Papers by Thomas Moschakis
Two probiotic strains of Lactobacillus paracasei subsp. paracasei (E6) and Lactobacillus paraplan... more Two probiotic strains of Lactobacillus paracasei subsp. paracasei (E6) and Lactobacillus paraplantarum (B1), isolated from traditional Greek dairy products, were microencapsulated by complex coacervation using whey protein isolate (WPI, 3% w/v) and gum arabic (GA, 3% w/v) solutions mixed at different polymer ratio (1:1, 2:1 and 4:1). The effect of total biopolymer concentration on cell viability was assessed using WPI and GA solutions of 1, 3 and 6% w/v at a constant ratio of 2:1. Several parameters were examined for optimization of the microcapsule formation, such as inoculum concentration and the effect of ionic strength. The viability of the bacterial cells during heat treatment and under simulated gut conditions was evaluated. Among the different WPI/GA weight ratios tested (1:1, 2:1 and 4:1), the highest survival rate was observed for the coact:rvate structures made with the ratio of2:1. The protection efficiency at low pH values was influenced by both concentration and the rat...
Langmuir, 2006
This work investigates surface properties of a protein particle gel and effects of polysaccharide... more This work investigates surface properties of a protein particle gel and effects of polysaccharide on the surface microstructure of such a protein gel. Whey protein isolate (WPI) was used as the primary gelling agent, and a polysaccharide (xanthan) was investigated for its surface smoothing effects. The surface properties of heat-set WPI gels with and without the presence of xanthan (0, 0.05, and 0.25%) were characterized using a surface friction technique. The surface friction force of a gel against a stainless steel substrate was found to be highly dependent on the sliding speed for all three gel samples, and the addition of xanthan caused a general reduction of surface friction. The gel containing no xanthan has the largest surface friction and behaved in the most load-dependent manner, whereas the gel containing 0.25% xanthan has the lowest surface friction and showed the least load dependency. It was inferred that the WPI gel containing no xanthan has the roughest surface among the three samples and the presence of xanthan leads to a smoother surface with probably a thinner layer of surface water. Surface features derived from surface friction tests were confirmed by surface microstructure observation from confocal laser scanning microscopy (CLSM) and environmental electron scanning microscopy (ESEM). Surface profiles from CLSM images were used to quantify the surface roughness of these gels. The mean square root surface roughness R(q) was calculated to be 3.8 +/- 0.2, 3.0 +/- 0.2, and 1.5 +/- 0.2 microm for gels containing 0, 0.05, and 0.25% xanthan, respectively. The dual excitation images of protein and xanthan from CLSM observation and images from ESEM observation indicate a xanthan-rich layer at the surfaces of the xanthan-containing gel samples. We speculate that the creation of the outer surface of a particle gel is based on a different particle aggregation mechanism from that leading to network formation in the bulk.
Current Opinion in Colloid & Interface Science, 2013
Particle tracking microrheology, an emerging experimental technique, which utilizes the Brownian ... more Particle tracking microrheology, an emerging experimental technique, which utilizes the Brownian motion of embedded particles to probe local dynamics of soft materials, is presented. Particle tracking microrheology is a powerful technique that enables the measurement of viscoelastic responses in small sample volumes, which are inaccessible to macrorheology and to spatially map structural heterogeneities at a microlevel. Therefore, particle tracking microrheology has considerable potential in food emulsions and gels, since these systems are commonly inhomogeneous. Recent advances and achievements are discussed, including the basic principles, operating regimes and limitations of the technique. The application of the technique in the field of food gels and emulsions to study the evolving dynamics of inhomogeneous at microscale length systems and during sol-gel transition is highlighted.
