Staff Publications

Staff Publications

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    '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.

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    Efficacy of l-glutamic acid, N,N-diacetic acid to improve the dietary trace mineral bioavailability in broilers
    Boerboom, Gavin M. ; Busink, Ronald ; Smits, Coen H. ; Hendriks, Wouter H. ; Martín-Tereso, Javier - \ 2020
    Journal of Animal Science 98 (2020)12. - ISSN 0021-8812
    broiler - L-glutamic-acid-N-N-diacetic-acid - trace mineral - zinc

    Trace minerals are commonly supplemented in the diets of farmed animals in levels exceeding biological requirements, resulting in extensive fecal excretion and environmental losses. Chelation of trace metal supplements with ethylenediaminetetraacetic acid (EDTA) can mitigate the effects of dietary antagonists by preserving the solubility of trace minerals. Lack of EDTA biodegradability, however, is of environmental concern. l-Glutamic acid, N,N-diacetic acid (GLDA) is a readily biodegradable chelating agent that could be used as a suitable alternative to EDTA. The latter was tested in sequential dose-response experiments in broiler chickens. Study 1 compared the effect of EDTA and GLDA in broilers on supplemental zinc availability at three levels of added zinc (5, 10, and 20 ppm) fed alone or in combination with molar amounts of GLDA or EDTA equivalent to chelate the added zinc, including negative (no supplemental zinc) and positive (80 ppm added zinc) control treatments. Study 2 quantified the effect of GLDA on the availability of native trace mineral feed content in a basal diet containing no supplemental minerals and supplemented with three levels of GLDA (54, 108, and 216 ppm). In study 1, serum and tibia Zn clearly responded to the increasing doses of dietary zinc with a significant response to the presence of EDTA and GLDA (P < 0.05). These results are also indicative of the equivalent nutritional properties between GLDA and EDTA. In study 2, zinc levels in serum and tibia were also increased with the addition of GLDA to a basal diet lacking supplemental trace minerals, where serum zinc levels were 60% higher at the 216 ppm inclusion level. Similar to the reported effects of EDTA, these studies demonstrate that dietary GLDA may have enhanced zinc solubility in the gastrointestinal tract and subsequently enhanced availability for absorption, resulting in improved nutritional zinc status in zinc-deficient diets. As such, GLDA can be an effective nutritional tool to reduce supplemental zinc levels in broiler diets, thereby maintaining health and performance while reducing the environmental footprint of food-producing animals.

    The Relationship Between Energy Intake and Weight Loss in Bariatric Patients
    Schoemacher, Louella A.H.M. ; Boerboom, Abel B. ; Thijsselink, Monique M.R. ; Aarts, Edo O. - \ 2019
    Obesity Surgery 29 (2019)12. - ISSN 0960-8923 - p. 3874 - 3881.
    Bariatric surgery - Dietary intake - Gastric sleeve - LSG - Macronutrients - Micronutrients - Redo - Roux-en-Y gastric bypass - RYGB - Weight loss

    Introduction: There is a huge variation in weight loss outcomes between bariatric patients, possibly due to differences in caloric intake and changes in the amount physical activity. However, the association between the change in energy intake and weight loss has not yet been the subject of an extensive investigation. Objective: To explore the relationship between total energy intake and % total body weight loss (%TBWL) over a period of 4 years post-surgery. Methods: Of the 466 patients who were asked to participate, a total 135 patients were included in this study. They all underwent bariatric surgery, 54 with primary Roux-en-Y Gastric Bypass, 43 redo Roux-en-Y gastric bypass after laparoscopic gastric banding and 38 laparoscopic sleeve gastrectomy. Pre- and post-operative dietary intake and physical activity were collected for both a weekday and a weekend day. The main analysis was performed using multiple regression analyses and was adjusted for age at surgery, BMI at baseline, obstructive sleep apnoea syndrome, type of eating behaviour, change in physical activity and protein intake (g/kg body weight). Results: %TBWL over time, post-operative energy intake and change in physical activity did not differ between the different procedure groups (p = 0.312, p = 0.988 and p = 0.050, respectively). Change in energy intake did differ between different procedure groups (p = 0.031) and linear regression showed that this was related to total body weight loss for the fully adjusted model (β = − 0.004, p = 0.014). Conclusion: This study showed a higher decrease in energy intake to be related with a higher %TBWL.

