Ultrasonics Sonochemistry xxx (2014) xxx–xxx
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Applications of ultrasound in food and bioprocessing Muthupandian Ashokkumar ⇑,1 School of Chemistry, University of Melbourne, VIC 3010, Australia Chemistry Department, King Abdulaziz University, Jeddah, Saudi Arabia
a r t i c l e
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Article history: Received 5 August 2014 Accepted 13 August 2014 Available online xxxx Keywords: Ultrasound Acoustic cavitation Food processing Viscosity modification Encapsulation
a b s t r a c t Improving the quality and nutritional aspects of food is one of the key issues for healthy life of human beings. The stability during storage is an important parameter in quality assurance of food products. Various processing techniques such as high pressure, thermal, pulsed electric field and microwave have been used to prolong the shelf-life of food products. In recent years, ultrasound technology has been found to be a potential food processing technique. The passage of ultrasound in a liquid matrix generates mechanical agitation and other physical effects due to acoustic cavitation. Owing to its importance, a number of review articles and book chapters on the applications of ultrasound in food processing have been published in recent years. This article provides an overview of recent developments in ultrasonic processing of food and dairy systems with a particular focus on functionality of food and dairy ingredients. More specifically, the use of high frequency ultrasound in fat separation from milk and viscosity modification in starch systems and the use of low frequency ultrasound in generating nutritional food emulsions, viscosity modification and encapsulation of nutrients have been highlighted. The issues associated with the development of large scale ultrasonic food processing equipment have also been briefly discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Acoustic cavitation generates extreme physical and chemical environment in liquids. Mechanical agitation, microjets, shear forces, microstreaming, hot spots and shockwaves are some of the physical forces that have been effectively used in several applications including emulsification and extraction [1–4]. The extreme thermal environment generated within cavitation bubbles leads to the generation of a variety of chemical reactions including the generation of highly reactive radicals [5–10]. Depending upon the nature of application, either the physical or chemical or both effects of acoustic cavitation can be selectively used. For example, extraction and emulsification processes primarily use the physical forces generated during acoustic cavitation [11,12]. For the generation of nanomaterials and the degradation of organic pollutants in aqueous environment, reducing or oxidising radicals are necessary [13–15]. In specific applications such as emulsion polymerisation and exfoliation of graphene sheets both the physical and chemical effects are necessary [16,17]. The physical forces are used to generate droplets of the monomer in the aqueous phase and the radicals are necessary to induce polymerisation of the monomer [14,16].
⇑ Address: School of Chemistry, University of Melbourne, VIC 3010, Australia. 1
E-mail address: [email protected] Adjunct Professor.
Similarly, exfoliation of graphene oxide sheets requires the shear and the reduction of graphene oxide is achieved by the reducing radicals generated during acoustic cavitation [17]. It has generally been observed that low frequency high intensity ultrasound generates strong shear and mechanical forces. Frequency in the range 20–40 kHz and applied power intensities >10 W/cm2 have been widely used [18]. Such systems have been found useful in generating oil-in-water emulsions in the size range 40–200 nm [19]. Other applications include reduction in molecular weight of polymers, extraction of a variety of bioactive and organic compounds and cleaning [21–25]. The amount of reactive radicals produced in such systems is significantly low [26]. Despite the cavitation activity is localised (see Fig. 1 left), the physical forces cause reasonably homogeneous effect in most applications (see Fig. 1 right). As can be seen in Fig. 1, the cavitation activity (probed using sonochemiluminescence, SCL) is confined close to the horn tip whereas the emulsification process is homogeneous throughout the sonicated liquid. In contrary, high frequency low power density ultrasound is widely used in applications where a high amount of redox radicals is necessary. For example, reduction of metal ions to generate metal nanoparticles and oxidation of organic pollutants in aqueous environment need large quantities of reducing and oxidising radicals, respectively [13,15]. High frequency ultrasound, in the range 200 kHz to less than 1 MHz, has been found useful for such
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Fig. 1. Left: Sonochemiluminescence generated at the tip of a 20 kHz flat horn demonstrating the localised cavitation activity; Right: An oil-in-water emulsion homogeneously generated throughout the reactor.
