence of the soft shell allows for easier interfacial adsorption of core-shell particles compared to the hard core particles only. To test this hypothesis, we prepared core-shell particles comprising a solid polystyrene (PS) core and a soft poly-N-isopropylacrylamide (PNIPAM) shell. To ascertain the effect of shell thickness, we prepared a range of core-shell particles with different shell thicknesses, containing identical core particles. We found that core-shell particles are intrinsically surface active and can generate high surface pressures at the air-water interface and oil-water interfaces, whereas core particles seemed to experience a large energy barrier for interfacial adsorption and did not lower the surface tension. We also confirmed by microscopy that core-shell particles are actually adsorbing to the fluid interface and form densely packed interfacial layers. Further, we found that a certain critical thickness of the soft shell is necessary in order to ensure facile interfacial adsorption. If the PNIPAM shell on top of the core particles is well above 100nm thick, particle adsorption at the air-water interface was found to be diffusion limited.
By gentle hand-shaking we were able to produce dispersion of air bubbles and emulsion droplets solely stabilized by core-shell particles. The resulting bubbles still underwent Ostwald ripening, albeit slowly. For oil-in-water emulsions of hexane and toluene, both of which have a relatively high solubility in the continuous phase, we found that core-shell particles can stop Ostwald ripening. The resulting emulsion droplets adopted pronounced non-spherical shapes, indicating a high elasticity of the interface. The high stability and the remarkable non-spherical shape of the emulsion droplets stabilized by core-shell particles were features we also observed for fluid dispersion stabilized by hard particles. This shows that in terms of emulsion stability core-shell particles behave similar to hard particles as interfacial stabilizer.
As to why the differences between the stability of bubble and oil dispersions arise could not be finally answered. Yet, microscopic analysis of the interfacial configuration of core-shell particles at the air-water interface reveals some peculiar insights which may suggest that core-shell particles adsorb in a polymer-like fashion with the soft PNIPAM shells adsorbing to the air-water interface only, while the hard PS cores reside in the continuous phase.
In summary, we showed that core-shell particles with a hard core and a soft shell can indeed combine the advantageous properties of hard and soft particles. The soft shell enables spontaneous adsorption to a variety of fluid interfaces. Despite their spontaneous adsorption, core-shell particles strongly anchor and do not spontaneously desorb from the fluid interface again. Further, the hard core provides enough rigidity to the core-shell particles to allow the establishment of a stress bearing interfacial particle network. This network eventually stops Ostwald ripening in oil-in-water emulsions. Our results therefore show that in the case of oil-water interfaces, core-shell particles can perform better than solely hard particles as interfacial stabilizers.
Complex coacervates and microgels for emulsions : robust, responsive, reversible
Monteillet, H.J.M. - \ 2015
Wageningen University. Promotor(en): Frans Leermakers, co-promotor(en): Mieke Kleijn; Joris Sprakel. - Wageningen : Wageningen University - ISBN 9789462574526 - 147
emulsies - gels - stabilisatie - elektrolyten - scheiding - emulsions - gels - stabilization - electrolytes - separation
The use of ionic liquids (ILs) as replacement of organic solvents in liquid-liquid extractions has shown great promise due to their low volatility, flammability and toxicity, tunable solvency to a wide variety of extractable compounds and mild- ness to delicate compounds such as biomolecules for pharmaceutical applications. However, the efficiency of extractions using ionic liquids is limited as the inher- ently high viscosity of ILs slows down the mass transfer. Increasing the interfacial area between the immiscible phases is an efficient way to increase the efficiency of liquid extractions; typically done by formulating emulsions, dispersions of fluid droplets suspended in a second immiscible continuous phase. While strategies to formulate stable emulsions from conventional apolar solvents, such as aliphatic or halogenated oils, in water are abundant, the peculiar properties of ionic liquids requires the exploration of new strategies to formulate stable emulsions; for exam- ple, common surfactant stabilization leads to rapid Ostwald ripening due to the inherent water solubility of many ionic liquids. Moreover, while the intended ionic liquid-in-water emulsions must be stable at operating temperatures for prolonged times, it should be possible to break the emulsion on-demand to recover the ex- tracted product. Also, the interfacial layer used for stabilization should not hinder the transfer of the intended product to the droplet phase. To increase the sus- tainability of extraction processes, recovery of both ionic liquid and stabilizer for re-use in a subsequent extraction step is highly desired. Aimed to establish new ways of stabilizing emulsions in general, and ionic liquid emulsions in specific, this thesis describes investigations into two novel stabilizers: interfacial electrostatic complexes and soft colloidal microgels.
In Part I, we focussed on how oppositely charged polyelectrolytes interact and form complexes across an oil-water interface. In Chapter 2, we demonstrated a new method for emulsion stabilization, in which electrostatic complexes formed across a liquid interface between two polyelectrolytes, one dissolved in the aqueous phase, the other in the oil phase. Using tensiometry we followed the polyelectrolyte adsorption at the oil-water interface; while the presence of either polyelectrolyte alone leads to interfacial depletion, the presence of both species leads to strong adsorption at the interface. This was further confirmed using confocal fluorescence microscopy where the colocalization of both species at the interface was observed; the strong overlap of peak intensities at the interface suggests a strongly intermixed layer. Using this approach, we prepared stable emulsions, which could be reversibly broken and reformed by simple pH and salt triggers. Interestingly, oil-in-water but also water-in-oil emulsions could be produced. This is the first demonstration of using selective associative phase separation to stabilize a segregating system.