    Led's do it in the dark : Boerboom Stekcultures bewortelt boomkwekerijstek daglichtloos
    Jacobs, R. ; Dalfsen, P. van - \ 2012
    Boom in business 3 (2012)2. - ISSN 2211-9884 - p. 10 - 11.
    plantenkwekerijen - vasteplantenkwekerijen - geconditioneerde teelt - kunstlicht - lichtgevende dioden - vermeerderingsmateriaal - plantmateriaal - proeven - nurseries - perennial nurseries - conditioned cultivation - artificial light - light emitting diodes - propagation materials - planting stock - trials
    Boereboom Stekcultures in Eindhoven bewortelt met led onder daglichtlozen omstandigheden stekmateriaal en weefselkweek van boomkwekerijgewassen. Een primeur de boomkwekerijwereld. Naast Philips en Cultus is ook PPO projectdeelnemer. De rol van PPO in het onderzoeksproject ligt op het vlak van wetenschappelijke ondersteuning en begeleiding van het praktijknetwerk, uitgevoerd door Pieter van Dalfsen.
    Potency of isothiocyanates to induce luciferase reporter gene expression via the electrophile-responsive element from murine glutathione S-transferase Ya
    Vermeulen, M. ; Boerboom, A.M.J.F. ; Blankvoort, B.M.G. ; Aarts, J.M.M.J.G. ; Rietjens, I. ; Bladeren, P.J. van; Vaes, W.H.J. - \ 2009
    Toxicology in Vitro 23 (2009)4. - ISSN 0887-2333 - p. 617 - 621.
    consensus sequence - mercapturic acids - identification - vegetables - sulforaphane - activation - mechanism - enzymes - fruit - risk
    Isothiocyanates are electrophiles that are able to induce phase II biotransformation enzyme gene expression via an electrophile-responsive element (EpRE) in the gene regulatory region. To study the potency of different isothiocyanates to induce the expression of EpRE-regulated genes, a Hepa-1c1c7 luciferase reporter cell line was exposed to structurally different isothiocyanates. The reporter cell line, EpRE(mGST-Ya)–LUX, contains the EpRE from the regulatory region of the mouse glutathione S-transferase Ya gene. Isothiocyanates containing a methyl-sulfur side chain, e.g. sulforaphane, showed a lower EC50 (0.8–3.2 µM) and a comparable induction factor (17–22.4) compared to the structurally different isothiocyanates containing an alkyl or aromatic side chain, e.g. allyl and phenylethyl isothiocyanate (EC50 3.9–6.5 µM, induction factor 17.5–23). After 24 h of exposure, on average (±SD) 23 ± 5% of the isothiocyanate was found in the cells and 77% in the cell medium. Isothiocyanates prove to be strong inducers of electrophile-responsive element-mediated gene expression at physiological concentrations. The here described luciferase reporter cell line is a suitable assay to measure the potency of compounds to induce EpRE-regulated gene expression
    The influence of fruit and vegetable consumption and genetic variation on NAD(P)H: quinone oxidoreductase (NQO1) phenotype in an endoscopy-based population
    Tijhuis, M.J. ; Boerboom, A.M.J.F. ; Visker, M.H.P.W. ; Camp, E.B.G. op den; Nagengast, F.M. ; Tan, A.C.I.T.L. ; Rietjens, I.M.C.M. ; Kok, F.J. ; Aarts, J.M.M.J.G. ; Kampman, E. - \ 2008
    Nutrition and Cancer 60 (2008)2. - ISSN 0163-5581 - p. 204 - 215.
    glutathione s-transferases - dt-diaphorase activity - mitomycin-c treatment - colorectal-cancer - transcription factors - activator protein-1 - c609t polymorphism - brussels-sprouts - promoter region - cell-lines
    NAD(P)H:quinone oxidoreductase (NQO1) is an inducible detoxification enzyme relevant for colorectal cancer biochemoprevention. We evaluated the influence of recent fruit and vegetable (F&V) consumption and polymorphisms in NQO1 and transcription factor NFE2L2 on rectal NQO1 phenotype and also whether white blood cell (WBC) NQO1 activity reflects rectal activity. Among 94 sigmoidoscopy patients, we assessed F&V consumption by dietary record and determined the NQO1 c.609C > T and g.-718A > G and NFE2L2 g.-650C > A, g.-684G > A, and g.-686A > G polymorphisms. NQO1 mRNA level was measured in rectal biopsies and NQO1 activity in rectal biopsies and WBC. Consumption of F&V did not yield higher mRNA level or activity but rather appeared to have a repressive effect. Rectal activity was higher among NQO1 609CC-genotypes as compared to 609CT-genotypes (P <0.0001; 609TT-genotypes were absent), whereas mRNA was higher among 609CT-genotypes (P <0.001). mRNA and activity correlated among NQO1 609CC-genotypes (r = .50, P = 0.0001) but not among 609CT-genotypes (r = .14, P = 0.45). The NFE2L2-684A-allele was associated with higher mRNA levels (P = <0.05). The other polymorphisms did not affect phenotype significantly. WBC and rectal activity did not correlate. In conclusion, genetic variation, especially the NQO1 609C > T polymorphism, is a more important predictor of rectal NQO1 phenotype than F&V consumption. WBC NQO1 activity is not a good surrogate for rectal activity.
    Glutathione S-transferase phenotypes in relation to genetic variation and fruit and vegetable consumption in an endoscopy-based population
    Tijhuis, M.J. ; Visker, M.H.P.W. ; Aarts, J.M.M.J.G. ; Peters, W.H.M. ; Roelofs, H.M.J. ; Camp, E.B.G. op den; Rietjens, I.M.C.M. ; Boerboom, A.M.J.F. ; Nagengast, F.M. ; Kok, F.J. ; Kampman, E. - \ 2007
