Application of Ultrasound

P676757-Ch13.qxd 6/20/05 5:02 PM Page 323 Application of 13 Ultrasound Timothy J Mason1, Enrique Riera2, Antonio Vercet3 and Pascual Lopez-Buesa3 1Coventry University, Sonochemistry Centre, School of Science and the Environment, Coventry, UK 2Instituto de Acústica, CSIC, Ultrasonics Department, Madrid, Spain 3Universidad de Zaragoza, Departamento de Producción Animal y Ciencia de los Alimentos, Facultad de Veterinaria, Zaragoza, Spain Nowadays, power ultrasound is considered to be an emerging and promising technology for food processing in industry. In this chapter a review of the most recent uses of power ultrasound in the food industry will be presented. The potential use of this novel technology to produce permanent changes in the material will be discussed in liquid systems (by the production of intense cavitation) and in gases (by the generation of high-intensity acoustic fields). Mechanical, chemical and biochemical effects produced by the propagation of high- intensity ultrasonic waves through the medium will be discussed. The inactivation of micro- organisms and enzymes for food preservation or decontamination by ultrasonic irradiation will demonstrate the benefits of ultrasound (alone or combined with heat and high-pressure techniques) as a food preservation tool. In addition, the increasing number of industrial processes that employ power ultrasound as a processing aid will be described including the mixing of materials; foam formation or destruction; agglomeration and precipitation of airborne powders; the improvement in efficiency of filtration, drying and extraction techniques in solid materials and the enhanced extraction of valuable compounds from vegetables and food prod- ucts. Finally, the effect of ultrasound on food properties such as flavour, colour and texture will be analysed. 1 Introduction Probably the first question that might be asked about applications of ultrasound in food technology is, why use ultrasound? For the answer to this we need only think of two properties of sound to appreciate the possibilities. The first is the use of sound as a diagnostic tool, e.g. in non-destructive evaluation and the second is the use of sound as a source of energy, e.g. in sonochemistry. These applications involve different frequency ranges of ultrasound (Figure 13.1) and the uses of both ranges within the food industry continue to be an active subject for research and development (Povey and Mason, 1998). Emerging technologies for food processing Copyright © 2005 Elsevier Ltd ISBN: 0-12-676757-2 All rights of reproduction in any form reserved P676757-Ch13.qxd 6/20/05 5:02 PM Page 324 324 Application of Ultrasound 010102 103 104 105 106 107 Human hearing 16 Hz–18 kHz Conventional power ultrasound 20 kHz–100 kHz Extended for special applications 20 kHz–1 MHz Diagnostic ultrasound 5 MHz–10 MHz Figure 13.1 Frequency ranges of sound. Table 13.1 Some uses of power ultrasound in food processing Mechanical effects crystallization of fats, sugars etc degassing destruction of foams extraction of flavourings filtration and drying freezing mixing and homogenization precipitation of airborne powders tenderization of meat Chemical and biochemical effects bactericidal action effluent treatment modification of growth of living cells alteration of enzyme activity sterilization of equipment Until recently the majority of applications of ultrasound in food technology involved non-invasive analysis with particular reference to quality assessment. Such applications use techniques that are similar to those developed in diagnostic medicine, or non-destructive testing, using high frequency (Ͼ1 MHz) low power (Ͻ1 W/cm2) ultrasound (Mulet et al., 2002). Examples of the use of such technologies are to be found in the location of foreign bodies in food (Cho and Irudayaraj, 2003), the analysis of droplet size in emulsions of edible fats and oils (Coupland and McClements, 2001) and the determination of the extent of crystallization in dispersed emulsion droplets (Hindle et al., 2002). In recent years food technologists have discovered that it is possible to employ a more powerful form of ultrasound (Ͼ5 W/cm2) at a lower frequency (generally around 40 kHz). This is usually referred to as power ultrasound and its history can be traced P676757-Ch13.qxd 6/20/05 5:02 PM Page 325 Fundamentals of ultrasound 325 back to 1927 when a paper was published entitled ‘The chemical effects of high frequency sound waves: a preliminary survey’, which described the development of power ultrasound for use in a range of processing including emulsification and surface cleaning (Richards and Loomis, 1927). By the 1960s the uses of power ultrasound in the processing industries were well accepted and this interest has continued to develop (Abramov, 1998; Mason, 2000; Mason and Lorimer, 2002). In this chapter we will concentrate on possible applications of power ultrasound in the food industry, an indication of the breadth of which is shown in Table 13.1. 