reactions. This is due to the generation of large number of activity cavitation bubbles as shown in Fig. 2, which shows the SCL activity throughout the reactor. Various studies have looked at maximising cavitation activity in such systems by varying experimental parameters that include frequency, power, etc. The use of very high frequencies (above 1 MHz) show a relatively lower cavitation (chemical) activity due to lower bubble temperatures and insufficient time for solvent evaporation into the cavitation bubbles [26,27]. In recent years, ultrasound has been used in food and bioprocessing applications, which include quality control, extraction, meat processing, crystallisation, functionality modification, separation, deactivation of microbes, etc. Several review articles are available in the literature [25,28–30]. In quality control applications, the changes to properties of sound waves when passing through processing solutions and food products have been used to evaluate their quality. For example, 4 MHz ultrasound has been used to monitor the moisture content in fish fillets [31]. The velocity of sound waves was found to vary with moisture contents in the fillets. Similarly, chemical composition of fermented sausages was evaluated using 1 MHz ultrasound [28]. Other quality control applications include the use of ultrasonic velocimetric technique to monitor the flow rates of processing liquids in juice industry and pulsed echo Doppler technique to monitor the rheological properties of various food materials. A detailed review on the use of ultrasound in food quality control is available [28]. Ultrasonic extraction is a widely used well established technique ([28,29] and references therein). The mechanical and shear forces generated help increase mass transfer allowing the diffusion of solvent into the crushed parts of plant materials. In addition, microjets and shear forces can also break the cell walls leading to effective releasing of the cell contents into solvents. For example, in supercritical extraction of gingerol from ginger [11], sonication resulted in an increase in the disruption of cell wall leading to the release of a higher amount of the compound. It has been
Fig. 2. SCL observed in a high frequency (440 kHz) reactor. Transducer is located at the bottom of the reactor.
suggested that the cell disruption due to 20 kHz sonication resulted in an increase in intra-particle diffusivity of the solvent. Ultrasonic preparation of food emulsions is also a well established technique. It is generally desired that the emulsion droplets are blow 50 nm and homogeneous in food emulsions. Leong et al. [12] have used 20 kHz ultrasound and Tween surfactant to generate stable triglyceride emulsion droplets in water. The delivery of nutritional oil in a dairy system has recently been reported [32,33]. Further details on this system will be elaborated later. The rate of crystallisation and the properties of crystals were found to be influenced by acoustic cavitation. Such process, commonly referred to as sonocrystallisation, is used in improving the quality of ice and fat crystals [34,35]. The above mentioned applications are well documented in the literature as books and review articles [25,28–35]. The main aim of this article is to provide an overview of the current developments of specific applications in selected food and dairy systems. While the article focuses on selected systems, the concepts and procedures are applicable to any food system in general. 2. High frequency ultrasound High frequency ultrasound has not been extensively used in food processing except for food quality monitoring and diagnostic purposes as mentioned above. One of the earlier studies explored the possibility of enhancing the antioxidant properties of phenolic compounds using acoustic cavitation generated OH radicals at an ultrasonic frequency of 358 kHz [26]. It is well known that OH radicals can be generated within cavitation bubbles due to the homolysis of H2O molecules under the high temperature conditions generated. The sonication of phenol in an aqueous solution has led to the hydroxylation of phenol producing catechol, hydroquinone and trihydroxy phenols. While phenol on its own does not show any antioxidant property, the hydroxylated phenols showed significant antioxidant efficiency. However, the sonochemical hydroxylation of cyanidin 3-glucoside was found to decrease the antioxidant capacity of this compound. It has been suggested that the position of OH radical addition within the aromatic ring of this compound is crucial to increase in its activity. OH addition in nonpreferred positions may decrease the antioxidant capacity of these compounds. Since sonochemical hydroxylation is not position specific, controlled hydroxylation is difficult to be achieved. In the same study, authors have indicated that sonochemical hydroxylation could be restricted using radical scavengers such as ascorbic acid to prevent such undesirable reactions during ultrasonic processing of food ingredients. A recent study [36] has used high frequency ultrasound (400 kHz and 1.6 MHz) for the separation of fat from milk (Fig. 3). The basic concept here is the establishment of standing waves where the acoustic force can move fat particles to nodal planes where they coalesce to generate larger low density particles
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This study has shown the sonication can significantly affect the pasting properties of starch, which may have potential application in food industry.