The experimental results triggered questions on the nature of the interfacial layer, which was too thin to be ascertained in detail using microscopy. Therefore,
we turned to self-consistent field (SCF) modelling to develop a deeper understand- ing of the structure and thermodynamics of this interfacially-templated complex- ation, as presented in Chapter 3. In analogy with our experiments, we use the Scheutjens-Fleer lattice method to consider mixtures of two solvents, an anionic oil- soluble polyelectrolyte, a cationic water-soluble polyelectrolyte, their counterions and additional indifferent monomeric electrolyte. We first considered a two-phase system with only one polyelectrolyte and salt. We found that the polyelectrolyte adsorption depends on its concentration. For polyelectrolyte concentrations lower than the salt concentration, the polyelectrolyte is depleted from the oil-water in- terface while for polyelectrolyte concentrations higher than the salt concentration, the polyelectrolyte adsorbs at this interface. This transition from depletion to ad- sorption originates from a competition between small ion and macroion adsorption, governed by the overall ionic strength. Upon introducing a second polyelectrolyte in the immiscible second solvent, a new phase spontaneously formed at the inter- face between oil and water. Surprisingly, our calculations showed that ion release entropy is not the driving force for complexation, as it often is in bulk complex coacervation; co-assembly is governed by enthalpic contributions. This is due to the solvent-selectivity of the polyelectrolytes in this scenario, which leads to low solvent content in the coacervate layer, hence close approach of the opposite charges resulting in a relatively large Coulombic enthalpy. Finally, we examined systems with asymmetric composition of the two polyelectrolytes within the same theoret- ical approach. This revealed an unusual pseudo-partial wetting scenario, due to interactions occurring at different length scales. When the electrostatic interactions are short ranged, the microscopically thin wetting film transitions to a mesoscopic thin film. However, charges built up on either side of the coacervate layer restrict the growth of the film to macroscopic dimensions. In our experiments we observe that the coacervate layer becomes turbid over time, suggesting structures on op- tical length scales, much larger than the typical dimensions of the polymer coils. This may be explained by the pseudo-partial wetting scenario due to the coexis- tence of a mesoscopic film with interfacial liquid droplets nucleating due to thermal fluctuations.
In the second part of this thesis, Part II, we studied the adsorption and or- ganization of colloidal microgels at a variety of liquid interfaces. These soft and deformable hydrogel colloids have gained a lot of interest in recent years due to their excellent ability to stabilize emulsions. As a result of their polymeric nature and osmotic equilibrium with the bulk solution, microgels exhibit an interesting duality between colloidal properties and polymeric behaviour. Microscopic research into their interfacial behaviour is often made difficult as they offer little refractive index contrast to the continuous phase and covalent attachment of fluorophores is known
to drastically alter their interactions. To overcome this problem, in Chapter 4 we introduce composite microgels, in which a solid fluorescent core is embedded in the centre of a soft and tunable hydrogel shell, thereby decoupling the imaging features of these microgels with the tunability of their softness, size, solvent-responsivity and interactions. We surprisingly find that while these microgels adsorb sponta- neously, without any energy barrier which is usual for the Pickering adsorption of micron-sized colloids, their anchoring at the liquid interface is irreversibly strong. Due to the high adsorption energy, saturated interfacial layers of these microgels show mild compression of the particles, increasing their packing density at the cost of elastic deformation. Moreover, we showed that these particles are able to stabilize a wide variety of oil-water interfaces and due to their spontaneous adsorp- tion allow the fabrication of Pickering droplets using microfluidics, which is usually hindered by the adsorption barrier for solid particles.
In Chapter 5, we arrive at the ultimate aim of this thesis, i.e. to provide proof- of-concept for a fully sustainable extraction process based on IL-in-water emulsions. We first show how microgels are able to create emulsions of a wide variety of ILs in water and prevent their Ostwald ripening, resulting in extended stability at room temperature. Upon heating and applying centrifugal compression, the emulsion can be rapidly broken, with all of the microgels returning the aqueous phase which can then be re-used in a secondary extraction step. Finally, we demonstrated that through the use of a paramagnetic ionic liquid, the concentration and breaking step can be performed without energy input with a simple permanent magnet, rendering the process sustainable from start to end.
Finally, in Chapter 6, we studied the adsorption and conformation of these composite microgels at solid-liquid interfaces. We first demonstrate how conven- tional sample preparation for studying microgels at solid interfaces, often involving a drying step, induces strong sample artefacts. We therefore developed a method to study the adsorption and conformation of microgels in-situ using liquid-state confocal and atomic force microscopy. Our results showed how the packing density for particle adsorption is governed by particle-particle repulsion, as adsorption en- ergies are typically very high. Using Quantitative Nanomechanical Mapping, the spatially-resolved mechanical analysis of surfaces using atomic force microscopy, we find that the degree of spreading of microgels during adsorption at a solid interface is governed by adsorption energy and particle softness as expected. This leads us to conclude that the unique properties of microgels at interfaces results from a subtle interplay between adsorption energy and internal elasticity.