Soft Matter, 2015
The aim of the present study was to explore the potential use of chitin nanocrystals, as colloida... more The aim of the present study was to explore the potential use of chitin nanocrystals, as colloidal rod-like particles, to stabilize aqueous foams. Chitin nanocrystals (ChN) were prepared by acid hydrolysis of crude chitin and foams were generated mainly by sonicating the respective dispersions. The foamability of the chitin nanocrystals was evaluated and the resulting foams were assessed for their stability, in terms of foam volume reduction and serum release patterns, during storage. Additionally, the samples were studied with light scattering and optical microscopy in order to explore the bubble size distribution and morphology of the foam. Nanocrystal concentration and charge density was varied to alter the packing of the crystals at the interface. At low concentrations of ChNs, foams were stable against coalescence and disproportionation for a period of three hours, whereas at higher concentrations, the foams were stable for several days. The enhanced stability of foams prepared with ChNs, compared to surfactant-stabilized foams, can be mainly attributed to the irreversible adsorption of the ChNs at the air-water interface, thereby providing Pickering stabilization. Both foam volume and stability of the foam were increased with an increase in ChNs concentration, and at pH values around the chitin's pKa (pH 7.0). Under these conditions, the ChNs show minimal electrostatic repulsion and therefore a higher packing of the nanocrystals is promoted. Moreover, decreased electrostatic repulsion enhances network formation between the ChNs in the aqueous films, thereby providing additional stability by gel formation. Overall, ChNs were proven to be effective in stabilizing foams, and may be useful in the design of Pickering-stabilized food grade foams.
Food Hydrocolloids, 2006
Extensive research works have been carried out in investigating the microstructure of heat-set wh... more Extensive research works have been carried out in investigating the microstructure of heat-set whey protein gels and their fractal nature, but little has been done on the surface studies of these systems due to the lack of suitable technique for surface characterization of wet and deformable food gels. This work intended to explore the possibility of applying confocal laser scanning
Procedia Food Science, 2011
In the present study, the interactions between negatively charged sodium stearoyl 2-lactylate (SS... more In the present study, the interactions between negatively charged sodium stearoyl 2-lactylate (SSL) and positively charged chitosan were studied in solution and on oil-water interfaces. Phase diagrams of the SSL/chitosan systems were obtained at both pH 4.0 and 5.7 by preparing concentrated solutions and mixing them at different ratios. Optical microscopy revealed that the complexes formed at pH=4.0 were smaller than those formed at pH=5.7 and that the size of the complexes decreased as the SSL/chitosan ratio was reduced. Emulsions (10% w/w oil) were prepared at pH=5.7 by using the complexes of a constant SSL concentration (0.4% w/w) and at different SSL/chitosan weight ratios. The droplet size of the emulsions that were formed with the complexes was greater than those containing only SSL. Using complexes of low SSL/chitosan ratio as emulsifying agent resulted in the production of emulsions with enhanced viscosity as determined by steady shear rheometry. However, none of the samples showed a gel-like behaviour since in all cases the G' (storage modulus) was lower that G'' (loss modulus). The presence of polysaccharide at the interface resulted in an increased stability of the emulsions subjected to environmental stresses, such as heat treatment and a freeze thawing process.
The effect of the microstructure of the medium on the growth of the food-borne pathogen Listeria ... more The effect of the microstructure of the medium on the growth of the food-borne pathogen Listeria monocytogenes was studied. The pathogen's growth kinetics was evaluated using liquid substrates and gels formed from different concentrations of sodium alginate (3.0% w/w) and gelatin (0-30.0% w/w). These results were further verified using a model dairy product with solid concentrations varying from 10.0 to 40.0% w/w. The pathogen's growth was faster in the liquid media than in the gels regardless of the gelling agent employed. The substrate's microstructure, apart from altering the growth pattern from planktonic to colonial, resulted in microbial growth suppression; however, each system affected the microorganism's growth in a different way. The suppressing effect of the substrate's microstructure on microbial growth was also dependent on temperature, while the presence of glucose in the solid medium accelerated microbial growth, thus reducing substantially the difference in growth kinetics between the gels and the liquid media. Any increase in the hydrocolloid concentration, which was also reflected in the rheological properties of the structured samples, resulted in a reduction of growth rate and in an increase of the lag phase of the pathogen. Overall, the gelation of the medium was found to exert a stress on the microorganism since the sol-gel transition, when the pathogen was already at the exponential growth phase, resulted in an additional lag phase or a decrease in the growth rate. The relationship between maximum specific growth rate and loss tangent of the gels (tanδ = G″/G′) was explored, pointing to the possible use of a single structural parameter to describe food matrix effects on microbial growth kinetics.
