J.M. Franco

On slip effects in steady-state flow measurements of oil-in-water food emulsions

The problem of the wall slip in steady-state flow measurements of oil-in-water (o/w) food emulsions has been investigated for different sensor system geometries and surface profiles. Experimental results demonstrated that slip effects are strongly dependent on the type of emulsion studied. This may be related to the emulsion microstructure. Thus, for instance, a highly structured gel-like continuous phase dampens wall slip effects, as opposed to emulsions in which creaming appears as a mechanism of instability, because of the formation of particulate flocs. The occurrence of wall slip is more clearly observed if the flow curves are obtained from stress sweep experiments. Although the recovery of the steady-state viscosity of a presheared sample is generally complete (similar values at low and high shear rates), the appearance of slip effects may be favoured by emulsion preshearing.

Flow behaviour and stability of light mayonnaise containing a mixture of egg yolk and sucrose stearate as emulsifiers

Abstract In this paper the influences of sucrose stearate concentration, fraction of dispersed phase and processing parameters on the stability of oil-in-water emulsions with a chemical composition similar to commercial light mayonnaise, but using a mixture of egg yolk and a sucrose stearate, with high hydrophilic—lipophilic balance, have been studied. Emulsions have been characterized by carrying out steady flow tests and droplet size distribution measurements as a function of ageing. From the experimental results obtained we may conclude that the stability of the emulsions is improved by increasing energy input and oil and sucrose stearate concentrations, using processing temperatures ~50°C. All these factors yield higher values of steady-state viscosity. The emulsions studied show both linear viscoelastic functions and droplet size distribution values quantitatively similar to those reported for other commercial mayonnaises. Moreover, a significant flocculation process takes place up to around six days after manufacture. Nevertheless, the application of a steady shear on the samples produces marked structural destruction.

Development of new green lubricating grease formulations based on cellulosic derivatives and castor oil

Environmentally friendly lubricating greases may be produced by solely replacing the mineral base oil for vegetable oil. However, the substitution of traditional metallic soaps by biodegradable and renewable thickeners is, up to now, much less considered. This work is focused on the development of new oleogels, using castor oil and cellulose derivatives, which could be potentially used as biodegradable lubricating greases. Thermal and thermo-rheological behaviours of these materials were characterised by means of TGA analysis and SAOS measurements, in order to evaluate the evolution of oleogel microstructure with temperature. Moreover, both roll-stability and leakage tendency standard tests, usually performed in the grease industry, were used to evaluate the mechanical resistance of each sample. The evolution of biogrease linear viscoelasticity functions with frequency is quite similar to that found for traditional lithium lubricating greases. However, the influence of temperature on biogreases linear viscoelasticity functions is less important than that found for traditional greases. In general, the biogrease samples studied show both slightly lower mechanical stability and higher leakage tendency than traditional lubricating greases. The use of a blend of ethyl and methyl cellulose as thickener provides a mechanical stability comparable to that found for commercial greases.

Influence of the Geometry on the Transient and Steady Flow of Lubricating Greases

The objective of this paper is to investigate the influences that geometry, roughness of the sensor system and gap size exert on the rheological measurements of a commercial lubricating grease. With this aim, creep and stress growth experiments, stepped strain rate ramps and dynamic measurements were performed using different geometries and roughness. A significant influence of gap size was found in transient and steady-state experiments in a certain range of strain rate. Roughening the surface of the geometry increases considerably the values of the stress in comparison to the measurements performed with smooth surfaces. The experimental results are discussed taking into account the wall slip phenomenon. In addition to this, different grooves made in the surfaces of the tools seem to eliminate or considerably decrease these wall slip effects. However, the influence of geometry or roughness was not detected in oscillatory tests performed inside the linear viscoelastic range.

Influence of composition of emulsifier blends on the rheological properties of salad dressing-type emulsions

This paper deals with the influence that composition of emulsifier blends exerts on the rheological properties of low-in-fat salad dressing-type emulsions. Binary blends of egg yolk and different types of amphiphilic molecules (low-molecular weight and macromolecules) were used in several proportions to stabilize emulsions by keeping constant the total amount of emulsifier. The different emulsifiers added to egg yolk were pea protein, sodium caseinate, polyoxyethylene(20)-sorbitan monolaurate (Tween 20) and sucrose distearate. Steady state flow tests and small-amplitude oscillatory measurements within the linear viscoelasticity region were carried out. Rheological tests were complemented with droplet size distribution measurements and observation of physical stability against creaming of these emulsions. It was pointed out that rheological properties, droplet size and physical stability of the emulsions studied depended on the weight ratio of emulsifiers in the binary blends, although the emulsifier total concentration remained constant, as well as the nature of the substance blended with egg yolk. These results have been explained on the basis of the relationship among rheological properties, droplet size distribution, continuous phase characteristics and interactions among different emulsifier molecules.

