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Here we present Positron Annihilation Doppler Broadening Spectroscopy (PADBS) as a powerful method to analyse the origin and development of defect processes in porous silicon structures as a result of alloying with lithium for the use in battery anode applications. Several prepared anodes were lithiated (discharged against Li+/Li) and de-lithiated (charged) with different capacities followed by a distinct treatment procedure and an analysis using the Delft Variable Energy Positron Beam. The results presented here show that we can distinguish two different processes attributed to (1) structural changes in silicon as a result of the alloying process, and (2) the formation of defects that initiate degradation of the material. The limit at which the porous material can be used for at least the first two cycles without the occurrence of damage can thus be accurately determined by using the PADBS technique., RST/Storage of Electrochemical Energy, RST/Neutron and Positron Methods in Materials

Water sorption and dynamical properties of bread crust have been studied using gravimetric sorption experiments. Water uptake and loss were followed while relative humidity (RH) was stepwise in- or decreased (isotherm experiment) or varied between two adjusted values (oscillatory experiment). Experimental results were compared with the Fickian diffusion model and empirical models like the exponential and power-law model. Sorption curves of isotherm experiments could be best described by the Fickian diffusion model for low RH and by the exponential model for large RH. Isotherm equilibrium values could be best described by the GAB equation. Transport rates depend on moisture content and show a maximum around RH=70%. Adsorption and desorption curves from oscillatory sorption experiments could be described best by the exponential model. From comparison of experimental sorption curves and the power-law model for short times it followed for all bread crust that the diffusional coefficient n is close to one. Normally this is associated to so-called case II diffusion and water transport that is limited by relaxation of the solid material. However, additional observations suggest that this may not be a valid explanation and that a kinetic barrier for water transfer to the solid matrix may explain the observed exponential behavior

The perception of complex textures in food is strongly related to the way food is processed during eating, and is modulated by other basic characteristics, such as taste and aroma. An understanding at the colloidal level of the basic processes in the mouth is essential in order to link the composition and structure of food products to more complicated texture attributes.

Very little is known on the rate dependency of the fracture behavior of crispy products as a function of water activity (Aw). Therefore, the effect of deformation speed on instrumental and sensory crispness was studied as a function of Aw. Deformation speed clearly affects the transition Aw range from crispy to non-crispy. At low deformation speed, the critical Aw was 0.40 ± 0.02 and at high deformation speed between 0.5 and 0.6. The transition was completed at an Aw = 0.56 ± 0.10 and 0.74 ± 0.00 for the low and high speed, respectively. The transition of sensory crispness at regular biting speed started at Aw = 0.48 and was completed at Aw = 0.75. The deformation speed effect is assumed to be caused by the viscoelastic nature of the material in the transition region causing it to behave locally relatively more viscous at low deformation speed and relatively more elastic at high deformation speed. This behavior depends on the plasticizer content, thus, Aw. For the product used in this study, the instrumental deformation speed that best correlate to sensory data is 10 to 40 mm/s. Sensory tests show that crispy attributes are perceived more intense when biting at higher speed

Establishing relationships between physical and sensorial properties of semi-solid foods is essential to develop tailored products. Whey protein/polysaccharide mixed gels were used to model both natural and fabricated semi-solid foods. The presence of various polysaccharides modulated the microstructure and large deformation properties of the mixed gels. The gels exhibited a large spectrum of sensorial properties as evaluated by panellists in a quantitative descriptive analysis. Mouthfeel attributes that discriminated best between the gels were wateriness, crumbliness, and spreadability. Wateriness strongly correlated with the amount of exuded phase (serum) measured during uniaxial compression. Serum release may have a positive effect on, for instance, the juiciness of a product. Large deformation measurements showed that highly crumbly gels fracture readily via a free-running crack. Low serum release is a requirement for that. Low crumbly gels fracture slowly, often releasing a high amount of serum. Spreadability related to the occurrence of multiple microcracks during deformation as observed by confocal laser scanning microscopy, which resulted in a large number of pieces after oral processing

A batter is provided comprising flour, water and optionally starch, wherein the batter comprises glutenin particles having a volume surface averaged particle size smaller than 10 [mu]m. A batter according to the invention comprises flour and optionally starch, and optionally baking powder and salt. At least 95 vol. % of all particles comprised in the batter have a particle size of 100 [mu]m or less. It appears that such a batter provides very crispy coatings to (deep-)fried products.