Temporal stabilizability and compensatability of time-varying linear discrete-time systems with white stochastic parameters
Willigenburg, L.G. van; Koning, W.L. De - \ 2015
European Journal of Control 23 (2015). - ISSN 0947-3580 - p. 36 - 47.
optimal compensation - stabilization - detectability - design - delays
This paper reveals that apart from changes of system structure vital system properties such as stabilizability and compensatability may be lost temporarily due to the stochastic nature of system parameters. To that end new system properties called temporal mean-square stabilizability (tms-stabilizability) and temporal mean-square compensatability (tms-compensatability) for time-varying linear discrete-time systems with white stochastic parameters (multiplicative white noise) are developed. When controlling such systems by means of (optimal) state feedback, tms-stabilizability identifies intervals where mean-square stability (ms-stability) is lost temporarily. This is vital knowledge to both control engineers and system scientists. Similarly, tms-compensatability identifies intervals where ms-stability is lost temporarily in case of full-order (optimal) output feedback. Tests explicit in the system matrices are provided to determine each temporal system property. These tests compute measures of the associated temporal system properties. Relations among the new system properties as well as relations with associated existing system properties are investigated and established. Examples illustrating principal applications and practical importance are provided.
The ecosystem engineer Crassostrea gigas affects tidal flat morphology beyond the boundary of their reef structures
Walles, B. ; Salvador de Paiva, J. ; Prooijen, B. van; Ysebaert, T. ; Smaal, A.C. - \ 2015
Estuaries and coasts 38 (2015)3. - ISSN 1559-2723 - p. 941 - 950.
wadden sea - sediment dynamics - pacific oysters - native mussels - habitat - estuary - stabilization - enhancement - communities - adaptation
Ecosystem engineers that inhabit coastal and estuarine environments, such as reef building oysters, do not only stabilise the sediment within their reefs, but their influence might also extend far outside their reefs, affecting tidal flat morphology and protecting the surrounding soft-sediment environment against erosion. However, quantitative information is largely missing, and the spatially extended ecosystem engineering effects on the surrounding soft-sediment largely unstudied. To quantify this, we measured elevations around eleven natural Crassostrea gigas reefs occurring on tidal flats in the Oosterschelde estuary (the Netherlands). These tidal flats experience strong erosion as a consequence of human interventions in the system. Various reef sizes were chosen to test the proportional effects of reefs on tidal flat morphology. Measurements were used to create 3-dimensional surface maps to obtain properties of the reefs and the surrounding soft-sediment environment. The area of the oyster reefs ranged from 2 to 1,908 m2. Reef length varied between 1 and 61 m, reef width between 1 and 45 m, and reef height between 0.20 and 1.08 m. Reefs varied in shape, going from round shape structures to more elongated ones. We observed elevated areas (>5 cm elevation from the background intertidal slope) on the lee side of all reefs, caused by the interaction between the reef’s structure and locally prevailing wave conditions. The elevated area (i.e. the spatially extended ecosystem engineering effect) affected by the reef was of the same order of magnitude as the reef area. The elevated area was related to reef properties such as reef length, width, and height. Reef length, however, appeared to be the best predictor. These findings contribute to management solutions for coastal adaptation and protection. Our study clearly showed that oyster reefs not only protect the tidal flat under their footprint, but as well an area beyond the boundary of the reef
Protein redesign by learning from data
Berg, B.A. van den; Reinders, M.J.T. ; Laan, J.M. van der; Roubos, J.A. ; Ridder, D. de - \ 2014
Protein Engineering, Design & Selection 27 (2014)9. - ISSN 1741-0126 - p. 281 - 288.
computational enzyme design - aspergillus - stabilization - optimization - generation - prediction - secretion - hydrolase - peptides - tools
Protein redesign methods aim to improve a desired property by carefully selecting mutations in relevant regions guided by protein structure. However, often protein structural requirements underlying biological characteristics are not well understood. Here, we introduce a methodology that learns relevant mutations from a set of proteins that have the desired property and demonstrate it by successfully improving production levels of two enzymes by Aspergillus niger, a relevant host organism for industrial enzyme production. We validated our method on two enzymes, an esterase and an inulinase, creating four redesigns with 5-45 mutations. Up to 10-fold increase in production was obtained with preserved enzyme activity for small numbers of mutations, whereas production levels and activities dropped for too aggressive redesigns. Our results demonstrate the feasibility of protein redesign by learning. Such an approach has great potential for improving production levels of many industrial enzymes and could potentially be employed for other design goals.
