|
|
|
|
Historical perspective
The use of ultrasound to modify the properties of proteins is of considerable current interest in food technology. It can be traced back to 1931 when solutions of egg albumin were exposed to sonication and became turbid as the albumin is precipitated out in fine shreds. The mechanism of this ultrasonic method of protein coagulation was one of the first to be investigated [1]. One of the oldest applications of ultrasound in food processing is emulsification. For food processing the development of the “Pohlmann whistle” or jet edge transducer proved to be an important new technology for efficient emulsification [2]. It showed how hydrodynamic cavitation in liquids could be used for industrial purposes. In 1960 a series of experiments was undertaken to compare four methods then in common usage for the emulsification of mineral oil, peanut oil and safflower oil [3]. The results proved that a homogeniser, which operated via a liquid whistle, was superior to three other types of apparatus, namely a colloidal mill and two types of sonicator, one of which employed a quartz crystal and the other a barium titanate transducer. Improvements in food preservation by drying and freezing using ultrasound has been the subject of considerable research. Its use in freezing has two benefits (a) through a reduction in the ”dwell time” incurred while the material remains at a steady low temperature while the whole mass freezes before the temperature reduces further (due to latent heat considerations) and (b) a reduction in ice crystal size due to ultrasonic seeding of crystallisation, a benefit when freezing soft fruit since smaller crystals rdeuce the problems of cell wall damage. A patent was secured for ultrasonic freezing [4]. Some years earlier in 1959 ultrasound had been investigated in terms of defrosting although at that time it was not found to be very efficient [5]. The origins of ultrasonically assisted drying was suggested in the same year [6] Current applications span a range of food technologies and provide many advantages over conventional methods including more effective mixing and micro-mixing, faster energy and mass transfer, selective extraction and in some cases a reduction in the number of processing steps [7] [8]
1. Wu, H. and S.-C. Liu, Coagulation of Egg Albumin by Supersonic Waves. Proceedings of the Society for Experimental Biology and Medicine, 28(8): p. 782-784 (1931) 2. Janowsky, W. and R. Pohlmann, Sonic and ultrasonic generation in liquids for industrial purposes. Z fur angewandte Physik, 222 (1948) 3. Singiser, R.E. and H.M. Beal, Emulsification with ultrasonic waves II. Evaluation of three ultrasonic generators and a colloid mill. Journal of the American Pharmaceutical Association, 49(7): 482-487 (1960) 4. Acton, E. and G.J. Morris, Method and apparatus for the control of solidification in liquids. (1992). 5. Brody, A.L. and J.N. Antonevich, Ultrasonic defrosting of foods. Food Technology,13, 109-112 (1959) 6. Boucher, R.M.G., Drying by airborne ultrasonics. Ultrason. News, 111, 8-9 and 14-16 (1959)
General Overview
Nowadays, power ultrasound is considered to be an emerging and promising technology for industrial food processing. 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 and the uses of both ranges in the food industry are an active subject for research and development. 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 (>1 MHz) low power (<1 W/cm2) ultrasound. Examples of the use of such technologies are to be found in the location of foreign bodies in food the analysis of droplet size in emulsions of edible fats and oils and the determination of the extent of crystallization in dispersed emulsion droplets. In recent years food technologists have discovered that it is possible to employ a more powerful form of ultrasound (>5 W/cm2) at a lower frequency (generally around 40 kHz) for food processing. By the 1960's the uses of power ultrasound in the processing industries were well accepted and this interest has continued to develop and in 1998 the first book dealing with both diagnostic and power ultrasound applied in the food industry was published [1]. . Here we will examine the breadth of possible applications as shown in the Table below
The potential use of this novel technology to produce permanent changes in food materials in liquid systems is through the generation of intense cavitation. This can lead to the inactivation of microorganisms and enzymes for food preservation or decontamination by ultrasonic irradiation demonstrates the benefits of ultrasound (alone or combined with heat and high-pressure techniques) as a food preservation tool. In addition there are an increasing number of industrial processes that employ power ultrasound as a processing aid including the mixing 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 products. In recent years here in Coventry we have tended to concentrate our efforts on a smaller number of topics of which one has remained active for very many years - the extraction of bioactive materials from plants. More recently this has concentrated on the isolation of neutriceuticals such as antioxidants since the interest in such materials seems to be attracting considerable current interest [2, 3, 4]. Other areas of food research are collaborative with other groups e.g. the University of Zagreb, Croatia on vegetable drying and protein modification [5.6]; with the University of Avignon, France on food sterilisation [7]; with the Institute of Acoustics, Madrid on defoaming [8]; with the University of Chihuaua, Mexico on meat marination [9]. In these days, when minimal and green processing is a driver for food preparation, power ultrasound can provide useful possibilities for the food technologist.
References
|
|
|
