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Synergistic effect of glycyrrhizic acid and cellulose nanocrystals for oil-water interfacial stabilization
Lulu Ma, Pascal Bertsch, Zhili Wan, Xiaoquan Yang and Peter Alfons Fischer
Food Hydrocolloids, vol. 120, pp. 106888, Amsterdam: Elsevier, 2021.

In this work, the interfacial and bulk properties of oil-in-water emulsions stabilized by glycyrrhizic acid (GA), structurally a saponin and cellulose nanocrystals (CNC) as a co-stabilizer are investigated at different ionic strengths. The influence of GA, CNC, and NaCl concentration on the bulk stabilization of the emulsion as well as on the adsorption behavior and dilatational viscoelasticity at a model oil-water interface are discussed. At the oil-water interface GA adsorbs with appropriate CNC concentration due to salt-induced charge screening, but desorbs from the interface into bulk by excessive increase of the CNC concentration. An interfacial synergistic effect between GA and CNC was found at GA concentrations below interfacial saturation, attributed to the formation of interfacial GA-CNC complexes. In emulsions, the synergistic stabilization concentrations of the compounds are found at higher concentration of 1 wt% GA, 1 wt% CNC, and 20 mM NaCl. FTIR measurements suggest complexation of GA and CNC via hydrogen bonds and intermolecular interactions. The results provide evidence of network formation due to GA-CNC complexes at the interface and aggregation in the continuous phase, which eventually stabilize the emulsion systems.

Rheology of cocoa butter
Kim Mishra, Lucas Kohler, Nico Kummer, Simon Zimmermann, Silas Ehrengruber, Fabian Kämpf, Damien Dufour, Gustav Nyström, Peter Fischer and Erich J. Windhab
Journal of Food Engineering, vol. 305, pp. 110598, Amsterdam: Elsevier, 2021.

The rheology of β cocoa butter crystal melt suspensions (CB CMS) was investigated directly after crystallization using in-line capillary rheometry (UVP-PD) as well as after tempering at zero deformation using off-line capillary rheometry (CR) and rotational rheometry (RR). CB CMS thixotropy was quantified by RR revealing that structure build up processes at zero deformation were not at equilibrium after 18 h, whereas structure decay occurred mainly in the first 10 s of measurement at γ˙ = 60 s . CR and UVP-PD were identified as preferable methods to probe thixotropic materials due to the short measurement time t and uniform sample deformation history for every γ˙-step. The scaling of the relative viscosity η and the (apparent) yield stress τ /τ as function of deformation history revealed two different regimes. Directly after the crystallization, the CB CMS behaved like a suspension of anisotropic crystals with a fractal dimension D≈1.57, an intrinsic viscosity [η] of 20 and a maximal packing fraction Φ of 0.17 leading to a scaling of η =(1−(Φ /Φ )) . After tempering at zero deformation, crystal aggregation increased the fractal dimension to D=2.6 and the scaling of the relative viscosity η reduced to ~Φ , analogously to a jammed system of elastic particles. V/VI M rel 0 0 max rel SFC max rel SFC −1 app −3.4 2

Influence of local convective heat flux on solidification, contraction and wall detachment behavior of molded chocolate during air cooling
Lucas Grob, Konstantina Papadea, Peter Braun and Erich J. Windhab
Innovative Food Science and Emerging Technologies, vol. 68, pp. 102629, Amsterdam: Elsevier, 2021.

The cooling process is an important step during chocolate production. It influences final product quality characteristics such as gloss, texture and melting behavior. Furthermore, it is a high energy consuming operation and its optimization leads to an increase in energy efficiency of the chocolate production. Dark chocolate was cooled in a pilot-scale cooling tunnel with cooling air temperatures of Tair = 2, 12 and 18 °C and air mean velocities of vin = 1.5, 3.5, and 6.0 m s−1. Convective heat transfer coefficients at the top and the bottom of the mold were received from model calculations applying a CFD model. Crystallization and detachment behavior from the mold walls were newly analyzed by measuring the damping characteristics of ultrasound waves transferring the filled molds during cooling. In addition, for this-like treated well-tempered dark chocolate, the crystallization and detachment behavior were analyzed in further detail. The convective heat transfer from the bottom of the mold increases in flow direction due to the existence of a typical mold geometry-dependent recirculating zone of the cooling air below the chocolate mold and also dependent on the local air velocity. It was shown that depending on intensity and homogeneity of the heat transfer in the air-cooling phase, the structure density of the chocolate can be increased which has a positive impact on resulting product quality characteristics. Moreover, the detachment of the chocolate from the mold wall was demonstrated to have an optimum for typical chocolate plate formats with 125 g weight at an apparent heat flux of 550 W/m2, for which the time until detachment reaches a minimum.

