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In this paper, we investigate whether the rheology of plant proteins is following similar scaling rules, as recently found for maltodextrins. The aim is to construct master curves of the rheology measurements, such that in combination with the scaling rules, the rheology can also be extrapolated to relevant practical conditions, that are not accessible with rheometers. We have analysed 4 different plant proteins, which are currently used in the manufacturing of meat analogs, on which we have performed frequency and amplitude sweeps for a wide range of temperatures and moisture contents. The frequency sweeps show that for all investigated temperatures and moisture contents the protein doughs show elastic behaviour for small strains and low frequencies, which maps on to a single master curve. Yet, the linear elastic modulus is governed by the ratio of the (moisture-dependent) glass transition temperature and the actual processing temperature (Tg/T). Furthermore, we have constructed master curves for the strain sweeps, characterized by a critical strain that is independent of moisture content and temperature. Beyond the critical strain, the protein dough is showing strain thinning behaviour. Also with the use of literature data, we have found for pea a similar scaling exponent for shear thinning, strain thinning, and the moduli in the transition zone. This seems to confirm that plant protein doughs adhere to the Cox-Merz and Rutgers-Delaware rule, allowing translation to practical steady flow conditions as occurring during manufacturing.

In this paper, we investigate the functionality of potato-based ingredients present in indirectly expanded snacks via careful analysis of their transformation during processing. This research is driven by the desire of industry to develop similar snacks for upcoming markets, where the potato-based ingredients are replaced by other starch sources, which are locally available and at a lower cost. For a range of reformulated snacks, the transformations of starchy ingredients are analysed with a wide variety of experimental methods, like DSC, XRD, and XRT. Our analysis shows that ingredients undergo little transformations during extrusion, which is indeed intended to be mild. During frying native tuber starches (potato and tapioca starch) fully gelatinize, while cereal starches show little gelatinization and swelling. Despite the gelatinization of tuber starches, the particulate character of ingredients is retained. Replacement of pregelatinized potato starch with other starches shows little change in structure. The evolution of the structure of the reformulated snacks are analysed with the CDS formalism. We conclude that gel formers and hard fillers present in the analysed formulations had little functionality regarding texture or structure. For texture, it appears to be required that the matrix composes of a bicontinuous structure of soft fillers, namely gelatinized tuber starches and potato dehydrates. Both these ingredients can be replaced by other tuber-starch sources if the aggregation of the two soft fillers can be prevented. Commercial availability of tuber flours can still be an issue.

International audience; Using literature data we have studied the rheological behaviour of concentrated maltodextrins and starches. We show that much of their rheology, like zero shear viscosity and shear thinning behaviour, appears to be governed by the ratio of the glass transition temperature and actual temperature, T-g/T, as the scaling parameter. Via this scaling, we can apply time-temperature-solvent superposition principle, which is also validated for linear dynamic viscoelastic experiments at different temperatures, compositions, and moisture content. Furthermore, we show that the dynamic viscoelastic experiments follow the Marin-Graessley model, indicating that concentrated maltodextrins and starches behave as transient (entangled) networks.

We have studied the rheological behaviour of concentrated maltodextrins varying in dextrose equivalence value (DE) over a range of temperatures (≤70 °C) and concentrations (≥62% (w/w) solids) using small amplitude oscillatory experiments. Rheological data could be mapped onto master curves for storage and loss modulus as described with a model for transient or entangled polymeric networks. Maltodextrins DE5 and DE12 showed a rubber-to-glass transition, where the rubber-like behaviour is hypothesized to be the consequence of physical entanglements at sufficiently high concentrations of polysaccharides. Maltodextrins DE21 and DE38 mostly consisting of small sugars and oligomers exhibited viscous behaviour over a large range of frequencies and only at high frequencies the onset of a cross-over was visible which could signify the vicinity of the glass transition. An effective diffusion drying model validated with sessile single droplet drying experiments was coupled to rheological models to estimate rheological behaviour during droplet drying. The predicted rheological properties of the skin formed at the surface of the droplet could be related to the occurrence of surface instabilities later during drying, including cavitation for elastic skins and wrinkling for viscous skins. Insight in rheological behaviour at high concentrations may therefore offer help in predicting the morphology of droplets generated with spray drying.

In this paper I review the production of frozen vegetables and fruits from a chain perspective. I argue that the final quality of the frozen product still can be improved via (a) optimization of the complete existing production chain towards quality, and/or (b) introduction of some promising novel processing technology. For this optimization, knowledge is required how all processing steps impact the final quality. Hence, first I review physicochemical and biochemical processes underlying the final quality, such as water holding capacity, ice crystal growth and mechanical damage. Subsequently, I review how each individual processing step impacts the final quality via these fundamental physicochemical and biochemical processes. In this review of processing steps, I also review the potential of novel processing technologies. The results of our literature review are summarized via a causal network, linking processing steps, fundamental physicochemical and biochemical processes, and their correlation with final product quality. I conclude that there is room for optimization of the current production chains via matching processing times with time scales of the fundamental physicochemical and biochemical processes. Regarding novel processing technology, it is concluded in general that they are difficult to implement in the context of existing production chains. I do see the potential for novel processing technology combined with process intensification, incorporating the blanching pretreatment—but which involves quite a change of the production chain.

