Single droplet drying with stepwise changing temperature-time trajectories: Influence on heat sensitive constituents.

Optimization procedures for industrial spray drying processes mainly rely on empirical understanding. Mechanistic understanding of the process is limited, but can be enhanced by studying the drying of single droplets. We here report on a new sessile single droplet drying platform, using two air streams to represent the inlet and outlet air of a spray dryer to simulate changing conditions in a spray dryer. Accurate temperature measurements confirmed the temperature profiles and their imposition onto a drying droplet. Single droplets of solutions containing ß-galactosidase and maltodextrin were dried with different temperature-time trajectories, with the inactivation of the enzyme as indicator for the thermal load on the droplet. The locking point is found to be an important parameter: the air temperature before this point does not influence the enzyme inactivation much, but a high air temperature after the locking point results in significant inactivation. The ß-galactosidase inactivation was also successfully predicted with a coupled drying and inactivation model.

Food proteins from yeast-based precision fermentation: Simple purification of recombinant ß-lactoglobulin using polyphosphate.

Proteins produced through precision fermentation are often purified through chromatographic methods. Faster and more cost-effective purification methods are desired for food application. Here, we present a simple method for purification of protein produced from yeast, using ß-lactoglobulin secreted from Pichia pastoris as an example. The food-grade salt hexametaphosphate (HMP) was used to precipitate the protein at acidic pH, while the impurities (extracellular polysaccharides; mainly mannan) remained soluble. After re-solubilization of the protein-HMP complex by neutralization, excess HMP was selectively precipitated using calcium chloride. The protein content of the crude sample increased from 26 to 72 wt% (comparable to purification with anion exchange chromatography), containing only residual extracellular polysaccharides (9 wt%) and HMP (1 wt%). The established method had no significant impact on the structural and functional properties (i.e., ability to form emulsions) of the protein. The presented method shows potential for cost-effective purification of recombinant proteins produced through yeast-based expression systems.

Mild Fractionation for More Sustainable Food Ingredients.

With the rising problems of food shortages, energy costs, and raw materials, the food industry must reduce its environmental impact. We present an overview of more resource-efficient processes to produce food ingredients, describing their environmental impact and the functional properties obtained. Extensive wet processing yields high purities but also has the highest environmental impact, mainly due to heating for protein precipitation and dehydration. Milder wet alternatives exclude, for example, low pH-driven separation and are based on salt precipitation or water only. Drying steps are omitted during dry fractionation using air classification or electrostatic separation. Benefits of milder methods are enhanced functional properties. Therefore, fractionation and formulation should be focused on the desired functionality instead of purity. Environmental impact is also strongly reduced by milder refining. Antinutritional factors and off-flavors remain challenges in more mildly produced ingredients. The benefits of less refining motivate the increasing trend toward mildly refined ingredients.

Electrode Surface Potential-Driven Protein Adsorption and Desorption through Modulation of Electrostatic, van der Waals, and Hydration Interactions.

When proteins in aqueous solutions are exposed to solid substrates, they adsorb due to the dynamic interplay of electrostatic, van der Waals, and hydration interactions and do so in a rather irreversible fashion, which makes protein recovery troublesome. Here, we use a gold electrode as the solid substrate and modulate the surface potential to systematically induce protein adsorption as well as partial desorption. We use different methods such as surface plasmon resonance, atomic force microscopy, and electrowetting and show that biasing the electrode to more negative potentials (by -0.4 V compared to the open-circuit potential at pH 6) results in an increased adsorption barrier of 6 kJ mol-1 for the negatively charged protein ß-lactoglobulin. Further, we clearly demonstrate that this is due to an increased double layer potential of -0.06 V and an increase in hydration repulsion. This indicates that an electric potential can directly influence surface interactions and thus induce partial ß-lactoglobulin desorption. These observations can be the basis for biosensors as well as separation technologies that use only one trigger to steer protein ad- and desorption, which is low in energy requirement and does not generate large waste streams, as is the case for standard protein separation technologies.

