::: Application Report

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1 Interfacial Shear Rheology of Coffee Samples Explore the secrets of a perfect crema! This application report presents typical measurements on the film formation and on the interfacial rheological properties of the final film of coffee samples. Anton Paar s Modular Compact Rheometer MCR 302 with IRS (Interfacial Rheology System) is the instrument for this charactarisation. 1 Introduction 1.1 General A lot of food products are either in the form of emulsions or foams for which interfacial properties are important to achieve stability of the products. Interfacial properties play a major role to prevent for example coalescence of emulsions or film drainage of foams. Various additives, which are often present or are added to improve the stability of the products, such as proteins, lipids or biopolymers, are due to their amphiphilic nature surface active and adsorbing at the air/water interface. Coffee is a complex dispersion, which for many coffee drinks is topped by a foam structure of tiny bubbles, e.g. the espresso foam - also called espresso cream or "crema". The "crema" is a polyphasic system consisting of dispersed tiny gas bubbles of carbon dioxide and water vapor surrounded by surfactant films, droplets of emulsified oils, and solid fragments smaller than 5µm [1]. Interfacial rheology does not probe the foam itself, but measures the adsorption of the amphiphilic ingredients and their network formation at the liquid surface. Higher values of the interfacial properties and a faster film formation are expected to correlate with a better foam stability. Some simple qualitative interfacial shear rheology measurements on coffee samples have been published and confirm the correlation between foam stability and interfacial rheology [2]. 1.2 Measuring Principles Different techniques have been used to measure interfacial shear properties. However, a combination of a suitable geometry with a rotational rheometer, which is both extremely sensitive in torque and angular resolution, offers the largest flexibility with respect to the various test possibilities and measuring ranges. The biconical bob geometry used for this work is depicted in Figure 1. The bicone geometry can be used for measurements of interfaces between two liquids or at the interface between one liquid and air, i.e. at the liquid surface. The edge of the bicone bob is located in the interfacial region between two immiscible liquids or at the surface of a single liquid. In case surface active substances (e.g. surfactants, lipids, or proteins) are present, due to their amphiphilic nature, they will adsorb at the interface forming a thin film, which often consists of just one monolayer. With the biconical bob being the rotor located at the interface and the stationary outer cup the biconical geometry acts as a "two-dimensional concentric cylinder geometry". As illustrated in Figure 2 the rotation of the bob leads to a shear gradient or shear flow in the monolayer film adsorbed at the interface. H 1 = 22,5mm H 2 = 45mm R = 40mm R 2 = 34,14mm 2 = 10 Fig. 1 The biconical geometry with the respective dimensions. C22IA108EN-B 1