In this paper, we present the rich physics and chemistry of the gastric digestion of protein gels. Knowledge of this matter is important for the development of sustainable protein foods that are based on novel proteins sources like plant proteins or insects. Their digestibility is an important question in the design of these new protein foods. As polyelectrolyte gels, they can undergo volume changes upon shifts in pH or ionic strengths, as protein gels experience when entering the gastric environment. We show that these volume changes can be modelled using the Flory-Rehner theory, combined with Gibbs-Donnan theory accounting for the distribution of electrolytes over gel and bath. In-vitro experiments of soy protein gels in simulated gastric fluid indeed show intricate swelling behaviour, at first the gels show swelling but at longer times they shrink again. Simulations performed with the Flory-Rehner/Gibbs-Donnan theory reproduce qualitatively similar behaviour. In the final part of the paper, we discuss how the model must be extended to model realistic conditions existing in the in-vivo gastric environment., Highlights • Physical-chemistry of gastric digestion of protein gels is presented. • A numerical model based on Flory-Rehner can qualitatively explain swelling of gels. • Protein gels in gastric environments can swell and shrink when put in gastric acid. • For ion transport it is important to recognize the finite volume of the gastric juice.

In this paper we present a sugar replacements strategy, derived from physical theory, which we think is applicable for a wide range of food categories. The strategy is based on the hypothesis that reformulated foods must mimic the values of two physical characteristic numbers, related to the plasticizing and hygroscopic properties of sugar, to reproduce the texture of a sugar-rich product. We will show the validity of the strategy for reformulated biscuits, using experimental determination of physical properties of dough and baked biscuits, as well as sensorial evaluation by a trained panel. Our investigations shown that the majority of these physical and sensorial attributes correlate with the two physical characteristic numbers. Furthermore, multiple reformulations can be defined which are close to the reference product (in terms of the two physical characteristic numbers), which are indeed scored similar by the trained sensory panel. Hence, our strategy also leaves room for further optimization of the reformulated food towards dietary fiber content, laxative properties or sweetness.

In this paper we show that the shift of the gelatinization temperature of starch in sugar and polyol solutions is explained by n O H , e f f , the volumetric density of hydrogen bonds in the solutions. n O H , e f f is computed using the dry glass transition temperatures of the low molecular weight carbohydrates. This correlation of starch gelatinization temperature to n O H , e f f is shown for 19 different sugar and polyol compounds in solutions at different concentrations, as measured in an earlier study by Allan et al. (2018). The earlier study found that the measured viscosity of the solutions best correlated to starch gelatinization temperature, but it was assumed that there is a more fundamental property of the sweetener that alters both the viscosity and the starch gelatnization behaviour. Here, it is shown that n O H , e f f is this fundamental property responsible for controlling both the viscosity and starch gelatinization temperature differences in the used sugar and polyol solutions. Because n O H , e f f is also related to T g , the glass transition temperature of the carbohydrate solutions, the viscosity of a wide variety of carbohydrate solutions can be mapped to a single master curve if plotted against T g / T , the ratio of glass transition and the actual temperature. Older hypotheses concerning the shift of the starch gelatinization temperature in carbohydrate solutions have explained it in terms of water activity. However, we show that n O H , e f f relates to water activity only for carbohydrates with similar molecular weights. We conclude that sugar and polyol solutions can be viewed effectively as a single solvent, which is characterized by n O H , e f f . This measure for volumetric density of hydrogen bonds in these solutions can be used to predict the starch gelatinization temperature in different formulations.

We review the functionality of sucrose during the manufacture of biscuits from the perspective of sugar replacement. Besides to providing sweetness, sucrose has important functionalities concerning structure and texture formation. These functionalities also need to be mimicked in reformulated biscuits. First, we review the hypotheses concerning the development of structure and texture of biscuits during manufacturing, which are conveniently summarized in a qualitative way using the Complex Dispersed Systems methodology. Subsequently, we represent the changes of the state of the biscuit during manufacturing in the supplemented state diagram, which indicates the important phase transitions occurring during mixing and baking. We propose that when reformulated biscuits follow similar paths in the state diagram, similar structures and textures can be obtained. Physical theories exist for predicting these phase transitions for existing sucrose-rich biscuits and also reformulated biscuits containing extensive sweeteners as sugar replacers. More accurate predictions of structure and texture can be eventually obtained if they are combined with computational models, including heat and moisture transfer.