    Carcinogenesis 28 (2007)4. - ISSN 0143-3334 - p. 848 - 857.
    brussels-sprouts - aldehyde dehydrogenase - cruciferous vegetables - colorectal-cancer - colon tissue - polymorphisms - expression - alpha - risk - pi
    High glutathione S-transferase (GST) activity may contribute to colorectal cancer prevention. Functional polymorphisms are known in the GSTM1, GSTT1, GSTA1 and GSTP1 genes. The influence of these GST polymorphisms and recent fruit and vegetable consumption on GST levels and activity has not been investigated simultaneously in a human population. Also, it is not clear if blood GST activity reflects rectal GST activity. Therefore, we determined GST polymorphisms in 94 patients scheduled for sigmoidoscopy. Rectal GST isoenzyme levels (GSTM1, GSTM2, GSTT1, GSTA and GSTP1) were measured by quantitative Western blotting, and rectal and white blood cell total GST activities were measured spectrophotometrically using 1-chloro-2,4-dinitrobenzene (CDNB) as a substrate. Vegetable and fruit consumption was assessed by dietary record. As expected, the GSTM1 and GSTT1 deletion polymorphisms, and the GSTA1 g.-69C>T polymorphism significantly affected the respective isoenzyme levels. Also, rectal GST isoenzyme levels differed between those with and without recent consumption of Alliaceae, Cucurbitaceae, Apiaceae and citrus fruit. Rectal GST activity, however, was not clearly influenced by fruit and vegetable consumption. It was most significantly determined by the GSTP1 c.313A>G polymorphism; compared to the 313AA genotypes, the 313AG and 313GG genotypes showed 36 and 67 nmol/ protein (p
    Assessment of the health-promoting activity of flavonoids for the application in food
    Lee-Hilz, Y.Y. ; Boerboom, A.M.J.F. ; Westphal, A.H. ; Berkel, W.J.H. van; Rietjens, I.M.C.M. ; Aarts, J.M.M.J.G. - \ 2006
    Pro-Oxidant Activity of Flavonoids Induces EpRE-Mediated Gene Expression.
    Lee-Hilz, Y.Y. ; Boerboom, A.M.J.F. ; Westphal, A.H. ; Berkel, W.J.H. van; Aarts, J.M.M.J.G. ; Rietjens, I.M.C.M. - \ 2006
    Chemical Research in Toxicology 19 (2006)11. - ISSN 0893-228X - p. 1499 - 1505.
    antioxidant response element - protein-kinase-c - gamma-glutamylcysteine synthetase - oxidative stress - transcriptional activation - anticancer properties - quinone reductase - phase-2 response - cell-lines - nrf2
    Flavonoids are important bioactive dietary compounds. They induce electrophile-responsive element (EpRE)-mediated expression of enzymes, such as NAD(P)H-quinone oxidoreductase (NQO1) and glutathione S-transferases (GSTs), which are major defense enzymes against electrophilic toxicants and oxidative stress. The induction of EpRE-mediated gene transcription involves the release of the transcription factor Nrf2 from a complex with Keap1, either by a direct interaction of the inducer with Keap1 or by protein kinase C (PKC)-mediated phosphorylation of Nrf2. The inhibition of PKC in Hepa1c1c7 cells, stably transfected with human NQO1-EpRE-controlled luciferase revealed that PKC is not involved in flavonoid-induced EpRE-mediated gene transcription. However, the ability of flavonoids to activate an EpRE-mediated response correlates with their redox properties characterized by quantum mechanical calculations. Flavonoids with a higher intrinsic potential to generate oxidative stress and redox cycling are the most potent inducers of EpRE-mediated gene expression. Modulation of the intracellular glutathione (GSH) level showed that the EpRE-activation by flavonoids increased with decreasing GSH and vice versa, supporting an oxidative mechanism. In conclusion, the pro-oxidant activity of flavonoids can contribute to their health-promoting activity by inducing important detoxifying enzymes, pointing to a beneficial effect of a supposed toxic chemical reaction
    Mechanism of flavonoid-induced EPRE-mediated gene transcription
    Lee-Hilz, Y.Y. ; Boerboom, A.M.J.F. ; Westphal, A.H. ; Berkel, W.J.H. van; Rietjens, I.M.C.M. ; Aarts, J.M.M.J.G. - \ 2006
    Activation of cancer protective enzymes by flavonoids
    Lee, Y.Y. ; Boerboom, A.M.J.F. ; Martin, A.H. ; Berkel, W.J.H. van; Rietjens, I.M.C.M. ; Aarts, J.M.M.J.G. - \ 2006
    In: Abstracts of the Dutch toxicology days, 13th-14th June 2006 - p. 170 - 170.
    Newly constructed stable reporter cell lines for mechanistic studies on electrophile-responsive element-mediated gene expression reveal a role for flavonoid planarity
    Boerboom, A.M.J.F. ; Vermeulen, M. ; Woude, H. van der; Bremer, B.I. ; Lee, Y.Y. ; Kampman, E. ; Bladeren, P.J. van; Rietjens, I.M.C.M. ; Aarts, J.M.M.J.G. - \ 2006
    Biochemical Pharmacology 72 (2006)2. - ISSN 0006-2952 - p. 217 - 226.
    transcription factor nrf2 - protein-kinase-c - antioxidant activity - nad(p)h-quinone oxidoreductase - consensus sequence - subunit gene - chemopreventive agents - detoxify carcinogens - inducible expression - colorectal-cancer
