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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    Kinetics of Ne-(carboxymethyl)lysine formation in aqueous model
    Nguyen, T.T.H. ; Fels, H.J. van der; Boekel, T. van - \ 2016
    Food Chemistry 192 (2016). - ISSN 0308-8146 - p. 125 - 133.
    This study investigated the formation of Nε-carboxymethyllysine (CML) in two caseinate solutions containing: (1) glucose, (2) lactose, each heated at 120 °C and 130 °C. At both heating temperatures, CML concentration in lactose-caseinate solution was higher than in glucose-caseinate solution. In both solutions, more CML was formed at 130 °C than at 120 °C. Using multiresponse modelling, two degradation routes for the sugars were confirmed: (1) isomerisation of glucose or lactose and subsequent degradation via Lobry de Bruyn–Alberda van Ekenstein (LA) arrangement; (2) the Maillard reaction between the reducing sugar and lysine residues. Modelling results suggested that CML was not formed from oxidation of the reducing sugars, but from the Maillard reaction via the Amadori rearrangement product. Since CML appeared to be thermally unstable under the current study conditions, it may not be a perfect indicator for heat damage of processed foods. This is the first study in which CML formation was linked to available information on the Maillard reaction via multiresponse modelling
    Modelling the formation of heat-induced contaminants during thermal processing of food
    Nguyen, H.T. - \ 2015
    Wageningen University. Promotor(en): Tiny van Boekel, co-promotor(en): Ine van der Fels. - Wageningen : Wageningen University - ISBN 9789462574588 - 172
    voedsel - maillard-reactie - voedselbereiding - acrylamiden - voedselbesmetting - gezondheidsgevaren - modellen - food - maillard reaction - food preparation - acrylamides - food contamination - health hazards - models

    Many of our food products have undergone a heat-treatment before consumption, either at home or at the food industry. Heat treatments not only bring out desired characteristics of the food products such as flavour, texture, taste and safety aspects but also leads to the formation of undesired compounds that may have negative impacts for human health. Such undesired compounds that are generated from the Maillard reaction are neo-formed food contaminants (NFC). NFC are present in many common heat-processed foods, such as potatoes-based products, cereal-based products, baby foods, and dairy products. Therefore, effective mitigation measures are being developed to minimize the generation of such undesired compounds while maintaining the organoleptic attributes of the food products as consumer’s demands. This thesis aimed at understanding the mechanistic pathways for the formation of three neo-formed food contaminants: acrylamide, 5-hydroxymethylfurfural (HMF), and -(carboxymethyl)lysine (CML), using multireponse kinetic modelling.

    First, the topic of the Maillard reaction, occurrence and human exposure to the three NFC in heated foods, and multiresponse kinetic modelling is introduced. Then, scientific literature on analytical methods, formation pathways, occurrence in processed foods, and health impacts of CML was reviewed. Based on the literature, an experiment was set up to understand the formation pathway for CML in caseinate-lactose/glucose solutions, each heated at 120oC and 130oC. According to the best fitting mechanistic model, the formation of CML in the two model solutions originated from the reaction between lactose/glucose and lysine residues via the Amadori rearrangement product formation. Moreover, glucose and lactose were degraded via Lobry de Bruyn-Alberda van Ekenstein (LA) arrangement. CML seems to be not thermally stable, and may thus not be an optimal indicator for heat damage of foods.

    Another experiment was done for gaining insights into the formation of acrylamide and HMF in biscuits during baking at 200oC. Four biscuit recipes were prepared with three sugar types: (1) sucrose (35 g), (2) glucose (17.5 g) and fructose (17.5 g), (3) fructose (17.5 g), and (4) glucose. The molar ratio of total glucose and fructose to asparagine in each type of biscuit was higher than 1. The concentrations of acrylamide and HMF were lowest in the sucrose-prepared biscuits. Kinetic modelling results suggested that during baking of these four kinds of biscuits, acrylamide was formed via the specific amino acid route, i.e., a reducing sugar reacts with asparagine to form the Schiff base without the Amadori product formation (not via Strecker degradation), and that HMF was formed via caramelisation. Fructose played a key role in the formation of both acrylamide and HMF.

