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 - 11 / 11

    • help
    • print

      Print search results

    • export

      Export search results

    • alert
      We will mail you new results for this query: q=Denkov
    Check title to add to marked list
    Origin of the extremely high elasticity of bulk emulsions, stabilized by Yucca Schidigera saponins
    Tsibranska, Sonya ; Tcholakova, Slavka ; Golemanov, Konstantin ; Denkov, Nikolai ; Arnaudov, Luben ; Pelan, Eddie ; Stoyanov, Simeon D. - \ 2020
    Food Chemistry 316 (2020). - ISSN 0308-8146
    Drop aggregation - Drop-drop adhesion - Emulsion - Emulsion elasticity - Interfacial elasticity - Non-purified oil - Phytosterols - Saponin

    We found experimentally that the elasticity of sunflower oil-in-water emulsions (SFO-in-W) stabilized by Yucca Schidigera Roezl saponin extract, is by >50 times higher as compared to the elasticity of common emulsions. We revealed that strong specific interactions between the phytosterols from the non-purified oil and the saponins from the Yucca extract lead to the formation of nanostructured adsorption layers which are responsible for the very high elasticity of the oil-water interface and of the respective bulk emulsions. Remarkably, this extra high emulsion elasticity inhibits the emulsion syneresis even at 65 vol% of the oil drops – these emulsions remain homogeneous and stable even after 30 days of shelf-storage. These results demonstrate that the combination of saponin and phytosterols is a powerful new approach to structure oil-in-water emulsions with potential applications for formulating healthier functional food.

    Role of interfacial elasticity for the rheological properties of saponin-stabilized emulsions
    Tsibranska, Sonya ; Tcholakova, Slavka ; Golemanov, Konstantin ; Denkov, Nikolai ; Pelan, Eddie ; Stoyanov, Simeon D. - \ 2020
    Journal of Colloid and Interface Science 564 (2020). - ISSN 0021-9797 - p. 264 - 275.
    Emulsion - Emulsion rheology - Interfacial rheology - Saponin - Surface elasticity - Surface rheology

    Hypothesis: Saponins are natural surfactants which can provide highly viscoelastic interfaces. This property can be used to quantify precisely the effect of interfacial dilatational elasticity on the various rheological properties of bulk emulsions. Experiments: We measured the interfacial dilatational elasticity of adsorption layers from four saponins (Quillaja, Escin, Berry, Tea) adsorbed on hexadecane-water and sunflower oil-water interfaces. In parallel, the rheological properties under steady and oscillatory shear deformations were measured for bulk emulsions, stabilized by the same saponins (oil volume fraction between 75 and 85%). Findings: Quillaja saponin and Berry saponin formed solid adsorption layers (shells) on the SFO-water interface. As a consequence, the respective emulsions contained non-spherical drops. For the other systems, the interfacial elasticities varied between 2 mN/m and 500 mN/m. We found that this interfacial elasticity has very significant impact on the emulsion shear elasticity, moderate effect on the dynamic yield stress, and no effect on the viscous stress of the respective steadily sheared emulsions. The last conclusion is not trivial, because the dilatational surface viscoelasticity is known to have strong impact on the viscous stress of steadily sheared foams. Mechanistic explanations of all observed effects are described.

    Foamability of aqueous solutions: Role of surfactant type and concentration
    Petkova, B. ; Tcholakova, S. ; Chenkova, M. ; Golemanov, K. ; Denkov, N. ; Thorley, D. ; Stoyanov, S. - \ 2020
    Advances in Colloid and Interface Science 276 (2020). - ISSN 0001-8686
    Dynamic surface tension - Foam - Foaminess - Surface modulus - Surfactant