    The electrophile-responsive element (EpRE) is a transcriptional enhancer involved in cancer-chemoprotective gene expression modulation by certain food components. Two stably transfected luciferase reporter cell lines were developed, EpRE(hNQO1)-LUX and EpRE(mGST-Ya)-LUX, based on EpRE sequences from the human NAD(P)H:quinone oxidoreductase (hNQO1) and the mouse glutathione-S-transferase Ya (mGST-Ya) gene, containing one and two tandem EpRE core sequences, respectively. The standard inducer tert-butylhydroquinone (tBHQ), the electrophile benzyl isothiocyanate (BITC), and the antioxidant flavonoid quercetin were found to induce luciferase expression, thereby validating these newly developed reporter cell lines. For tBHQ and BITC, but not for quercetin, higher maximum luciferase induction was found under control of the mGST-Ya EpRE as compared to the hNQO1 EpRE, pointing at different induction mechanisms. Furthermore, we investigated the structure-activity relationship for induction of luciferase expression by flavonoids in EpRE(mGST-Ya)-LUX cells, and also the relation between luciferase induction and flavonoid antioxidant potency. Five different flavonoids with a planar molecular structure were found to induce various levels of luciferase activity, whereas taxifolin, a non-planar flavonoid, did not induce luciferase activity. This suggests that a stereospecific molecular interaction may be important for EpRE-mediated gene activation, possibly with Keap1, a regulator of EpRE-controlled transcription, or with another effector or receptor protein. No consistent relation between luciferase induction level and flavonoid antioxidant potential was observed. Altogether, these results point to differences in induction mechanism between the various chemoprotective compounds tested. The newly developed stably transfected reporter cell lines provide a validated tool for future screening and mechanistic studies of EpRE-mediated gene transcription.
    Pantanal-Taquari: tools for decision making in integrated water management
    Jongman, R.H.G. ; Eupen, M. van; Makaske, B. ; Rooij, S.A.M. van; Groenveld, G.H. ; Querner, E.P. ; Jonker, R.N.J. ; Padovani, C. ; Tomás, W. ; Kawakami de Resende, E. ; Capella, A. ; Soriano, B. ; Galdino, S. ; Salis, S. ; Mosselman, E. ; Kappel, B. van; Ververs, M. ; Stolker, C. ; Haasnoot, M. ; Maathuis, B. ; Boerboom, L. ; Hein, S. ; Berends, H.A. - \ 2006
    Wageningen : Alterra (Alterra-rapport 1295) - 215
    waterbeheer - besluitvorming - inundatie - biodiversiteit - savannen - wetlands - modellen - brazilië - integraal waterbeheer - water management - decision making - flooding - biodiversity - savannas - wetlands - models - brazil - integrated water management
    Final report (draft) of the project carried out in the framework of the Dutch involvement of World Water Forum; Alterra in cooperation with ITC, Arcadis, WL Delft Hydraulics and Regenboog Advies
    A physiological threshold for protection against menadione toxicity by human NAD(P)H : quinone oxidoreductase (NQO1) in Chinese hamster ovary (CHO) cells
    Haan, L.H.J. de; Boerboom, A.M.J.F. ; Rietjens, I.M.C.M. ; Capelle, D. van; Ruijter, A.J.M. de; Jaiswal, A.K. ; Aarts, J.M.M.J.G. - \ 2002
    Biochemical Pharmacology 64 (2002). - ISSN 0006-2952 - p. 1597 - 1603.
    NAD(P)H:quinone oxidoreductase 1 (NQO1) has often been suggested to be involved in cancer prevention by means of detoxification of electrophilic quinones. In the present study, a series of Chinese hamster ovary (CHO) cell lines expressing various elevated levels of human NQO1 were generated by stable transfection. The level of NQO1 over-expression ranged from 14 to 29 times the NQO1 activity in the wild-type CHO cells. This panel of cell lines, allowed investigation of the protective role of NQO1 in quinone cytotoxicity. It could be demonstrated that menadione toxicity was significantly reduced in all NQO1-transfected CHO clones compared to the wild-type cells, but the clones did not show differences in their level of protection against menadione. This observation pointed at a critical threshold concentration of NQO1 above which a further increase does not provide further protection against quinone cytotoxicity. Additional studies in which the NQO1 activity was inhibited by dicoumarol showed that only dicoumarol concentrations of about five times the 50 for NQO1 inhibition were able to reduce NQO1 levels below the apparent threshold, making the cells more sensitive. The level of this threshold was estimated to be in the range of base line NQO1 activities observed in several tissues and species. Thus, the results of the present study indicate that beneficial effects of NQO1 induction by, for example, cruciferous vegetables might be absent or present depending on the NQO1 activity threshold for optimal protection and the basal level of NQO1 expression in the tissue and species of interest.
    Proteins and protein/surfactant mixtures at interfaces in motion
    Boerboom, F.J.G. - \ 2000
    Agricultural University. Promotor(en): A. Prins; M.A. Cohen Stuart. - S.l. : S.n. - ISBN 9789058082817 - 213
    eiwitten - oppervlakten - oppervlaktespanningsverlagende stoffen - eigenschappen - surfaces - surfactants - foaming - properties