The aim of study is to improve the rehydration quality of dried shiitake mushrooms for their instant food manufacturers and fast restaurants. Serial combined drying methods were investigated to achieve this objective: either instant controlled pressure drop drying (DIC) or freeze drying (FD) was used as the treatments for microstructure improvement, and they were applied either before or after an additional drying step at low (35 °C) or high (65 °C) temperatures. Dried mushrooms were assessed for quality indicators like relative volume, rehydration rate, dry matter loss and sensory scores. Microstructure properties were inferred to understand the physical mechanisms of quality changes. Principal component analysis (PCA) was used to cluster treatments and to identify combinations of drying techniques, rendering improved quality. Consequently, it was shown that DIC treatment before hot air drying at 35 °C was shown to be the most promising combined drying method to enhance the rehydration quality, leading to a high volume recovery ratio, low dry matter loss after rehydration, and high rehydration rates. This good performance could be explained by the retention of pore interconnectivity resulting from the slight expansion of porous structure during DIC and the retention of cell membrane integrity.

In this paper, we review the problem of clumping due to frost formation on frozen vegetables, like par-fried potato products. This problem has been very scarcely investigated in the scientific literature. Yet in the industry it is a significant problem, as evident by the various patents on this topic. Thanks to the enormous, recent growth of scientific literature on frost formation on engineered, structured surfaces, we have drawn a multitude of hypotheses of factors governing the clumping and frost formation of frozen foods, which can also be viewed as a structured surface.

In this paper we investigate whether the Flory-Huggins theory can describe the thermodynamics of solutions of simple carbohydrates, like sugars and polyols. In particular, we focus on the description of the solubility of the carbohydrates in water. This is investigated for both binary and ternary mixtures, having two types of these carbohydrates. This research question arises especially in the case of bakery products, where one seeks to replace sucrose with other simple carbohydrates - which are often polyols. Based on the model parameters obtained from fitting the theory to the experimental data of binary solutions, we show that the theory can predict (a) solubility data for ternary mixtures, over a broad range of concentrations and temperatures, and (b) the deliquescence point of binary mixtures of carbohydrate crystals as a function of temperature. The theory can even be applied to carbohydrates, which form hydrate crystals. Together with our earlier theories on the thermodynamics of complex food mixtures, we have now a complete thermodynamic framework to describe the phase and state transitions of food materials as confectionery and bakery products, where the question of sucrose replacement is urgent.

In this paper we perform direct numerical simulation (DNS) on the problem of fluid flow through a filter plate backed by a packed bed of spheres, resembling a cake layer on top of a membrane. For both the complete problem, and its single components (the filter plate and a bed of spheres of finite height) we have observed three flow regimes, depending on the Reynolds number. In each regime the flow resistance is showing a different scaling with the Reynolds number. In the Stokes flow regime the total flow resistance can be decomposed in linear independent components. The interior flows inside the filter holes and inside the packed bed follow the same correlations as hold for the single component. However, at the transition zone between filter plate and packed bed, there is a diverging flow in the first row of the packed bed, whose contribution in the flow resistance scales with the fractional hole to the power −1.5. Similar scaling exponent has been found earlier for the viscous-inertial regime with Reynolds numbers larger than 10, which has been modelled using the porous medium approach. Our findings suggest that also in the Stokes flow and the weakly flow regime the problem can also be solved with the same porous medium approach using the Navier-Stokes equation having Darcy–Brinkman terms incorporated. This investigation provides a good basis for developing more accurate analytical models for the flow resistance of membrane filters with a cake layer on top.

In this paper, we investigate whether the Flory-Huggins-Free-Volume (FHFV) theory can describe the rich thermodynamics of the ternary mixtures of starch, polyol, and water. These systems exhibit 1) non-monotonic moisture sorption with increasing plasticizer concentration, 2) phase separation, and 3) antiplasticization. After extending the FHFV theory with 1) the proper formulation of the chemical potential of water and polyol, and 2) the proper composition dependency of the interaction parameter between starch and water, the theory is well able to describe the above described complex thermodynamic behavior, showing good agreement with experimental data. Furthermore, our analysis shows that phase separation can already occur when the ternary mixture is still in the glassy state. Overall, the phase separation happens after the antiplasticization/plasticization transition, which can be linked to the minimum in moisture sorption, when increasing the polyol concentration at equal water activity. We think that the extended theory will become an important tool for analysis and design of complex food materials, pharmaceutical systems, and biopolymeric films having carbohydrates as plasticizers.