2 interfacial velocity gradient shear direction used. In general, however, a more comprehensive analysis of the flow field in the rheometer geometry is desirable, yielding an interfacial velocity distribution that accounts for bulk contributions to the interfacial shear stress. Such analyses for the biconical bob rheometer have been published already in the seventies of the last century, but apparently, due to the complexity of the numerical calculations involved, they have not been used on a broader scale. A software analysis package based on the solution of the full flow field treatment for the biconical bob geometry is available. This allows the calculation of the absolute interfacial rheological properties for steady and dynamic shear conditions. This full flow field analysis is unique to the biconical geometry. Fig. 2 Schematic illustration of amphiphilic substances adsorbed at the interface and the interfacial shear gradient acting onto the interfacial films. In combination with a suitable rheometer the biconical geometry can be used for performing all kinds of rheological test methods permitted by the rheometer. This means not just rotational or steady flow curve measurements but also oscillatory or dynamic testing are possible with the biconical geometry. Moreover, creep and creep recovery, stress relaxation, start up of steady shear, etc. can be performed. The ability of doing steady flow and dynamic measurements with the same geometry is extremely valuable, since it allows to do all test types on the same sample filling. The film forming process can take a few seconds for low molecular weight surfactants to as long as days in the case of adsorbing proteins The two-dimensional interfacial shear stress is defined as: i i, with i being the interfacial shear viscosity ([ i ] = = N s/m), and being the shear rate, respectively. Other interfacial shear properties like the interfacial shear storage modulus and the interfacial shear loss modulus can be defined accordingly. Often when disks, rings, bicones, or similar geometries are used as interfacial rheometers, the interfacial flow is assumed to be completely decoupled from the bulk phase flow, i.e., dissipation of interfacial stresses into the bulk phases is considered negligible. In that case, such a rheometer may be treated as a two-dimensional Couette device, and the interfacial shear viscosity is easily calculated from the torque with standard procedures. For highly viscous interfacial films, e.g., protein adsorption layers studied at low shear rates, this expression has been Further details of the instrumental setup of the bicone rheometer and a detailed description of the analysis of the flow field for the biconical geometry can be found in the article of Erni et al. [3] and in the references therein. 2 Experimental Setup 2.1 Sample A commercial soluble coffee powder was mixed in different concentrations with distilled water. 2.2 Instrument and Methods An MCR 301 rheometer (predecessor of the MCR 302) with a Peltier temperature device (P-PTD200/80I) and the Interfacial Rheology System (IRS) accessory with the bicone (BiC68-5) geometry was used. The measurement temperature was set to 23 C and the Direct Strain Oscillation (DSO) method was used in oscillatory testing. For film formation measurements the bicone was first positioned by a normal force assisted surface detection to the water/air interface. After that the geometry was lifted up again and the coffee powder was mixed into the water. Immediately afterwards the geometry was moved back to the interface position. The special interfacial analysis software package is used to take into account the film subphase coupling and to calculate the absolute interfacial properties. 3 Results and Discussion Figure 3 shows the film formation for the same coffee sample at three different concentrations. Measured at a constant strain and a constant frequency it is possible to follow the adsorption and network formation of the surface C22IA108EN-B 2

3 active ingredients at the liquid/air interface. For higher concentrations the film shows elasticity already after a shorter time. In the case of the lowest concentration the moduli are increasing over a longer time reaching the final values not even after 13h (780min). Within the first 100min no elasticity is detected at all and the loss modulus is starting from very low values of about 10-5 Pa m, which is also the value for the pure water/air interface at the selected frequency. The interfacial shear viscosity and the interfacial shear stress as a function of the applied shear rates of a medium concentration (0.15g/114ml) coffee film measured 14h after the onset of the film formation is shown in Figure 4. A strong shear thinning behavior can be observed. Within the measured shear rate range the interfacial viscosity increases towards lower shear rates indicating a typical behavior for a fluid with an apparent yield stress. At shear rates higher than 30s -1 turbulence of the subphase starts to influence the results leading to a constant or even slightly increasing viscosity reading % 100 Strain Fig. 3 Film formation measurement at a constant strain (0.1%) and a constant frequency (1Hz) for three different concentrations (0.3g coffee powder / 114ml water (red), 0.15g coffee powder / 114ml water (green) and 0.05g coffee powder / 114ml water (blue)). i /s 100 Shear Rate Fig. 4 Interfacial shear viscosity and interfacial shear stress as a function of the shear rate of a coffee film (0.15g coffee powder / 114ml water) 14h after the start of the film formation. i Fig. 5 Amplitude sweep of a coffee film (0.3g coffee powder / 114ml water) at the frequency f = 1Hz. The interfacial storage modulus Gi and interfacial loss modulus Gi are plotted versus the applied strain. In Figure 5 an amplitude sweep is presented with logarithmically increasing strain at a constant frequency. At low strains the interfacial shear storage modulus Gi is higher than the interfacial shear loss modulus Gi indicating a gel-like structure at rest. The point where Gi starts to decrease is generally defined as the limiting value of the so-called linear visco-elastic (LVE) range. In order to evaluate the behavior of the interfacial film it is useful to look at the moduli as a function of the interfacial shear stress. In Figure 6 the same data as in Figure 5 are plotted versus the interfacial shear stress. The stress value at which the interfacial storage modulus is starting to decrease can be attributed as the apparent yield point. At higher shear stresses a crossover of the Gi and Gi curves can be seen. At strains higher than the crossover point the viscous part, i.e. Gi, is higher compared to the elastic part, i.e. Gi, indicating the interfacial film is liquid-like now showing flow behavior. Therefore the crossover point is also called flow point. The stress value at the flow point (2.6 Pa m) is clearly higher compared to the stress value at the end of the LVE-range (5.9 Pa m). C22IA108EN-B 3