    The research described in this thesis covers a number of aspects of the relation between surface properties and foaming properties of proteins, low molecular surfactants and mixtures thereof. This work is the result of a question of the industrial partners if it is possible to understand the foaming properties of protein hydrolysates. As there are many aspects of the surface properties that can be responsible for the foaming behaviour a number of problems were defined by which we can obtain a better understanding of the relation between surface properties and foam formation and stability in relation to the type of surface active substance.

    In this thesis an important question a priori has been: How can we understand the foaming properties from the properties of the surface active species. We presumed that the molecular properties cannot be translated forthwith to foaming properties but that a number of translation steps are necessary. First of all the molecular properties need to be translated into surface properties which manifest themselves in mechanical properties of surfaces and films. In addition there is a relation between these mechanical properties and the foaming properties.

    An important consideration a priori was that the way in which the surface is deformed is important for finding the relevant relation between the mechanical properties and the foaming properties. Here we made a distinction in two types of deformation being deformations caused by forces applied parallel and perpendicular to the surface. A force that is applied perpendicular to the surface (pressure) generally leads to a homogeneous deformation of the surface. The properties of the surface change with time or time scale but at the surface there are no differences with respect to surface tension. surface concentration or relative rate of expansion. Forces applied perpendicular to the surface generally lead solely to enlargement or reduction in surface area. Forces applied parallel to the surface (shear forces) can also lead to enlargement or reduction in surface area. These forces are generally due to viscous friction with the surface. In addition to a change in surface area these forces also cause a redistribution of surface active material over the surface. Hence the surface concentration. the surface tension as well as the relative rate of expansion vary over the surface. This leads to a surface tension gradient that is necessarily equal to the viscous drag at the surface. It is striking that in literature only homogeneous deformations are studied in detail. This fact can be attributed to the difficult experimental accessibility of surfaces subjected to shear forces. Nevertheless shear forces may play a role in the foaming properties such as in drainage and bubble break-up. An important part of this thesis will be devoted to the relation between viscous friction and surface motion.