The Flory-Huggins-Free-Volume (FHFV) theory, describing the moisture sorption of carbohydrates, is extended towards a wider range of compounds. Earlier application of the FHFV theory has been to carbohydrate/water mixtures, as can be found in foods. Now, we have extended the theory towards polyols (or polyalcohols) which are investigated in as proxies for secondary organic aerosols, whose behaviour is important for understanding climate behaviour. The investigated polyols are characterized by the ratio of the number hydroxyl groups N O H and the number of carbon atoms N C , which is often lower than ratios found in carbohydrates in food materials. We have found that the value of the Flory-Huggins interaction parameter is a function of the solute molecular properties, namely its chain length N C and the ratio N O H / N C of the solute. The deviation of this ratio from 1 2 can be viewed as a measure for its hydrophilicity. For food science, the extension of the theory has also significant implications, as the interaction parameter of newly investigated ingredients (rich in hydroxyl groups) can be estimated by means of the molecular properties. One must think of insoluble food fibers like xylan-glucans or arabinoxylans, or modified biopolymers based on starch or cellulose.

Understanding moisture diffusivity behaviour over a wide range of moisture contents is pivotal for optimising drying operations. Generally, data on moisture diffusivity are scarce and the effect of matrix composition on moisture diffusivity at relevant temperature for drying processes is not yet well described. In this paper moisture diffusivity in protein-carbohydrate films is systematically investigated for a wide range of moisture contents at 80 °C. Diffusion data are obtained from controlled thin film drying experiments following the regular regime method and compared to theoretical models. Moisture diffusivity for binary maltodextrin-water and whey protein-water systems appeared similar and were reasonably well described with the Darken relation. Diffusivity was lower for casein-water systems at moisture contents above 0.15 kg water/kg, which may be explained by compartmentalization of water in the casein micelles. At low moisture contents all binary systems showed universal behaviour, which may be explained by random coil behaviour leading to similar water-molecule interactions. This behaviour could be well described by free-volume theory. In mixed systems of proteins and carbohydrates moisture diffusivity appeared strongly influenced by the presence of casein, probably due to their high voluminosity. Finally, it was surprisingly observed that diffusivity in multicomponent systems decreased sharply at lower water contents when compared to binary systems. This might be explained by a denser molecular packed system in the dry regime for multicomponent systems or water trapping by protein-carbohydrate complexes.

In this paper, we investigate the potential of zwitterions like amino acids as alternative (bulking) sugar replacers. Commonly, polyols or oligosaccharides are taken as sugar placers, but they are perceived as less attractive due to laxative problems. The choice of amino acids as alternative sugar replacers is inspired by their presence in Natural Deep Eutectic Solvents (NADES) and compatible solutes, next to carbohydrates. In these compounds, hydrogen bonding plays an essential role similar to sugar (replacers) in food. Because of the novelty of the topic, we first discuss at length NADES as a potential source of alternative plasticizers/sugar replacers. Subsequently, we perform a detailed analysis of their thermodynamic properties, to investigate whether amino acids and related zwitterions indeed have the appropriate properties for sugar replacement. Our analysis shows that several zwitterions indeed have proper thermodynamic properties, which can make them good plasticizers and humectants. However, the use of several zwitterions can be restricted by their poor solubility, except for L-proline. Solubility problems can be circumvented by their use in mixtures of several sugar replacers, which enhances their solubility. The poor solubility makes it also difficult to characterize the glass transition temperatures of the pure solute T g , g . Consequently, we have estimated them via extrapolation of the viscosity curves, which is shown to scale with T g ∕ T . We have taken T g of the plasticizer solution as a measure of the hydrogen bond density, n O H , e f f . That measure is shown to explain the melting behaviour of biopolymers like egg white and starch in glycine and L-proline solutions, similar to sugar (replacers). However, our measurements of the biopolymer melting show indications of phase separation at high moisture contents for both amino acid solutions as for carbohydrate solutions. Overall, the amino acids and other compatible zwitterions have very similar thermodynamic behaviour as sugars and their carbohydrate-based replacers. In proper combinations with alternative plasticizers, the zwitterionic compatible solutes can well be used in sugar replacement strategies for foods.

Particle morphology development during spray drying is critical to powder properties. The aim of this study was to investigate whether the dextrose equivalence (DE) of maltodextrins can be used as an indicator for the final particle morphology. Maltodextrins were characterized on glass transition temperature (Tg) and viscosity, where low DE-value maltodextrins exhibited higher Tg and viscosity than high DE maltodextrins (≥21). A new custom-built sessile single droplet dryer was used to analyse morphology development of minute maltodextrin droplets (R0 ~ 100 μm) at 60 °C and 90 °C. Droplets with low DE showed early skin formation (2–5 s) and developed smoothly shaped particles with large cavities. Rheology on low DE maltodextrin films at dry matter of 82% (w/w) suggested that drying droplets acquired elasticity after locking providing resistance against surface compression. After locking morphology development is probably halted as the glassy state is approached. On the contrary, rheology on high DE maltodextrin (≥21) films at dry matter of 93% (w/w) suggested that drying droplets with high DE developed viscous skins, which are susceptible to surface deformations, leading to wrinkling, folding or creasing particle morphologies. The results demonstrated that DE-value may be used as an indicator for particle morphology development when interpreted in view of the process conditions.

