M.B.J. Meinders

Prediction of the state diagram of starch water mixtures using the Flory–Huggins free volume theory

In this paper we analyse the phase and state transitions of starch and other glucose homopolymers and oligomers using the free volume extension of the Flory–Huggins theory by Vrentas and Vrentas, combined with the Couchman–Karasz theory for the glass transition. Using scaling relations of model parameters with molar weight we have obtained accurate predictions of moisture sorption and the freezing, boiling, and melting data obtained from literature for starch, dextrans, pullulan and maltodextrins. With the estimated model parameters we can construct the complete state diagram for starch, which can now be used as a quantitative tool for design and analysis of food structuring processes.

Structure and dynamics of egg white ovalbumin adsorbed at the air/water interface.

The molecular properties of egg white ovalbumin adsorbed at the air/water interface were studied using infrared reflection absorption spectroscopy (IRRAS) and time-resolved fluorescence anisotropy (TRFA) techniques. Ovalbumin adsorbed at the air/water interface adopts a characteristic partially unfolded conformation in which the content of the β-sheet is 10% lower compared to that of the protein in bulk solution. Adsorption to the interface leads to considerable changes in the rotational dynamics of ovalbumin. The results indicate that the end-over-end mobility of the ellipsoidal protein becomes substantially restricted. This is likely to reflect a preferential orientation of the protein at the interface. Continuous compression of surface layers of ovalbumin causes local aggregation of the protein, resulting in protein–network formation at the interface. The altered protein–protein interactions contribute to the strong increase in surface pressure observed.

Moisture diffusivity in food materials

This paper investigates whether moisture diffusion can be predicted for food materials. We focus especially on mixtures of glucose homopolymers and water. The predictions are based on three theories: (1) the Darken relation, linking the mutual diffusivity to the self diffusivities, (2) the generalised Stokes-Einstein relation for the solute self diffusivity, and (3) the free volume theory for water self diffusivity. Using literature data obtained for the whole class of glucose homopolymer, we show that these theories predict the moisture diffusivity for the whole range of volume fractions, from zero to one, and a broad range of temperatures. Furthermore, we show that the theories equally holds for other hydrophilic biopolymers one finds in food. In the concentrated regime, all experimental data collapse to a single curve. This universal behaviour arises because these biopolymers form a hydrogen bonded network, where water molecules move via rearrangement of the free volume.

Water content or water activity: What rules crispy behavior in bread crust?

A dry crust loses its crispness when water migrates into the crust. It is not clear if it is the amount of water absorbed or the water activity ( a w) that leads to a loss of crispness. The hysteresis effect observed when recording a water sorption isotherm allowed us to study the effects of a w and moisture content separately. All experiments were carried out on model bread crusts made from Soissons bread flour. The effect of water content and water activity on the glass transition of model bread crusts was studied in detail using two complimentary techniques: phase transition analysis (PTA) and nuclear magnetic resonance (NMR). The results were compared with sensory data and results from a puncture test, which provided data on acoustic emission and fracture mechanics during breaking of the crusts. The water content of the crust was found to be decisive for the transition point as measured by PTA and NMR. However, both water content and water activity had an effect on perceived crispness and number of force and sound peaks. From this may be concluded that the distribution of the water in the samples with a history of high water content is more inhomogeneous, which results in crispy and less crispy regions, thus making them overall more crispy than samples with the same water content but higher a w.

IRRAS, a new tool in food science

This report illustrates that infra-red reflection absorption spectroscopy (IRRAS) is a new powerful and promising technique to obtain detailed molecular information of biomolecules at and near the air/water interface of complex food solutions. Here it is demonstrated that in combination with spectral simulation, this technique can provide information on local concentration, conformation and orientation of different constituents simultaneously. Furthermore, dynamics of, e.g. protein accumulation and unfolding at the interface, can be studied on-line. Molecular properties can be measured together with system parameters, such as surface pressure, pH and ionic strength, and as a function of process, like expansion or compression the surface.

Proteins at air-water interfaces studied using external reflection circular dichroism

In this report we describe the first attempts to record external reflection circular dichroism (ERCD) spectra of beta-lactoglobulin solutions. It is shown that the accumulated proteins at and near the air-water interface can be detected using ERCD and that the signals obtained contain information on the conformational properties and concentration of the proteins residing at the interface. The local protein concentration and its conformation are in full agreement with previous observations using external reflection infrared spectroscopy. The ERCD signals are dominated by linear dichroism (LD) due to non-ideal behavior of the instrumental optics, but can be explained for using the theoretical description of chiral reflection. This allows the analysis of ERCD spectra of protein solutions. The measured ERCD signals are described accurately in the region between 190 and 220 nm, but poor resemblance is obtained at higher wavelengths. We are however confident that improvement of experimental conditions and theoretical description will allow that in the near future, external reflection circular dichroism (CD) can be a valuable tool that complements the application of external reflection infrared spectroscopy to study interfacial systems.