    In a similar experiment, the effects of different types of wheat flour on acrylamide and HMF formation in sucrose-prepared biscuits during baking at 200oC were investigated. Four types of wheat flour, which had the most different concentrations of asparagine, and total glucose and fructose (the reducing sugar), were selected for the preparation of four kinds of biscuits. Out of four wheat flour types, two had the molar ratio of reducing sugars to asparagine lower than 1, and the other two had a ratio higher than 1. Results showed that those different molar ratios in wheat flour did not have effects on the pathways leading to the formation of acrylamide and HMF in all four types of biscuits. Acrylamide was formed via the specific amino acid route, and HMF was formed via caramelisation. No clear correlation was found between the concentration of either acrylamide or HMF in biscuits and the concentration of asparagine or the reducing sugars in wheat flour. Asparagine was not a limiting factor for acrylamide generation in biscuits.

    The outcomes of this thesis give insights into the actual reaction pathways for the formation of acrylamide and HMF in biscuits during baking at 200oC and for the formation of CML in the model solutions. These modelling results may help to control the formation of these NFCs in a quantitative way.

    Improved applicability of nisin in novel combinations with other food preservation factors
    Pol, I.E. - \ 2001
    Wageningen University. Promotor(en): F.M. Rombouts; E.J. Smid. - S.l. : S.n. - ISBN 9789058083821 - 95
    voedselbewaring - nisine - listeria monocytogenes - bacillus cereus - food preservation - nisin - listeria monocytogenes - bacillus cereus

    General discussion

    Modern consumers nowadays, have a preference for more natural, mildly preserved food products with a fresh appearance over traditionally preserved products. Mild preservation techniques applied singly are usually not sufficient to control microbial outgrowth and combinations of measures are needed to ensure complete safe products (16). Bacteriocins, produced by lactic acid bacteria have been successfully used as biopreservatives in a number of food products to inhibit the growth of pathogenic and spoilage organisms (27). Up till now, nisin is the only bacteriocin that has been approved by the WHO to be used as a food preservative. Due to its restricted inhibition spectrum and the decreased solubility and heat sensitivity at neutral pH, application is still limited (10). The study described in this thesis aimed to increase the practical application of nisin by combinations with other biopreservatives or mild preservation techniques.

    Nisin and essential oils

    Essential oils, derived from plants, are known for their flavor characteristics. Many of the compounds found in essential oils possess antimicrobial activity (4, 9, 14, 22), and therefore are suitable candidates for mild food preservation in combination with nisin. The essential oils dramatically enhance the bactericidal activity of nisin at concentrations, which alone do not affect the bacterial cell counts of the foodborne pathogens Listeria monocytogenes and Bacillus cereus (chapter 2). Adaptation of these cells to lower temperatures resulted in an increased sensitivity towards nisin, possibly due to an altered membrane composition leading to a change in membrane fluidity or to an increased electrostatic interaction of nisin with phospholipids in the membrane caused by an increase in negative charges (8, 18 - 21, 31). Alternatively a decrease in lipid II content as a result of changes in the membrane composition might explain the decreased activity of nisin (5). Lowering the temperature had a negative influence on the synergy between nisin and the essential oils, which might result from the lower sensitivity of the cells towards essential oils at lower temperatures (28).

    The exact mechanism underlying this synergy is not exactly known. Both nisin and carvacrol cause a dissipation of the proton motive force as well as depletion of the internal ATP pool (6, 12, 23, 26, 30, 32, chapter 3). In combination, carvacrol enhances the membrane potential dissipating effect of nisin, at concentrations which do not affect the viable count of B. cereus . Apparently cells are able to cope with low concentrations of nisin and carvacrol. When concentrations increase, cells are no longer able to compensate for loss of membrane integrity and a synergistic reduction of the pH gradient and depletion of the intracellular ATP pool were observed. The reduction in internal ATP is not proportional to the increase in external ATP and no additional increase in external ATP was observed upon simultaneous exposure to nisin and carvacrol. This observation excludes increased leakage of ATP as an explanation for the synergistic depletion of the intracellular ATP pool. Consequently, the underlying mechanism of the synergistic inactivation of B. cereus is most likely not the increased poreforming ability of nisin by carvacrol. Presumably, the rate of ATP hydrolysis is increased upon simultaneous addition of nisin and carvacrol or the internal ATP pool is exhausted in an attempt to reenergize the membrane (1, 23, 29). Alternatively, the disturbance of the membrane permeability by carvacrol and nisin might lead to impairment of membrane bound enzymes like ATPase, resulting in a decreased ATP synthesis (15, 26).