Background Betaine (trimethylglycine) lowers plasma homocysteine, a possible risk factor for cardiovascular disease. However, studies in renal patients and in obese individuals who are on a weight-loss diet suggest that betaine supplementation raises blood cholesterol; data in healthy individuals are lacking. Such an effect on cholesterol would counteract any favourable effect on homocysteine. We therefore investigated the effect of betaine, of its precursor choline in the form of phosphatidylcholine, and of the classical homocysteine-lowering vitamin folic acid on blood lipid concentrations in healthy humans. Methods and Findings We measured blood lipids in four placebo-controlled, randomised intervention studies that examined the effect of betaine (three studies, n = 151), folic acid (two studies, n = 75), and phosphatidylcholine (one study, n = 26) on plasma homocysteine concentrations. We combined blood lipid data from the individual studies and calculated a weighted mean change in blood lipid concentrations relative to placebo. Betaine supplementation (6 g/d) for 6 wk increased blood LDL cholesterol concentrations by 0.36 mmol/l (95% confidence interval: 0.25¿0.46), and triacylglycerol concentrations by 0.14 mmol/l (0.04¿0.23) relative to placebo. The ratio of total to HDL cholesterol increased by 0.23 (0.14¿0.32). Concentrations of HDL cholesterol were not affected. Doses of betaine lower than 6 g/d also raised LDL cholesterol, but these changes were not statistically significant. Further, the effect of betaine on LDL cholesterol was already evident after 2 wk of intervention. Phosphatidylcholine supplementation (providing approximately 2.6 g/d of choline) for 2 wk increased triacylglycerol concentrations by 0.14 mmol/l (0.06¿0.21), but did not affect cholesterol concentrations. Folic acid supplementation (0.8 mg/d) had no effect on lipid concentrations. Conclusions Betaine supplementation increased blood LDL cholesterol and triacylglycerol concentrations in h

Background: A high plasma concentration of total homocysteine (tHcy) is associated with increased risk of cardiovascular disease. A high protein intake and hence a high intake of methionine¿the sole dietary precursor of homocysteine¿may raise plasma tHcy concentrations. Objectives: We studied whether high intake of protein increases plasma concentrations of tHcy in the fasting state and throughout the day. Design: We conducted a randomized, dietary controlled, crossover trial in 20 healthy men aged 18¿44 y. For 8 d, men consumed a controlled low-protein diet enriched with either a protein supplement [high-protein diet (21% of energy as protein)] or an isocaloric amount of short-chain glucose polymers [low-protein diet (9% of energy as protein)]. After a 13-d washout period, treatments were reversed. On days 1 and 8 of each treatment period, blood was sampled before breakfast (fasting) and throughout the day. Results: Fasting tHcy concentrations did not differ significantly after the 1-wk high-protein and the 1-wk low-protein diets. The high-protein diet resulted in a significantly higher area under the 24-h homocysteine-by-time curves compared with the low-protein diet, both on day 1 (difference: 45.1 h · µmol/L; 95% CI: 35.3, 54.8 h · µmol/L; P <0.0001) and on day 8 (difference: 24.7 h · µmol/L; 95% CI: 15.0, 34.5 h · µmol/L; P <0.0001). Conclusions: A high-protein diet increases tHcy concentrations throughout the day but does not increase fasting tHcy concentrations. As previously shown, the extent of the tHcy increase is modified by the amino acid composition of the protein diet. The clinical relevance of this finding depends on whether high concentrations of tHcy¿particularly postprandially¿cause cardiovascular disease