Dextrose equivalence of maltodextrins determines particle morphology development during single sessile droplet drying.

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.

Visualizing the hydrodynamics in sieve-based lateral displacement systems.

Deterministic lateral displacement (DLD) systems structure suspension flow in so called flow lanes. The width of these flow lanes is crucial for separation of particles and determines whether particles with certain size are displaced or not. In previous research, separation was observed in simplified DLD systems that did not meet the established DLD geometric design criteria, by adjusting the outflow conditions. We here investigated why these simplified DLD systems are able to displace particles, by experimentally investigating the hydrodynamics in the device. Flow lanes were visualized and the local flow velocities were measured using µPIV and compared with 2D fluid dynamics simulations. The size of the flow lanes strongly correlates with the local flow velocity (Vy and Vx), which depends on the hydrodynamics. Therefore, the geometric design criteria of DLD devices is in fact just one method to control the local hydrodynamics, which may also be influenced by other means. These findings give a new perspective on the separation principle, which makes the technique more flexible and easier to translate to industrial scale.

Morphology development during single droplet drying of mixed component formulations and milk.

We report on the influence of selected components and their mixtures on the development of the morphology during drying of single droplets and extend the results to the morphology of whole milk powder particles. Sessile single droplet drying and acoustic levitation methods were employed to study single droplet drying. The influence of carbohydrates (lactose and maltodextrin DE12) and proteins (micellar casein or whey protein) on morphology development is very different, since upon concentration protein systems will jam and undergo a colloidal glass transition, whereas carbohydrate systems will gradually increase in viscosity as a consequence of the concentration. Whey protein gives relatively rigid shells due to jamming of the "hard sphere" proteins, while casein micelles behave as "soft spheres" that can deform after jamming, which gives flexibility to the shell during drying. The influence of the carbohydrates on the final morphology was found much larger than the influence of the proteins. Caseins influenced morphology only in mixtures with lactose at higher concentrations due to its high voluminosity. Similar observations were done for whole milk, where fat appeared to have no influence. With maltodextrin the influence of the casein was again observed in the shape and smoothness of wrinkles. Both sessile and levitated droplet drying methods provide a similar and consistent view on morphology development.

Effect of pre-treatment on in vitro gastric digestion of quinoa protein (Chenopodium quinoa Willd.) obtained by wet and dry fractionation.

Quinoa protein was isolated from quinoa seeds using wet fractionation that resulted in a protein isolate (QPI) with a high protein purity of 87.1% (w/dw) and a protein yield of around 54%, and a dry fractionation method delivered a quinoa protein concentrate (QPC) with a purity of 27.8% (w/dw) and yield of around 47%. The dry fractionation process only involves milling and sieving and keeps the protein in its natural, native state. The aim was to study the in vitro gastric digestibility of both protein. Attention was paid to thermal pre-treatment of QPI and QPC. QPC showed significantly higher (p < .05) digestibility than QPI samples. The results were interpreted with a simple double exponential model. The fraction of easily digested protein in QPC is higher than for QPI. The better digestibility of the QPC was explained by the prevention of the formation of large aggregates during pre-heating of the protein.

Reducing the critical particle diameter in (highly) asymmetric sieve-based lateral displacement devices.

Deterministic lateral displacement technology was originally developed in the realm of microfluidics, but has potential for larger scale separation as well. In our previous studies, we proposed a sieve-based lateral displacement device inspired on the principle of deterministic lateral displacement. The advantages of this new device is that it gives a lower pressure drop, lower risk of particle accumulation, higher throughput and is simpler to manufacture. However, until now this device has only been investigated for its separation of large particles of around 785 µm diameter. To separate smaller particles, we investigate several design parameters for their influence on the critical particle diameter. In a dimensionless evaluation, device designs with different geometry and dimensions were compared. It was found that sieve-based lateral displacement devices are able to displace particles due to the crucial role of the flow profile, despite of their unusual and asymmetric design. These results demonstrate the possibility to actively steer the velocity profile in order to reduce the critical diameter in deterministic lateral displacement devices, which makes this separation principle more accessible for large-scale, high throughput applications.