Proteins from microalgae for the stabilization of fluid interfaces, emulsions, and foams
Pascal Bertsch, Lukas Böcker, Alexander Mathys and Peter Alfons Fischer
Trends in Food Science & Technology, vol. 108, pp. 326-342, Amsterdam: Elsevier, 2021.
Micro-computed tomography study on bread dehydration and structural changes during ambient storage
Yi Chen, Annapaola Parrilli, Florian Jaedig, Adrian Fuhrmann, Christian Staedeli, Peter Fischer and Erich J. Windhab
Journal of Food Engineering, vol. 296, pp. 110462, Amsterdam: Elsevier, 2020.

This study investigated the structural changes in bread during ambient storage. The structural changes in bread were studied using X-ray micro-computed tomography. Fresh bread had a porosity of 53.7 ± 1.6% and a high pore connection value, suggesting that most pores in the bread are extensively interconnected. Moreover, fresh bread had cell walls with an average thickness of about 0.2 mm and a wide pore size distribution. During storage, the bread underwent substantial water-loss, leading to volume shrinkage and microstructural alterations. Significant increases in total porosity and in the number of small pores as well as a significant decrease in average cell wall thickness were observed over 14 storage days. Grayscale intensity obtained from the X-ray images provided additional insights into the changes in the network density, which is expected to be correlated with water distribution and starch recrystallization during staling.

Effect of Arthrospira platensis microalgae protein purification on emulsification mechanism and efficiency
Lukas Böcker, Pascal Bertsch, David Wenner, Stephanie Teixeira, Jotam Bergfreund, Severin Eder, Peter Alfons Fischer and Alexander Mathys
Journal of Colloid and Interface Science, vol. 584, pp. 344-353, Orlando, FL: Elsevier, 2020.

In light of environmental concerns and changing consumer demands, efforts are increasing to replace frequently used animal-based emulsifiers. We demonstrate the interfacial network formation and emulsifying potential of Arthrospira platensis protein extracts and hypothesize a mechanistic change upon progressing purification. A microalgae suspension of A. platensis powder in phosphate buffer solution (pH 7, 0.1 M) was homogenized and insoluble components separated by centrifugation. Proteins were precipitated at the identified isoelectric point at pH 3.5 and dialyzed. In interfacial shear rheology measurements, the build-up of an interfacial viscoelastic network was faster and final network strength increased with the degree of purification. It is suggested that isolated A. platensis proteins rapidly form an interconnected protein layer while coextracted surfactants impede protein adsorption for crude and soluble extracts. Emulsions with 20 vol% Medium chain triglycerides (MCT) oil could be formed with all extracts of different degrees of purification. Normalized by protein concentration, smaller droplets could be stabilized with the isolated fractions. For potential applications in food, pharma and cosmetic product categories, the enhanced functionality has to be balanced against the loss in biomass while purifying microalgae proteins or other alternative single cell proteins.

Viscoelastic characterization of the crosslinking of β-lactoglobulin on emulsion drops via microcapsule compression and interfacial dilational and shear rheology
Matthew D. Biviano, Lukas Böni, Joseph D. Berry, Peter Alfons Fischer and Raymond R. Dagastine
Journal of Colloid and Interface Science, vol. 583, pp. 404-413, Amsterdam: Elsevier, 2020.

Hypothesis Interfacial rheology provides insight into the mechanical properties of adsorption layers on liquid–liquid interfaces, which mediates the stability of emulsion droplets. The use of capsule compression at the scale of an emulsion droplet to probe the interfacial rheology may open up the possibility of testing the interfacial rheological properties of droplets with complex histories and extremely small volumes found in many applications. Experiments The time dependent interfacial rheological behavior of β-lactoglobulin adsorption layers on an oil/water interface in the native and crosslinked state was extracted using small oscillatory indentation with atomic force microscopy (AFM). The results of this novel model and experimental approach were compared to the well-established techniques of interfacial shear rheology (ISR) and dilational pendant drop tensiometry that were performed on analogous interfaces. Findings The tan δ measured between the ISR and AFM measurements provide similar results in an overlapping frequency range, but the viscoelastic moduli G’ and G’’ differ by several orders of magnitude. This is most likely the result of the different flow fields and the low deformation of the AFM measurements compared to dilational and shear flow fields. © 2020 Elsevier Inc.