Background: A high plasma total homocysteine (tHcy) concentration is a risk factor for cardiovascular disease. The increase in tHcy induced by methionine, the sole dietary precursor of homocysteine, might be modulated by other amino acids present in dietary proteins. Objectives: Our objectives were to compare the postprandial effect of free and dietary methionine on plasma tHcy concentrations and to investigate whether serine and cystine modify the effect of free methionine on tHcy. Design: We conducted a randomized crossover trial in 24 healthy men. Each subject ingested 4 meals on separate days, which were separated by 1 wk. tHcy concentrations were measured in the fasting state and at 2, 4, 6, 8, 10, and 24 h after meal ingestion. The meals were 1) a low-protein meal fortified with 30 mg methionine/kg body wt (reference, denoted by "Met"), 2) meal I additionally fortified with 60.6 mg serine/kg body wt (MetSer), 3) meal I additionally fortified with 12.3 mg cystine/kg body wt (MetCys), and 4) a protein-rich meal containing 30 mg methionine, 60.6 mg serine, and 12.3 mg cystine per kg body wt (Protein). Results: The mean (SD) fasting tHcy concentration was 9.1 +/- 2.7 mumol/L. Mean peak tHcy concentrations were 17.9 +/- 4.5, 14.3 +/- 3.3, 14.8 +/- 3.9, and 11.2 +/- 3.1 mumol/L after Met, MetSer, MetCys, and Protein, respectively. Compared with the mean 24-h area under the tHcy-by-time curve after Met, the mean curves after MetSer, MetCys, and Protein were 37%, 32%, and 77% smaller, respectively (all P <0.0005). Conclusions: Dietary methionine increases tHcy much less than does free methionine. Serine and cystine attenuate the tHcy-raising effect of free methionine. Thus, dietary proteins with a high content of serine or cystine relative to methionine may lead to lower postprandial tHcy responses.

Brownian Dynamics (BD) simulations have been performed to study structure and rheology of particle gels under large shear deformation. The model incorporates soft spherical particles, and reversible flexible bond formation. Two different methods of shear deformation are discussed, namely affine and non-affine deformation, the second being novel in simulation studies of gels. Also two dynamic descriptions of the model are presented, with and without inertia effects. Non-affine deformation resulted in a slower increase of the stress at small deformation than affine deformation. At large deformation both models gave similar stress responses, although the inertia model resulted in lower stresses. The particle gels, regardless of the model used, were observed to fracture into lumps that compactified due to local reorganization. A reversible yielding transition, as observed in polymer gels, was not found. Fractal properties of the gels were irreversibly lost at large deformation

Load-extension tests on flour dough are widely used by plant breeders, millers and bakers. The 'Kieffer dough and gluten extensibility rig' is a small-scale version of the Brabender extensograph, in which test pieces of about 0.4 g are extended. With the Kieffer rig, lower strain rates can be applied than in the Brabender extensograph and the experimental data can be expressed in terms of stress and strain. In this paper the performance of the Kieffer rig is illustrated by measurements on a weak and a strong dough. Formulas are given for the calculation of fundamental rheological parameters from the results of measurements with the Kieffer rig. Sagging and bending of the test pieces before measurements could be started, caused difficulties in the determination of the exact starting point of extension. The deformation was not purely uniaxial extension, because a shear component was also observed. The amount of dough that is extended did not increase throughout the test. This is probably due to the occurrence of a shear component fracture which occurred mainly near the hook. A relatively large variation in stress and strain at fracture was observed. The maximum in stress represents the strain at which the sample fractures macroscopically better than the maximum in force. Variation in deformation history and volume of the test pieces have a negative effect on the reproducibility.

Interfacial rheological properties and their suitability for foam production and stability of two vegetable proteins were studied and compared to ß-casein. Proteins used ranged from flexible to rigid/globular in the order of ß-casein, gliadin and soy glycinin. Experiments were performed at pH 6.7. Network forming properties were characterised by the surface dilational modulus (determined with the ring trough) and the critical falling film length (Lstill) at which a stagnant protein film will break. Gliadin had the highest dilational modulus, followed by glycinin and ß-casein, whereas glycinin formed the strongest film against fracture in the overflowing cylinder. The rate of decrease in the surface tension was studied at the air–water (Wilhelmy plate method) and the oil–water interface (bursting membrane) and the dynamic surface tension during compression and expansion in the caterpillar. Gliadin had the lowest equilibrium interfacial tensions and ß-casein the lowest dynamic surface tension during expansion. Hardly any foam could be formed at a concentration of 0.1 g/l by shaking. At a concentration of 1.4 g/l most foam was formed by ß-casein, followed by gliadin and glycinin. It seems that in the first place the rate of adsorption is important for foam formation. For the vegetable proteins, adsorption was slow. This resulted in lower foamability, especially for glycinin.