A first principles microfiltration model based on shear-induced diffusion is compared to experiments performed on the clarification of beer. After performing an identifiability and sensitivity analysis, the model parameters are estimated using global minimization of the sum of least squares. The model is compared to different series of experiments, where either crossflow or permeate flux is varied. This study is concluded with a parameter study on the scaling of the filtration time with various model parameters. We have found that the filtration time primarily depends on two dimensionless numbers, namely the normalized critical distance for cake layer formation, and the dimensionless time required to plug all pores in the selective layer. We have found that there is an optimal setting of these parameters, rendering a maximal amount of filtrated beer in one cycle.

In this paper we present hyperelastic models for swelling elastic shells, due to pressurization of the internal cavity. These shells serve as model systems for cells having cell walls, as can be found in bacteria, plants and fungi. The pressurized internal cavity represents the cell vacuole with intact membrane at a certain turgor pressure, and the elastic shell represents the hydrated cell wall. At pressurization the elastic shell undergoes inhomogeneous deformation. Its deformation is governed by a strain energy function. Using the scaling law of Cloizeaux for the osmotic pressure, we obtain approximate analytical expressions of the cell volume versus turgor pressure - which are quite comparable to numerical solutions of the problem. Subsequently, we have simulated the swelling of shells - where the cell wall material is embedded with microfibrils, leading to strain hardening and anisotropic cell expansion. The purpose of our investigations is to elucidate the contribution of cell membrane integrity and turgor to the water holding capacity (hydration) of plant foods. We conclude with a discussion of the impact of this work on the hydration of food material, and other fields like plant science and the soft matter physics of responsive gels.

In this work, the Flory Huggins Free Volume theory is used to interpret the sorption isotherms of broccoli from its composition and using physical properties of the components. This theory considers the mixing properties of water, biopolymers and solutes and has the potential to describe the sorption isotherms for varying product moisture content, composition and temperature. The required physical properties of the pure components in food became available in recent years and allow now the prediction of the sorption isotherms with this theory. Sorption isotherm experiments have been performed for broccoli florets and stalks, at two temperatures. Experimental data shows that the Flory Huggins Free Volume (FHFV) theory represents the sorption isotherm of fresh and blanched broccoli samples accurately. The results also show that blanching affects the sorption isotherm due to the change of composition via leaching solutes and the change of interaction parameter due to protein denaturation. © 2013 Springer Science+Business Media New York.

Models of fouling during beer clarification, previously validated for dead-end filtration, are combined with a model describing the build-up of the cake layer of yeast cells via shear-induced diffusion induced by crossflow. For describing shear-induced diffusion, we have taken the most recent model, which is based on the effective temperature concept. Via scale analysis we show that (1) effects of polydispersity are negligible and (2) the cake layer growth model still be reduced to the earlier model of Romero and Davis, albeit with different parameter values. The adapted Romero and Davis model has been coupled to the earlier reviewed models of fouling by aggregates and macromolecules, for which we have performed computer simulations. In this paper we report on striking results on the downstream movement of the critical point, xcr, where the cake layer starts building up. We show that this is due to strong coupling between the cake layer build-up and the other fouling modes. Hence, this means that the concept of critical flux can not be viewed any more as a time-invariant value. This finding has implications beyond the application of beer, and especially for biotechnological broths which have similar composition of beer.

We show that the glass transition of a multitude of mixtures containing hydrogen bonding materials correlates strongly with the effective number of hydroxyl groups per molecule, which are available for intermolecular hydrogen bonding. This correlation is in compliance with the topological constraint theory, wherein the intermolecular hydrogen bonds constrain the mobility of the hydrogen bonded network. The finding that the glass transition relates to hydrogen bonding rather than free volume agrees with our recent finding that there is little difference in free volume among carbohydrates and polysaccharides. For binary and ternary mixtures of sugars, polyols, or biopolymers with water, our correlation states that the glass transition temperature is linear with the inverse of the number of effective hydroxyl groups per molecule. Only for dry biopolymer/sugar or sugar/polyol mixtures do we find deviations due to nonideal mixing, imposed by microheterogeneity.

We review the functionality of particulate ingredients in edible brittle foams, such as expanded starchy snacks. In food science and industry there is not a complete awareness of the full functionality of these filler ingredients, which can be fibers, proteins, starch granules and whole grains. But, we show that much can be learned about that from the field of synthetic polymeric foams with (nano)fillers. For edible brittle foams the enhancement of mechanical strength by filler ingredients is less relevant compared to the additional functionalities such as 1) the promotion of bubble nucleation and 2) cell opening - which are much more relevant for the snack texture. The survey of particulate ingredients added to snack formulations shows that they cannot be viewed as inert fillers, because of their strong hygroscopic properties. Hence, these fillers will compete with starch for water, and that will modify the glass transition and boiling point, which are important factors for snack expansion. Filler properties can be modified via extrusion, but it is better if that processing step is decoupled from the subsequent processing steps as mixing and expansion. Several filler ingredients are also added because of their nutritional value, but can have adverse effect on snack expansion. These adverse effects can be reduced if the increase of nutritional value is decoupled from other filler functionality via compartmentalization using micropellets.