Effect of thermal denaturation of lupin protein on its emulsifying properties

The effect of the protein-thermal treatment on the rheological behaviour, texture, surface hydrophobicity, droplet size distribution and colour of oil-in-water emulsions stabilised by a vegetable protein isolated from Lupinus albus seeds was studied. Protein isolates dispersions were treated at different temperatures, 50-90°C, whilst the thermal treatment time was varied between 10 and 40 min. The study was carried out according to an experimental design based on the response surface methodology (RSM). It was found that protein thermal treatment increases the values of the rheological and textural parameters and the protein surface hydrophobicity, and decreases the Sauter diameter of oil droplets.

Enhancement of gel strength by application of thermal treatments in highly flocculated emulsions

Thermal-induced changes of the rheological behavior of flocculated emulsions through the optimization of in situ thermorheological treatments, as well as the effect that several compositional variables (oil fraction, egg yolk concentration or cholesterol reduction level) exert on the enhancement of gel strength, were investigated. With this aim, oil-in-water emulsions were prepared using a spray-dried egg product. Different temperature ramps under oscillatory shear and droplet size distribution measurements were carried out. An increase in temperature produces, first of all, an increase in the storage modulus up to a critical temperature, which depends on heating rate. Subsequently, a dramatic decrease in the viscoelastic functions occurs, due to emulsion breakdown. However, the application of upward/downward temperature cycles, setting the maximum temperature at 67 °C, avoids emulsion breakdown and yield significantly higher values of the rheological functions in comparison to those found with fresh emulsions, in spite of the thermal-induced droplet coalescence observed. The final values of linear viscoelastic functions and droplet size depend on the type of cycles applied, which are based on different heating rates or times at the maximum temperature, and emulsion composition.

Rheology and thermal degradation of isocyanate-functionalized methyl cellulose-based oleogels.

The -NCO-functionalization of methyl cellulose with HMDI and its application to chemically gel the castor oil is explored in this work by analyzing the influence of functionalization degree on the rheological and thermogravimetric behavior of resulting chemical oleogels. With this aim, different methyl cellulose chemical modifications were achieved by limiting the proportion of HMDI and, subsequently, oleogels were obtained by dispersing these polymers in castor oil and promoting the reaction between those biopolymers and the hydroxyl groups located in the ricinoleic fatty acid chain. -NCO-functionalized methyl cellulose-based oleogels were characterized from themogravimetric and rheological points of view. Suitable thermal resistance and rheological characteristics were found in order to propose these oleogels as promising bio-based alternatives to traditional lubricating greases based on non-renewable resources. In general, -NCO-functionalized methyl cellulose thermally decomposed in three main steps whereas resulting oleogels thermal decomposition takes place in one main single stage which comprises the thermal degradation of both the polymer and the castor oil. Temperature range for thermal degradation is broadened when using highly -NCO-functionalized methyl cellulose. A cross-linked viscoelastic gel was obtained with methyl cellulose functionalized in a relatively low degree (around 6% -NCO molar content). The rheological properties of highly functionalized methyl cellulose-based oleogels evolve during several months of aging, but mainly during the first week, due to the progress of the reaction between -NCO functional groups and castor oil -OH groups. SAOS functions analyzed and oleogel relative elasticity increase with the functionalization degree. Oleogel linear viscoelastic response is also extremely dependent on NCO-functionalized methyl cellulose concentration.

Effect of rheological behaviour of lithium greases on the friction process

Purpose – The aim of this work is to investigate the relationship among rheology, tribology and traditional standardized technological parameters of lithium lubricating greases.Design/methodology/approach – Lubricating greases having the same composition but differing in processing protocols have been manufactured and characterized in order to isolate the rheological behaviour from the formulation.Findings – Some successful empirical correlations between rheological (viscous and viscoelastic) and technological standardized parameters, with the friction factor obtained from a ball‐disc tribometer, have been established in order to elucidate the role of the rheological behaviour of lubricating greases on the friction process. In addition to this, an energetic evaluation of the structural degradation of greases during the friction process has been carried out by performing stress‐growth experiments. Thus, the storage energy density, which is related to the grease capacity to accumulate energy in the elastic ...

Chemical modification of methyl cellulose with HMDI to modulate the thickening properties in castor oil

This work deals with the selective incorporation of reactive isocyanate groups into methyl cellulose in order to be used as reactive thickening agent in castor oil. Resulting gel-like dispersions may have potential applications as green lubricating greases formulated from renewable resources. Two different isocyanate-functionalized methyl cellulose-based polymers were obtained by reaction of methyl cellulose with 1,6-hexamethylene diisocyanate. The functionalization degree, from fully functionalized to a certain number of free hydroxyl groups (58:36:6 ratio between –OMe, –NCO and free –OH groups), was controlled by modifying the reagents molar ratio. These polymers were characterized through nuclear magnetic resonance of protons (1H-NMR), Fourier transform infrared spectroscopy and thermogravimetric analysis (TGA). Thermal and rheological responses of oleogels prepared by dispersing these polymers in castor oil were studied by means of TGA analysis and small-amplitude oscillatory shear measurements. The evolution of linear viscoelasticity functions with frequency of the oleogel containing isocyanate-functionalized methyl cellulose with lower –NCO content is quite similar to that found for traditional lithium lubricating greases. In relation to long-term stability of these oleogels, the values of viscoelastic functions significantly increase during the first 7 days of ageing and then remain almost constant.