Langmuir, 2006
This work investigates surface properties of a protein particle gel and effects of polysaccharide... more This work investigates surface properties of a protein particle gel and effects of polysaccharide on the surface microstructure of such a protein gel. Whey protein isolate (WPI) was used as the primary gelling agent, and a polysaccharide (xanthan) was investigated for its surface smoothing effects. The surface properties of heat-set WPI gels with and without the presence of xanthan (0, 0.05, and 0.25%) were characterized using a surface friction technique. The surface friction force of a gel against a stainless steel substrate was found to be highly dependent on the sliding speed for all three gel samples, and the addition of xanthan caused a general reduction of surface friction. The gel containing no xanthan has the largest surface friction and behaved in the most load-dependent manner, whereas the gel containing 0.25% xanthan has the lowest surface friction and showed the least load dependency. It was inferred that the WPI gel containing no xanthan has the roughest surface among the three samples and the presence of xanthan leads to a smoother surface with probably a thinner layer of surface water. Surface features derived from surface friction tests were confirmed by surface microstructure observation from confocal laser scanning microscopy (CLSM) and environmental electron scanning microscopy (ESEM). Surface profiles from CLSM images were used to quantify the surface roughness of these gels. The mean square root surface roughness R(q) was calculated to be 3.8 +/- 0.2, 3.0 +/- 0.2, and 1.5 +/- 0.2 microm for gels containing 0, 0.05, and 0.25% xanthan, respectively. The dual excitation images of protein and xanthan from CLSM observation and images from ESEM observation indicate a xanthan-rich layer at the surfaces of the xanthan-containing gel samples. We speculate that the creation of the outer surface of a particle gel is based on a different particle aggregation mechanism from that leading to network formation in the bulk.
Langmuir, 2006
The emulsification processes, during which acylglycerols/zinc stearate emulsifier, water, and oil... more The emulsification processes, during which acylglycerols/zinc stearate emulsifier, water, and oil phase formed ternary systems, such as water-in-oil (W/O) emulsions, oil-in-water (O/W) dispersions, and unstable oil-water mixtures, were investigated in order to characterize the progressive transformations of the dispersed systems. The type, structure, and phase transitions of the systems were found to be determined by temperature and water phase content. Crystallization of the emulsifier caused the destabilization and subsequent phase inversion of the emulsions studied, at a temperature of 60-61 degrees C. The observed destabilization was temporary and led, at lower temperature, to W/O emulsions, "O/W + O" systems, or O/W dispersions, depending on the water content. Simultaneous emulsification and cooling of 20-50 wt % water systems resulted in the formation of stable W/O emulsions that contained a number of large water droplets with dispersed oil globules inside them ("W/O + O/W/O"). In water-rich systems (60-80 wt % of water), crystallization of the emulsifier was found to influence the formation of crystalline vesicle structures that coexisted, in the external water phase, with globules of crystallized oil phase. Results of calorimetric, rheological, and light scattering experiments, for the O/W dispersions obtained, indicate the possible transition of a monostearoylglycerol-based alpha-crystalline gel phase to a coagel state, in these multicomponent systems.
Langmuir, 2006
Brownian diffusion of fluorescent microspheres (0.21, 0.5, and 0.89 microm diameter) in conjuncti... more Brownian diffusion of fluorescent microspheres (0.21, 0.5, and 0.89 microm diameter) in conjunction with confocal microscopy has been used to monitor the microrheology of phase-separated regions in a protein-stabilized oil-in-water emulsion containing various low concentrations of a nonadsorbing polysaccharide, xanthan gum. The sensitivity and reliability of the technique has been demonstrated in test experiments on (i) aqueous glycerol solutions and (ii) concentrated surfactant-stabilized emulsions (30-60 vol % oil, 1-2 wt % Tween 20). From particle tracking measurements on the caseinate-stabilized emulsions (30 vol % oil, 1.4 wt % sodium caseinate, pH 7) containing xanthan (0.03-0.07 wt %), the apparent viscosity in the oil-droplet-rich regions has been estimated to be up to 10(3) times higher than that in the phase-separated xanthan-rich regions. This means that our previously determined shape relaxation times for xanthan-containing blobs in the same systems can be attributed to the dominant viscoelasticity of the surrounding regions of concentrated oil droplets and not to the rheology of the xanthan-rich blobs themselves. These data provide clear and unequivocal evidence for the dominant role of the interconnected depletion-flocculated network of oil droplets in the physicochemical mechanism by which hydrocolloid thickeners control the creaming instability of concentrated oil-in-water emulsions.