    Nisin and PEF treatment

    In addition to essential oils, Pulsed Electric Field treatment was also found to improve the antimicrobial action of nisin against B. cereus. Synergy was only found when PEF treatment was spread over a period of 10 minutes to match the relevant inactivation time scale of nisin's action. The additional stress imposed by PEF treatment possibly facilitates the incorporation of nisin into the cytoplasmic membrane resulting in more or larger pores or pores with a longer lifetime (chapter 4). Further reduction of the intensities of the treatments was achieved by adding carvacrol as a third hurdle to the combination of nisin and PEF treatment (chapter 5).

    The fact that synergy was found between the three treatments renders the combination very interesting for mild food preservation. However, extrapolation of the results from labscale experiments in buffer systems to food model matrices is usually difficult and the influence of food ingredients on the efficiency of preservation techniques are not fully understood. The efficiency of PEF treatment against vegetative cells of B. cereus is not affected by proteins in skimmed milk (20 %). However, the proteins do have a negative influence on the nisin activity, either as a result of a decreased bioavailability of nisin due to binding of the molecule to proteins or because of protection of the microorganisms by the proteins. As a consequence, the synergy between nisin and PEF treatment is less pronounced in skimmed milk (20 %).

    In sharp contrast to the improved bactericidal activity found in HEPES buffer, carvacrol is not able to enhance the synergy between nisin and PEF treatment in diluted milk (only in high concentrations (1.2 mM)). Possibly, carvacrol binds to the proteins, reducing the availability of the molecule. However, this is not consistent with the fact that carvacrol increases the antimicrobial activity of PEF treatment in milk. Therefore, the absence of synergy between nisin, PEF treatment and carvacrol is more likely explained by the decreased bioavailability of nisin, thereby decreasing the extent of synergy between nisin and carvacrol and consequently between all three treatments. The influence of PEF treatment on the behavior of proteins is not exactly known. Proteins can carry electric charges and might behave as dipoles when subjected to PEF treatment, which cause the macromolecules to reorient or deform (such as protein unfolding and denaturation), and possibly some breakdown of covalent bonds or casein micelles may occur (3). These PEF induced changes in the structure of proteins may play a role in the existence of synergy between carvacrol and PEF. Dilution of the milk to 5 % still provides enough proteins to stimulate synergy between carvacrol and PEF treatment (chapter 5).

    Before such novel techniques can replace currently used thermal processes, more insight into spore inactivation is needed (chapter 6). Nisin and PEF treatment do not directly inactivate or damage spores of B. cereus , however germinated spores can be inactivated by nisin or PEF treatment to a certain extent. The PEF resistance of the germinated spores is lost 50 minutes after the onset of germination. Nisin resistance was lost immediately in parallel to heat resistance, suggesting that loss of nisin resistance might be ascribed to changes in the dehydrated state of the core. Sulfhydryl groups in the membrane, not available in ungerminated spores, were suggested to be the natural target for nisin and therefore access to the membrane is a prerequisite for inactivation (17, 24, 25). In addition, the increase in availability of the membrane-anchored cell wall precursor Lipid II upon germination could also play a role in the loss of nisin resistance (5). Apparently, nisin has gained access to the membrane by penetrating the coat, which was made more permeable upon germination or alternatively, the protective coat was degraded by spore lytic enzymes, allowing nisin to reach the cytoplasmic membrane. The late loss of PEF resistance can be explained by its dependence on the degradation of the spore coat. To exert antimicrobial inactivation by PEF treatment, free migration of ions is needed to increase the transmembrane potential of the spores. Formation of pores occurs after compression of the membrane and reorientation of the phospholipids in the membrane. In spores the ions are immobilized by proteins or DPA, restricting their mobility (7, 13) and subsequently the build up of an increased transmembrane potential is prevented. Secondly, the spore core is surrounded by several rigid protecting layers limiting the compression and reorientation of the phospholipids (2).

