Ganesan Narsimhan

Effect of composition and pore structure on binding energy and effective diffusivity of moisture in porous food

Abstract Food samples of different compositions were formulated by extruding mixtures of starch and gluten of different compositions, whereas samples of varying pore structure were obtained through extrusion of durum semolina at 57 and 137°C. Experimental measurement of desorption isotherm of these samples at different temperatures in the range 25–40°C indicated that the water binding ability was fairly insensitive to the pore structure of food samples and was found to be lower for extruded and pregelatinized samples as well as for samples of higher gluten content. Experimental desorption isotherm data were represented by the Oswin equation in order to evaluate moisture binding energy as a function of moisture content and temperature by employing the Clausius-Clapeyron equation. Binding energy was found to be negligible at high moistures (above 0·2 d.b.), increasing at lower moisture contents, lower temperatures and higher starch contents. Effective diffusivity of moisture through the porous food at different moisture contents and temperatures was inferred from drying curves in the temperature range 60–105°C. Effective diffusivity (D eff ) was higher in pregelatinized samples and was found to be much higher through porous puffed pasta than regular pasta. The effect of composition on D eff was confounded by the effect of pore structure as a result of variations in pore size distribution in the extruded samples of different compositions. The decrease in D eff at lower moisture contents was postulated to be due to the decrease in the available free water for diffusion and was explained through a simple model which related the available free moisture to the binding energy.

Droplet breakage in stirred dispersions. Breakage functions from experimental drop-size distributions

Transient breakage drop-size distributions have been experimentally measured using an image analysis technique. The transient distributions show self-similar behavior. The breakage rate and daughter-drop distribution functions have been determined using an inverse-problem approach which takes advantage of this self-similarity. The inverse-problem results show that the breakage rate is not a power law function of the drop size. The breakage rate is found to increase sharply with the drop size and the stirrer speed while decreasing sharply with increase in the interfacial tension. It is also found to decrease with increase in the dispersed phase viscosity, though the dependence on the viscosity is weaker than on the other variables. The daughter drop distribution was found to be relatively insensitive to the stirrer speed and interfacial tension, but was found to depend on the dispersed phase viscosity. As the drop viscosity increases, the breakage becomes more erosive in nature, leading to a broader size distribution of daughter drops. Generalized correlations for the breakage rate and daughter-drop distribution which account for the effect of physical properties and experimental conditions are presented. These relations will be very useful in predicting the drop-size distributions in stirred dispersions. Models for the breakage functions are compared with those determined in this study and the model predictions of the transient-size distributions are compared with the experimental data.

Designing carbohydrate nanoparticles for prolonged efficacy of antimicrobial peptide

In this work, carbohydrate nanoparticles were created to prolong the efficacy of antimicrobial peptide against pathogens. Nisin and Listeria monocytogenes were used as the peptide and pathogen models, respectively, and phytoglycogen (PG)-based nanoparticles were developed as carriers of nisin. PG from su1 mutant maize was subjected to β-amylolysis as well as subsequent succinate or octenyl succinate substitutions. The goal was to minimize the loss of peptide during storage and meanwhile realize an effective release in the presence of bacteria. The capabilities of PG derivatives as carriers of nisin were evaluated using centrifugal ultrafiltration, zeta-potential, and the initial availability of nisin against L. monocytogenes. All methods indicated that nisin loading was favored by a high degree of substitution (DS), presence of hydrophobic octenyl moiety, and β-amylolysis of PG nanoparticles. To evaluate the prolonged nisin efficacy, preparations containing nisin and PG derivatives were loaded into a BHI-agar deep-well model (mimicking nisin depletion at the nutrient-containing surface). The residual inhibitory activities of preparations against L. monocytogenes were monitored during 21 days of storage at 4 °C. The results showed that all PG derivatives led to the prolonged retention of nisin activity and the longest retention was associated with high DS, β-amylolysis, and octenyl succinate. Evidently, both electrostatic and hydrophobic interactions are the driving forces of nisin adsorption, and the glucan structure at the nanoparticle surface also affects nisin loading and retention during storage.