2 Fundamentals of ultrasound 2.1 The physics and chemistry of ultrasound 2.1.1 Power ultrasound in liquid systems The major mechanical effects of ultrasound are provided when the power is sufficiently high to cause cavitation. Like any sound wave, ultrasound is propagated via a series of compression and rarefaction waves induced in the molecules of the medium through which it passes. At sufficiently high power the rarefaction cycle may exceed the attractive forces of the molecules of the liquid and cavitation bubbles will form. Such bubbles grow by a process known as rectified diffusion, i.e. small amounts of vapour (or gas) from the medium enters the bubble during its expansion phase and is not fully expelled during compression. The bubbles, distributed throughout the liquid, grow over the period of a few cycles to an equilibrium size for the particular frequency applied. If the bubbles were only subject to that particular frequency they would remain as oscillating bubbles, however, the acoustic field that influences an individual bubble among the many thousands generated in a cavitating fluid is not uniform. Each bubble will slightly affect the localized field experienced by neigh- bouring bubbles. Under such circumstances the irregular field will cause the cavitation bubble to become unstable and collapse. It is this collapse that generates the energy for chemical and mechanical effects. For example, in aqueous systems at an ultrasonic frequency of 20 kHz, each cavitation bubble collapse acts as a localized ‘hotspot’ generating temperatures of about 4000 K and pressures in excess of 1000 atmospheres. This bubble collapse, distributed through the medium, has a variety of effects within the system depending upon the type of material involved. 2.1.1.1 Homogeneous liquid-phase systems It is not absolutely correct to describe any system within which cavitation occurs as homogeneous since cavitation bubbles must be present. However, it is logical to refer to systems in the state that they were in before ultrasound is introduced. There are two major zones in which cavitation collapse can influence such systems: in the bulk liquid immediately surrounding the bubble where the rapid collapse of the bubble generates shear forces which can produce mechanical effects and in the bubble itself where any species introduced during its formation will be subjected to extreme conditions of temperature and pressure on collapse, leading to chemical effects. P676757-Ch13.qxd 6/20/05 5:02 PM Page 326 326 Application of Ultrasound In order for cavitation to occur it is necessary to pull molecules of the liquid apart to produce a hole (cavity). A pure liquid would require very high power levels to initiate cavitation, too high to be achieved by normal ultrasonic equipment. However, most normal liquids contain some discontinuities, such as gas bubbles or dust motes, which act as weak spots and allow the bubbles to form. Ultrasonic degassing is normally per- formed before cleaning equipment is used because it increases the efficiency of cavitation by removing air bubbles which absorb acoustic energy and dampen sonication. After degassing continued sonication will provide continuous and more powerful cavitation that will have other effects. The cavitation bubble does not contain a vac- uum; during the process of growth in an acoustic field vapour from the liquid medium or dissolved volatile reagents will have entered the bubble. On collapse, these vapours will be subjected to extremely large increases in temperature and pressure, resulting in molecular fragmentation. In the case of water the extreme conditions are sufficient to cause rupture of the O-H bond, leading to the production of small quantities of oxygen gas and hydrogen peroxide. 2.1.1.2 Solid–liquid systems Unlike cavitation bubble collapse in the bulk liquid, collapse of a cavitation bubble on or near to a surface is asymmetrical because the surface provides resistance to liquid flow from that side. The result is an inrush of liquid predominantly from the side of the bubble remote from the surface, resulting in a powerful liquid jet being formed, targeted at the surface. The effect is equivalent to high pressure jetting and is the rea- son that ultrasound is used for cleaning. This effect can also increase mass and heat transfer to the surface by disruption of the interfacial boundary layers. Acoustic cavitation can also produce dramatic effects on powders suspended in a liquid. Surface imperfections or trapped gas can act as the nuclei for cavitation bubble formation on the surface of a particle and subsequent surface collapse can then lead to shock waves which break the particle apart. Cavitation bubble collapse in the liquid phase near to a particle can force it into rapid motion. Under these circumstances the general dispersive effect is accompanied by interparticle collisions that can lead to erosion, surface cleaning and wetting of the particles and particle size reduction. 2.1.1.3 Liquid–liquid systems The general mechanical effects of cavitation at or close to a liquid–liquid interface lead to very effective emulsification/homogenization.

Load more