    Combining nisin and PEF treatment did not result in additional inactivation of the germinating spores. Since loss of PEF resistance occurs only after 50 minutes of germination and loss of nisin resistance seems to be an early event in spore germination, synergy would therefore be less likely due to different time scales of action. Furthermore, the incomplete germination of the spores reduces the margins to observe synergy. Ideally, complete and synchronized germination is needed to quantify the inactivation by nisin or PEF treatment and determine precisely the onset of loss of nisin or PEF resistance.

    One of the main problems associated with the use of antimicrobial compounds is the development of tolerance or resistance to certain compounds. Adaptation of cells to carvacrol was correlated to a decrease in membrane fluidity as demonstrated by Ultee et al. (30). In addition, they observed a change in phospholipid composition of the membrane. Cells adapted to carvacrol exhibited an increased sensitivity towards nisin compared to control cells (chapter 6). A decrease in the membrane fluidity is not expected to increase nisin's action, but a change in the head group composition, with an increase in negatively charged lipids, might stimulate the electrostatic binding of nisin and in this way enhance nisin's action (8, 18 - 21, 31). Alternatively an increase in lipid II content in carvacrol-adapted cells as a result of changes in the membrane composition might explain the increased activity of nisin (5). A decrease in the membrane fluidity did not change the susceptibility towards a PEF treatment. A more rigid membrane is less likely to be compressed by accumulating charges as a result of applied field strength and the ordered state of the phospholipids in the membrane decreases the chance of reorientation, which would reasonably lead to a decreased inactivation by PEF treatment. Although the bactericidal activity of nisin was increased by adaptation to carvacrol, the synergy between nisin and PEF treatment was not influenced by a change in membrane fluidity and membrane composition. Attemps to change the membrane composition of spores by adaptation of vegetative cells to carvacrol prior to and during sporulation did not lead to inactivation of spores by either nisin or PEF treatment.


    Combinations of nisin with essential oils or PEF treatment have been successful in overcoming the restrictions in practical application of nisin. For instance, the inhibition spectrum of nisin can be widened by combination with other preservation technologies like PEF treatment. In addition, the limited activity of nisin at higher temperatures can be complemented by the increased synergy between nisin and essential oils.

    The application of multiple hurdles has great potential to be used as a mild food preservation technology. The occurrence of synergy between nisin and essential oils or PEF technology allows for a reduction in the intensities of the treatments demonstrating the suitability for mild preservation. Increasing the number of hurdles (lysozyme) improves the observed synergy and further increases the mildness of the preservation technology (chapter 1).

    Consumer's acceptation of these combination techniques in case of the essential oils is not expected to meet difficulties. This combination meets with present preference for more natural and mild preservation methods. Herbs and spices, of which essential oils are the active components, are already used for centuries as flavoring agents and in homeopathic products and medicines. Currently, carvacrol is Generally Recognized As Safe (GRAS) and has been approved by the Code of Federal Regulation (CFR) to be used as a flavoring agent (11). However, when the essential oils are used for their antimicrobial activity, they will be regarded as new food additives and subsequently require a non-toxicity report (27). To circumvent these problems, the original herbs and spices can be used as food flavoring agents, while at the same time advantage can be taken of their antimicrobial activity. However, the producer has to take into account the low concentration of the active compound in herbs and spices. Furthermore, the essential oils have a strong and specific flavor and can only be applied in products where this aroma is appreciated.

    Acceptance of PEF technology is expected to give more problems and introduction of this technology has to be handled carefully. Consumers might associate PEF treated foods with residual electromagnetic raditation, just like radiated foods are associated with radioactivity. Only when PEF technology is introduced carefully and the consumers are supplied with the right information, they will accept this technology as mild preservation.

    At the moment, not enough information is known about PEF technology and its mechanism of action. Evidently, more research needs to be done to verify the influence of other food ingredients including fat particles on the antimicrobial activity. Furthermore the influence of PEF treatment on the product quality needs to be investigated. The fresh-like appearance, color and the vitamin content are seemingly unaffected however, the influence of PEF treatment on proteins, polysaccharides macromolecules, or lipids is not exactly known.