    A device which enables the quantification of the relation between viscous drag and motion of the surface in relation to the surface properties is the overflowing cylinder. This device consists of an inner cylinder surrounded by an outer cylinder with a larger diameter. In this inner cylinder liquid is pumped up which flows over the rim into the space between the inner and outer cylinder. At the top surface we find a continuously expanding surface of which the relative expansion rate remains approximately constant over the surface in the vicinity of the centre of the cylinder. The expansion rate of the surface can be influenced by changing the length of the falling film. Hence within certain limits the expansion rate at the top surface c5n be varied. If we consider the falling film at the leading edge of the inner cylinder the falling motion of the liquid in the film causes the deformation of the surface parallel to the surface. If we would be able to measure the properties of the falling film we could learn how the surface properties vary with distance. However the surface of the falling film is experimentally not accessible. Therefore in this thesis the changes in surface properties of the falling film have been studied by measuring the surface tension of the top and bottom surface at a fixed place. This provides a reasonable measure of the surface tension difference over the falling film of the overflowing cylinder.

    In order to interpret this difference, the conditions at the falling film have been approximated by means of simple hydrodynamic theory. From this approximation we could conclude that there is a relation between the relative expansion rate, the length of the falling film and the surface tension difference. The surface of the falling film is propelled by the falling motion of the liquid which causes a surface tension gradient at the surface as a consequence of the viscous drag. If we compare the calculations with experimental data we can find that this approach is in agreement with the experiments. Hence the difference in surface tension over the falling film can be considered to be a measure of the resistance to deformation of the surface to forces applied parallel to the surface.

    The surface tension gradients which could be generated by means of different surface active species appeared to differ significantly from each other. Especially the difference between low molecular surfactants and proteins could be shown to be large. This can be ascribed to the sensitivity of the surface tension of these substances to expansion and compression of the surface. Adsorbed layers of proteins have a high surface tension in expansion. This can be explained by the time required for unfolding at a surface of these substances. In compression low surface tensions are found for proteins due to the slow desorption of proteins. The surface tension of low molecular surfactants is less sensitive to compression and expansion. In expansion the relatively short diffusion length causes the surface tension to deviate much less from the equilibrium surface tension. In compression these substances desorb easily causing the surface tension to be close to the equilibrium surface tension as well. Hence the surface tension gradient that can be generated by proteins is much larger the for low molecular surfactants. If mixtures of low molecular surfactants and proteins (Tween 20 andβ-casein) are considered it is found that in expansion the surface tension is influenced by both species. In compression however the surface tension is around the equilibrium surface tension of the low molecular surfactant. From this we can conclude that the low molecular surfactant determines the surface tension in compression. Most probably there the affinity of the surface active substance for the surface is important which causes the preferential desorption of proteins. This means that the surfaces of these mixtures have a low resistance to deformation by forces applied parallel to the surface.