Improved emulsion stability by succinylation of patatin is caused by partial unfolding rather than charge effects
Delahaije, R.J.B.M. ; Wierenga, P.A. ; Giuseppin, M.L.F. ; Gruppen, H. - \ 2014
Journal of Colloid and Interface Science 430 (2014). - ISSN 0021-9797 - p. 69 - 77.
in-water emulsions - protein-exposed hydrophobicity - beta-lactoglobulin - drop size - adsorption - flocculation - interface - stabilization - ph - dependence
This study investigates the influence of succinylation on the molecular properties (i.e. charge, structure and hydrophobicity) and the flocculation behavior of patatin-stabilized oil-in-water emulsions. Patatin was succinylated to five degrees (0% (R0) to 57% (R2.5)). Succinylation not only resulted in a change of the protein charge but also in (partial) unfolding of the secondary structure, and consequently in an increased initial adsorption rate of the protein to the oil–water interface. The stability against salt-induced flocculation showed two distinct regimes, instead of a gradual shift in stability as expected by the DLVO theory. While flocculation was observed at ionic strengths > 30 mM for the emulsions stabilized by the variants with the lowest degrees of modification (R0–R1), the other variants (R1.5–R2.5) were stable against flocculation ¿ 200 mM. This was related to the increased initial adsorption rate, and the consequent transition from a protein-poor to a protein-rich regime. This was confirmed by the addition of excess protein to the emulsions stabilized by R0–R1 which resulted in stability against salt-induced flocculation. Therefore, succinylation of patatin indirectly results in stability against salt-induced flocculation, by increasing the initial adsorption rate of the protein to the oil–water interface, leading to a shift to the protein-rich regime.
Common and rare single nucleotide polymorphisms in the LDLR gene are present in a black South African population and associate with low-density lipoprotein cholesterol levels
Zyl, T. ; Jerling, J.C. ; Conradie, K.R. ; Feskens, E.J.M. - \ 2014
Journal of Human Genetics 59 (2014). - ISSN 1434-5161 - p. 88 - 94.
coronary-heart-disease - plasma-lipid levels - receptor gene - familial hypercholesterolemia - messenger-rna - human genome - mutations - risk - stabilization - expression
The LDL receptor has an essential role in regulating plasma LDL-C levels. Genetic variation in the LDLR gene can be associated with either lower or moderately raised plasma levels of LDL-C, or may cause familial hypercholesterolemia. The prevalence of single-nucleotide polymorphisms (SNPs) in the LDLR in the black South African population is not known and therefore, we aimed to determine the genotypic variation of the LDLR in the study population as well as to define the association of the different genotypes with plasma LDL-C levels. A random selection of 1860 apparently healthy black South African volunteers aged 35–60 years was made in a cross-sectional study. Novel SNPs were identified in a subset of 30 individuals by means of automated sequencing before screening the entire cohort by means of the Illumina VeraCode GoldenGate Genotyping Assay on a BeadXpress Reader system. Twenty-five SNPs were genotyped, two of which were novel. A very rare SNP, rs17249141, in the promoter region was significantly associated with lower levels of LDL-C. Four other SNPs (rs2738447, rs14158, rs2738465 and rs3180023) were significantly associated with increased levels of LDL-C. We can conclude that some of the various SNPs identified do indeed associate with LDL-C levels
Bifunctional immobilization of a hyperthermostable endo ß 1,3 glucanase
Przybysz, A. ; Volmer, A.A. ; Westphal, A.H. ; Berkel, W.J.H. van - \ 2014
Applied Microbiology and Biotechnology 98 (2014)3. - ISSN 0175-7598 - p. 1155 - 1163.
pyrococcus-furiosus - enzyme immobilization - epoxy supports - proteins - stabilization - adsorption - reagent - acid
Laminarinase A (LamA) from Pyrococcus furiosus is a hyperthermostable endo-ß-1,3-glucanase (EC 22.214.171.124) belonging to the glycosyl hydrolase family GH16. Here, we report the two-step immobilization of LamA on macroporous acrylic epoxy beads, extra-functionalized with disulfide groups. To facilitate initial immobilization via thiol–disulfide exchange, we introduced, by site-directed mutagenesis, a superficial cysteine residue near the protein C-terminal end. The thus-obtained S296C variant showed similar catalytic properties as native LamA. The activity of immobilized S296C displayed an inverse relationship with particle size. Use of conventional beads (150–300 µm in diameter) obstructed the catalytic efficiency due to pore diffusion limitation of the polysaccharide substrate. Bifunctional attachment to milled beads (20–40 µm) resulted in high enzyme load and outstanding catalytic features. Bifunctional immobilized S296C showed extreme pH stability and could be repeatedly used at 60 °C without significant activity loss.
