R. Hans Tromp

Visualisation of biopolymer mixtures using confocal scanning laser microscopy (CSLM) and covalent labelling techniques

Abstract Confocal scanning laser microscopy (CSLM) has been used to study the behaviour of mixtures of proteins, gelatine, whey proteins and β-lactoglobulin, and polysaccharides, dextran, gellan gum, carrageenan, gum Arabic, and starch. CSLM proved to be a suitable technique to visualise the microstructure of these (phase separated) mixtures in two and three-dimensional images. Contrast through fluorescence is obtained either by covalent labelling (polysaccharides and proteins) or non-covalent labelling (proteins and starch). Double and triple labelling allows the visualisation of individual components in a complex mixture of biopolymers.

Non-equilibrium cluster states in colloids with competing interactions

Cluster formation and gelation are studied in a colloidal model system with competing short-range attractions and long-range repulsions. In contrast to predictions by equilibrium theory, the size of clusters spontaneously formed at low colloidal volume fractions decreases with increasing strength of the short-range attraction. Moreover, the microstructure and shape of the clusters sensitively depend on the strength of the short-range attraction: from compact and crystalline clusters at relatively weak attractions to disordered and quasi-linear clusters at strong attractions. By systematically varying attraction strength and colloidal volume fraction, we observe gelation at relatively high volume fraction. The structure of the gel depends on attraction strength: in systems with the lowest attraction strength, crowding of crystalline clusters leads to microcrystalline gels. In contrast, in systems with relatively strong attraction strength, percolation of quasi-linear clusters leads to low-density gels. In analyzing the results we show that nucleation and rearrangement processes play a key role in determining the properties of clusters and the mechanism of gelation. This study implies that by tuning the strength of short-range attractions, the growth mechanism as well as the structure of clusters can be controlled, and thereby the route to a gel state.

A neutron diffraction and computer modeling study of the interatomic structure of phosphoric acid

Wide angle neutron diffraction in combination with H/D substitution was used to determine the inter- and intramolecular structure of 100% phosphoric acid (H3PO4, PA). From radial distribution functions gHH(r), gHX(r), and gXX(r) (where X is either O or P) the hydrogen bonds were found to be characterized by a very short O…H distance (1.54 A). Within a molecule, the orientation of an OH group was found to be preferably in one of the three O–P–O planes. In the interpretation of the radial distribution functions, use was made of preliminary results of molecular dynamics simulations. Temperature effects on the structure of PA were only found in the hydrogen bond structure, which becomes somewhat less well defined when heating up from room temperature to 60 °C. Polyphosphates could not be detected, probably due to the small degree of polymerization.

Protein-Polysaccharide Interactions to Alter Texture

The sensory perception of texture is an important contributor of our general appreciation of foods. Food texture is mainly described in terms of mouthfeel and afterfeel attributes. The role of oral processing in the perception of texture and the role of microstructure therein have been reviewed regularly over recent years (Chen & Engelen 2012, Foegeding et al. 2011, Stieger & van de Velde 2013) and are not, therefore, addressed in this review. The scope of this review relates to the molecules that underlay the texture of foods. Protein, carbohydrate, and fat are the major structuring components in foods. In this review we focus on the physical interactions between proteins and polysaccharides that form the basis for the microstructure and texture of these foods. In general, food products are classified in four categories by their sensory and rheological properties: liquids, semisolids, soft solids, and hard solids (van Vliet et al. 2009). These four categories provide a useful classification framework, although they are not precisely defined by specific rheological properties. The current review focuses on semisolid and soft-solid foods.

Donnan Potentials in Aqueous Phase-Separated Polymer Mixtures

A promising approach to texturize water is by the addition of mutually incompatible polymers, leading to phase separation. Here, we demonstrate that the phase stability of aqueous polymer solutions is affected not only by chemical differences between the polymers but also by their electric charge. Direct electrochemical measurements are performed of the electric potential difference between two coexisting phases in aqueous solutions of the charged protein fish gelatin (nongelling) and the uncharged polysaccharide dextran. Charge counteracts demixing because of the entropic cost of confining the counterions to one phase, resulting in a strong shift of the critical point upon an increase of the charge on one of the polymers. Upon phase separation, the charged polymer is spatially confined, and due to the Donnan effect, an interfacial electric potential is developed. A direct proportionality is found between this Donnan potential and the difference in gelatin concentration in the two phases, for which we propose a theoretical explanation. The electrostatics may provide a new handle in the development of stable water-in-water emulsions.