3. Low frequency ultrasound Low frequency (primarily 20 kHz) high intensity ultrasound has extensively been used in emulsification processes. For example, 20 kHz horn-type sonicators have been used to synthesis polymer latex particles using emulsion polymerisation [20]. As mentioned earlier, both the physical shear forces and the radicals generated are important for such polymerisation reactions. The shear forces
Fig. 3. Ultrasonically separated fat globules (a dye is added for visual clarification) using 400 kHz ultrasound. Adapted from Ref. [36].
that would ultimately float to the liquid surface where it could be skimmed off. One of the advantages in the ultrasonic fat process is that it does not need acoustic cavitation avoiding radical generation and no shear forces are generated. The viscosity of food systems is one of the critical functional properties difficult to be controlled during processing. Most food ingredients/products undergo heat and other shear processes prior to making the final products. For example, to prepare starch-based gel products, starch granules are subjected to heat processing leading to the formation of a paste. During such processing, the viscosity of starch paste can significantly increase and may provide processing difficulties. In some cases, a substantially higher temperature is required that may also affect the quality of the gel. A few studies have reported on the effect of sonication on the physicochemical properties of starch systems [37–41]. Various effects observed include pitting on starch granules due to acoustic cavitation generated microjets, reduction in the molecular weight of starch molecules, swelling of starch granules, etc. Zuo et al. [40,41] reported on the reduction in the viscosity of starch pastes by sonication at 211 kHz. It can be seen in Fig. 4 that the viscosity of waxy starch paste increases as a function of temperature. However, the sonicated samples show a significant reduction in the viscosity.
(a)
(b)
Fig. 4. Viscosity as function of pasting time for non-sonicated (open symbols) and sonicated (solid symbols) waxy rice starch dispersions. Suspensions were heated at 25 °C (h,j) or 63 °C (s,d) for 30 min. Sonication conditions are: power intensity of 0.18 W/cm2 and a frequency of 211 kHz. Dashed line is the heating profile. Adapted from Zuo et al. [40,41].
(c) Fig. 5. Experimental set-up for the glass cell with a flat tip horn (a), a flow-through horn (b) and a sealed chamber with a flat tip horn (c).
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Fig. 6. Photograph of 7% OM emulsions (20 kHz US; 176 W) with Sudan (III) dye processed for different sonication times on Day 9. 1 min (left) 3 min (middle) and 6 min (right). Adapted from Ref. [32].