The use of enzymes such as peroxidases or glucose oxidase instead of chemical oxidants is a very interesting option for improving breadmaking performance of doughs. In this study the effect of such enzymes on bulk rheological properties of dough was quantified and their influence on the polymer network in dough deduced. Small deformation oscillation and relaxation tests (strain 0·001) are not suitable for discriminating between doughs prepared in the presence of the different enzymes. Flow relaxation tests at high deformation and long relaxation times showed a clear distinction between the effect of peroxidase and glucose oxidase. Peroxidase increases only the number or lifetime of transient bonds, whereas glucose oxidase additionally produced cross-links that were permanent on time scales up to 3 h. Peroxidase probably introduced a second, more transient structure (arabinoxylan network) through the gluten network, whereas glucose oxidase may also have strengthened the gluten network. A higher water addition could not compensate for the effect of peroxidase; on longer time scales the stress remained at a higher level. Similar results were obtained in large deformation biaxial and uniaxial extension tests. Peroxidases only increased stress levels. The addition of glucose oxidase resulted in a higher stress and more intense strain hardening. Only in biaxial extension was an influence of pH observed. An increase in stress level was accompanied by a decrease in fracture strain, making predictions of the effects on bread structure complicated. Author Keywords: dough rheology, relaxation, peroxidase, glucose oxidase

Background: A high plasma total homocysteine concentration is associated with increased risk of cardiovascular disease. Consumption of unfiltered or filtered coffee raises total homocysteine concentrations in healthy volunteers. The responsible compound, however, is unknown. Objective: The objective was to determine whether caffeine explains the homocysteine-raising effect of coffee. Design: Forty-eight subjects aged 19–65 y completed this randomized crossover study with 3 treatments, each lasting 2 wk. Subjects consumed 6 capsules providing 870 mg caffeine/d (test treatment), 0.9 L paper-filtered coffee providing 870 mg caffeine/d, or 6 placebo capsules. Blood samples were drawn fasting and 4 h after consumption of 0.45 L coffee or 3 capsules. Results: The mean fasting plasma homocysteine concentration after the placebo treatment was 9.6 ± 3.1 ?mol/L. The caffeine and coffee treatments increased fasting homocysteine by 0.4 ?mol/L (95% CI: 0.1, 0.7; P = 0.04), or 5%, and by 0.9 ?mol/L (95% CI: 0.6, 1.2; P = 0.0001), or 11%, respectively, compared with placebo. The increase in homocysteine concentrations 4 h after consumption of 0.45 L coffee relative to consumption of 3 placebo capsules was 19% (P = 0.0001). Caffeine treatment had a much weaker acute effect on homocysteine (4%; P = 0.09). Effects of caffeine were stronger in women than in men, but the effects of coffee did not differ significantly between men and women. Conclusions: Caffeine is partly responsible for the homocysteine-raising effect of coffee. Coffee, but not caffeine, affects homocysteine metabolism within hours after intake, although the effect is still substantial after an overnight fast. Key Words: Caffeine … paper-filtered coffee … homocysteine … B vitamins … crossover experiment