    Unfolding behaviour of proteins at interfaces

    The most important class of surface active substances which have been studied in this thesis are proteins due to the similarities in structure and properties with protein hydrolysates. Proteins consist of 20 different amino acids which vary in residual group. Despite the similarity in the basic structure of these substances the difference in foaming properties between proteins is large. In literature this difference in foaming properties is ascribed to the difference in unfolding rate during adsorption at air/water interfaces of these polymers. There are no reliable data on the unfolding rate of proteins however. The overflowing cylinder technique can be used to determine the unfolding rates of proteins since the top surface is in a steady state while the relative expansion rate is finite. The relative expansion rate can be seen as a characteristic time scale of the surface.

    In order to measure the unfolding rate of the proteins, at the top surface, the surface tension, the relative rate of expansion and the surface concentration were determined using the Wilhelmy plate technique, laser Doppler anemometry and ellipsometry respectively. With these three parameters the surface can be characterised completely. The reasoning behind the characterisation is as follows: If proteins need time for the unfolding at an interface, the relative expansion rate determines the mean degree of unfolding of the proteins at the interface. As a function of the relative rate of expansion and at equal adsorbed amounts the surface tension will vary due to a difference in the mean degree of unfolding.

    If we would know the surface tension and surface concentration for different bulk concentrations as a function of the relative rate of unfolding then we would be able to establish the influence of the unfolding rate of the protein on the surface tension. Since this is difficult to determine on the basis of the raw date, a simple model was used to express this influence of unfolding in an unfolding and a refolding parameter. In this model the transport to and the unfolding at the top surface of an overflowing cylinder has been described. The unfolding and refolding has been described by means of a first order reaction. In essence we assume that the degree of unfolding can be seen as an average over many stages of unfolding over a large number of molecules. By dividing the interface into a large number of concentric rings and by carrying out the calculations for the transport and unfolding for each concentric ring the surface properties can be calculated as a function of the distance to the centre. By varying the relative rate of expansion as a function of the distance to the centre in the same way as takes place at the top surface of the overflowing cylinder also the surface tension gradient can be determined. Since there is a fixed relation between the surface tension gradient and the maximum relative rate of expansion, the maximum relative rate of expansion can be determined by means of iteration.

    In this thesis the unfolding behaviour of the proteins:β-casein,β-lactoglobulin, BSA and lysozyme has been determined experimentally. In literature these proteins have been characterised well with respect to adsorption and unfolding behaviour. The unfolding rates of these proteins differ several orders of magnitude.β-Casein andβ-lactoglobulin were shown to unfold most rapid. The characteristic time scales of unfolding of these proteins is in the order of a tenth forβ-casein to a few tenths forβ-lactoglobulin. Lysozyme hardly unfolds within the time scale of the experiment which indicates that the unfolding takes more than 100 seconds. The model was shown not to apply for BSA since the change in surface tension proceeds in two steps. Nevertheless it could be calculated that the unfolding of BSA takes place in a time scale in the order of 20 seconds.

    The model and the overflowing cylinder technique have also been applied to mixtures of proteins and low molecular surfactants. The systemβ-casein/Tween 20 was chosen because in this system no complications are known such as aggregation in he bulk phase, or electrostatic interactions at the surface. At expanding interfaces these systems were shown to behave in a more or less additive manner. The surface tension of the mixture appeared to be lower than the addition of the decrease in surface tension of each species individually. This can be attributed to the fact that both species occupy part of the space at the interface. In static conditions it was found in literature that low molecular surfactants can displace proteins from the interface. This difference in behaviour is caused by the fact that in expansion the surface tension is controlled mainly by transport to and unfolding at the surface while in equilibrium time scale is irrelevant and the affinity of the substances to the surface determines the behaviour. Despite certain deviations the measured properties were indicated to be consistent with the calculations by the model. Higher order effects such as the presence of the surfactants in micelles and preferential adsorption were shown to have little effect of these mixtures at expanding surfaces.

