Food Science and Technology International

Food Science and Technology International http://fst.sagepub.com

Power Ultrasound H. Feng, W. Yang and T. Hielscher Food Science and Technology International 2008; 14; 433 DOI: 10.1177/1082013208098814 The online version of this article can be found at: http://fst.sagepub.com/cgi/content/abstract/14/5/433

Published by: http://www.sagepublications.com

On behalf of:

Consejo Superior de Investigaciones CientÃ-ficas (Spanish Council for Scientific Research)

Additional services and information for Food Science and Technology International can be found at: Email Alerts: http://fst.sagepub.com/cgi/alerts Subscriptions: http://fst.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.co.uk/journalsPermissions.nav Citations http://fst.sagepub.com/cgi/content/refs/14/5/433

Downloaded from http://fst.sagepub.com at UNIV OF ILLINOIS URBANA on March 3, 2009

Power Ultrasound H. Feng,1,* W. Yang2 and T. Hielscher3 1

Department of Food Science and Human Nutrition, University of Illinois, Urbana IL 61801, USA 2 Alabama A&M University, Huntsville, AL 35811, USA 3 Hielscher USA Inc., Ringwood, NJ 07456, USA

The use of ultrasound as a processing aid has been explored in various industrial sectors for over fifty years, but the application of this physical energy in food processing is relatively new. In this article, the current research and development activities of power ultrasound centered on food and bioprocessing applications are summarized. The mode of action of ultrasonication is attributed to several mechanical and chemical actions caused by cavitation, which include localized hot spots, formation of shock waves, microsteaming, and liquid jets, as well as the production of chemical species that will also impact the process kinetics and food quality attributes. Means to enhance cavitation activity is addressed. The research needs for the future development of ultrasound food processing are also discussed. Key Words: Ultrasound, cavitation, inactivation, scale up

INTRODUCTION The term ‘Ultrasound’ designates those sonic waves with frequencies that are above the range audible to humans. It can be divided into two categories: highfrequency ultrasound and power ultrasound. The former uses high frequencies of 2–20 MHz with low sound intensity (0.1–1 W/cm2) and mainly has applications in food quality analysis, medical imaging, and nondestructive inspection. Power ultrasound, also known as high-intensity ultrasound, refers to sound waves with low frequencies (20–100 kHz) and high sound intensity (10–1000 W/cm2). Applications of power ultrasound have been tested for microbial and enzyme inactivation, bio-component separation, emulsification, interface heat and mass transfer enhancement, cutting, crystallization enhancement, and extraction of bioactive component(s) in foods and plants. Due to new developments in ultrasound generation techniques, as well as increased understanding of cavitation phenomena, interest has increased in recent years to examine the use of power ultrasound as an alternative food processing and preservation tool. The combination of sonication with

*To whom correspondence should be sent (e-mail: [email protected]). Received 3 March 2008; revised 8 August 2008. Food Sci Tech Int 2008;14(5):0433–436 ß SAGE Publications 2008 Los Angeles, London, New Delhi and Singapore ISSN: 1082-0132 DOI: 10.1177/1082013208098814

other treatments such as pH, mild heat, and low pressure has been found to enhance the efficacy of an ultrasound treatment. Usually an additive and even synergistic effect can be observed for microbe and food enzyme inactivation in a thermosonication or manothermosonication (MTS) treatment. The food industry nevertheless harbors lingering concerns about the use of ultrasound for food processing, which include concerns about the quality of the food treated with ultrasound as well as scale-up and economic issues. This manuscript aims to review the progress made in recent years in ultrasound technology and in understanding the mechanisms of how ultrasound works, and to address the research needs for the advancement of this technology.

STATE-OF-ART OF THE TECHNOLOGY Microbial and Enzyme Inactivation Currently, power ultrasound can treat a liquid in three modes: sonication, thermosonication, and MTS. The impacts of the three modes on the efficacy of an ultrasonic inactivation operation can be summarized as follows: 1. When sonicating a liquid food at sublethal temperatures (sonication), the microbial inactivation is caused by ultrasound alone and the kill rate is relatively low, especially at low acoustic energy density (AED).


