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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Hydrolysis rate constants at 10–25 °C can be more than doubled by a short anaerobic pre-hydrolysis at 35 °C
Zhang, L. ; Gao, R. ; Naka, A. ; Hendrickx, T.L.G. ; Rijnaarts, H.H.M. ; Zeeman, G. - \ 2016
Water Research 104 (2016). - ISSN 0043-1354 - p. 283 - 291.
Anaerobic digestion - Cellulose - Hydrolysis rate constant - Low temperature - Tributyrin

Hydrolysis is the first step of the anaerobic digestion of complex wastewater and considered as the rate limiting step especially at low temperature. Low temperature (10–25 °C) hydrolysis was investigated with and without application of a short pre-hydrolysis at 35 °C. Batch experiments were executed using cellulose and tributyrin as model substrates for carbohydrates and lipids. The results showed that the low temperature anaerobic hydrolysis rate constants increased by a factor of 1.5–10, when the short anaerobic pre-hydrolysis at 35 °C was applied. After the pre-hydrolysis phase at 35 °C and decreasing the temperature, no lag phase was observed in any case. Without the pre-hydrolysis, the lag phase for cellulose hydrolysis at 35–10 °C was 4–30 days. Tributyrin hydrolysis showed no lag phase at any temperature. The hydrolysis efficiency of cellulose increased from 40 to 62%, and from 9.6 to 40% after 9.1 days at 15 and 10 °C, respectively, when the pre-hydrolysis at 35 °C was applied. The hydrolysis efficiency of tributyrin at low temperatures with the pre-hydrolysis at 35 °C was similar to those without the pre-hydrolysis. The hydrolytic activity of the supernatant collected from the digestate after batch digestion of cellulose and tributyrin at 35 °C was higher than that of the supernatants collected from the low temperature (≤25 °C) digestates.

Accurate determination of the Ca2+ activity in milk-based systems by Ca-ISE: Effects of ionic composition on the single Ca2+ activitiy coefficient and liquid junction potentials
Gao, R. ; Leeuwen, H.P. van; Valenberg, H.J.F. van; Boekel, M.A.J.S. van - \ 2011
Food Chemistry 129 (2011)2. - ISSN 0308-8146 - p. 619 - 623.
selective electrode - calcium-ion - ultrafiltrate
Calcium ion selective electrode (Ca-ISE) was found to underestimate the actual Ca2+ ion activity in simulated milk ultrafiltrate (SMUF) and milk. It is shown that the ionic compositional difference between conventional calibration solutions and milk type samples had a significant effect on the single Ca2+ activity coefficient, which generates the erroneous estimate of Ca2+ activities in SMUF and milk. This study tests new standards with ionic profiles similar to SMUF, aiming at the reduction of the errors generated by the compositional difference between conventional standards and milk samples. As a result, the new standards showed a significant improvement in the accuracy of Ca2+ activity and Ca2+ activity coefficient over the conventional standards. The systematic error is reduced from 20% to 5% for SMUF and from 44% to 15% for milk. In addition, the new standards generate liquid junction potentials that are practically insignificant
Ion speciation in milk-like systems
Gao, R. - \ 2010
Wageningen University. Promotor(en): Tiny van Boekel, co-promotor(en): Herman van Leeuwen. - [S.l. : S.n. - ISBN 9789085857228 - 250
melkproducten - ionenbalans - ionen - ionenuitwisseling - wiskundige modellen - milk products - ion balance - ions - ion exchange - mathematical models
The research described in this thesis is about ion behaviour in milk systems as a function of changing conditions (pH, ionic strength, addition of electrolytes and non-electrolytes). The focus was on (i) quantification of the ion equilibria in milk systems based on experimental conditions, (ii) development of quantitative models describing the ion equilibria. The so-called Donnan Membrane Technique (DMT) was successfully applied to determine the concentrations of free Ca2+, Mg2+, Na+ and K+ ions in milk-like systems. Next, models were developed to describe ion equilibria in simulated milk ultrafiltrate solution (SMUF) as a first step to build models for milk systems. Freshly prepared SMUF appeared not to be in thermodynamic equilibrium regarding the supersaturation of calcium phosphate. Therefore, the equilibrium ion speciation (EIS) model and the dynamic ion speciation (DIS) model were developed for SMUF at equilibrium and at initial state, respectively. Both EIS and DIS models gave satisfactory predictions in ion equilibria in SMUF enriched with different electrolytes. Also, addition of non-electrolytes such as various disaccharides appeared to remarkably increase the Ca2+ and H+ ion activities in SMUF and milk. The observed results could be explained by a theory called Mean Spherical Approximation (MSA), which suggests that excluded volume effects due to addition of sugars account for the increase in Ca2+ and H+ ion activity coefficients and thus on Ca2+ and H+ ion activities. Based on the EIS and DIS models, a model named Milk Ion Speciation (MIS) was built to describe ion equilibria in milk. The MIS model was able to predict the ion composition in milk and milk enriched with different electrolytes. In addition, the methodologies of Ca2+ activity measurement were reviewed. We found that the calcium ion selective electrode (Ca-ISE) underestimated the Ca2+ activity at ionic strength lower than 0.1 mol/kg water compared to DMT. It appeared that the ionic composition of conventional calibration standards significantly affects the Ca ion activity measurement. A series of new calibration standards, having similar composition to milk serum, was developed and this remarkably improved the Ca2+ activity measurement in milk-like systems. The developed ion speciation models were carefully evaluated. Particularly, the current limitations of the models such as the nature of CCP and the effect of adding polyphosphate were intensively discussed. Moreover, the models were tested to calculate ion composition in cheese. It turns out that the models offer great potential to correlate ion properties with the functional properties of dairy products.