Interfacial properties and emulsification performance of thylakoid membrane fragments.

Thylakoids membranes are sophisticated, dynamic structures found in plant leaves, composed of protein complexes in a dynamic lipid matrix. The interfacial absorption dynamics and viscoelasticity of thylakoid membranes fragments were measured to assess the properties of the interfacial layer and to elucidate an emulsifying mechanism that includes the role of thylakoid's composition and 3D structure. Thylakoid membranes were extracted from sugar beet leaves by a series of buffer washing, filtration and centrifugation. The extract containing the intact thylakoid membranes was suspended in water through high-pressure homogenisation, which disrupted the structure into membrane fragments. Thylakoid fragments showed surface and interfacial behaviour similar to soft particles or Pickering stabilizers with slow adsorption kinetics. After adsorption, an elastic and stable thin film was formed, indicating formation of new interactions between adjacent thylakoid fragments. In an emulsion, thylakoid fragments stabilised oil droplets against coalescence, despite droplet aggregation occurring already during emulsification. Droplet aggregation occurred by steric and electrostatic bridging, which in turn forms a 3D network where the oil droplets are immobilised, preventing further droplet coalescence or aggregation. It was concluded that both composition and structure of thylakoid fragments determine their emulsifying properties, conferring potential for encapsulation systems, where the search for natural materials is gaining more attention.

Cross-flow deep fat frying and its effect on fry quality distribution and mobility.

Conventional industrial frying systems are not optimised towards homogeneous product quality, which is partly related to poor oil distribution across the packed bed of fries. In this study we investigate an alternative frying system with an oil cross-flow from bottom to top through a packed bed of fries. Fluidization of rectangular fries during frying was characterised with a modified Ergun equation. Mixing was visualized by using two coloured layers of fries and quantified in terms of mixing entropy. Smaller fries mixed quickly during frying, while longer fries exhibited much less mixing, which was attributed to the higher minimum fluidization velocity and slower dehydration for longer fries. The cross-flow velocity was found an important parameter for the homogeneity of the moisture content of fries. Increased oil velocities positively affected moisture distribution due to a higher oil refresh rate. However, inducing fluidization caused the moisture distribution to become unpredictable due to bed instabilities.

Crust morphology and crispness development during deep-fat frying of potato.

Crust formation is an important factor in determining the crispness of French fries. This study aimed at unravelling detailed structural and textural properties of the crust in relation to crispness during frying as a function of the process temperature and time. X-ray tomography showed a larger overall pore volume at higher frying times, while a lower final moisture content mainly resulted in an increase in the amount of large pores. Texture analysis revealed that the increase in porosity, due to the increased formation of pores, results in a more crispy behaviour after frying with oil of up to 180°C. At temperatures above 180°C crispness is actually found to decrease again, which is explained by the increased plastic behaviour of the crust. This may be related to the reduced glass transition temperature of the crust because of increased sugar degradation at a very high temperature.

The potential of aqueous fractionation of lupin seeds for high-protein foods.

Aqueous fractionation of protein from lupin seeds was investigated as an alternative to the conventional wet fractionation processes, which make use of organic solvents. The effect of extraction temperature was studied and the consequences for downstream processing were analysed. Omitting the extraction of oil with organic solvents resulted in a protein isolate that contained 0.02-0.07 g oil g(-1) protein isolate, depending on the exact extraction conditions. Nevertheless, the protein functionality of the aqueous fractionated lupin protein isolate was similar to the conventional lupin protein isolate. The protein isolate suspension could be concentrated to 0.25 g mL(-1) using ultrafiltration, which provides a relevant concentration for a range of high-protein products. Based on the results, we conclude that aqueous fractionation can be a method to lower the environmental impact of the extraction of proteins from legumes that contain water- and dilute salt-soluble proteins.