Composition, concentration and charge profiles of water–water interfaces

The properties of interfaces are discussed between coexisting phases in phase separated aqueous solutions of polymers. Such interfaces are found in food, where protein-rich and polysaccharide-rich phases coexist. Three aspects of such interfaces are highlighted: the interfacial profiles in terms of polymer composition and polymer concentration, the curvature dependence of the interfacial tension, and the interfacial potential, arising when one of the separated polymers is charged. In all three cases a theoretical approach and methods for experimental verification are presented.

Milk gelation studied with small angle neutron scattering techniques and Monte Carlo simulations.

The sol-gel transition of fat-free milk by acidification was studied with neutron scattering experiments and Monte Carlo simulations. Spin-echo small angle neutron scattering (SESANS) and ultrasmall angle neutron scattering (USANS) experiments were performed to measure the static structure of milk and yogurt, as well as the aggregation kinetics. Colloidal gelation was simulated from a reaction limited domain (RLCA) to the diffusion limited regime (DLCA) as cluster-cluster aggregation of adhesive, hard spheres on a three-dimensional lattice. Comparisons were drawn between experimental and numerical correlation functions. Milk was modeled as a suspension of casein micelles in water, and its structure was described with a dilute system of solid spheres with a log-normal distribution of sizes. The structure and formation of yogurt were described with a self-affine model, used for systems containing heterogeneities with a wide range of sizes. Simulation speed was increased by 1 order-of-magnitude using a new algorithm to eliminate dead time. Observations by SESANS and USANS of milk particle sizes and yogurt length scales were consistent and agreed well with literature. Kinetic USANS data yielded reliable information about the growth of typical length scale during aggregation. The simulation model predicted the measurement data qualitatively best staying close to the RLCA regime until large structures had formed. Correlation lengths were in good quantitative agreement, but longest simulated length scales were a of factor 2(1)/(2) below experimental findings. We conclude that small, mobile aggregates are formed during the first 3 h, mostly influencing the dimensionality of the system and that large, inert structures are formed from 2 up to 8 h, which determine the typical length scale.

Carbon-13 nuclear magnetic relaxation in supercooled liquid and glassy maltose

13 C longitudinal relaxation rates (T1-1) in highly viscous liquid and solid amorphous maltose, its mixtures with water and methanol, and also crystalline maltose monohydrate, have been measured as a function of temperature, above and below the calorimetric glass transition temperatures of the amorphous materials. From the results it is concluded that, at temperatures up to 60°C below the glass transition temperature, the carbon atoms in the exocyclic hydroxymethyl groups of maltose are more mobile than the endocyclic carbon atoms. A few percent of water is sufficient to considerably enhance amorphous maltose mobility. At temperatures close to the glass transition methanol in amorphous maltose–methanol mixtures retains a high degree of rotational mobility which is decoupled from the bulk viscosity.

Water–Water Interfaces

The theory and experimental properties of interfaces between phase separated aqueous polymer solutions are discussed. These interfaces are characterized by a very low interfacial tension (0.01–1 μN/m), accumulation of solvent and in some cases a spontaneous curvature and an electric potential (Donnan potential). The procedure of calculating interfacial tension of both flat and curved interfaces is presented, and an overview of the methods to measure this interfacial tension and interface potential is given.

Analysis of light scattered by turbid media in cylindrical geometry.

The angle dependence of the transmitted light through a cylindrical turbid sample (latex suspension, developing milk gel, draining/coarsening milk, and protein foams) in a standard light scattering setup was analyzed in terms of the transport mean free path length or scattering length l* (a measure for the turbidity) and the absorption length labs. By variation of the concentration of an absorbing dye, the independence of l* and labs was demonstrated. The resulting value of the specific extinction coefficient of the dye was found to be in fair agreement with direct spectroscopic determination and practically identical in milk and latex suspensions. The validity of this technique for obtaining l* was demonstrated by monitoring the acid-induced gelation of milk. The possibility to simultaneously determine l* and labs was used to follow the time development of a draining and coarsening protein foam which contained an absorbing dye. It was shown that labs can be used as a measure for the volume fraction of air in the foam. This method of monitoring the transmission of multiple light scattering provides an easy way to determine l* and, specifically for foams, quantitative data dominated by the bulk of the foam.