433

434

H. FENG

2. When sonicating at lethal temperatures (thermosonication), the inactivation is a result of a combined action of heat and ultrasound, and usually additive or even synergistic effects can be observed. In a milk thermosonication test at 60 8C, the decimal reduction (D) value of Listeria monocytogenes is 0.3 min, which represents a seven-fold increase in inactivation rate compared to thermalonly treatment at the same temperature (Earnshaw et al., 1995). However, an upper temperature limit exists for each microorganism beyond which the application of ultrasound to a food system does not introduce additional inactivation (Ugarte et al., 2007). 3. When low pressure (200–500 kPa) is introduced into a thermosonication system to achieve MTS, a further increase in the inactivation rate has been observed. In tomato pectinmethylesterase (PME) and polygalacturonase (PG) inactivation tests, a 52.9-and 26.3-fold increase in the inactivation rate for thermoresistant PGI and PGII, compared to thermal treatment alone, has been reported (Lopez et al., 1998). Several theories have been proposed to elucidate the inactivation mechanism of an ultrasonic treatment. When ultrasonic waves travel through a liquid in the form of longitudinal waves comprised of a series of compression and rarefaction portions, negative pressure at the rarefaction portions will – if the acoustic pressure is large enough – tear the liquid apart and form small cavities. The formation, growth, and implosion of gas – or vapor-filled cavities in a liquid is called cavitation. There are two types of cavitation, transient and stable cavitation. In transient cavitation, a bubble or cavity only lasts for several acoustic cycles before it collapses, while a stable cavitating bubble would stay in a liquid for tens or even hundreds of acoustic cycles. The collapse of transient bubbles creates both mechanical and chemical effects in the liquid. The mechanical effects include localized high pressure (1000 atm), high temperature (5000 K), a high rate of heating and cooling (109 K/s), and the formation of shock waves. These shock waves can cause cell envelope rupture and facilitate sonochemical reactions. The chemical effects include formation of OH and Hþ species and hydrogen peroxide. These species have important bactericidal properties. At solid and liquid interfaces, the asymmetric implosion of transient cavitating bubbles generates liquid jets that can have a liquid in-flow velocity of up to 156 m/s, which can cause severe cell envelope damage, clean a solid surface, and promote bioseparation. On the other hand, a stable cavitating bubble produces micro-streaming alongside the bubble and creates high hydrodynamic shear stresses, which are mechanical actions that can be utilized in many processing applications.

ET AL.

Microbial inactivation tests are usually conducted at 20 kHz, a frequency at the low end of the power ultrasound frequency spectrum. It has been found that spores, and gram-positive and coccal cells, are more resistant to ultrasound treatment than vegetative, gram-negative, and rod-shaped bacteria. For the same organism, the resistance to ultrasound treatment varies among different strains. The inactivation kinetics at sub-lethal temperatures usually exhibit a log-linear relation, while at lethal temperatures the inactivation kinetics have been reported as nonlog-linear. Shoulders are considered to be caused by cell disaggregation, while the presence of tails has been related to a progressive loss in cavitation intensity during sonication treatment, which may be true for an open system where air entrainment may occur and cavitation intensity is hence reduced. Cell injury studies have demonstrated that thermosonication causes extensive physical damage on a cell envelope in the form of wrinkles, ruptures, and perforations (Ugarte et al., 2006). In sonication tests assisted with elevated external pressure, the survival gram-positive and gram-negative cells recovered in media with sodium chloride added are virtually identical to those recovered in a nonselective medium (Pagań, et al., 1999). The absence of sub-lethally injured cells in current studies has been attributed to irreversible physical damage to the outer membrane (Manãs and Pagań, 2005). More ultrasound inactivation data are needed for vegetative cells, yeast, mold, spores, viruses, and food toxins. Existing inactivation data are often difficult to compare because of the lack of a well-defined control parameter and because of the diverse testing protocols and ultrasound systems used. In addition, most microbial inactivation tests have been conducted in liquid media. Although surface decontamination of solid objects with air-borne ultrasound has been proven effective in the inactivation of a virus, more studies are needed before one can draw general conclusions. Ultrasonic inactivation of food enzymes mainly focuses on those endogenous enzymes more resistant to a thermal treatment than foodborne pathogens. Therefore, inactivation tests have been conducted to reduce enzyme activity in citrus, tomato, and dairy products. Inactivation of enzymes at sublethal temperatures has not proved very effective (Raviyan et al., 2004). Most experiments have been conducted at temperatures elevated high enough to cause microbial inactivation. The most effective inactivation is achieved with pressure-assisted sonication treatments (Lopez et al., 1998). Enzyme inactivation caused by ultrasound has been attributed to different mechanisms. It is generally agreed that sonication depolymerizes macromolecules. The shear stress generated by stable cavitation is considered