    Foam formation and foam stability

    In order to quantify the experimental values and techniques of this research for practical situations, foaming experiments have been performed for a number of relevant systems. In these experiments, the bubble size and the drainage rate of the foams have been determined. Subsequently the results of these experiments were related to mechanical surface properties.

    The experiments indicated that the surface tension difference over the falling film in the overflowing cylinder correlated with the bubble size and the rate of drainage. For the formation of foam this means that not only the amount of energy supplied is important for the bubble size but that also characteristic properties of the surface being the maximal viscous drag that can be transferred is important for the break-up of foam bubbles. The explanation that can be given for this is that the shear stress generated at the surface provides the deformation that leads to an unstable shape of the bubbles which finally leads to break-up. Surface active species that enable the generation of a high shear stress therefore promote the generation of small bubbles.

    In addition it was demonstrated that there is a relation between drainage and viscous friction at the bubble surface. The rate of drainage expressed in the decrease of the characteristic film thickness decreases when a higher surface tension difference between top and bottom surface in the overflowing cylinder is present at a rather arbitrarily chosen relative rate of expansion of 1 s -1 .

    In chapter 6 the foaming properties of protein hydrolysates were discussed. The most important reason that protein hydrolysates have good foaming properties is that in these systems the low molecular components do not displace high molecular components. This causes the surface tension difference between a compressed and expanded surface to be high. This supports the creation of small bubbles and the resistance to drainage. It is possible that the good foamability of protein hydrolysates compared to proteins is caused by the lower surface tension in equilibrium.

    In general it can be said that surface tension gradients play a larger role in foam formation and foam stability than the attention in literature would suggest. More attention for the properties which determine the resistance against deformation of surfaces, parallel to the surface would lead to a better insight in the reasons why different surface active substances exhibit different foaming behaviour.

    The use of overflowing cylinder technique to measure stagnant surface behaviour.
    Prins, A. ; Boerboom, F.J.G. ; Kalsbeek, H.K.A.I. van - \ 1998
    Colloids and Surfaces (1998). - ISSN 0166-6622 - p. 395 - 401.
    Bulk and surface rheological behaviour of aqueous milk protein solutions. A comparison.
    Boerboom, F.J.G. ; Groot-Mostert, A.E.A. de; Prins, A. ; Vliet, T. van - \ 1996
    Netherlands Milk and Dairy Journal 50 (1996). - ISSN 0028-209X - p. 183 - 198.
    Skin formation on liquid surfaces under non-equilibrium conditions.
    Prins, A. ; Jochems, A.M.P. ; Kalsbeek, H.K.A.I. van; Boerboom, J.F.G. ; Wijnen, M.E. ; Williams, A. - \ 1996
    Progress in Colloid and Polymer Science 100 (1996). - ISSN 0340-255X - p. 321 - 327.
    De vegetatie van een tropisch hooggebergte (Venezuela).
    Boerboom, J.H.A. - \ 1992
    Unknown Publisher (Hinkeloord Reports 1) - 14 p.
    Ecophysiology of 8 woody multipurpose species from semiarid northeastern Mexico.
    Boerboom, J.H.A. ; Smits, M.P. ; Stienen, H. ; Reid, N. ; Landa, J. - \ 1990
    In: Proc. Int. Symp. Forest tree physiology, E. Dreyer, G. Aussenac, M. Bonnet-Masimbert (eds.). Nancy, France, 1988. Ann. Sci. Forestières 46, suppl. (1989) 454s-458s
    Mesures a les zones agrícoles amb finalitats de conservació del medi ambient, especialment el sòl i l'aigua.
    Boerboom, J.H.A. - \ 1990
    In: Jornades d'Estudi Sobre Les Zones de Muntanya, J. Ainud de Lasarte, J. Ganyet, R. Majoral, Ll. Prats (eds.). Dept. Cultura Generalitad Catalunya, Barcelona - p. 233 - 240.
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