    The development of tolerance or resistance to the PEF treatment or the combination treatments is not clear and should receive more attention, since microorganisms generally adapt to environmental stress factors. Increased tolerance towards nisin and carvacrol has been studied in more detail (8, 18 - 21, 31) however, no such research has been conducted concerning PEF technology. Combining preservation technologies in which the microorganism is attacked from different sides should reduce the development of tolerance to a minimum. Inactivation of spores is another challenge to be overcome before such combination technologies can be implemented in current preservation strategies.

    In conclusion, these combination techniques are a welcome alternative to currently used pasteurization methods. The current limitations in the application of nisin can be complemented by the inhibition spectrum of the combination treatment. In addition, the synergy observed between the different preservation techniques allows for a reduction of the used intensities increasing the suitability for mild preservation.


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    Adaptation of the food-borne pathogen Bacillus cereus to carvacrol
    Ultee, A. ; Kets, E.P.W. ; Alberda, M. ; Hoekstra, F.A. ; Smid, E.J. - \ 2000
    Archives of Microbiology 174 (2000). - ISSN 0302-8933 - p. 233 - 238.
    Carvacrol, a natural antimicrobial compound present in the essential oil fraction of oregano and thyme, is bactericidal towards Bacillus cereus. A decrease of the sensitivity of B. cereus towards carvacrol was observed after growth in the presence of non-lethal carvacrol concentrations. A decrease of the melting temperature (Tm) of membranes from 20.5 °C to 12.6 °C was the immediate effect of the addition of carvacrol. Cells adapted to 0.4 mM carvacrol showed a lower membrane fluidity than non-adapted cells. Adaptation of 0.4 mM carvacrol increased the Tm from 20.5 °C to 28.3 °C. The addition of carvacrol to cell suspensions of adapted B. cereus cells decreased Tm again to 19.5 °C, approximately the same value as for the non-adapted cells in the absence of carvacrol. During adaptation, changes in the fatty acid composition were observed. The relative amount of iso-C13:0, C14:0 and iso-C15:0 increased and cis-C16:1 and C18:0 decreased. The head-group composition also changed, two additional phospholipids were formed and one phospholipid was lacking in the adapted cells. It could be concluded that B. cereus adapts to carvacrol when present at non-lethal concentrations in the growth medium by lowering its membrane fluidity by changing the fatty acid and head-group composition
    Metabolic dysfunction and unabated respiration precede the loss of membrane integrity during dehydration of germinating radicles
    Leprince, O. ; Harren, F.J.M. ; Alberda, M. ; Hoekstra, F.A. - \ 2000
    Plant Physiology 122 (2000). - ISSN 0032-0889 - p. 597 - 608.
    This study shows that dehydration induces imbalanced metabolism before loss of membrane integrity in desiccation-sensitive germinated radicles. Using a photoacoustic detection system, responses of CO2 emission and fermentation to drying were analyzed non-invasively in desiccation-tolerant and -intolerant radicles of cucumber (Cucumis sativa) and pea (Pisum sativum). Survival after drying and a membrane integrity assay showed that desiccation tolerance was present during early imbibition and lost in germinated radicles. However, tolerance could be re-induced in germinated cucumber radicles by incubation in polyethylene glycol before drying. Tolerant and polyethylene glycol (PEG)-induced tolerant radicles exhibited a much-reduced CO2 production before dehydration compared with desiccation-sensitive radicles. This difference was maintained during dehydration. In desiccation-sensitive tissues, dehydration induced an increase in the emission of acetaldehyde and ethanol that peaked well before the loss of membrane integrity. Acetaldehyde emission from sensitive radicles was significantly reduced when dehydration occurred in 50␘2 instead of air. Acetaldehyde/ethanol were not detected in dehydrating tolerant radicles of either species or in polyethylene glycol-induced tolerant cucumber radicles. Thus, a balance between down-regulation of metabolism during drying and O2 availability appears to be associated with desiccation tolerance. Using Fourier transform infrared spectroscopy, acetaldehyde was found to disturb the phase behavior of phospholipid vesicles, suggesting that the products resulting from imbalanced metabolism in seeds may aggravate membrane damage induced by dehydration.