Higher Salt Hydrophobicity Lengthens Ionic Wormlike Micelles and Stabilizes Them upon Heating
Stéphane Isabettini, Lukas Böni, Mirjam Baumgartner, Kaita Seito, Simon Kuster, Peter Alfons Fischer and Viviane Lutz-Bueno
Langmuir, vol. 37: no. 1, pp. 132-138, Washington, DC: American Chemical Society, 2020.

Tuning the rheological properties of surfactant solutions by charge screening is a convenient formulation tool in cosmetic, household, oil recovery, drag-reduction, and thickening applications. Surfactants self-assemble in water, and upon charge screening and core shielding, they grow into long wormlike micelles (WLMs). These are valuable model systems for soft matter physics, and the most explored formulation is hexadecyl-trimethylammonium bromide (CTAB) and sodium salicylate (NaSal). Replacing NaSal with aromatic salts of altered hydrophobicity results in different penetration of the additive in the CTAB micellar core. This altered penetration depth will determine the anisotropic micellar growth that tailors the viscoelastic response. Sodium 4-methylsalicylate (mNaSal) is a higher hydrophobicity alternative to NaSal, requiring less additive to induce strong changes in the viscoelastic properties. Herein, we provide a comparative study of the mNaSal/CTAB system with the reference NaSal/CTAB over a range of temperatures and salt concentrations. The findings from the well-known NaSal/CTAB pair are transferred to the mNaSal/CTAB system, revealing the origins of the WLM solution’s viscoelastic properties by discerning contributions from charge screening and micellar core shielding upon small differences in hydrophobicity.

Tea interfacial rheology
Caroline Giacomin and Peter Alfons Fischer
Abstract Book of the 18th International Congress on Rheology (ICR 2020), pp. 168-168, s.l.: ICR, 2020.

Tea is consumed daily worldwide and is present in many culturally significant activities. This includes, among others, British afternoon tea, yerba mate rituals of South America, and traditional tea ceremonies in Japan or China. When tea is left to steep, a thin film at the air-water interface can form. In certain conditions, this film is observable by naked eye and, when disturbed, visibly cracks like sea ice. The properties of this interfacial film are assessed using interfacial shear rheology and surface tension measurements. Layer properties are distinguished between tea varieties and water conditions. Water hardness, acidity, presence of sugar or milk, tea concentration, and brewing temperature all affect the formation of this layer. Interfaces formed in hard water (200 mg/L CaCO3) exhibit increased elastic modulus when compared to those in both moderately hard water (100 mg/L CaCO3) and soft water (0 mg/L CaCO3). All films formed in chemically hardened water exhibit yielding point behavior in an amplitude sweep. Conditions forming strongest layer may be industrially useful in packaged tea beverages for preferable shelf life stability and for milk tea beverages, emulsion stabilization.

Complex fluids in the animal kingdom
Patrick A. Rühs, Jotam Bergfreund, Pascal Bertsch, Stefan J. Gstöhl and Peter Alfons Fischer
Abstract Book of the 18th International Congress on Rheology (ICR 2020), pp. 129-129, s.l.: ICR, 2020.

Animals have evolved distinctive survival strategies in response to constant selective pressure. To demonstrate the importance of complex fluids and flow phenomena in the animal kingdom, species that utilize complex fluids as part of their survival strategy are introduced and discussed. In detail, we review how animals cope with and use exogen or endogen, biotic or abiotic complex fluids as a competitive advantage for movement, prey, defense, and reproduction. We demonstrate how animals learned to manipulate their habitat, e.g., sandfish and crabs that exploit the granular rheology of sand (exogen abiotic) and showcase exotic animals that produce unique bio-fluids (endogen biotic), from deep-sea hagfishes to subterranean velvet worms. For unknown and uncharted phenomena, potential rheological properties of the involved complex fluids are suggested. This contribution aims to assess complex flow behaviors used by animals in nature, compare and reveal the effect of material properties on biological behavior, and demonstrate the importance of rheology in nature.

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