We present the first model ever, that describes explicitly ice crystal growth in a sugar solution during freezing. This 2-D model uses the phase field method, supplemented with realistic, and predictive theories on the thermodynamics and (diffusion) kinetics of this food system. We have to make use of a novel type of phase field to obtain realistic, micron-sized ice crystals, and exclusion of sugar from the crystalline phase. Via simulation of a single ice crystal, we identify important time scales governing the growth. These times scales are also important for the coarsening of the ice morphology in freezing systems with multiple ice crystals. These simulations show that the average ice crystal size is governed by the freezing rate via a power law, similar to an empirical relation from literatures, which is deduced from experiment. The presented model is viewed as a good basis for even more realistic simulations of crystal growth in food.

Microfluidic devices are an emerging technology for processing suspensions in e.g. medical applications, pharmaceutics and food. Compared to larger scales, particles will be more influenced by migration in microfluidic devices, and this may even be used to facilitate segregation and separation. In order to get most out of these completely new technologies, methods to experimentally measure (or compute) particle migration are needed to gain sufficient insights for rational design. However, the currently available methods only allow limited access to particle behaviour. In this review we compare experimental methods to investigate migration phenomena that can occur in microfluidic systems when operated with natural suspensions, having typical particle diameters of 0.1 to 10 µm. The methods are used to monitor concentration and velocity profiles of bidisperse and polydisperse suspensions, which are notoriously difficult to measure due to the small dimensions of channels and particles. Various methods have been proposed in literature: tomography, ultrasound, and optical analysis, and here we review and evaluate them on general dimensionless numbers related to process conditions and channel dimensions. Besides, eleven practical criteria chosen such that they can also be used for various applications, are used to evaluate the performance of the methods. We found that NMR and CSLM, although expensive, are the most promising techniques to investigate flowing suspensions in microfluidic devices, where one may be preferred over the other depending on the size, concentration and nature of the suspension, the dimensions of the channel, and the information that has to be obtained. The paper concludes with an outlook on future developments of measurement techniques.

The internal moisture distribution that arise in food products during drying, is a key factor for the retention of quality attributes. To reveal the course of moisture content in a product, internal moisture profiles in broccoli florets are measured by MRI imaging during drying experiments with controlled air flow and temperature. The 3D images concern a matrix size of 64×64×64 elements. Signal intensity is converted to product moisture content with a linear relationship, while taking a minimum detectable moisture content of 0.3 kg water/ kg dry matter into account. Moisture content as a function of time is presented for a 2D cross sectional area in the middle of a broccoli sample. The average moisture contents for the cross sectional area obtained from the MRI imaging are compared with spatial model simulations for the moisture distribution. In that model the effective diffusion coefficient is based on the Free Volume Theory. This theory has the advantage that the changed mobility of water in the product during drying is taken into account and the theory also predicts the moisture transport in the porous broccoli floret. Key parameters for the Free Volume Theory are estimated by fitting to the experimental MRI results and the effective diffusion coefficient is given as a function of the product water content. Keywords: diffusion properties; MRI; convective drying; moisture profiles

Moisture diffusion in porous broccoli florets and stalks is modeled using the free volume and Maxwell-Eucken theories. These theories are based on the mobility of water and concern the variation of the effective diffusion coefficient for a wide range of temperature and moisture content during product drying. Mass and heat transport, shrinkage, and vitamin C degradation during drying of broccoli are simulated by a spatial model. The effective diffusion coefficient varies strongly with product moisture content and temperature. Vitamin C degradation is high at moisture contents around 2 kg water/kg dry matter. The influence of the size of broccoli on the drying rate is evaluated for several types of broccoli florets and stalks.

Via Lattice Boltzmann simulations we show there is a great deal of universality in the reduced mobility of spheres moving along the centre line of capillaries or rectangular microchannels. The reduced mobility follows a generalization of the Haberman-Sayre correlation, which is a function of the degree of confinement, e. This quantity e=d(p)/D(e), is defined as the ratio of the particle diameter and the length scale D(e), which is a weighted mean of the square root of the cross section area D(A), and the hydraulic diameter D(h). The force experienced by spheres in pressure driven flow can be described by a generalized Faxen theorem, combined with effective medium theory. This force can be decomposed in a drag force and a pressure gradient force, both inversely proportional to the reduced mobility.