Water uptake mechanism in crispy bread crust.

Crispness is an important quality characteristic of dry solid food products such as crispy rolls. Its retention is directly related to the kinetics of water uptake by the crust. In this study, a method for the evaluation of the water sorption kinetics in bread crust is proposed. Two different sorption experiments were used: an oscillatory sorption test and a sorption test in which the air relative humidity (RH) was increased stepwise. These two experiments had different time scales, which made it possible to get a better understanding of the mechanisms involved. Results show that the adsorption and desorption dynamics of the oscillatory sorption test could be described by a single exponential in time. The water uptake rate ( k) was one of the fitting parameters. A maximum in the water uptake rate was found for a RH value between 50 and 70%. The rate parameters of the experiment where RH was increased stepwise were around a factor 10 lower than those derived from oscillatory sorption experiments. This is an important factor when designing experiments for the determination of water uptake rates. In addition, also a parameter describing the time dependence of the rate parameters of the oscillatory sorption experiment was calculated (C), again by fitting a single exponential to the rate parameters. C was in the same range as the rate parameter of the isotherm experiment. This indicates that different (relaxation) processes are acting at the same time in the bread crust during water uptake.

Mesoscale models of dispersions stabilized by surfactants and colloids

In this paper we discuss and give an outlook on numerical models describing dispersions, stabilized by surfactants and colloidal particles. Examples of these dispersions are foams and emulsions. In particular, we focus on the potential of the diffuse interface models based on a free energy approach, which describe dispersions with the surface-active agent soluble in one of the bulk phases. The free energy approach renders thermodynamic consistent models with realistic sorption isotherms and adsorption kinetics. The free energy approach is attractive because of its ability to describe highly complex dispersions, such as emulsions stabilized by ionic surfactants, or surfactant mixtures and dispersions with surfactant micelles. We have classified existing numerical methods into classes, using either a Eulerian or a Lagrangian representation for fluid and for the surfactant/colloid. A Eulerian representation gives a more coarse-grained, mean field description of the surface-active agent, while a Lagrangian representation can deal with steric effects and larger complexity concerning geometry and (amphiphilic) wetting properties of colloids and surfactants. However, the similarity between the description of wetting properties of both Eulerian and Lagrangian models allows for the development of hybrid Eulerian/Lagrangian models having advantages of both representations.

Predictive model to describe water migration in cellular solid foods during storage

BACKGROUND Water migration in cellular solid foods during storage causes loss of crispness. To improve crispness retention, physical understanding of this process is needed. Mathematical models are suitable tools to gain this physical knowledge. RESULTS Water migration in cellular solid foods involves migration through both the air cells and the solid matrix. For systems in which the water migration distance is large compared with the cell wall thickness of the solid matrix, the overall water flux through the system is dominated by the flux through the air. For these systems, water migration can be approximated well by a Fickian diffusion model. The effective diffusion coefficient can be expressed in terms of the material properties of the solid matrix (i.e. the density, sorption isotherm and diffusion coefficient of water in the solid matrix) and the morphological properties of the cellular structure (i.e. water vapour permeability and volume fraction of the solid matrix). The water vapour permeability is estimated from finite element method modelling using a simplified model for the cellular structure. CONCLUSION It is shown that experimentally observed dynamical water profiles of bread rolls that differ in crust permeability are predicted well by the Fickian diffusion model.

Effect of Temperature and Pressure on the Stability of Protein Microbubbles

Protein microbubbles are air bubbles with a network of interacting proteins at the air-water interface. Protein microbubbles are commonly used in medical diagnostic and therapeutic research. They have also recently gained interest in the research area of food as they can be used as structural elements to control texture, allowing for the manufacture of healthier foods with increased consumer perception. For the application of microbubbles in the food industry, it is important to gain insights into their stability under food processing conditions. In this study, we tested the stability of protein microbubbles against heating and pressurization. Microbubbles could be heated to 50 °C for 2 min or pressurized to 100 kPa overpressure for 15 s without significantly affecting their stability. At higher pressures and temperatures, the microbubbles became unstable and buckled. Buckling was observed above a critical pressure and was influenced by the shell modulus. The addition of cross-linkers like glutaraldehyde and tannic acid resulted in microbubbles that were stable against all tested temperatures and overpressures, more specifically, up to 120 °C and 470 kPa, respectively. We found a relation between the storage temperatures of microbubble dispersions (4, 10, 15, and 21 °C) and a decrease in the number of microbubbles with the highest decrease at the highest storage temperature. The average rupture time of microbubbles stored at different storage temperatures followed an Arrhenius relation with an activation energy for rupture of the shell of approximately 27 kT. This strength ensures applicability of microbubbles in food processes only at moderate temperatures and storage for a moderate period of time. After the proteins in the shell are cross-linked, the microbubbles can withstand pressures and temperatures that are representative of food processes.