Effect of surface concentration on secondary and tertiary conformational changes of lysozyme adsorbed on silica nanoparticles

Kinetics of tertiary conformation of lysozyme adsorbed on 90 nm silica nanoparticles was inferred using tryptophan fluorescence for different surface concentrations (0.24 to 0.92 mg/m(2)), pH (4, 7 and 9), ionic strength (10 and 100 mM), 2,2,2-trifluoroethanol (TFE) (5, 15 and 30%) and Dithiothreitol (DTT) (0.5 mg/ml) concentrations. A rapid initial unfolding, followed by a much slower refolding and subsequent unfolding, were observed with the extent of unfolding being higher at lower surface concentration, higher ionic strengths, higher TFE and DTT concentrations and at pH 9. The rate of unfolding was found to be higher at lower surface concentrations, pH 4, higher ionic strengths, higher TFE and DTT concentrations. In contrast, earlier results showed that beta lactoglobulin unfolded slower and exhibited only an initial rapid and a subsequent slow unfolding phase. Circular Dichroism spectra showed that alpha helix content was lower for adsorbed lysozyme compared to bulk with a corresponding increase in beta sheet and random coil. This decrease in alpha helix was found to be more pronounced at lower surface concentrations. DTT decreased alpha helix with a corresponding increase in random coil while TFE was found to have negligible effect on secondary structure.

A new approach for the prediction of the rate of nucleation in liquids

Abstract A formalism is developed for the prediction of the rate of nucleation in liquids which as opposed to the usual quasi-thermodynamic treatment does not employ (i) the macroscopic concept of interfacial tension for small clusters and (ii) the principle of detailed balance. The rate of dissociation of a surface monomer from a cluster is calculated by using a first passage time analysis. An expression for the critical cluster size is obtained in terms of the characteristics of the interaction potential between the monomers of a molecular cluster which, for illustrative purposes, is taken here as the square well potential. A generalized Kelvin equation is derived to obtain the dependence of the solubility on the cluster size. Finally, an expression for the rate of nucleation is obtained on the basis of the kinetics of dissociation. The predicted nucleation rates for spherical clusters with uniform distribution of molecules are found to be much larger than those predicted by the classical theory. In contrast to the classical theory, the nucleation rates exhibit a gradual increase at very low supersaturations, increase catastrophically at a critical supersaturation, and taper off gradually at high supersaturations. The predicted nucleation rates for approximately spherical clusters with fee packing are found to be much smaller than those for spherical amorphous clusters and exhibit behavior qualitatively different from that of the latter. The classical theory is shown to overpredict the interfacial tension and hence underpredict the nucleation rates. Since the spherical amorphous cluster and a cluster with a definite structure such as fee are probably appropriate models for large and small clusters, respectively, the actual nucleation rates should lie between the two, coinciding with the former at low and the latter at high supersaturations. The classical theory for the rate of nucleation can be recovered from the present formalism when the critical cluster size is sufficiently large, i.e., larger than 10 6 monomers. Such a condition, however, is satisfied only for extremely small supersaturations.

A model for unsteady state drainage of a static foam

Abstract A model for the unsteady state drainage of a standing foam has been proposed. This model accounted for (i) the liquid drainage from plateau borders due to gravity as well as the gradient of plateau border suction and (ii) the movement of the foam-liquid interface due to the accumulation of drained liquid, and assumed that the foam bed consisted of dodecahedral bubbles of the same size. The simplifying assumption of a negligible fraction of liquid in thin films, employed in the present model, was justified through the comparison of the timescales of film and plateau border drainage. Typical evolutions of a liquid holdup profile as well as a foam-liquid interface are reported for foam generated by bubbling inert gas through a liquid pool at a constant gas flow rate in the form of bubbles of the same size. The predicted equilibrium liquid holdup profile has been shown to depend only on the density difference, bubble size and surface tension but not on viscosity and surface viscosity, and compared well with experimental data. Asymptotic expression for the equilibrium disjoining pressure indicated that larger density difference, larger bubble sizes and smaller surface tensions would tend to make the standing foam less stable.

Effects of kinetics of adsorption and coalescence on continuous foam concentration of proteins: Comparison of experimental results with model predictions

Protein enrichment and recovery were measured in a continuous foam concentration column for bovine serum albumin (BSA) for different pool heights, foam heights, superficial gas velocities, bubble sizes, feed flow rates, pH, and ionic strengths. Protein enrichment was found to decrease with an increase in pool height for low pool heights, reach a minimum at an intermediate pool height, and subsequently increase with pool height for sufficiently large pool heights eventually approaching an asymptotic value. Such a behavior was due to the combined effects of kinetics of adsorption of protein and coalescence. The increase in protein enrichment with pool height was due to the predominant effect of kinetics of adsorption of protein, whereas the opposite behavior at low pool heights was due to the predominant effect of coalescence in the foam. Protein enrichment was found to be higher for smaller feed concentrations, smaller gas velocities, larger bubble sizes, and larger foam heights. Enrichment at pH values different from the isoelectric point was found to be higher because of more coalescence. A model for foam concentration of proteins was employed to predict enrichment and recovery. The model predictions agreed well with the experimental data.

