Luis Elvira

On-line ultrasonic velocity monitoring of alcoholic fermentation kinetics

In this work, fundamental aspects on the ultrasonic velocity monitoring of alcoholic fermentations in synthetic broths (glucose, fructose and sucrose) and natural media (must and wort) are reported. Results are explained in terms of monosaccharide catabolism, polysaccharide hydrolysis, gas production and microorganism growth. The effect of each one of these subprocesses upon ultrasonic velocity has been independently studied. It is shown that, regarding the sound propagation, the simplest systems behave as ternary dissolutions of sugar and ethanol in water, where, in the course of time, substrates are transformed into metabolites according to the fermentation reaction. A semi-empirical approach, based on the excess volume concept and the density and velocity measurements of binary mixtures, has been used to calculate these magnitudes in the ternary mixtures and to obtain the concentrations of the main solutes throughout the fermentations, reaching a good correlation (especially for the media of simplest composition). In all the processes analyzed, the data obtained from the ultrasonic measurements followed the changes caused by the yeast metabolism, asserting the potential of mechanical waves to monitor fermentations and, in general, biotechnological processes.

Damage characterization using guided- wave linear arrays and image compounding techniques

In this work, a high-resolution imaging method for the inspection of isotropic plate-like structures using linear arrays and Lamb waves is proposed. The evaluation of these components is limited by the low dynamic range resulting from main lobe and side lobe field patterns, and from the narrowband nature of the Lamb waves. Based on a full matrix array, synthetic aperture technique using all emitter-receiver combinations, different images from the same object are obtained by using different apodization coefficients, which are related to a trade-off between main lobe width and relative side lobe level. Several image compounding strategies have been tested and a new algorithm, based on apodization and polarity diversities between signals, is proposed. However, some effects, such as the dead zone close to the array and reverberations caused by interactions of the wavefront and defects, still limit the quality of the images. The use of spatial diversity, obtained by an additional array, introduces complementary information about the defects and improves the results of the proposed algorithm, producing high-resolution, high-contrast images. Experimental results are shown for a 1-mm-thick isotropic aluminum plate with artificial defects using linear arrays formed by 30 piezoelectric elements, with the low dispersion symmetric mode S0 at the frequency of 330 kHz.

Two-step yielding in magnetorheology

We use particle level dynamic simulations, ultrasonic characterization, and rheomicroscopy to investigate the yielding behavior of magnetorheological (MR) fluids under oscillatory shear in both dilute and concentrated regimes. Dilute suspensions exhibit a single peak in the elastic stress that is associated to the breaking of the field-induced structures at the flow point (G′ = G″). On the other hand, more concentrated suspensions demonstrate a two-step yielding that is associated to the existence of short-ranged attractions between the particles, possibly coming from remnant magnetization or van der Waals forces. This two-step yielding is demonstrated by introducing additives in the formulation of the MR fluids and performing particle level simulations that include R-shifted Lennard-Jones potentials of interaction.

Ultrasonic measurement device for the characterization of microbiological and biochemical processes in liquid media

A measuring device for the characterization of liquid media based on the propagation of ultrasonic waves is presented. It is a four-channel system especially designed for monitoring microbiological and biochemical processes. The liquid samples are placed in commercial glass bottles which can be sterilized. The bottles have inlet and outlet tubes, which can be used for adding substances or extracting samples during the measuring process without interruption. Magnetic stirring can be used to keep the liquid agitated for homogenization purposes. Thermal control elements assure the temperature stability during the measurement. The liquid characterization is based on the detection of amplitude and time-of-flight changes in the sample under study. The main features, operation and performance of this ultrasonic device are analysed in this work, and some measurements and preliminary results are shown.

Ultrasonic velocity assay of extracellular invertase in living yeasts.