Journal of Texture Studies, 2005
The surface properties of heat-set whey protein gels (14 wt %) was studied by measuring the frict... more The surface properties of heat-set whey protein gels (14 wt %) was studied by measuring the friction at the gel's surface. A simple device was constructed that can be conveniently attached to a Texture Analyzer. Surface friction forces of gels with and without addition of salt were measured as a function of sliding speed and surface load. Surface friction strongly depended on the sliding speed for all three gel systems over the speed range 0.01 mm/s to 10 mm/s. The gel without salt addition showed the highest speed dependency, while the gel containing 200 mM NaCl had the lowest speed dependency. Surface load tests showed nearly linear relationships for both protein gels (with and without salt addition). Unlike solid materials, both protein gels exhibited a surface friction even as the surface load approached zero. Possible contributions of surface attraction and viscous flow to the measured forces are discussed. Results from surface friction tests were further confirmed by optical observation of the surface using a confocal laser scanning microscope (CLSM), where a very smooth surface was observed for the whey protein gel without salt addition, but a much rougher surface was observed for the gel containing 200 mM NaCl.
Journal of Food Engineering, 2011
A new technique was developed for measuring the thickness of can coatings applied to a metal subs... more A new technique was developed for measuring the thickness of can coatings applied to a metal substrate. A metal disc with a known radius creates a small indent in the coating, stopping exactly when the coating thickness is cut through. The indent length is employed to calculate the coating thickness. Accurate measurement of thin films is achieved as the indent is much larger than the respective film thickness. The thickness of epoxy phenolic coatings on tinplate were measured and the results were validated by micrographic cross section analysis and by confocal laser scanning microscopy (CLSM). The coating thicknesses measured using the indentation method coincide with the film thicknesses measured by CLSM, whereas discrepancies were observed using the micrographic cross section analysis. The new technique, presented in this paper, uses simple equipment, is inexpensive, fast and easy to use, and is suitable for film thickness measurements in laboratories and production plants.
Journal of Colloid and Interface Science, 2010
The sol-gel transition of a model dairy system (sodium caseinate solution) which undergoes gelati... more The sol-gel transition of a model dairy system (sodium caseinate solution) which undergoes gelation by acidification has been studied by conventional bulk rheology and particle tracking microrheology, via confocal microscopy. The Brownian diffusion of fluorescent microspheres (0.21, 0.32, 0.5, and 0.89 μm in diameter) with different surface coatings (polyethylene glycol, carboxylate groups and polystyrene) was used to probe spatial mechanical properties of the gels at the scale of microns. The microrheological results are compared with the macroscopic viscoelastic properties (storage and loss shear modulus) measured in a concentric cylinder rheometer (double gap, at shear strain of 0.005 and frequency of 1 Hz). At pH values close to pI of the caseins, where formation of a protein network, i.e., gelation, became obvious from the confocal microscopy and bulk rheological measurements, all the particles had a tendency to adhere to the network. In spite of this, the microrheological values of the moduli were only slightly lower than the macroscopically determined values and the gel points calculated via both techniques tended to be in good agreement. However, the particle tracking method has higher sensitivity and can detect changes in the structuring of the system before these are registered by the bulk rheological measurement.