The mechanical properties of short doughs of various composition were determined in small amplitude oscillatory experiments and in uniaxial compression. Regardless of composition, the linear region was very limited; beyond that, pronounced yielding and flow occurred. Conductimetry was also used to evaluate the effect of fat type and fat content on dough structure. Short doughs showed large differences in mechanical spectra, conductivity and apparent biaxial extensional viscosity, according to fat and sucrose contents, fat type and mixing time. It is concluded that short doughs are bicontinuous systems; reducing the fat content or changing the rheological properties of the fat relative to those of the non-fat phase results in fat-dispersed systems. The rheological properties of the non-fat phase are largely determined by intact flour particles present in a concentrated sucrose syrup. Sucrose delays, if not inhibits, gluten development through its effect on solvent quality and facilitates formation of a non-fat continuous phase via its effect on solvent quantity. Mixing promotes formation of a continuous fat phase.Mechanical properties of short-dough biscuits of various composition were determined in three-point bending tests. Increasing fat content or omitting sucrose from the recipe decreased the modulus and the fracture stress of the biscuits. The effect of fat content, however, was dependent on fat type. Temperature during dough preparation, dough water content and temperature during bending tests affected the mechanical properties of biscuits to an extent which depended on fat content. Diffusion of Sudan III into the biscuits indicated that low-fat biscuits are fat-dispersed systems and high-fat biscuits are bicontinuous. Differential scanning calorimetry showed that, irrespective of composition, starch gelatinisation was slight, if not absent, presumably due to the limited water content coupled with the low baking temperature. Under certain storage conditions, b

The mechanical properties of concentrated starch + water systems were studied during heating, cooling and storage. Methods used were a small-amplitude dynamic rheological test and compression between parallel plates. The mechanical properties were related to the structure of the gels. Information about the structure of the gels was obtained by electron and light microscopy and DSC. Starches used were from wheat and potato.During heating of starch suspensions at rest, storage moduli first increased and subsequently decreased. This result is related to swelling of starch granules, melting of crystallites, separation of amylose and amylopectin, and loss of entanglements between starch molecules. Concentrated starch gels formed during heating at rest consist of partly swollen, irregularly shaped granules, which are tightly packed, with a thin amylose gel layer in between. The mechanical properties of these gels at large deformations are determined by the stiffness of the swollen granules, their shape and the mechanical properties of the thin amylose gel layer. Observed changes in Young modulus, and in the stress and strain at fracture during ageing are primarily ascribed to the increase in stiffness of the swollen granules. This increase is due to the formation of (semi) crystalline domains consisting of clusters of ordered double helices of short branches of amylopectin molecules.The mechanical properties of starch breads were measured in two successive compression/decompression cycles and the results were discussed by applying a theory developed for cellular solids. The mechanical properties of starch bread are determined by the mechanical properties and the dimensions of the condensed lamellae and beams, which have a structure comparable with that of concentrated starch gels, as well as the size and size distribution of the gas cells. GMS and SSL affect the mechanical properties of starch bread in two opposite ways; by affecting the properties of the lamellae and bea

Bread dough is a foam and the stability of the gas bubbles towards disproportionation and coalescence during the breadmaking process determines for a large part the final appearance of the baked bread with respect to crumb structure and loaf volume. Gas bubble behaviour in bread dough is determined by both surface and bulk rheological properties of dough (components). These properties were studied and their relevance to breadmaking was established.Surface dilational moduli of different dough components were determined. It appears that wheat lipids and added surfactants as SSL and DATEM can retard disproportionation to a large extent if these components are present in the right concentration and modification. Wheat proteins will hardly retard this foam instability mechanism.Dynamic measurements and biaxial extension tests on wheat and rye flour as well as wheat gluten doughs were performed. Both flour and gluten doughs show strain hardening at 20 and at 55°C. Wheat cultivars with good breadmaking performance exhibit stronger strain hardening properties than poor baking ones and rye. Next to strain hardening, biaxial stress and extensibility are important parameters determining bread quality.During mixing both the surface tension and the viscosity of the dough determine the (minimum) radii of the entrapped gas bubbles. Directly after mixing surface properties dominate primarily gas bubble behaviour by retarding disproportionation, especially if surfactants like SSL or DATEM are added. Already during first proof bulk properties, especially biaxial stress, extensibility and strain hardening, start to dominate gas bubble stability and this remains as such during almost the remainder of the breadmaking process. In the final stage of oven rise surface properties may contribute to the stability of some dough films that have become very thin.