    High critical temperature above Tg may contribute to the stability of biological systems
    Buitink, J. ; Dries, I.J. van den; Hoekstra, F. ; Alberda, M. ; Hemminga, M.A. - \ 2000
    Biophysical Journal 79 (2000). - ISSN 0006-3495 - p. 1119 - 1128.
    In this study, we characterized the molecular mobility around Tg in sugars, poly-L-lysine and dry desiccation-tolerant biological systems, using ST-EPR, 1H-NMR, and FTIR spectroscopy, to understand the nature and composition of biological glasses. Two distinct changes in the temperature dependence of the rotational correlation time (R) of the spin probe 3-carboxy-proxyl or the second moment (M2) were measured in sugars and poly-L-lysine. With heating, the first change was associated with the melting of the glassy state (Tg). The second change (Tc), at which R abruptly decreased over several orders of magnitude, was found to correspond with the so-called cross-over temperature, where the dynamics changed from solid-like to liquid-like. The temperature interval between Tg and Tc increased in the order of sucrose < trehalose < raffinose staychose < poly-L-lysine < biological tissues, from 17 to >50°C, implying that the stability above Tg improved in the same order. These differences in temperature-dependent mobilities above Tg suggest that proteins rather than sugars play an important role in the intracellular glass formation. The exceptionally high Tc of intracellular glasses is expected to provide excellent long-term stability to dry organisms, maintaining a slow molecular motion in the cytoplasm even at temperatures far above Tg.
    Changed properties of the cytoplasmic matrix associated with desiccation tolerance of dried carrot somatic embryos : An in situ Fourier transform infrared spectroscopic study
    Wolkers, W.F. ; Tetteroo, F.A.A. ; Alberda, M. ; Hoekstra, F.A. - \ 1999
    Plant Physiology 120 (1999). - ISSN 0032-0889 - p. 153 - 163.
    Tulipaline and tuliposide in cultured explants of tulip bulb scales.
    Rossum, M.W.P.C. van; Alberda, M. ; Plas, L.H.W. van der - \ 1998
    Phytochemistry 49 (1998). - ISSN 0031-9422 - p. 723 - 729.
    A Fourier transform infrared microspectroscopy study of sugar glasses: application to anhydrobiotic higher plant cells.
    Wolkers, W.F. ; Oldenhof, H. ; Alberda, M. ; Hoekstra, F.A. - \ 1998
    Biochimica et biophysica acta-protein structure and molecular enzymology 1379 (1998). - ISSN 0167-4838 - p. 83 - 96.
    Fourier transform infrared microspectroscopy (FTIR) was used to study glasses of pure carbohydrates and in the cytoplasm of desiccation tolerant plant organs. The position of the OH stretching vibration band (vOH) shifted with temperature. Two linear regression lines were observed in vOH against temperature plots. The temperature at the point of intersection between these two lines coincided with the glass transition temperature (T(g)), as determined by other methods. The temperature at the intersection point decreased with increasing water content, which further validates that, indeed, T(g) was observed. T(g) values that were determined for dry glucose, sucrose, maltose, trehalose and raffinose glasses were 27, 57, 91, 108 and 108°C, respectively. The shift of vOH with temperature, the wavenumber-temperature coefficient (WTC), was higher in sugar glasses having higher T(g). This suggests that glasses are more loosely packed when they have higher T(g). For Typha latifolia pollen and dried Craterostigma plantagineum leaves we obtained similar vOH vs. temperature plots as for carbohydrate glasses, indicating that a glass transition was observed. The T(g) in dry pollen was ca. 45°C and in dry plant leaves ca. 65°C, with WTC values comparable to those observed in the carbohydrates. The T(g) values in these tissues decreased with increasing water contents. Our data suggest that the carbohydrates that are present in the cytoplasm are primary factors contributing to the glassy state. We conclude that FTIR provides new insights in the structure of glasses in carbohydrates and in biological tissues.
    Properties of proteins and the glassy matrix in maturation-defective mutant seeds of Arabidopsis thaliana.