generated during acoustic cavitation disperse fine droplets of a liquid monomer in the aqueous phase (Fig. 1). The primary and secondary radicals generated enter the monomer droplets and initiate the polymerisation process and convert each droplet into a polymer particle. Such ultrasonic polymerisation process showed significant advantages compared to conventional radical polymerisation process in terms of faster reaction kinetics, control over molecular weight and particle size and functionalisation of the polymer particles [20,27]. Leong et al. [19] have studied the possibility of creating transparent food emulsions using flat-tip and flow through horns and a sealed chamber arrangement (Fig. 5). Using different horn types and reactor configurations, the authors have explored the effect of various experimental parameters on the size of emulsion droplets generated and observed that the minimum droplet size achieved is independent of equipment configuration but primarily due to the power density used. It has been demonstrated that acoustic cavitation is a viable process by which about 40 nm transparent triglyceride oil emulsions could be generated at high power densities in a sealed chamber (Fig. 5c). While the focus of the above and other studies in the literature is to generate transparent emulsion droplets in aqueous medium, a recent study has demonstrated the potential use of ultrasonic emulsification process for encapsulation and delivery of nutritional compounds in dairy-based drinks [32,33]. Ultrasonic emulsification was successfully used to incorporate up to 21% flax seed oil in pasteurised homogenised skim milk (PHSM). It has been shown in this study that the emulsions were stabilised by the partiallydenatured whey proteins and were stable for at least 9 days. It has been reported that the emulsion stability is dependent upon the power and duration of sonication. For example, the photographs shown in Fig. 6 suggest that 6 min processing time is necessary to prepare a stable emulsion at 176 W applied power. For clarification purpose, a dye was dissolved in flax seed oil and the separation of the dye from the matrix was used to diagnose the stability of the emulsion. It has been noted in this study that a shortterm stable emulsion could be generated in 3 min processing time. The above mentioned study also compared the stability of the flax seed oil emulsions generated by ultrasonic processing with that generated by a conventional Ultraturrax (UT) at similar energy densities to that of US. UT did not produce stable emulsions until 20 min of processing suggesting the superiority of US emulsification process (Fig. 7). The reason for the stability of ultrasonically generated emulsion is suggested to be due to acoustic cavitation. The shear forces generated during acoustic cavitation partially denatures about 20% of the whey proteins leading to an increase in the hydrophobicity of these protein molecules [32,42]. The adsorption of such denatured proteins on the flaxseed oil emulsion provides the stability to the
Fig. 7. Flagseed oil emulsion generated by UT (left) and ultrasound (right). The flagseed oil droplets coalesced and separated from dairy matrix within a few minutes of treatment by UT. Whereas, ultrasonically prepared emulsion was stable for several days.
droplets from coalescence and Ostwald ripening. UT does not generate cavitation and hence whey proteins remain in their native state and do not stabilize the emulsion droplets. In a following report, Shanmugam and Ashokkumar [33] have also explored the functional properties of the flaxseed oil–dairy emulsions. The gels made of ultrasonically generated emulsions showed better gelation characteristics, viz., decreased gelation time, increased elastic nature, decreased syneresis and increased gel strength. For example, syneresis of ultrasonically prepared samples is about 70% lower than that prepared conventionally. Similarly, the strength of the gels prepared using ultrasonically generated emulsions is about 3 times stronger. The enhanced functional properties are also due to the presence of partially-denatured whey proteins that resulted in a strong gel network. Overall, the above mentioned studies have demonstrated that nutritional compounds could be dispersed as stable emulsion droplets in dairy based drinks leading to an increase in the nutritional value of the drinks that have potential health benefits to the human beings. It is well known that milk and dairy proteins undergo heat processing to improve their stability during storage. For example the storage stability of milk is enhanced by ultrahigh temperature treatment. Milk undergoes extensive heat treatment during the preparation of dairy-based powder products including infant milk powders. During such processes, proteins in milk and other dairy products undergo heat-induced aggregation resulting in an increase in viscosity that often affects the manufacturing process of several dairy products. For this reason, heat stability of dairy systems is one of the major issues in dairy industry. It has been reported that the sonication of pre-heated whey protein solutions showed heat stability towards aggregation and viscosity changes [43,44]. Sonication of a pre-sonicated protein solution resulted in
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5
150
Viscosity, cP
preheat postheat
Preheat-Sonicate
100
Preheat-SonicatePostheat
preheat only 50
Fig. 9. Gels generated by TSPP in 5 wt% micellar casein system. Left: sonication after the gelation was induced; Right: sonication prior to the addition of TSPP.