    In this paper we study the main surface characteristics which control the foamability of solutions of various surfactants. Systematic series of experiments with anionic, cationic and nonionic surfactants with different head groups and chain lengths are performed in a wide concentration range, from 0.001 mM to 100 mM. The electrolyte (NaCl) concentration is also varied from 0 up to 100 mM. For all surfactants studied, three regions in the dependence of the foamability, VA, on the logarithm of surfactant concentration, lgCS, are observed. In Region 1, VA is very low and depends weakly on CS. In Region 2, VA increases steeply with CS. In Region 3, VA reaches a plateau. To analyse these results, the dynamic and equilibrium surface tensions of the foamed solutions are measured. A key new element in our interpretation of the foaming data is that we use the surface tension measurements to determine the dependence of the main surface properties (surfactant adsorption, surface coverage and surface elasticity) on the surface age of the bubbles. In this way we interpret the results from the foaming tests by considering the properties of the dynamic adsorption layers, formed during foaming. The performed analysis reveals a large qualitative difference between the nonionic and ionic surfactants with respect to their foaming profiles. The data for the nonionic and ionic surfactants merge around two master curves when plotted as a function of the surface coverage, the surface mobility factor, or the Gibbs elasticity of the dynamic adsorption layers. This difference between the ionic and nonionic surfactants is explained with the important contribution of the electrostatic repulsion between the foam film surfaces for the ionic surfactants which stabilizes the dynamic foam films even at moderate surface coverage and at relatively high ionic strength (up to 100 mM). In contrast, the films formed from solutions of nonionic surfactants are stabilized via steric repulsion which becomes sufficiently high to prevent bubble coalescence only at rather high surface coverage (> 90%) which corresponds to related high Gibbs elasticity (> 150 mN/m) and low surface mobility of the dynamic adsorption layers. Mechanistic explanations of all observed trends are provided and some important similarities and differences with the process of emulsification are outlined.

    Role of surface properties for the kinetics of bubble Ostwald ripening in saponin-stabilized foams
    Tcholakova, Slavka ; Mustan, Fatmegul ; Pagureva, Nevena ; Golemanov, Konstantin ; Denkov, Nikolai D. ; Pelan, Edward G. ; Stoyanov, Simeon D. - \ 2017
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 534 (2017). - ISSN 0927-7757 - p. 16 - 25.
    Adsorption layer - Foam coarsening - Ostwald ripening - Saponin - Surface rheology
    Bubble Ostwald ripening (OR) leads to a gradual increase of the mean bubble size in foams with time. The rate of OR can be reduced significantly or even arrested completely using appropriate solid particles and/or surfactants as foam stabilizers. In the current paper, we show that saponins, a widespread class of natural surfactants, can reduce significantly the rate of OR in foams. To reveal the reasons for the reduced rate of OR in saponin-stabilized foams, we performed measurements of the rate of bubble diminishing, for single air bubbles placed below a solution surface, with a series of saponin bio-surfactants. These saponin surfactants form adsorption layers with surface elasticity, spanning a very wide range - from almost zero up to several thousand mN/m. The measured rate of bubble OR showed no correlation with the surface elastic modulus (dilatational or shear), as measured at 0.1. Hz frequency of surface oscillations. A reasonable correlation was observed only with the surface stress (deviation from the equilibrium surface tension), measured at very slow rate of surface deformation, which mimics much better the actual processes of bubble OR in foams - higher surface stress corresponds to lower OR rate. New theoretical expression, accounting for the out-of- equilibrium surface tension during bubble shrinkage and for the gas flux across the meniscus regions surrounding the foam films, was derived and used to calculate theoretically the rate of bubble diminishing. The comparison of the theoretical predictions with the experimental data shows clearly that the main reason for the reduced rate of OR in the studied systems is the high resistance to gas transfer of the saponin adsorption layers. The deviations from the equilibrium surface tension, although noticeable, have smaller effect. The complementary experiments with actual foams showed that the rate of OR is even lower (compared to the rate measured with single bubbles) which is explained with the thicker non-equilibrium foam films, formed between the neighboring bubbles in saponin-stabilized foams.
    Surface properties of adsorption layers formed from triterpenoid and steroid saponins
    Pagureva, N. ; Tcholakova, S. ; Golemanov, K. ; Denkov, N. ; Pelan, E. ; Stoyanov, S.D. - \ 2016
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 491 (2016). - ISSN 0927-7757 - p. 18 - 28.
    Air-water interface - Dilatation - Saponins - Shear - Surface rheology