The purpose of this study is to gain insight into the conditions determining whether small particles in a liquid are able to jointly occupy the total volume thus forming a gel network. In order to build a network the colloidal particles have to be 'sticky', unstable. In the unassociated state the particles move at random through the liquid due to collisions with solvent molecules. This movement, called thermal or Brownian motion or diffusion, depends on the temperature and on the size of the particles. By thermal motion particles may meet and subsequently stick, thus forming clusters or floes. This process is called perikinetic aggregation. Other transport mechanisms that lead to aggregation involve velocity gradients in the dispersion (orthokinetic aggregation) or sedimentation of the flocs.A model describing the formation of a gel out of aggregating floes is derived in chapter II. Floes that are formed by aggregation have in general a fractal geometry. This implies that repetitive levels of detail exist on all length scales between the size of the primary particles and the size of the floe. A floe is built up of smaller floes that, on their turn, are built of still smaller floes, etc. Each separate fractal floe has its own geometry, different from that of any other floe. However, all floes share a similar average structure characterized by a stochastic fractal nature, and are in this respect scale invariant. The efficiency with which floes fill the available space is expressed by the fractal dimensionality, D, which is the exponent in the power-law relation between the number of particles in a floe and the size of the floe. A low value of D implies a small number of particles needed to build up a floe of certain size, and thus a high spacefilling efficiency. In a three- dimensional system, D may attain values between 1 and 3. Computer simulations of the aggregation process yield D = 1.8 if all collisions lead to attachment (diffusion - limited cluster aggregation)

Previous studies have indicated that consumption of boiled coffee raises total and low density lipoprotein (LDL) cholesterol, whereas drip-filtered coffee does not. We have tested the effect on serum lipids of consumed coffee that was first boiled and then filtered through commercial paper coffee filters. Sixty-four healthy volunteers consumed six cups per day of this boiled and filtered coffee for 17 days. Then, they were randomly divided into three groups, which, for the next 79 days, received either unfiltered boiled coffee (lipid content, 1.0 g/l), boiled and filtered coffee (0.02 g lipid/l), or no coffee. Serum total cholesterol levels rose by 0.42 mmol/l (16 mg/dl; 95% confidence interval [CI], 0.14-0.71), LDL cholesterol levels by 0.41 mmol/l (16 mg/dl; 95% CI, 0.16-0.66), and apolipoprotein B levels by 8.6 mg/dl (95% CI, 3.8-13.4) in those who consumed boiled coffee relative to those who consumed boiled and filtered coffee. Responses of triglycerides, high density lipoprotein cholesterol, and apolipoprotein A-I did not differ significantly among these groups. No significant effects on serum lipid levels were found in the boiled and filtered coffee-consuming group compared with those who drank no coffee. In subjects who drank boiled coffee, serum campesterol level, an indicator of cholesterol absorption, remained constant. The serum lathosterol level, an indicator of cholesterol synthesis, increased by 11% (p less than 0.05), but the lathosterol to cholesterol ratio did not change. We propose that paper filters of the type used for drip-filtered coffee retain the lipid present in boiled coffee and in that way remove the hypercholesterolemic factor.

The aim of this study was to gain more insight into the factors, determining the inter- and intramolecular interactions between adsorbed macromolecules. To that end several experimental and theoretical approaches were followed, using well-defined systems. It was shown that these interactions could conveniently be studied by measurements on emulsions and thin free liquid films. Two different macromolecules were used: a nonionic one: polyvinyl alcohol (PVA) and an ionic one: a copolymer of methacrylic acid and the methyl ester of methacrylic acid (PMA-pe) in the molar ratio 2:1.The characterization of the used materials has been described in chapter 2. The conformational transition, occurring in dissolved polymethacrylates was briefly discussed. At low pH, the molecules occur in a compact form, the hypercoiled form, or a-conformation. At pH above ~ 6 the molecules occur in the common. more extended b-conformation. From viscometry on PVA solutions conformational parameters, such as the root mean square end-to-end distance, the length of a statistical chain element and the linear expansion factor were determined. These conformational parameters were determined in a I M aqueous glycerol solution because in the film experiments 1 M glycerol was present in the PVA solutions in order to lower the water vapour pressure.In chapter 3 the experimental methods have been described. In the first part attention was paid to the preparation of the emulsions and to the determination of basic properties, such as specific area and adsorbed amount. A variety of rheological measurements were described in the second part. A more detailed description was given of the apparatus for the dynamic measurements (the rheometer) and of that for creep measurements.The end of chapter 3 deals with the thickness measurements of polymerstabilized free liquid films. First, a description of the apparatus and the experimental procedure was given. Subsequently, a discussion followed of the calculation of th