Deterministic lateral displacement (DLD) ratchets are microfluidic devices, which are used for size-based sorting of cells or DNA. Based on their size, particles are showing different kinds of motion, leading to their fractionation. In earlier studies, so-called zigzag and displacement motions are observed, and in recent study by our group (Kulrattanarak et al., Meas Sci Technol, 2010a; J Colloid Interface Sci, 2010b), we have shown that also mixed motion occurs, which is an irregular alternation of zigzag and displacement motion. We have shown that the mixed motion is due to asymmetry of the flow lane distribution, induced by the symmetry breaking of the oblique primitive lattice cell (Kulrattanarak et al. 2010b). In this study, we investigate mixed motion in depth by numerical and experimental analysis. Via 3D simulations, we have computed explicit particle trajectories in DLD, and are able to show that there are two critical length scales determining the type of motion. The first length scale d f,1 is the first flow lane width, which determines the transition between zigzag motion and mixed motion. The other length scale, d f,c , determines the transition between mixed motion and displacement motion. Based on our experimental and numerical results we have been able to correlate the migration angle of particles showing mixed motion to the particle size, relative to the two critical length scales d f,1 and d f,c

This paper presents the design, fabrication and testing of deterministic ratchets which are used for fractionation purpose. Ratchets with different configuration are prepared to determine particle trajectories with different sizes. Silicon based MEMS technology is used to fabricate devices containing arrays of obstacles with varying size, spacing and number. The arrays of high aspect ratio, 7-20μm wide, silicon pillars are defined and etched in 60 to 80μm deep microchannels using Deep Reactive Ion Etching (DRIE). The working modes of the ratchets are demonstrated using colloidal suspensions of latex particles which have an average diameter ranging between 2 and 6μm.

An adaptive optimal scheduling and controller design is presented that attempts to improve the performance of beer membrane filtration over the ones currently obtained by operators. The research was performed as part of a large European research project called EU Cafe with the aim to investigate the potential of advanced modelling and control to improve the production and quality of food. Significant improvements are demonstrated in this paper through simulation experiments. Optimal scheduling and control comprises a mixed integer non-linear programming problem (MINLP). By making some suitable assumptions that are approximately satisfied in practice, we manage to significantly simplify the problem by turning it into an ordinary non-linear programming problem (NLP) for which solution methods are readily available. The adaptive part of our scheduler and controller performs model parameter adaptations. These are also obtained by solving associated NLP problems. During cleaning stages in between membrane filtrations enough time is available to solve the NLP problems. This allows for real-time implementation.

For micro-machined membranes, it is possible to choose pore size, pore geometry and membrane porosity, within certain limits. Different pore geometries (circular, square, slit shaped and triangular pores), particle size to pore size ratios, pore edges and membrane porosities were evaluated with lattice-Boltzmann computer simulations and torque balance considerations for various modes of operation. We focused on hydrodynamic interactions and assumed uncharged neutral surfaces of the particle and the pore. However, the model can easily be extended with additional relations for such interactions in practical systems with defined properties. It was concluded that pore geometry can have a large effect on the flux (up to 60%). Further, the effect of shielding could be quantified. Above a surface coverage of 0.05, the particles effectively shield each other from the flow field, therewith necessitating either a higher cross flow velocity or a lower transmembrane pressure for particle removal. Based on the simulations, an extended criterion for the critical flux was developed, which includes the effects of pore geometry, particle to pore size ratio and membrane porosity. Different optimal membrane choices follow for processes aimed at retention of all particles, and for processes aimed at fractionation of particles into different fractions.

During cross-flow membrane emulsification, droplet formation at a micro-engineered membrane with uniform pores was visualized by microscopy. Although the membrane pores were uniform in size, the obtained emulsions were polydisperse. Observations showed this to be due to steric hindrance of droplets forming simultaneously. At some pores close together, the droplet formation frequencies were almost identical (0.86 and 0.88 s −1 ). This was confirmed by the cross-correlation value (around 0.8). Because the droplets hindered each other during growth, the droplet diameter decreased with an increasing number of active pores. In spite of the uniform pore size and shape, the number of active pores only gradually increased upon increasing transmembrane pressure. We suggest this to be due to the dynamics of the flow of the to-be-dispersed phase towards and through the membrane. Under the process conditions used, interactions can only be prevented by using a membrane with a lower porosity.

This work concerns the degradation of the nutritional compounds glucoraphanin (GR) and vitamin C (Vc), and the inactivation of the enzyme myrosinase (MYR) in broccoli ( Brassica oleracea var. italica ) during drying with air temperatures in the range of 30–60 °C. Dynamic optimization is applied to find the optimized temperature trajectories that minimize degradation and inactivation. Simulation and experimental results for optimized temperature trajectories are compared to constant inlet air temperature drying at 40 and 50 °C. The results show that with the optimized temperature trajectories the retention of GR, MYR and Vc improved significantly. Moreover, the experiments show that degradation and inactivation during drying is slower than expected from kinetic studies. The deviation is explained from the difference in the physical state of the samples used in the drying experiments, i.e. original plant tissue versus the grounded state of the plant tissue used in the experiments for the kinetic studies. The results indicate that besides temperature and moisture content the physical state is also an important aspect in the degradation of nutritional compounds and enzymes.