    Wolkers, W.F. ; Alberda, M. ; Koornneef, M. ; Leon-Kloosterziel, K.M. ; Hoekstra, F.A. - \ 1998
    The Plant Journal 16 (1998). - ISSN 0960-7412 - p. 133 - 143.
    Wit, C.T. de - \ 1992
    In: Food from dry lands. An integrated approach to planning af agricultural development / Alberda, Th., Dordrecht : Kluwer Academic Publishers (Systems approaches for sustainable agricultural development 1) - ISBN 9780792318774 - p. 193 - 200.
    Wit, C.T. de; Seligman, N.G. - \ 1992
    In: Food from dry lands : an integrated approach to planning of agricultural development / Alberda, Th., Dordrecht : Kluwer Academic Publishers (Systems approaches for sustainable agricultural development 1) - ISBN 9780792318774 - p. 1 - 5.
    Modelling agricultural development strategy.
    Spharim, I. ; Spharim, R. ; Wit, C.T. de - \ 1992
    In: Food from dry lands. An integrated approach to planning of agricultural development. / Alberda, Th., van Keulen, H., Seligman, N.G., de Wit, C.T., Dordrecht : Kluwer Academic Publishers (Systems approaches for sustainable agricultural development 1) - ISBN 9780792318774 - p. 159 - 192.
    Food from dry lands. An integrated approach to planning of agricultural development.
    Alberda, Th. ; Keulen, H. van; Seligman, N.G. ; Wit, C.T. de - \ 1992
    Dordrecht : Kluwer Academic Publishers (Systems approaches for sustainable agricultural development 1) - ISBN 9780792318774
    dry farming - bedrijfssystemen - landgebruik - modellen - natuurlijke hulpbronnen - nomadisme - pampa's - pastoralisme - ruimtelijke ordening - bescherming - herstel - onderzoek - hulpbronnengebruik - plattelandsontwikkeling - plattelandsplanning - semi-aride klimaatzones - sociale economie - steppen - duurzaamheid (sustainability) - zonering - economische planning - regenafhankelijke landbouw - dry farming - farming systems - land use - models - natural resources - nomadism - pampas - pastoralism - physical planning - protection - rehabilitation - research - resource utilization - rural development - rural planning - semiarid zones - socioeconomics - steppes - sustainability - zoning - economic planning - rainfed agriculture
    This book centers around an analysis of the options for the agronomic development of semiarid regions with winter rains (i.e. Mediterranian regions). Crop and sheep husbandry systems were designed and integrated into agro-pastoral systems in which small-grain crops act as a buffer for feed production. These systems served as a basis for rational planning of regional agricultural development under alternative development objectives.
    Production and water use of several food and fodder crops under irrigation in the desert area of Southwestern Peru
    Alberda, T. - \ 1984
    Wageningen : Pudoc (Agricultural research reports no. 928) - 50
    irrigatie - veldgewassen - akkerbouw - landbouw met irrigatie - woestijnen - peru - aride klimaatzones - irrigation - field crops - arable farming - irrigated farming - deserts - peru - arid zones
    Distribution of dry matter and nitrogen between the different plant parts in intact and depodded soyabean plants after flowering.
    Alberda, T. ; Boer, J.M.W. de - \ 1983
    Netherlands Journal of Agricultural Science 31 (1983)2. - ISSN 0028-2928 - p. 171 - 179.
    The influence of length of growing period, nitrogen fertilization and shading on tillering of perennial ryegrass (Lolium perenne L.).
    Alberda, T. ; Sibma, L. - \ 1982
    Netherlands Journal of Agricultural Science 30 (1982)2. - ISSN 0028-2928 - p. 127 - 135.
    The effect of cutting frequency and nitrogen fertilizer rates on dry matter production, nitrogen uptake and herbage nitrate content.
    Sibma, L. ; Alberda, T. - \ 1980
    Netherlands Journal of Agricultural Science 28 (1980)4. - ISSN 0028-2928 - p. 243 - 251.
    Crop photosynthesis : methods and compilation of data obtained with a mobile field equipment
    Alberda, T. - \ 1977
    Wageningen : Pudoc, Centre for Agricultural Publishing and Documentation (Agricultural research reports 865) - 46
    agronomie - fotosynthese - plantkunde - agronomy - photosynthesis - botany
    Laar, H. van; Kremer, D. ; Wit, C.T. de - \ 1977
    In: Crop photosynthesis: methods and compilation of data obtained with a mobile field equipment / Alberda, T., Wageningen : Pudoc (Agricultural research reports 865) - ISBN 9789022006283 - p. 12 - 21.
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