The rheological properties of rennet-induced skim milk gels, which are viscoelastic materials, were studied under various conditions.Dynamic and stress relaxation experiments were performed at small deformations of the gel network, whereas constant stress (creep) experiments were performed at large deformations. Stress relaxation moduli calculated from the dynamic moduli agreed fairly well with stress relaxation moduli determined by means of stress relaxation experiments, implying that true material properties were determined.The effects of important variables, such as casein concentration, temperature, pH, calcium, phosphorus and ionic strength on the mechanical properties of the gels were studied. The results are discussed in relation to the types of bond present in the network. Special attention was paid to the time scale at which processes, especially relaxation of bonds, occurred., Om meer inzicht te krijgen in de factoren die van belang zijn voor de structuur en de mechanische eigenschappen van met leb gestremde ondermelkgelen werd het reologisch gedrag van deze gelen bij verschillende omstandigheden onderzocht. Tevens werd nagegaan in hoeverre bepaalde relaties, die voor visco-elastische polymeren zijn afgeleid, ook toepasbaar zijn op melkgelen. 3 soorten reologische meetmethoden werden gebruikt: dynamische metingen met een "Den Otter"-reometer, spanningsrelaxatie-metingen met een Weissenberg-reogoniometer, en constante-spanningsmetingen (bij grote vervorming) met een Deer-reometer

The structure of acid casein gels was studied by rheology, permeametry, electron microscopy and pulse NMR. Gels were mainly formed by heating at rest of casein solutions acidified in the cold (0- 2 °C). The structure of such gels involves both the spatial distribution of the structural elements and the interaction forces between them. An acid casein gel has an irregular particulate structure. Gelation results from the aggregation of casein particles, which themselves have a complex internal structure due to the association of numerous different casein molecules. Dense areas of coagulated particles are separated by large pores. Gelation at pH=4.6 appears to be subject to an activation Helmholtz energy, which is largely due to the stabilizing effect of the glycomacropeptide part of K-casein. At ageing temperatures above 283 K, the dynamic moduli G' and G'' linearly increase with the logarithm of time. Their absolute values primarily depend on the heterogeneity of the network. The moduli decrease with increasing time scale of measurement. From the effect of variation of experimental conditions (pH, ageing and measuring temperatures, ionic strength and ionic composition) it may be concluded that electrostatic interactions and hydrophobic bonding are involved in keeping the network together. Moreover, Van der Waals attraction, steric interactions and possibly hydrogen bonds may play a part. Differences in the internal structure of the casein particles appear to be paramount in determining differences in the number and character of the interparticle bonds; this was in agreement with results on the spin-spin relaxation time of water protons. Addition of rennet enables gel formation in a much broader range of temperature and pH; the character of the gels changes considerably around pH=5.2.

The rheological and fracture behaviour of Gouda cheese was studied. Methods for determining these properties of visco-elastic materials are described. Application of the theory of fracture mechanics, after modification and expansion, to visco-elastic materials with a low or no yield stress is discussed. For such materials, of which Gouda cheese is an example, the flow properties greatly affect the fracture behaviour.From the effect of variation in composition (fat, water, NaCl and Ca content, pH) and maturation on the behaviour of Gouda cheese, it may be concluded that this cheese may be considered as a composite material. Fat particles act as a filler in a swollen protein matrix. The amount of fat and the rigidity of the fat particles affect the rigidity of the cheese. Factors like pH, water and NaCl content, that change the properties of the protein matrix, clearly affect the -rheological and fracture behaviour, e.g. the rigidity and the shortness, of the cheese. The trends of these changes on the protein matrix are similar to those on rennet and acid skimmilk gels under various conditions .As an example of the importance of the experimental results for cheese manufacturing, the relation between the pH of cheese and eye or slit formation was studied., Een studie naar de vervormings- en breukeigenschappen van kaas. Compressie-, rek-, en buigproeven werden toegepast om reologische en breukeigenschappen bij grote vervormingen te bepalen, en tevens werd de invloed van de samenstelling op deze eigenschappen van Goudse kaas beschreven