Power Ultrasound

important, which can cause degradation of high molecular weight polymers even without the presence of bubble collapse. Bio-component Separation The use of power ultrasound for separation of food components/constitutes has been investigated in recent years. In starch-protein separation experiments, sonication can recover 97.3–99.5% of the total starch from degermed corn flour (67.5% total starch) and hominy feed (46.4% total starch) – two low value dry-milling byproducts. The quality of the resulting starch is comparable to regular commercial cornstarch (Zhang et al., 2005a). When applying ultrasonication to the fine fiber stream in a quick germ/quick fiber process, a process in which no SO2 is added during steeping to enhance starch separation, starch yield (66.93–68.72%) was almost as high as that of a traditional wet milling operation (68.92%). The quality of starch from the ultrasound treatment was comparable to or better than conventional wet milling starch, as evidenced by lower protein content, comparable color, and similar pasting properties measured with a Rapid Visco-Analyzer (RVA) (Zhang et al., 2005b). In a corn wet milling process, power ultrasound was used to rapidly remove corn pericarp prior to steeping, which resulted in a reduction in steeping time and improved the isolated starch gelatinization and pasting properties (Liu, 2002; Yang et al., 2002a). In an ultrasound-assisted tomato peeling test, with a 2% lye solution, peel loss was reduced by 3%, compared to peeling under commercial conditions, where a lye solution of 10% was used (Lee et al., 2005, personal communication). Power ultrasound treatment was used to extend the shelf life of roasted peanuts by removing oils on peanut kernel surfaces. A 10-min sonication removed, as can be discerned from microscopic examination, all the surface oil and increased the shelf life by up to 17% (Yang et al., 2005). Maximizing Cavitation Activity Most power ultrasound applications are based on the activity of cavitation. Measures to increase cavitation activity will increase the efficiency of an ultrasound treatment. To increase the cavitation activity, basically two approaches can be used. One is to increase the number of cavitating bubbles, while maintaining the power of implosion, which can be realized by lowering the cavitation threshold or using multiplefrequency techniques. The other is to increase the power of bubble implosion by either applying an external static pressure during sonication or increasing AED (W/mL). Insonating a liquid generates bubbles having a size distribution over a large range. Not all

435

bubbles are capable of producing cavitational effects. The greatest coupling of ultrasonic energy will occur when the natural resonance frequency of a bubble equals the applied ultrasound frequency (Mason and Lorimer, 1999). In theory, the traditional single-frequency ultrasound technique may only mobilize bubbles with certain sizes to generate the cavitation effect. In recent years, studies into the application of multi-frequency sonication to enhance cavitation have received increasing interest. The hypothesis behind the use of a multiple frequency technique is that multiple frequency sonication will allow bubbles with a wide range of sizes to generate cavitation and hence increase the cavitation intensity. In a two-frequency system, the cavitation enhancement as described by iodine release is more than additive (Ciuti et al., 2003). In the case of a sonication treatment assisted by external pressure, an increase in bubble implosion intensity can increase cavitation activities and hence result in a boost in kill rate (Pagań et al., 1999). Scale-up and Commercial Applications The scale-up of an ultrasonic treatment system to large-scale operations can be achieved by integrating flow-cell modules, either in series or in parallel, in order to facilitate a large throughput. A flow cell can be manufactured by inserting probes or transducers into a tube. The transducers can also be fixed to the external surface of the flow cell. Another method entails providing acoustic energy for the treatment by means of the coaxial insertion of a radially emitting bar into a pipe. A few commercial applications have used power ultrasound to perform homogenization, cutting, and extraction in the processing of food and bio-products (Feng and Yang, 2005).

RESEARCH NEEDS Today, research should concentrate on the following: 1. Study the cavitation phenomenon, including factors affecting the cavitation intensity, the means for increasing or maximizing cavitation, methods of quantifying cavitation activity in both model and real food systems, and the correlation between cavitation activity and an operational parameter (power draw from the generator, acoustic power density, etc.) in an ultrasonic processing system. 2. Investigate the microbial and enzyme inactivation mechanism and inactivation kinetics. 3. Examine the effect of ultrasound on quality attributes of food products. Studies on the effect of ultrasound on food quality are scarce. There are no reports on sensory tests for ultrasound treated


436

4. 5.

6.

7. 8.