Effect of Disaccharides on Ion Properties in Milk-Based Systems
Gao, R. ; Leeuwen, H.P. van; Temminghoff, E.J.M. ; Valenberg, H.J.F. van; Eisner, M.D. ; Boekel, M.A.J.S. van - \ 2010
Journal of Agricultural and Food Chemistry 58 (2010)10. - ISSN 0021-8561 - p. 6449 - 6457.
mean spherical approximation - nuclear-magnetic-resonance - activity-coefficients - primitive model - aqueous systems - sugar solutions - caprine casein - calcium-ion - electrolytes - sucrose
The mean spherical approximation (MSA) theory is used to explain the impact of sugars on ion properties in milk-based systems by taking into account electrostatic interactions and volume exclusion effects. This study first focuses on the changes in Ca2+ activity and pH in a solution consisting of CaCl2, KCl, and K3citrate, as a function of sucrose concentration. MSA model calculations were compared with experimental results, and the model satisfactorily describes the ion properties. The excluded volume effects appear to account for a considerable increase in activity coefficient of the ions. This offers a sufficient explanation for the increase in Ca2+ activity and the decrease in pH in milk-based systems with added disaccharides. In addition, hydration of milk proteins seems to enhance ion pair formation in milk. All disaccharides lead to similar modification of the thermodynamic properties of milklike systems, confirming that the observed effects are primarily due to volume exclusion effects
Modelling ion composition in simulated milk ultrafiltrate (SMUF). I: Influence of calcium phosphate precipitation.
Gao, R. ; Halsema, F.E.D. van; Temminghoff, E.J.M. ; Leeuwen, H.P. van; Valenberg, H.J.F. van; Eisner, M.D. ; Giesbers, M. ; Boekel, M.A.J.S. van - \ 2010
Food Chemistry 122 (2010)3. - ISSN 0308-8146 - p. 700 - 709.
donnan membrane technique - salt solution - ph - 50-degrees-c - equilibria - speciation - additives
Freshly prepared simulated milk ultrafiltrate (SMUF) is a solution that is supersaturated with respect to various calcium phosphate phases that precipitate in time. As a consequence, the ion composition of equilibrated SMUF was found to be significantly different from that of freshly prepared SMUF. This study proposes a thermodynamic ion-speciation model that is able to describe ion equilibria in SMUF. Moreover, it is also able to describe calcium phosphate precipitation in fresh SMUF on its way to equilibrium by using an apparent solubility product for CaHPO4·2H2O as a function of time. The model was validated by experiments in which CaCl2 and Na2HPO4 were added to freshly prepared SMUF. The changes in calcium activity and pH were followed and the precipitates were characterised by X-ray diffraction. The model was able to predict the observed changes
Modelling ion composition in simulated milk ultrafiltrate (SMUF) II. Influence of pH, ionic strength and polyphosphates.
Gao, R. ; Halsema, F.E.D. van; Temminghoff, E.J.M. ; Leeuwen, H.P. van; Valenberg, H.J.F. van; Eisner, M.D. ; Boekel, M.A.J.S. van - \ 2010
Food Chemistry 122 (2010)3. - ISSN 0308-8146 - p. 710 - 715.
cyclic phosphate anions - physicochemical properties - biological structure - casein micelles - bovine-milk - nacl - water
This study aims to determine whether the dynamic ion speciation (DIS) model, as introduced in part I, can predict the ion composition in freshly prepared simulated milk ultrafiltrate (SMUF) under various conditions, e.g. pH, ionic strength and presence of various polyphosphates. Experiments were carried out in freshly prepared SMUF enriched with different salts. The model predictions were in satisfactory agreement with experimental results for all conditions. Moreover, specific ion effects were taken into account in the model, where sodium ions were not counted as equivalent to potassium ions due to differences in hydration properties. Polyphosphates, in particular sodium hexametaphosphate (SHMP), were well integrated into the DIS model with reasonable association constants and solubility products. The DIS model suggests that SHMP displays chain polyphosphate properties, which provides better understanding of its interaction with cations. The DIS model appears to be a robust and versatile tool to describe ion equilibria in SMUF
Modelling the ion composition in simulated milk ultrafiltrate under the influence of pH, ionic strength and calcium chelating agents
Gao, R. ; Eisner, M. ; Boekel, M.A.J.S. van - \ 2009
European Dairy Magazine 7 (2009). - ISSN 0936-6318 - p. 12 - 15.
Simultaneous determination of free calcium, magnesium, sodium and potassium ion concentrations in simulated milk ultrafiltrate and reconstituted skim milk using the Donnan Membrane Technique
Gao, R. ; Temminghoff, E.J.M. ; Leeuwen, H.P. van; Valenberg, H.J.F. van; Eisner, M.D. ; Boekel, M.A.J.S. van - \ 2009
International Dairy Journal 19 (2009)8. - ISSN 0958-6946 - p. 431 - 436.
selective electrode - salt solution - speciation - stability - minerals
This study focused on determination of free Ca2+, Mg2+, Na+ and K+ concentrations in a series of CaCl2 solutions, simulated milk ultrafiltrate and reconstituted skim milk using a recently developed Donnan Membrane Technique (DMT). A calcium ion selective electrode was used to compare the DMT results. The study showed that the free Ca2+ concentrations measured by the DMT agreed well with calcium electrode data. Concentrations of free Ca2+, Mg2+, Na+ and K+ measured by the DMT agreed with concentrations predicted by existing ion speciation models. It is concluded that the DMT is suitable to measure various free metal ion concentrations simultaneously in complex milk-type systems
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