Solution of inverse problems in population balances-II. Particle break-up

Abstract A mathematical and computational procedure called the inverse problem is developed to extract quantitative information from transient particle size distribution measurements The population balance framework describes the evolution of these transient size distributions. This paper describes an inverse problem for the determination of the size specific breakage rates and daugher drop distributions from transient size distribution measurements when these distributions evolve to a “self-preserving” or similarity distribution. The experimental time scaled with respect to the timescale of breakage is used as the similarity variable. A test for the existence of similarity is developed. This test uses only the available transient size distribution data. The result of this test is also used to determine breakage rate information The determination of the daughter drop distribution is an ill-posed problem. The ill-posedness is overcome by using the property that the distribution is a monotone function. Analysis shows that the asymptotic behavior of the daughter drop distribution can be determined from the experimental similarity distribution. Incorporating this additional information into the solution strategy has resulted in significantly improved solutions of the inverse problem. The optimum solution is chosen such that the similarity distribution predicted using this solution has error of the same order of magnitude as the error in the experimental similarity distribution. Several examples of the inverse problem are outlined

Adsorption dynamics and interfacial properties of α-lactalbumin in native and molten globule state conformation at air–water interface

Abstract The effect of conformation of α-lactalbumin on the dynamics of adsorption as well as on the properties of adsorbed layer at air–water interface were investigated. The surface hydrophobicity of molten globule conformation of α-lactalbumin (prepared by lowering pH to 2) was found to increase by 15-fold compared to the protein in its native form (pH 7) with increased flexibility as evidenced by a much smaller hysteresis area of spread surface pressure isotherms. This higher surface hydrophobicity resulted in (i) higher surface activity of α-lactalbumin in its molten globule conformation compared to the native form inspite of a much higher electrical energy barrier due to much higher net charge and (ii) diffusion controlled adsorption for much longer times and surface pressures than the native form. The diffusion coefficient of α-lactalbumin in molten globule state was lower than that for native form (2.22×10−10 m2 s−1 vs 4.65×10−10 m2 s−1) and increased for the former at higher ionic strength. The area per molecule during the dynamics of adsorption was found to be the same at a fixed surface pressure for both the conformations and lower than the corresponding value for fully denatured protein thus indicating that α-lactalbumin does not fully unfold at the interface in both the conformations. Even though α-lactalbumin adsorbed much more in its molten globule conformation, it gave poor interfacial rheological properties compared to the native form. Ionic strength did not influence the relationship between the interfacial elasticity and surface concentration for both the conformations.

The Brownian coagulation of aerosols over the entire range of Knudsen numbers: Connection between the sticking probability and the interaction forces

Abstract The existing models for Brownian coagulation of aerosols account for the effect of interparticle forces through a phenomenological sticking probability, i.e., the probability of coagulation upon collision. This probability is usually assumed to be equal to unity. A model for Brownian coagulation of equal-sized electrically neutral aerosol particles is proposed, which takes into account explicitly the van der Waals attraction and Born repulsion, instead of the phenomenological sticking probability. In this model, the relative motion between two particles in the vicinity of the sphere of influence is considered to be free molecular, the thickness of this region is taken to be equal to the average correlation length for the relative Brownian motion. The relative motion of the particles outside this region is described by the Fokker-Plank equation. The sticking probability predicted by the model, in terms of the interaction potential between two particles, becomes vanishingly small for very small particles. The collisions with the medium that a particle experiences during its escape from the potential well and the corresponding energy dissipation, neglected in this model, will increase the coagulation coefficient, in particular for the large particles because they stay longer in the region in which the interaction potential is acting. For this reason the model is likely to be valid for sufficiently small particles and to provide a lower bound for the coagulation coefficient of the larger particles. However, for large particles the interaction potential is deep and the decay length of the potential is small compared to the particle sizes. As a result, the region of the interaction potential can be replaced by a sink. This is equivalent to considering the sticking probability as equal to unity. In contrast to the earlier models, the expression derived for the coagulation coefficient in the latter case displays the proper continuum and free molecular limits and agrees well with the Fuchs empirical formula. It yields an upper bound for sufficiently small particles but can be exceeded for particles of intermediate sizes. Comparison with the experimental data seem to indicate the validity of this upper bound for particles as small as 0.01 μm.