H. FENG

food products, mainly due to the fact that food grade ultrasound treatment systems are still not available. Study the effect of ultrasonic treatment on food components (starch, protein, lipids, biopolymers, etc.). Identify the most resistant pathogenic microorganism(s) in selected food systems to a power ultrasound treatment. Develop new noncontact ultrasonic food processing systems (food grade) with fixed frequencies and variable frequency techniques. Develop air-borne ultrasound techniques for surface treatment of solid foods. Examine the economic feasibility of an ultrasonic processing and preservation ultrasound technology for a targeted application.

CRITERIA FOR ESTABLISHING RESEARCH PRIORITIES Emphasis should be placed on studies aimed at understanding the mode of action of an ultrasound treatment, to establish a knowledge base for the application of power ultrasound technology, and to evaluate the advantages and disadvantages of the power technology in comparison with traditional and other emerging food processing technologies. Suggested Direction for Public Funds To promote ultrasound technology, a wise use of public research dollars is critical. Instead of providing US grants to individual and sporadic projects, a concerted effort that draws from the key research forces in the United States to form an ultrasound research center might be the best and most economical way to utilize public dollars. A joint research initiative from multiple governmental agencies could provide funds for application-oriented projects with a component for fundamental research. Industry-university-federal Agency Partnerships Grass-root efforts supported by USDA and other government agencies are needed to establish an ultrasound research center with members from industry. Industry members from both food companies

ET AL.

and ultrasound instrument companies should be included.

REFERENCES Ciuti P., Dezhkunov N.V., Francescutto A., Calligaris F. and Sturman F. (2003). Study into mechanisms of the enhancement of multibubble sonoluminescence emission in interacting fields of different frequencies. Ultrasonics Sonochemistry 19(6): 337–341. Earnshaw R.G., Appleyard J. and Hurst R.M. (1995). Understanding physical inactivation processes: combined preservation opportunities using heat, ultrasound and pressure. International Journal of Food Microbiology 28: 197–219. Feng H. and Yang W. (2005). Power ultrasound. In: Hui Y.H. (ed.), Handbook Food Science, Technology and Engineering. New York: Marcel Dekker. Lee J.W. and Feng H. (2004). Tomato peeling with ultrasound. Unpublished work. University of Illinois. Liu Z. (2002). Ultrasound enhanced corn pericarp separation process. M.S. thesis. Dept. of Food Science, University of Arkansas, Fayetteville, AR. Lopez P., Vercet A., Sanchez A.C. and Burgos J. (1998). Inactivation of tomato pectin enzymes by manothermosonication. Zeitschrift Fur Lebensmittel-Untersuchung und-Forschung A 207: 249–252. Manãs P. and Pagań R. (2005). Microbial inactivation by new technologies of food preservation. Journal of Applied Microbiology 98: 1387–1399. Mason T.J. and Lorimer J.P. (1999). Applied Sonochemistry. Darmstadt: Wiley-VCH. Pagań R., Manãs P., Palop A. and Sala F.J. (1999). Resistance of heatshocked cells of Listeria monocytogenes to mano-sonication and mano-thermo-sonication. Letters Applied Microbiology 28: 71–75. Raviyan P., Zhang Z., and Feng H. (2005). Ultrasonication for food enzyme inactivation: effect of cavitation intensity and temperature on inactivation. Journal of Food Engineering 70(2): 189–196. Ugarte E., Feng H., and Martin E.S. (2007). Inactivation of Shigella and Listeria monocytogenes with power ultrasound at sub-lethal and lethal temperatures. Journal of Food Science 72(4): M103–M107. Ugarte E., Feng H., Martin E.S. and Cadwallader K.R. (2006). Inactivation of Escherichia coli with power ultrasound in apple cider. Journal of Food Science. 71(2): E102–E108. Yang W., Siebenmorgen T.J. and Liu Z. (2002). Rapid debranning of corn with power ultrasound. In: Proceedings of the 2002 AACC Annual Conference, Montreal, Canada, October 13–17, 2002. Yang W., Wambura P. and Williams L. (2005). Extending the capability of power ultrasound to cereal and oilseed processing for food and non-food applications. In: Proceedings IFT Annual Meeting, New Orleans, LA July 16–20, 2005. Zhang Z., Feng H., Niu Y. and Eckhoff S.R. (2005a). Starch recovery from degermed corn flour and hominy feed using power ultrasound. Cereal Chemistry 82(4): 447–449. Zhang Z., Niu Y., Eckhoff S.R. and Feng H. (2005b). Sonication enhanced starch separation in a milling process and its effect on the resulting starch. Starch 57: 240–245.