Journal of Colloid and Interface Science, 2005
The time-dependent evolution of the phase-separated microstructure of a caseinate-stabilised emul... more The time-dependent evolution of the phase-separated microstructure of a caseinate-stabilised emulsion containing xanthan gum added before emulsification has been investigated by confocal laser scanning microscopy, image analysis and rheology. Moderately low levels of xanthan addition lead to depletion flocculation and gravity-induced phase separation. Increasing the polysaccharide concentration causes immobilisation of the microstructure due to an increase in the local viscoelasticity: that is, the emulsion structure cannot easily rearrange to expel xanthan-enriched aqueous serum phase because a weak gel-like network is generated. The effect of xanthan on the evolving microstructure of phase-separated regions, which reflects indirectly the local emulsion micro-rheology, has been estimated from image analysis of time sequences of confocal micrographs. A comparison has been made between object shape analysis using four different shape descriptors. The roundness parameter has been found to be a convenient descriptor for reliably quantifying the structural change in terms of the relaxation rate of xanthan-rich aqueous drops. The Taylor parameter has been used to link the kinetics of drop relaxation to the timedependent small-deformation rheological behaviour. The analysis of the combined experimental data reveals the difficulty of relating the evolving microstructure to bulk rheological measurements. 2004 Elsevier Inc. All rights reserved.
Journal of Colloid and Interface Science, 2012
The sol-gel transition of barley isolated β-glucan solutions which undergo gelation with ageing h... more The sol-gel transition of barley isolated β-glucan solutions which undergo gelation with ageing has been studied by conventional bulk rheology, phase contrast microscopy and particle tracking microrheology. Also, characterization of primary structure of the β-glucan isolate was carried out by HPLC. The Brownian diffusion of fluorescent microspheres (0.75 µm diameter, carboxylate-coated particles) was used to probe spatial mechanical properties of the gels at the scale of microns. The potential use of passive particle tracking as a new method of studying food systems that present spatial heterogeneities is explored. For the β-glucan gels cured at 25 o C both the microrheology and the bulk rheology revealed that with increasing concentration of the polysaccharide the gelation time decreased, while the gelation rate and gel strength of the barley β-glucan gels increased. Moreover, the melting point increased with increasing concentration of the β-glucans indicating a better organization of the ordered domains in the network structure. The particle tracking method had higher sensitivity and could map molecular ordering and structural heterogeneities at a micro level. Furthermore, this method could detect changes in the structuring of the system before such events can be registered by the bulk rheological measurements. For these reasons, the timescales of the microscopic dynamic behavior do not always seem to match the timescales of the overall macroscopic behavior.
International Dairy Journal, 2012
Oil-in-water emulsions (10%, w/w, oil) were prepared at pH 5.7 by using electrostatically formed ... more Oil-in-water emulsions (10%, w/w, oil) were prepared at pH 5.7 by using electrostatically formed complexes of 0.5% (w/w) sodium caseinate (Na-CAS) and 0e0.6% (w/w) chitosan. Emulsions stabilized by complexes with increased levels of chitosan (>0.2% w/w) had a smaller average droplet size and exhibited greater stability during storage. All chitosan containing emulsions exhibited a slight shear thinning behaviour with increased high-shear-rate viscosity at higher chitosan levels. Even though complexing of Na-CAS with chitosan resulted in a delay of the adsorption of the surface active components at the oil/water interface, the presence of chitosan did not significantly influence the rheological properties of the formed interfaces. Acid-induced aggregation was observed only for the emulsion that was stabilized solely by Na-CAS when the pH dropped close to the isoelectric point of the protein component, resulting in flocculation and large increase in the storage modulus.
Food Hydrocolloids, 2014
The impact of oat b-glucan fortification (1.4%) on the acidification and gelation kinetics of fer... more The impact of oat b-glucan fortification (1.4%) on the acidification and gelation kinetics of fermented skimmed milk (12% total solids) as well as the effects of inclusion of a probiotic strain (Lactobacillus paracasei subsp. paracasei B117) into the yogurt starter culture (Lactobacillus delbrueckii subsp. bulgaricus Y 6.15 and Streptococcus thermophilus Y 4.10) and fermentation temperature (36, 38 and 40 C) have been explored by dynamic rheometry, pH measurements and microbial enumeration. The viability/growth of the mixed culture bacteria was also evaluated upon storage. Incorporation of b-glucan to milk resulted in phase separation between proteins and the added polysaccharide, altering the entire gelation process as manifested by the development of a transient gel structure at the early stages, followed by significant (P < 0.05) retardation of both protein aggregation and acidification kinetics, and finally the formation of significantly weaker gels compared to control formulations. Furthermore, fortification of milk with bglucan led to a liquid-like structure (tan d > 1) at the end of fermentation (pH ¼ 4.6) when performed at 36 C. A higher fermentation temperature accelerated the acidification kinetics, yielding a shorter gelation time, but the storage modulus values of milk gels decreased. Inclusion of the probiotic strain into the yogurt starter culture seemed to increase the gelation rate without affecting the gelation time and gel strength. Viability of all three culture bacteria was enhanced by decreasing fermentation temperature and during storage. Moreover, the L. paracasei showed good compatibility with the yogurt starter culture and the addition of b-glucan enhanced the viability of the probiotic strain in the fermented products throughout cold storage (4 C).