    Saponins are natural surfactants with non-trivial surface and aggregation properties which find numerous important applications in several areas (food, pharma, cosmetic and others). In the current paper we study the surface properties of ten saponin extracts, having different molecular structure with respect to the type of their hydrophobic fragment (triterpenoid or steroid aglycone) and the number of sugar chains (1 to 3). We found that the triterpenoid saponins Escin, Tea Saponin and Ginsenosides have area per molecule in the range between 0.5 and 0.7nm2, and the adsorbed molecules are orientated perpendicularly to the interface. The comparison of the experimentally measured surface elasticities with theoretically estimated ones shows that the saponins with very high dilatational and shear elasticities (up to 2000mN/m) have molecular interaction parameter in the adsorption layers which is above the threshold value for two-dimensional phase transition. In other words, the highly elastic layers are in surface condensed state, due to strong attraction between the adsorbed molecules. Furthermore, these adsorption layers have non-linear rheological response upon expansion and contraction, even at relatively small deformation. Layers from the other studied saponins (steroids and crude mixtures of triterpenoid saponins), which are unable to form strong intermolecular bonds within the adsorption layer, have zero shear elasticity and viscosity and low dilatational elasticity and viscosity, comparable in magnitude to those reported in literature for protein adsorption layers. The comparison of the results, obtained by several independent experimental methods, allowed us to formulate the conditions under which the results from different interfacial rheology tests could be compared, despite the complex non-linear response of the saponin adsorption layers.

    Lowering of cholesterol bioaccessibility and serum concentrations by saponins : In vitro and in vivo studies
    Vinarova, Liliya ; Vinarov, Zahari ; Atanasov, Vasil ; Pantcheva, Ivayla ; Tcholakova, Slavka ; Denkov, Nikolai ; Stoyanov, Simeon - \ 2015
    Food & Function 6 (2015)2. - ISSN 2042-6496 - p. 501 - 512.

    Using an in vitro digestion model, we studied the effect of six saponin extracts on the bioaccessibility of cholesterol and saturated fatty acids (SFAs). In the absence of saponins, around 78% of the available cholesterol was solubilized in the simulated intestinal fluids. The addition of two extracts, Quillaja Dry (QD) and Sapindin (SAP), was found to decrease cholesterol bioaccessibility to 19% and 44%, respectively. For both extracts, the main mechanism of this effect is the displacement of cholesterol molecules from the bile salt micelles, leading to formation of cholesterol precipitates that cannot pass through the mucus layer of the intestine. QD decreased strongly the SFA bioaccessibility as well, from 69 to 9%, due to formation of calcium-SFA precipitates, while SAP had no effect on SFA. We studied the in vivo activity of QD and SAP extracts by measuring serum cholesterol in mice fed with experimental diets within a 7-day period. Both extracts were found to prevent dietary hypercholesterolemia in mice fed on a cholesterol-rich diet. The other saponin extracts did not show any significant effect in vitro and, therefore, were not studied in vivo. The cholesterol lowering ability of Sapindin extract is reported for the first time in the current study. This journal is