Modeling the vertical soil organic matter profile using Bayesian parameter estimation
Braakhekke, M.C. ; Wutzler, T. ; Beer, C. ; Kattge, J. ; Schrumpf, M. ; Ahrens, B. ; Schoning, I. ; Hoosbeek, M.R. ; Kruijt, B. ; Kabat, P. ; Reichstein, M. - \ 2013
Biogeosciences 10 (2013)1. - ISSN 1726-4170 - p. 399 - 420.
terrestrial ecosystem model - scots pine forest - carbon dynamics - radioactive isotopes - temperate soils - lead - pb-210 - stabilization - transport - turnover
The vertical distribution of soil organic matter (SOM) in the profile may constitute an important factor for soil carbon cycling. However, the formation of the SOM profile is currently poorly understood due to equifinality, caused by the entanglement of several processes: input from roots, mixing due to bioturbation, and organic matter leaching. In this study we quantified the contribution of these three processes using Bayesian parameter estimation for the mechanistic SOM profile model SOMPROF. Based on organic carbon measurements, 13 parameters related to decomposition and transport of organic matter were estimated for two temperate forest soils: an Arenosol with a mor humus form (Loobos, the Netherlands), and a Cambisol with mull-type humus (Hainich, Germany). Furthermore, the use of the radioisotope Pb-210(ex) as tracer for vertical SOM transport was studied. For Loobos, the calibration results demonstrate the importance of organic matter transport with the liquid phase for shaping the vertical SOM profile, while the effects of bioturbation are generally negligible. These results are in good agreement with expectations given in situ conditions. For Hainich, the calibration offered three distinct explanations for the observations (three modes in the posterior distribution). With the addition of Pb-210(ex) data and prior knowledge, as well as additional information about in situ conditions, we were able to identify the most likely explanation, which indicated that root litter input is a dominant process for the SOM profile. For both sites the organic matter appears to comprise mainly adsorbed but potentially leachable material, pointing to the importance of organo-mineral interactions. Furthermore, organic matter in the mineral soil appears to be mainly derived from root litter, supporting previous studies that highlighted the importance of root input for soil carbon sequestration. The Pb-210(ex) measurements added only slight additional constraint on the estimated parameters. However, with sufficient replicate measurements and possibly in combination with other tracers, this isotope may still hold value as tracer for SOM transport.
Implications of alternative assumptions regarding future air pollution control in scenarios similar to the Representative Concentration Pathways
Chuwah, C.D. ; Noije, T. van; Vuuren, D.P. van; Hazeleger, W. ; Strunk, A. ; Deetman, S. ; Beltran, A.M. ; Vliet, J. van de - \ 2013
Atmospheric Environment 79 (2013). - ISSN 1352-2310 - p. 787 - 801.
intercomparison project accmip - greenhouse-gas concentrations - atmospheric chemistry - tropospheric ozone - next-generation - climate-change - model - emissions - stabilization - simulations
The uncertain, future development of emissions of short-lived trace gases and aerosols forms a key factor for future air quality and climate forcing. The Representative Concentration Pathways (RCPs) only explore part of this range as they all assume that worldwide ambitious air pollution control policies will be implemented. In this study, we explore how different assumptions on future air pollution policy and climate policy lead to different concentrations of air pollutants for a set of RCP-like scenarios developed using the IMAGE model. These scenarios combine low and high air pollution variants of the scenarios with radiative forcing targets in 2100 of 2.6 W m(-2) and 6.0 W m(-2). Simulations using the global atmospheric chemistry and transport model TM5 for the present-day climate show that both climate mitigation and air pollution control policies have large-scale effects on pollutant concentrations, often of similar magnitude. If no further air pollution policies would be implemented, pollution levels could be considerably higher than in the RCPs, especially in Asia. Air pollution control measures could significantly reduce the warming by tropospheric ozone and black carbon and the cooling by sulphate by 2020, and in the longer term contribute to enhanced warming by methane. These effects tend to cancel each other on a global scale. According to our estimates the effect of the worldwide implementation of air pollution control measures on the total global mean direct radiative forcing in 2050 is +0.09 W m(-2) in the 6.0 W m(-2) scenario and -0.16 W m(-2) in the 2.6 W m(-2) scenario. (C) 2013 Elsevier Ltd. All rights reserved.
Land use and management effects on soil organic matter fractions in Rhodic Ferralsols and Haplic Arenosols in Bindura and Shamva districts of Zimbabwe
Mujuru, L. ; Mureva, A. ; Velthorst, E.J. ; Hoosbeek, M.R. - \ 2013
Geoderma 209-210 (2013). - ISSN 0016-7061 - p. 262 - 272.