Factors impeding enzymatic wheat gluten hydrolysis at high solid concentrations.

Enzymatic wheat gluten hydrolysis at high solid concentrations is advantageous from an environmental and economic point of view. However, increased wheat gluten concentrations result in a concentration effect with a decreased hydrolysis rate at constant enzyme-to-substrate ratios and a decreased maximum attainable degree of hydrolysis (DH%). We here identified the underlying factors causing the concentration effect. Wheat gluten was hydrolyzed at solid concentrations from 4.4% to 70%. The decreased hydrolysis rate was present at all solid concentrations and at any time of the reaction. Mass transfer limitations, enzyme inhibition and water activity were shown to not cause this hydrolysis rate limitation up to 50% solids. However, the hydrolysis rate limitation can be, at least partly, explained by a second-order enzyme inactivation process. Furthermore, mass transfer impeded the hydrolysis above 60% solids. Addition of enzyme after 24 h at high solid concentrations scarcely increased the DH%, suggesting that the maximum attainable DH% decreases at high solid concentrations. Reduced enzyme activities caused by low water activities can explain this DH% limitation. Finally, a possible influence of the plastein reaction on the DH% limitation is discussed.

Model for particle migration in bidisperse suspensions by use of effective temperature.

A model for the particle migration in a bidisperse flowing suspension is proposed and compared to experimental data. A mixture formulation, describing the suspension velocity and pressure and the concentrations of two solid fractions is derived from a multi-fluid model. In the multi-fluid model the liquid phase and both dispersed phases are interpenetrating phases. The closure relations are based on a mean field approach extending closure relations of a monodisperse suspension. The model is used to predict segregation based on particle size in channel flow where the particles are subjected to Brownian motion and shear-induced migration. The comparison of the model results with experimental data shows that particle migration is predicted well by the given formulation.

Suspension flow in microfluidic devices--a review of experimental techniques focussing on concentration and velocity gradients.

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.

Mixed motion in deterministic ratchets due to anisotropic permeability.

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.

Classification and evaluation of microfluidic devices for continuous suspension fractionation.

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.

Micrometer-sized fibrillar protein aggregates from soy glycinin and soy protein isolate.

Long, fibrillar semiflexible aggregates were formed from soy glycinin and soy protein isolate (SPI) when heated at 85 degrees C and pH 2. Transmission electron microscopy analysis showed that the contour length of the fibrils was approximately 1 microm, the persistence length 2.3 microm, and the thickness a few nanometers. Fibrils formed from SPI were more branched than the fibrils of soy glycinin. Binding of the fluorescent dye Thioflavin T to the fibrils showed that beta-sheets were present in the fibrils. The presence of the fibrils resulted in an increase in viscosity and shear thinning behavior. Flow-induced birefringence measurements showed that the behavior of the fibrils under flow can be described by scaling relations derived for rodlike macromolecules. The fibril formation could be influenced by the protein concentration and heating time. Most properties of soy glycinin fibrils are comparable to beta-lactoglobulin fibrils.

Lattice Boltzmann simulations of droplet formation in a T-shaped microchannel.

We investigated the formation of a droplet from a single pore in a glass chip, which is a model system for droplet formation in membrane emulsification. Droplet formation was simulated with the lattice Boltzmann method, a method suitable for modeling on the mesoscale. We validated the lattice Boltzmann code with several benchmarks such as the flow profile in a rectangular channel, droplet deformation between two shearing plates, and a sessile drop on a plate with different wetting conditions. In all cases, the modeling results were in good agreement with the benchmark. A comparison of experimental droplet formation in a microchannel glass chip showed good quantitative agreement with the modeling results. With this code, droplet formation simulations with various interfacial tensions and various flow rates were performed. All resulting droplet sizes could be correlated quantitatively with the capillary number and the fluxes in the system.