    Role of the hydrophobic phase for the unique rheologica properties of saponin adsorption layers
    Golemanov, K. ; Tcholakova, S. ; Denkov, N. ; Pelan, E.G. ; Stoyanov, S.D. - \ 2014
    Soft Matter 10 (2014)36. - ISSN 1744-683X - p. 7034 - 7044.
    oil/water interfaces - aqueous foams - surface rheology - water-interface - quillaja bark - air/water - shear - drainage - monolayers - emulsions
    Saponins are a diverse class of natural, plant derived surfactants, with peculiar molecular structure consisting of a hydrophobic scaffold and one or several hydrophilic oligosaccharide chains. Saponins have strong surface activity and are used as natural emulsifiers and foaming agents in food and beverage, pharmaceutical, ore processing, and other industries. Many saponins form adsorption layers at the air–water interface with extremely high surface elasticity and viscosity. The molecular origin of the observed unique interfacial visco-elasticity of saponin adsorption layers is of great interest from both scientific and application viewpoints. In the current study we demonstrate that the hydrophobic phase in contact with water has a very strong effect on the interfacial properties of saponins and that the interfacial elasticity and viscosity of the saponin adsorption layers decrease in the order: air > hexadecane » tricaprylin. The molecular mechanisms behind these trends are analyzed and discussed in the context of the general structure of the surfactant adsorption layers at various nonpolar phase–water interfaces.
    Remarkably high surface visco-elasticity of adsorption layers of triterpenoid saponins
    Golemanov, K. ; Tcholakova, S. ; Denkov, N. ; Pelan, E. ; Stoyanov, S.D. - \ 2013
    Soft Matter 9 (2013)24. - ISSN 1744-683X - p. 5738 - 5752.
    amplitude oscillatory shear - acacia-concinna saponins - steroidal saponins - beta-lactoglobulin - alkaline hydrolysate - sorbitan tristearate - interfacial rheology - tribulus-terrestris - plasma-cholesterol - panax-ginseng
    Saponins are natural surfactants, with molecules composed of a hydrophobic steroid or triterpenoid group, and one or several hydrophilic oligosaccharide chains attached to this group. Saponins are used in cosmetic, food and pharmaceutical products, due to their excellent ability to stabilize emulsions and foams, and to solubilize bulky hydrophobic molecules. The foam and emulsion applications call for a better understanding of the surface properties of saponin adsorption layers, including their rheological properties. Of particular interest is the relation between the molecular structure of the various saponins and their surface properties. Here, we study a series of eight triterpenoid and three steroid saponins, with different numbers of oligosaccharide chains. The surface rheological properties of adsorption layers at the air-water interface, subjected to creep-recovery and oscillatory shear deformations, are investigated. The experiments showed that all steroid saponins exhibited no shear elasticity and had negligible surface viscosity. In contrast, most of the triterpenoid saponins showed complex visco-elastic behavior with extremely high elastic modulus (up to 1100 mN m(-1)) and viscosity (130 N s m(-1)). Although the magnitude of the surface modulus differed significantly for the various saponins, they all shared qualitatively similar rheological properties: (1) the elastic modulus was much higher than the viscous one. (2) Up to a certain critical value of surface stress, sC, the single master curve described the dependence of the creep compliance versus time. This rheological response was described well by the compound Voigt model. (3) On increasing the surface stress above sC, the compliance decreased with the applied stress, and eventually, all layers became purely viscous, indicating a loss in the layer structure, responsible for the elastic properties. The saponin extracts, showing the highest elastic moduli, were those of Escin, Tea saponins and Berry saponins, all containing predominantly monodesmosidic triterpenoid saponins. Similarly, a high surface modulus was measured for Ginsenosides extracts, containing bidesmosidic triterpenoid saponins with short sugar chains.
    Surface Shear Rheology of Saponin Adsorption Layers
    Golemanov, K. ; Tcholakova, S. ; Denkov, N. ; Pelan, E. ; Stoyanov, S.D. - \ 2012
    Langmuir 28 (2012)33. - ISSN 0743-7463 - p. 12071 - 12084.
    physico-chemical properties - quillaja bark saponin - air-water-interface - thin liquid-films - plant saponins - chromatographic determination - phospholipid monolayers - yucca-schidigera - acid saponins - cholesterol