conservation agriculture - carbon sequestration - no-tillage - microbial biomass - density fractions - term changes - dynamics - impact - pools - stabilization
Soil organic carbon (SOC) is a major attribute of soil quality that responds to land management activities which is also important in the regulation of global carbon (C) cycling. This study evaluated bulk soil C and nitrogen (N) contents and C and N dynamics in three soil organic matter (SOM) fractions separated by density. The study was based on three tillage systems on farmer managed experiments (conventional tillage (CT), ripping (RP), direct seeding (DS)) and adjacent natural forest (NF) in Haplic Arenosols (sandy) and Rhodic Ferralsols (clayey) of Zimbabwe. Carbon stocks were significantly larger in forests than tillage systems, being significantly lower in sandy soils (15 and 14 Mg C ha- 1) than clayey soils (23 and 21 Mg C ha- 1) at 0–10 and 10–30 cm respectively. Nitrogen content followed the same trend. At the 0–10 cm depth, SOC stocks increased under CT, RP and DS by 0.10, 0.24, 0.36 Mg ha- 1 yr- 1 and 0.76, 0.54, 0.10 Mg ha- 1 yr- 1 on sandy and clayey soils respectively over a four year period while N stocks decreased by 0.55, 0.40, 0.56 Mg ha- 1 and 0.63, 0.65, 0.55 Mg ha- 1 respectively. SOM fractions were dominated by mineral associated heavy fraction (MaHF) which accounted for 86–93% and 94–98% on sandy and clayey soils respectively. Tillage systems on sandy soils had the smallest average free light fraction (fLF) and occluded light fraction (oLF) C stocks (25.3 ± 1.3 g m- 2 and 7.3 ± 1.2 g m- 2) at 0–30 cm when compared with corresponding NF (58.4 ± 4 g m2 and 18.5 ± 1.0 g m- 2). Clayey soils, had the opposite, having all fLF C and N in tillage systems being higher (80.9 ± 12 g C m- 2 and 2.7 ± 0.4 g N m- 2) than NF (57.4 ± 2.0 g C m- 2 and 2.4 ± 0.3 g N m- 2). Results suggest that oLF and MaHF C and N are better protected under DS and RP where they are less vulnerable to mineralisation while fLF contributes more in CT. Thus, DS and RP can be important in maintaining and improving soil quality although their practicability can be hampered by unsupportive institutional frameworks. Under prevailing climatic and management conditions, improvement of residue retention could be a major factor that can distinguish the potential of different management practices for C sequestration. The exploitation of the benefits of RP or DS and the corresponding sustainability of systems need support for surface cover retention which should also be extended to conventional tillage
Oil bodies: An insight on their microstructure - maize germ vs sunflower seed
Nikiforidis, K. ; Kiosseoglou, V. ; Scholten, E. - \ 2013
Food Research International 52 (2013)1. - ISSN 0963-9969 - p. 136 - 141.
body emulsions - physicochemical stability - oxidative stability - physical stability - oleosins - proteins - extraction - stabilization
Storage triacylglycerols in oleaginous seeds are surrounded by a layer that consists of phospholipids and proteins, mainly oleosins. These entities are intracellular organelles, known as oil bodies. It is often reported that they have a spherical shape, but imaging using cryo-SEM analysis showed that they are rather elastic and their shape depends on their surrounding environment. In this research we have shown that oil bodies in maize germ, which has a relatively low moisture content, are taking the shape of the available space they have. On the other hand, oil bodies in sunflower seeds, which contain double amounts of water, appear with an almost spherical shape. Oil bodies can be extracted from oleaginous seeds using an aqueous alkaline extraction, which leads to a stable natural oil-in-water emulsion. As no additional energy is required, this method can be considered as sustainable and may find a lot of potential uses in the industry. Extraneous co-extracted proteins most likely form a second layer around the oil body surface, which protects the oil bodies from coalescence, even at high oil concentration. The extraneous proteins of maize germ oil body emulsions could be removed by applying aqueous washing steps, but not in the case of sunflower seed oil bodies.
Medium-term impact of tillage and residue management on soil aggregate stability, soil carbon and crop productivity
Paul, B.K. ; Vanlauwe, B. ; Ayuke, F. ; Gassner, A. ; Hoogmoed, M. ; Hurisso, T.T. ; Koala, S. ; Lelei, D. ; Ndabamenye, T. ; Six, J. ; Pulleman, M.M. - \ 2013
Agriculture, Ecosystems and Environment 164 (2013)1. - ISSN 0167-8809 - p. 14 - 22.
organic-matter dynamics - conservation agriculture - no-till - africa - systems - nitrogen - kenya - stabilization - protection - yields
Conservation agriculture is widely promoted for soil conservation and crop productivity increase, although rigorous empirical evidence from sub-Saharan Africa is still limited. This study aimed to quantify the medium-term impact of tillage (conventional and reduced) and crop residue management (retention and removal) on soil and crop performance in a maize–soybean rotation. A replicated field trial was started in sub-humid Western Kenya in 2003, and measurements were taken from 2005 to 2008. Conventional tillage negatively affected soil aggregate stability when compared to reduced tillage, as indicated by lower mean weight diameter values upon wet sieving at 0–15 cm (PT <0.001). This suggests increased susceptibility to slaking and soil erosion. Tillage and residue management alone did not affect soil C contents after 11 cropping seasons, but when residue was incorporated by tillage, soil C was higher at 15–30 cm (PT*R = 0.037). Lack of treatment effects on the C content of different aggregate fractions indicated that reduced tillage and/or residue retention did not increase physical C protection. The weak residue effect on aggregate stability and soil C may be attributed to insufficient residue retention. Soybean grain yields tended to be suppressed under reduced tillage without residue retention, especially in wet seasons (PT*R = 0.070). Consequently, future research should establish, for different climatic zones and soil types, the critical minimum residue retention levels for soil conservation and crop productivity.