A Lattice Boltzmann scheme is developed for the simulation of the heat and mass transfer during cooling of packed cut flowers. Using this problem as a case study, we investigate whether the Lattice Boltzmann scheme can be used as a generic tool in the design process of packages with agricultural products. The Lattice Boltzmann method has been chosen for its attractive simplicity and its appeal to physical intuition. The method maps the problem on a regular lattice, populated with lattice gas particles, which move across the lattice and collide with one another. Mathematical and numerical analysis have shown that our Lattice Boltzmann scheme is consistent with a convection–diffusion equation with heat and mass transfer, even up to high grid Peclet and Courant numbers. Comparison of the simulation results of the LB scheme with experimental data shows that the model applies quite well to cooling of packed cut flowers. Given the results of this study we conclude that the Lattice Boltzmann scheme is indeed suitable as a generic framework for the modelling of heat and mass transfer processes in packages of agricultural products.

Via Lattice Boltzmann simulations we have derived an empirical relation for the hydrodynamic interaction between a sedimenting sphere and an obstacle in narrow confining flow channels. This relation is of importance for particle tracking in microfluidic devices used for fractionation of suspensions. The hydrodynamic interaction is a function of the dimensionless gap, the degree of confinement, and the ratio of the principal curvatures of the sphere and obstacle. The degree of confinement is defined as the ratio of particle diameter and the length scale, which enters the mobility of the confined sphere, and is described by a generalisation of the Haberman–Sayre correlation. For gaps larger than a certain the screening length, the hydrodynamic interaction changes over from a 1/ η to a 1/ η 2 scaling. The screening length is dependent on the degree of confinement.

By extending the concept of an effective temperature, earlier introduced for sheared monodisperse suspensions, we propose a continuum theory for sheared bidisperse suspensions. We show the theory for sheared suspensions can be constructed from the theory for Brownian suspensions by replacing the temperature with the effective temperature. Furthermore, we explore the validity of closure relations based on mean field/free volume theory, by comparison with experimental data obtained for Brownian bidisperse suspensions. In a recent paper, we have shown that the new theory, combined with the discussed closure relations, is indeed a predictive theory.

Nowadays microfluidic devices are becoming popular for cell/DNA sorting and fractionation. One class of these devices, namely deterministic ratchets, seems most promising for continuous fractionation applications of suspensions (Kulrattanarak et al., 2008 [1]). Next to the two main types of particle behavior, zigzag and displacement motion as noted by the inventors (Huang et al., 2004 [2]) and (Inglis et al., 2006 [3]), we have shown recently the existence of a intermediate particle behavior, which we named ‘mixed motion’. In this paper we formulate the hypothesis that the occurrence of mixed motion is correlated with anisotropy in the permeability of the obstacle array. This hypothesis we base on the comparison of experimental observations of mixed motion and the flow lane distribution as obtained from 2-D flow simulations.

Membrane processes are well-known for separating and fractionating suspensions in many industries, but suffer from particle accumulation on the membrane surface. Currently, there are new developments using microfluidic devices for cell/DNA sorting and fractionation. We anticipate these devices are also applicable to fractionation of polydisperse and concentrated suspensions (e.g. foods), and may potentially have fewer problems with particle accumulation compared to membranes. This review article presents an overview of relevant microfluidic devices. We focus on their performance with respect to concentrated suspensions, as one finds in food industry. We give quantitative estimates on their yield, selectivity, and the potential for large-scale application. From this evaluation follows that deterministic ratchets seem most promising.

In this contribution we report on the initial steps in the development of a numerical scheme for flow through packed or suspended spheres. The spheres are semiresolved, meaning that their diameter is smaller than the grid spacing, but their excluded volume is taken into account. Flow in the fluid phase is solved using the volume-averaged equations. Particle motion is solved via Newtons law, taking into account drag force and lubrication forces only. Despite the low resolution of the flow field, the particle trajectories of two spheres colliding in shear flow can be reasonable reproduced

In the special case of relaxation parameter ω = 1 lattice Boltzmann schemes for (convection) diffusion and fluid flow are equivalent to finite difference/volume (FD) schemes, and are thus coined finite Boltzmann (FB) schemes. We show that the equivalence is inherent to the homology of the Maxwell–Boltzmann constraints for the equilibrium distribution, and the constraints for finite difference stencils as derived from Taylor series expansion. For convection–diffusion we analyse the equivalence between FB and the Lax–Wendroff FD scheme in detail. It follows that the Lax–Wendroff procedure is performed automatically in the finite Boltzmann schemes via the imposed Maxwell–Boltzmann constraints. Furthermore, we make some remarks on FB schemes for fluid flows, and show that an earlier related study can be extended to rectangular grids. Finally, our findings are briefly checked with simulations of natural convection in a differentially heated square cavity.