    Saponins are a wide class of natural surfactants, with molecules containing a rigid hydrophobic group (triterpenoid or steroid), connected via glycoside bonds to hydrophilic oligosaccharide chains. These surfactants are very good foam stabiliziers and emulsifiers, and show a range of nontrivial biological activities. The molecular mechanisms behind these unusual properties are unknown, and, therefore, the saponins have attracted significant research interest in recent years. In our previous study (Stanimirova et al. Langmuir 2011, 27, 12486-12498), we showed that the triterpenoid saponins extracted from Quillaja saponaria plant (Quillaja saponins) formed adsorption layers with unusually high surface dilatational elasticity, 280 +/- 30 mN/m. In this Article, we study the shear rheological properties of the adsorption layers of Quillaja saponins. In addition, we study the surface shear rheological properties of Yucca saponins, which are of steroid type. The experimental results show that the adsorption layers of Yucca saponins exhibit purely viscous rheological response, even at the lowest shear stress applied, whereas the adsorption layers of Quillaja saponins behave like a viscoelastic two-dimensional body. For Quillaja saponins, a single master curve describes the data for the viscoelastic creep compliance versus deformation time, up to a certain critical value of the applied shear stress. Above this value, the layer compliance increases, and the adsorption layers eventually transform into viscous ones. The experimental creep recovery curves for the viscoelastic layers are fitted very well by compound Voigt rheological model. The obtained results are discussed from the viewpoint of the layer structure and the possible molecular mechanisms, governing the rheological response of the saponin adsorption layers.
    In vitro study of triglyceride lipolysis and phase distribution of the reaction products and cholesterol: effects of calcium and bicarbonate
    Vinarov, Z. ; Petrova, L. ; Tcholakova, S. ; Denkov, N.D. ; Stoyanov, S.D. ; Lips, A. - \ 2012
    Food & Function 3 (2012)11. - ISSN 2042-6496 - p. 1206 - 1220.
    physical-chemical behavior - lipid-based formulations - drug-delivery systems - water-soluble drugs - adult human-beings - pancreatic lipase - fat digestion - intestinal digestion - aggregation states - lipophilic drugs
    We describe a relatively simple in vitro model for triglyceride (TG) lipolysis which mimics closely the conditions in the human stomach and small intestine. The main model advantages are: (1) as in vivo, sodium bicarbonate is used for buffering; (2) the pH-profile in the small intestine is closely matched; (3) the experimental procedure does not include complex equipment. To test its performance, the proposed in vitro model is applied to quantify the effects of Ca2+, pH, and bicarbonate on the degree of TG lipolysis and on the solubilization of the lipolysis products and cholesterol in the aqueous phase. We found that TG lipolysis passes through a shallow minimum at 3.5 mM Ca2+ when varying the calcium concentration between 1 and 11 mM, while the presence of bicarbonate and the increase of pH led to a higher degree of lipolysis. Centrifugation and filtration were used to separate the aqueous phase and to study the solubilisation of the lipophilic components in the aqueous phase. We found that the solubilized cholesterol increases linearly with the concentration of free fatty acids (FFA) which is evidence for co-solubilization of these two components in the bile micelles. At high Ca2+ concentrations, aggregates larger than 300 nm were observed by cryo-microscopy and light scattering, which solubilize well cholesterol and saturated FFA. In contrast, the monoglycerides were always predominantly solubilized in the small bile micelles with diameters around 4 nm.
    Modified Capillary Cell for Foam Film Studies Allowing Exchange of the Film-Forming Liquidwie
    Wierenga, P.A. ; Basheva, E.S. ; Denkov, N.D. - \ 2009
    Langmuir 25 (2009)11. - ISSN 0743-7463 - p. 6035 - 6039.
    anionic surfactant solutions - continuous-flow tensiometry - coalescence stability - protein adsorption - ionic-strength - drop size - emulsions - interfaces - forces - ph
    Many of the macroscopic properties of foams and emulsions are controlled by the mesoscopic properties of the thin films separating the bubbles or droplets. The properties of these films depend on contributions (1) from the adsorbed surface layers and (2) from the liquid that separates these adsorbed layers. To separate in the experimental studies the effects of these two contributions, we developed a new modified version of the capillary cell for foam film studies (originally developed by Scheludko and Exerowa (Scheludko, A.; Exerowa, D. Kolloid Z. 1959, 165, 148-151), which allows exchange of the film-forming liquid between the air-water surfaces. This modified cell allows one to distinguish between the role of the adsorbed species (e.g., proteins, particles, or long-chain synthetic polymers) and the species present in the film interior (e.g., particles, electrolytes, or surfactants). The film properties that can be studied in this way include film stability, rate of film thinning, and surface forces stabilizing the film. These properties are of significant interest in understanding and controlling the stability of dispersed systems. The experimental procedure and the capabilities of the modified cell are demonstrated in several examples.
    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.