A proposal for a new scenario framework to support research and assessment in different climate research communities
Vuuren, D.P. van; Riahi, K. ; Moss, R. ; Edmonds, J. ; Thomson, A. ; Nakicenovic, N. ; Kram, T. ; Berkhout, F. ; Swart, R.J. ; Janetos, A. ; Rose, S.K. ; Arnell, N. - \ 2012
Global environmental change : human and policy dimensions 22 (2012)1. - ISSN 0959-3780 - p. 21 - 35.
expert judgments - impact assessment - vulnerability - costs - stabilization - adaptation - strategies
In this paper, we propose a scenario framework that could provide a scenario "thread" through the different climate research communities (climate change - vulnerability, impact, and adaptation - and mitigation) in order to support assessment of mitigation and adaptation strategies and climate impacts. The scenario framework is organized around a matrix with two main axes: radiative forcing levels and socio-economic conditions. The radiative forcing levels (and the associated climate signal) are described by the new Representative Concentration Pathways. The second axis, socio-economic developments comprises elements that affect the capacity for mitigation and adaptation, as well as the exposure to climate impacts. The proposed scenarios derived from this framework are limited in number, allow for comparison across various mitigation and adaptation levels, address a range of vulnerability characteristics, provide information across climate forcing and vulnerability states and span a full century time scale. Assessments based on the proposed scenario framework would strengthen cooperation between integrated-assessment modelers, climate modelers and vulnerability, impact and adaptation researchers, and most importantly, facilitate the development of more consistent and comparable research within and across these research communities.
Does predator interference cause alternative stable states in multispecies communities?
Feng, J. ; Dakos, V. ; Nes, E.H. van - \ 2012
Theoretical Population Biology 82 (2012)3. - ISSN 0040-5809 - p. 170 - 176.
mutual interference - marine ecosystems - shallow lakes - regime shifts - thresholds - enrichment - paradox - stabilization - resilience - stability
Whereas it is well known that simple ecological mechanisms may promote stability in simple species models, their consequences for stability and resilience in multispecies communities are largely unexplored. Here, we studied the effect of predator interference on the occurrence of alternative attractors and complex dynamics in randomly constructed multispecies predator-prey communities. We studied three types of interference: random interference ("asymmetric"), random interference but symmetrical between pairs of predators ("symmetric"), and interference among only the same species ("conspecific"). In all cases predator interference increased the average number of alternative attractors, whereas at the same time it reduced the emergence of oscillatory or chaotic dynamics. Our findings demonstrate a contrasting effect of predator interference on the stability of a community: on the one hand it reduces cycles and chaos in the dynamics, on the other hand predator interference increases the likelihood that communities may undergo critical transitions between multiple stable states.
Organic matter of subsoil horizons under broadleaved forest: Highly processed or labile and plant-derived?
Vancampenhout, K. ; Vos, B. de; Wouters, K. ; Swennen, R. ; Buurman, P. - \ 2012
Soil Biology and Biochemistry 50 (2012)july. - ISSN 0038-0717 - p. 40 - 46.
pyrolysis-gc/ms - chemical-composition - carbon pool - soils - stabilization - spectroscopy - ecosystems - mechanisms - fractions - chemistry
Between 30 and 63% of the soil organic matter (SOM) is stored below 30 cm, making subsoil-SOM an important source and sink in the global carbon cycle. Nevertheless, detailed information on the composition of subsoil-SOM remains scarce. This study aims to evaluate the chemical composition of SOM in topsoil and subsoil horizons in broadleaved forests on acid loamy soils. Six sites were chosen in Northern Belgium under beech, oak and hybrid poplar, on Gleysols, Umbrisols, Cambisols and Albeluvisols on loamy Quaternary deposits. Analytical pyrolysis–gas chromatography/mass spectrometry (pyrolysis–GC/MS) was performed on the dialyzed alkaline extract, which represents between 41 and 90% of the total organic carbon for the selected sites. All extracts show a significant shift in chemical composition between the topsoil and the subsoil. While topsoil-SOM mainly differs according to input and nutrient status, subsoil-SOM shows high relative amounts of alkanes and alkenes or polysaccharides for coarse and fine textured soils respectively. Lignins, lignin-derived phenols or aromatics were not major contributors to subsoil-SOM, regardless of soil type. Furthermore, results show that very labile plant-derived molecules are present in the subsoil, i.e. long-chain aliphatics and (cellulose-derived) anhydrosugars. The organic matter signature of the subsoil samples was evaluated for typical indications of fresh material, decay, podzolisation and anaerobic processes, and indicates root input and stabilization of certain labile plant-derived compounds against microbial decay to be important in the subsoil.
Variation in soil carbon stocks and their determinants across a precipitation gradient in West Africa
Saiz, G. ; Bird, M.I. ; Domingues, T.F. ; Schrodt, F. ; Schwartz, M. ; Veenendaal, E.M. - \ 2012
Global Change Biology 18 (2012)5. - ISSN 1354-1013 - p. 1670 - 1683.
