Always an exciting technology, the uptake of ultrasonic spectroscopy has been hindered by its technical complexity and limited resolution. As Cormac Smyth, Breda O'Driscoll, Evgeny Kuryashov and Vitaly Buckin show, a new device has overcome these problems is finding applications in various dairy processes.
Ultrasonic spectroscopy is a non-destructive analytical technique based on the measurements of parameters of low energy ultrasonic waves propagating through analysed samples.
This technique is well known through its successful application in medicine and a number of fields of material analysis. However, in the past the limited resolution of the measurements and complicated cleaning and sample handling procedures prevented a broad spread of this technique in research and analytical laboratories.
High-resolution
The novel HR-US101 high-resolution spectrometer (Fig. 1) from Ultrasonic Scientific, which employs modern technologies, overcomes these limitations. As a result, commercial ultrasonic instruments are now available for a wide spectrum of analytical tasks.
This instrument can perform analysis in a broad range of sample volume (1ml typically, 0.03ml to 200ml and higher when custom made) and performs analysis in various regimes with a record resolution. It was employed at the Department of Chemistry at University College Dublin in the analysis of gelation processes in milks, melting of milk fat in milk and butter, composition analysis, particle sizing, creaming effects and coagulation in milks and milk based drinks. Some of these applications are discussed in this article.
Benefits of ultrasonic analysis
Non-destructive analysis of intrinsic properties of materials includes measurements of signals (waves) travelling through the analysed sample. Until now only one wave has dominated in the field of material analysis the electromagnetic wave. This wave, which probes electromagnetic properties of materials, is employed in optical spectroscopy and its variations, NMR, microwave and others.
Ultrasound provides an alternative wave, which is the same as an acoustical one, only at higher frequency (or tone in acoustical terms). In this wave oscillating pressure causes oscillation of compressions and therefore by its nature it is a rheological wave.
Compression in the ultrasonic wave changes the distances between the molecules of the sample, which responds by intermolecular repulsions. Therefore ultrasonic spectroscopy allows us to probe intermolecular forces in the sample, thus providing new information about its interior.
Amplitudes of deformations in the ultrasonic wave employed in analytical ultrasound are extremely small, making the ultrasonic analysis non-destructive. As most materials are ultrasonically transparent, ultrasound allows the analysis of a broad variety of samples including opaque materials. Another advantage of the technology employed in the HR-US101 ultrasonic spectrometer is the absence of large actuators as in dynamic rheology or bulky light sources and other optical parts or expensive consumables, thus representing a robust and multipurpose instrument, which performs a broad range of analytical functions of fast, non-destructive and non-expensive analysis.
Ultrasonic spectroscopy is based on the measurements of two independent parameters, ultrasonic attenuation and ultrasonic velocity. Ultrasonic attenuation is determined by the energy losses in ultrasonic waves and can be expressed in terms of viscosity of the medium or its longitudinal loss modulus. It allows analysis of kinetics of fast chemical reactions, microstructure of materials including particle sizing, aggregation, gelation, crystallization and other processes and characteristics.
Ultrasonic velocity is determined by the density and the elastic response of the sample to the oscillating pressure in the ultrasonic wave and can be expressed in terms of compressibility or storage modulus (longitudinal). This parameter is extremely sensitive to the molecular organisation, composition and intermolecular interactions in the analysed medium and is responsible for the major portion of applications of high-resolution ultrasonic spectroscopy for analysis of chemical properties of materials.
However wide application of ultrasonic velocity requires extremely high resolution of the measurements, which was a problem in the past. The HR-US 101 spectrometer, with a resolution down to 105 per cent, is the first commercial instrument which allows the user to enjoy the full potential of ultrasonic analysis.
Examples
Fig.2 illustrates ultrasonic monitoring of gelation process in milk. At time 0 the ultrasonic cell was filled with 1ml milk and bacterial culture was added. The gelation starts at one hour as indicated by a sharp increase in ultrasonic attenuation. Ultrasonic velocity increases at a pregelation stage showing a change in milk composition as a result of bacterial activity. This is followed by an additional increase in ultrasonic velocity at gelation stage. The levelling off of ultrasonic velocity after 1.7hours shows a decline in bacterial activity. The structure of the gel however changes until 2.5hours as can be seen from ultrasonic attenuation curves. In the time interval between 1.7 and 2.5hours the ultrasonic velocity curves are frequency dependent, which is a good indication of synerisis.
Fig.3 illustrates the application of HR-US101 for optimisation of composition of calcium-fortified milks. Calcium fortified milk drinks is one of the industry's responses to today's consumers needs for a healthy lifestyle. The addition of calcium changes the balance of colloid interactions in milk thus leading to coagulation during heat treatment or when it is added to hot beverages.
Dealing with this problem in a product development laboratory requires an analytical tool that can determine the coagulation point fast, precisely and easily. The insert in Fig.3 illustrates a typical temperature dependence of the ultrasonic attenuation and ultrasonic velocity in calcium fortified milk obtained in the temperature ramp regime of HR-US101 spectrometer. Coagulation is indicated by a sharp decrease in the ultrasonic velocity and increase in the ultrasonic attenuation.
This allows us to determine the coagulation temperature (0.1degree) with a resolution, which is even higher than required for developing the optimal milk composition (no coagulation at temperature below 100°C). The main part of Fig.3 illustrates the results of ultrasonic assessments of the effects of the addition of different ingredients/stabilisers on the coagulation temperature of milk. These curves allow an easy optimisation of composition of calcium-fortified milks.
Cormac Smyth and Breda O'Driscoll are with Ultrasonic Scientific Limited, GEC, Dublin. Ireland, www.ultrasonic-scientific.com. Evgeny Kuryashov and Vitaly Buckin are with the Department of Chemistry, University College Dublin, Ireland.