4 third interval a small amplitude oscillation with constant strain and constant frequency was applied, whereas in the second interval a rotation with a constant shear rate was preset. As Figure 8 indicates the structure of the interfacial film breaks up during the constant shear rate interval, but it recovers fully in about 300s to the initial values. Grafik oder 10-5 Bild Interfacial Shear Stress i Fig. 6 The interfacial storage modulus and interfacial loss modulus (same data as in Figure 5) are plotted versus the interfacial shear stress. The yield (end of LVE-range) and flow point (crossover point) are indicated as points on the curve (see text). A frequency sweep measured at a small constant strain of 0.01% as depicted in Figure 7 illustrates that is higher than, i.e. the film is showing a gel-like structure at rest, over a frequency range from 0.005Hz up to 50Hz. At frequencies higher than 10Hz the data are influenced by turbulence as well as by sample and instrument inertia effects. Therefore, similar to the steady flow curve, the viscosity increases again at higher frequencies Grafik oder Bild Hz 10 1 Frequency f 10 1 Fig. 7 Frequency sweep of a coffee film (0.3g coffee powder / 114ml water) at a strain of 0.01%. The interfacial storage modulus, the interfacial loss modulus and the complex interfacial shear viscosity I i *I are plotted versus the frequency. Beside the initial film formation and the stability of the final film, it is worth to look at the time dependence after a mechanical distortion of the film, i.e. the structure recovery or thixotropic behavior. Figure 8 shows results of a so-called 3 Interval Thixotropy Test (3ITT). In the first and Grafik 10oder -2 Bild s 400 Time t Fig. 8 Structure recovery with a 3ITT-Test of a coffee film (0.3g coffee powder / 114ml water): First and third Interval: Oscillation with 0.1% strain at 1Hz. Second Interval: 10s rotation with a shear rate of 10s Conclusion An Interfacial Rheology System (IRS) based on a biconical geometry in combination with the MCR rheometer allows all kinds of test methods including steady flow and dynamic measurements. The flow field analysis reveals absolute values for the interfacial properties. The Interfacial Rheology System allows the investigation of films formed at the water/air interface after solving coffee powder into the water subphase. Various different tests provide information on the time dependent dynamics of the film formation itself as well as on the rheological properties of the final film. Viscosity curves are describing the shear rate dependence of the films. Amplitude and frequency sweeps are used to characterize the rest structure of the films. Structure recovery tests allow the description of the thixotropic behavior of the interfacial film. The coffee films are fully recoverable after distortion by a steady shear flow, and the time dependence of this process can be illustrated exactly. Interfacial shear rheology with a biconical geometry is a sensitive tool for the investigation of interfacial films formed at the liquid s surfaces. Since foam stability correlates strongly with interfacial shear rheology also the C22IA108EN-B 4

5 stability of coffee foams, i.e. the expresso cream, can be studied using this Interfacial Rheology System based on the bicone geometry. After all, wishing you buon gusto and a fine crema when enjoying your next espresso. 5 Literature [1] The complexity of coffee, Illy E.; J. Scientific American, 86-91, 06/2002 [2] Rheological interfacial properties of espresso coffee foaming fractions, Piazza, L., Bulbarello, A. and Gigli, J; 13th World Congress of Food Science & Technology, Nantes, France (2006); IUFoST 2006, DOI: /IUFoST: [3] Stress and strain-controlled measurements of interfacial shear viscosity and viscoelasticity at liquid / liquid and gas / liquid interfaces, Erni P., Fischer P., Windhab E.J., Kusnezov V., Stettin H., Läuger J; J.Rev.Sci.Instr., 74(11), (2003) Text: Jörg Läuger, 2007 Measurement: Jörg Läuger, 2007 Layout: Margot Brandstätter, 2011 Contact Anton Paar GmbH: Fig. 9 The Anton Paar Modular Compact Rheometer MCR 302 Tel: Fax: info@anton-paar.com C22IA108EN-B 5