Food Hydrocolloids, 2014
The effect of sodium chloride (NaCl) and pH of sorbitol-plasticized whey protein isolate (WPI) an... more The effect of sodium chloride (NaCl) and pH of sorbitol-plasticized whey protein isolate (WPI) and sodium caseinate (NaCas) films on their mechanical and physical properties were examined. Moreover, antimicrobial films were prepared by incorporating different levels of potassium sorbate and natamycin in WPI films, plasticized with sorbitol, and the diffusion rates of the antimicrobial agents in a liquid medium were evaluated. The addition of NaCl resulted in a decline of Young modulus (E) of the edible films and an increase of the elongation at break upon extension (%EB). For WPI films, reducing the pH of the casting solution down to 5.0 resulted in creation of WPI films with greater flexibility than those at pH 7.0, while the highest %EB values were observed at pH 6.0. The moisture uptake behavior increased with the addition of sodium chloride and the films made from casting the WPI-sorbitol solution containing 200 mM NaCl showed a greater moisture adsorption capacity at a given a w . The addition of NaCl did not affect significantly the water vapor permeability with the exception of 300 mM NaCl. Whey protein films, made by casting a solution of pH 5.0 showed a significant increase in water vapor permeability (WVP), compared with films originated from solutions with pH 7.0 or 6.0. The addition of sodium chloride and the pH reduction of edible WPI films containing potassium sorbate resulted in reduction of the diffusion coefficient of the antimicrobial. In the case of films containing natamycin, adding sodium chloride did not affect the diffusion coefficient, whereas reducing the pH values of the film forming solution decreased this parameter. The diffusion rate of the two antifungal agents was inversely affected by the molecular shape/size of the component.
Food Hydrocolloids, 2014
Confocal microscopy Microstructure Particle tracking microrheology a b s t r a c t Cereal b-gluca... more Confocal microscopy Microstructure Particle tracking microrheology a b s t r a c t Cereal b-glucans are known to display functional properties with health benefits, such as reduction of plasma cholesterol and of postprandial serum glucose levels in humans and animals; such effects have been lately attributed to viscosity and gelation potential of these water soluble fibres. The local dynamics of b-glucan solutions differing in molecular size and the molar ratio of trimers to tetramers (DP3/DP4) chain segments, which undergo gelation upon ageing, was investigated. Confocal microscopy, particle tracking microrheology and conventional bulk shear rheological measurements have been employed to study the microstructure and mechanical properties of the polysaccharide networks on various length scales. The structural features of b-glucans, such as molecular weight and ratio of DP3/DP4, were found to be important determinants of their gelling properties and microstructure. For the b-glucan gels cured at 25 C both the microrheology and the bulk rheology revealed that with decreasing molecular weight and increasing DP3/DP4 molar ratio the gelation time decreased, while the gelation rate increased along with the storage modulus. In all samples studied, particulate clusters of b-glucan assemblies were generated with size w1e15 mm and the clusters were eventually interconnected to expand over the available space to form an elastic network. The pore size and the structural entities were found to increase in size at lower values of the DP3/DP4 molar ratio and with preparations of high molecular weight. The size and the compactness of the structural entities seem to play an important role in the network reinforcement. The embedded tracer particles were found to experience relatively homogeneous microenvironments at DP3/DP4 w2.1, reflecting a rather slow rate of chain aggregates structuring. The behaviour of the polysaccharide network dynamics at a microscopic level does not always seem to match the overall bulk macroscopic response.
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Papers by Thomas Moschakis