land-use change - organic-matter - biotic controls - cycle feedback - savanna soils - forest soils - sequestration - texture - senegal - stabilization
We examine the influence of climate, soil properties and vegetation characteristics on soil organic carbon (SOC) along a transect of West African ecosystems sampled across a precipitation gradient on contrasting soil types stretching from Ghana (15°N) to Mali (7°N). Our findings derive from a total of 1108 soil cores sampled over 14 permanent plots. The observed pattern in SOC stocks reflects the very different climatic conditions and contrasting soil properties existing along the latitudinal transect. The combined effects of these factors strongly influence vegetation structure. SOC stocks in the first 2 m of soil ranged from 20 Mg C ha-1 for a Sahelian savanna in Mali to over 120 Mg C ha-1 for a transitional forest in Ghana. The degree of interdependence between soil bulk density (SBD) and soil properties is highlighted by the strong negative relationships observed between SBD and SOC (r2 > 0.84). A simple predictive function capable of encompassing the effect of climate, soil properties and vegetation type on SOC stocks showed that available water and sand content taken together could explain 0.84 and 0.86 of the total variability in SOC stocks observed to 0.3 and 1.0 m depth respectively. Used in combination with a suitable climatic parameter, sand content is a good predictor of SOC stored in highly weathered dry tropical ecosystems with arguably less confounding effects than provided by clay content. There was an increased contribution of resistant SOC to the total SOC pool for lower rainfall soils, this likely being the result of more frequent fire events in the grassier savannas of the more arid regions. This work provides new insights into the mechanisms determining the distribution of carbon storage in tropical soils and should contribute significantly to the development of robust predictive models of biogeochemical cycling and vegetation dynamics in tropical regions.
Tuning of Collagen Triple-Helix Stability in Recombinant Telechelic Polymers
Silva, C.I. da; Skrzeszewska, P.J. ; Golinska, M.D. ; Werten, M.W.T. ; Eggink, G. ; Wolf, F.A. de - \ 2012
Biomacromolecules 13 (2012)5. - ISSN 1525-7797 - p. 1250 - 1258.
triblock copolymers - stabilization - hydrogels - gelatin - gels - hydroxylation - proteins - kinetics - network - water
The melting properties of various triblock copolymers with random coil middle blocks (100–800 amino acids) and triple helix-forming (Pro-Gly-Pro)n end blocks (n = 6–16) were compared. These gelatin-like molecules were produced as secreted proteins by recombinant yeast. The investigated series shows that the melting temperature (Tm) can be genetically engineered to specific values within a very wide range by varying the length of the end block. Elongation of the end blocks also increased the stability of the helices under mechanical stress. The length-dependent melting free energy and Tm of the (Pro-Gly-Pro)n helix appear to be comparable for these telechelic polymers and for free (Pro-Gly-Pro)n peptides. Accordingly, the Tm of the polymers appeared to be tunable independently of the nature of the investigated non-cross-linking middle blocks. The flexibility of design and the amounts in which these nonanimal biopolymers can be produced (g/L range) create many possibilities for eventual medical application.
Comparative performance of conservation agriculture and current smallholder farming practices in semi-arid Zimbabwe
Baudron, F. ; Tittonell, P.A. ; Corbeels, M. ; Letourmy, P. ; Giller, K.E. - \ 2012
Field Crops Research 132 (2012). - ISSN 0378-4290 - p. 117 - 128.
organic-matter - west-africa - soil - tillage - management - stabilization - saturation - crops
Conservation agriculture (CA) is currently promoted in sub-humid and semi-arid areas of sub-Saharan Africa as a means to increase crop water use efficiency and stabilize yields. In this study, conducted during three consecutive seasons in a semi-arid area of Zimbabwe, the short-term performance of CA and current farming practices (CP) were compared in two multi-locational experiments: (1) unfertilised on-farm trials with a cotton-sorghum rotation during three consecutive seasons, and (2) farmers' cotton fields receiving fertiliser provided on credit by cotton companies during two consecutive seasons. In both cases, residues for mulch were produced in situ. In addition to biophysical measurements, farmers' perceptions of the technology were appraised. CA did not affect cotton productivity during the first 2 years of the experiments, which received average or above average rainfall. During the drier 2009-2010 season CA had a negative effect on crop yield both in the on-farm trials (average yield of 730 and 820 kg ha(-1) under CA and CP, respectively) and in the farmers' cotton fields (average yield of 1220 and 1440 kg ha(-1) under CA and CP, respectively). There was no difference in water runoff between CA and CP on a relatively fine-textured soil, but significantly more runoff with CA on a coarser-textured soil (14 mm during the wetter 2008-2009 season), due to soil surface crusting and soil compaction. Most soils in the study area fall into this latter category. For this reason, farmers perceived ploughing as necessary during drier years to maximize water infiltration, but perceived CA as beneficial during wetter years as a means to 'shed water' and avoid water-logging. This is rather counterintuitive vis-a-vis the common description of CA as a water-harvesting technology. Soil crusting and compaction may be avoided by the production and retention of quantities of biomass greater than what was realised in this study (on average, only 770 kg ha(-1) of residues were retained as mulch in the on-farm trials). This may be achieved through better crop management (e.g. adequate fertilisation, timely planting, crop protection) in combination with intercropping. Increasing crop primary productivity (e.g. through adequate fertilisation, timely planting and crop protection) is a pre-requisite for the principles of CA to benefit smallholders under semi-arid conditions. Our results indicate that certain legume intercropping combinations may contribute to such an end.