The use of a low-intensity ultrasonic technique (noninvasive, nondestructive, on-line, and able to assess opaque samples) to monitor the kinetics of invertase hydrolysis is presented. Adiabatic compressibility has been shown to be sensitive to sugar species: ultrasonic velocity increasing as saccharose is transformed into glucose and fructose. The influence of initial sucrose mass concentration (2-60%), temperature (25-55 degrees C), pH (3.5-6.5), and number of microorganisms (10(5)-10(9) yeasts/ml) on the reaction rate, catalyzed by the extracellular invertases of intact Saccharomyces cerevisiae cells, has been measured. The results were proven to be in strict agreement with the optimal kinetic parameters of the enzyme. Ultrasonic velocity variations are explained in terms of changes of the solute concentrations in the mixture water-saccharose-glucose/fructose and calculated from the velocity of ultrasound in the corresponding pure sugar solutions. A linear relationship between the initial rate of ultrasonic velocity and the number of yeasts (enzymes) is pointed out.

Measuring the yield stress in magnetorheological fluids using ultrasounds

In this work, we propose a method to accurately determine the yield stress in magnetorheological (MR) fluids using ultrasounds. The setup is constructed, and experimental data are obtained on a model conventional MR fluid under steady shear stress ramp-up tests. By using video-microscopy, ultrasonic techniques, and rheometry simultaneously, it is possible to precisely determine the yield stress at experimentally accessible times.

Sound speed and density characterization of milk adulterated with melamine

Milk contaminated with melamine resulted in an important health hazard that affected many babies in China recently. Ultrasonic characterization of adulterated milk may detect gross levels of melamine contamination. Sound speed and density measurements were made in skim milk as a function of melamine adulteration. An ultrasonic measurement technique to implement milk quality control is discussed.

Concentration measurement of yeast suspensions using high frequency ultrasound backscattering.

This work proposes the use of an ultrasound based technique to measure the concentration of yeasts in liquid suspension. This measurement was achieved by the detection and quantification of ultrasonic echoes backscattered by the cells. More specifically, the technique was applied to the detection and quantification of Saccharomyces cerevisiae. A theoretical approach was proposed to get the average density and sound speed of the yeasts, which were found to be 1116 kg/m(3) and 1679 m/s, respectively. These parameters were needed to model the waves backscattered by each single cell. A pulse-echo arrangement working around 50 MHz, being able to detect echoes from single yeasts was used to characterize experimentally yeast solutions from 10(2) to 10(7)cells/ml. The Non-negative Matrix Factorization denoising technique was applied for data analysis. This technique required a previous learning of the spectral patterns of the echoes reflected from yeasts in solution and the base noise from the liquid medium. Comparison between pulse correlation (without denoising) and theoretical and experimental pattern learning was made to select the best signal processing. A linear relation between ultrasound output and concentration was obtained with correlation coefficient R(2)=0.996 for the experimental learning. Concentrations from 10(4) to 10(7)cells/ml were detected above the base noise. These results show the viability of using the ultrasound backscattering technique to detect yeasts and measure their concentration in liquid cultures, improving the sensitivity obtained using spectrophotometric methods by one order of magnitude.

Study of the effect of particle volume fraction on the microstructure of magnetorheological fluids using ultrasound: Transition between the strong-link to the weak-link regimes.

The effect of particle volume fraction on the microstructure of magnetorheological (MR) fluids has been studied using ultrasonic techniques. When no magnetic field is applied, they behave as slurry. However, when magnetic field is applied, important features regarding the change of the microstructure have been found with the help of ultrasonic waves propagating in the direction of the magnetic field. As the volume fraction increases, a rearrangement of particles which decrease the compressibility of the system is detected; nevertheless, the material behaves as a non-consolidated material. Three different particle volume fraction regions are found identifying a critical particle volume fraction predicted in the literature. Ultrasounds are confirmed as an interesting tool to study MR fluids in static conditions.

Sound attenuation in magnetorheological fluids

In this work, the attenuation of ultrasonic elastic waves propagating through magnetorheological (MR) fluids is analysed as a function of the particle volume fraction and the magnetic field intensity. Non-commercial MR fluids made with iron ferromagnetic particles and two different solvents (an olive oil based solution and an Araldite-epoxy) were used. Particle volume fractions of up to 0.25 were analysed. It is shown that the attenuation of sound depends strongly on the solvent used and the volume fraction. The influence of a magnetic field up to 212 mT was studied and it was found that the sound attenuation increases with the magnetic intensity until saturation is reached. A hysteretic effect is evident once the magnetic field is removed.