5 sec control
1 min 20 min 40 min
0
Treatment Fig. 8. Sonication of 9% whey protein concentrate (WPC) solution after preheating results in a heat-stable WPC [43,44]. Compared to the control sample, the viscosity increases significantly upon heating the WPC solution at 80 °C for 90 s. The viscosity of the preheated sample decreases substantially following 20 kHz sonication for 5 s to 1 min. The post heating of these sonicated solutions did not lead to an increase in the viscosity compared to unsonicated sample. The data also demonstrates that less than 1 min sonication is sufficient to provide heat stability, which is an important observation form energy consumption view point.
substantial reduction in the viscosity and subsequent heat stability as shown in Fig. 8. In an effort to understand the mechanism involved, the effect of sonication on whey protein solutions at low and high frequencies was studied. The results demonstrated that low frequency ultrasound is effective in lowering the viscosity compared to high frequency ultrasound. As mentioned previously, low frequency ultrasound generates strong shear forces and high frequency ultrasound generates significant amount of radicals. Based on the observed results, it has been concluded that the physical shear forces generated during acoustic cavitation is responsible for the reduction of viscosity and the radicals do not play any role in improving the heat stability of whey proteins. In order to explore other possible systems where ultrasound can be used, Chandrapala et al. have investigated the effect of ultrasound on various dairy products. WPC consists of 3:1 b-lactoglobulin (b-LG) and a-lactalbumin (a-LA). With an aim to
understand the heat stability observed by the ultrasonic treatment, Chandrapala et al. [45] studied the effect of sonication on pure and mixed b-LG and a-LA systems and observed interesting results. Sonication led to an increase in free thiol groups in b-LG and surface hydrophobicity in both b-LG and a-LA when they were processed individually. However, in the mixture, a slight increase in surface hydrophobicity followed by a decrease was observed. The slight increase in hydrophobicity might be responsible for the protein aggregation during sonication in the absence of pre-heat treatment leading to an increase in gel strength and decrease in syneresis [46]. Chandrapala et al. have also reported on the effect of sonication on the chemically induced gelation of micellar casein systems [47]. In cheese and yoghurt manufacturing processes, chemically induced gelation process is a critical step. In order to see if sonication of casein systems affects the gel properties of casein micelles, experiments were carried out on 5 wt% micellar casein solution. As shown in Fig. 9, sonication of micellar casein solution prior to the addition of tetra sodium pyrophosphate (TSPP, gelation agent) led to the formation of a firmer gel with a significant decrease in syneresis. This study has demonstrated that sonication can be an effective technique to alter the functionality of dairy systems and one should take care of the processing sequence. The same group has also studied the effect of sonication on the integrity of micelle units [48] and the structural characteristics of whey proteins [49] and concluded that sonication at 20 kHz does not affect the structural characteristics of dairy proteins. This is an important observation as it paves the way to implement ultrasonic technology in dairy industry. If sonication were to affect the
Fig. 10. (a) Schematic diagram showing a liquid core encapsulated by a polymer shell; (b) ultrasonically synthesised chitosan microspheres – encapsulated liquid (fish oil) can be released by opening the shell.
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physical or structural characteristics of proteins, it may have detrimental effects on the final dairy products. Zisu et al. have reported on the advantages of sonication of dairy systems that offer stability over storage in terms of age thickening [50] and pilot scale demonstration of ultrasonic dairy processing as a viable industrial process [51]. From above discussion, it can be concluded that the ultrasonic processing of dairy systems has the potential to improve the functional properties of dairy systems and their products without affecting their quality or nutritional values. High value nutrients can be delivered in a food matrix by encapsulation of such nutrients. Encapsulation of nutrients has several advantages over just mixing them in a food matrix that include minimising the flavour changes, avoiding any reaction between the nutrient and the food matrix that may reduce the functional properties of the nutrients, etc. While not much work has been reported on the ultrasonic encapsulation and delivery of nutritional compounds in food matrix, the concept is similar to that used in diagnostic and therapeutic medicine where specific drugs are encapsulated in a capsule and released at targeted tissues by functionalizing the carrier materials. In a model study, Zhou et al. [52] have encapsulated various organic liquids within protein shells (core–shell microspheres) using an ultrasonic methodology (Fig. 10a). The overall procedure is relatively simple. An aqueous solution containing a suitable polymer (protein, polysaccharide, etc.) is sonicated in the presence of an organic liquid to generate an emulsion of the organic liquid in the aqueous medium. In the case of protein–shelled systems, partially denatured proteins adsorb on the surface of the liquid droplets. Oxidative radicals generated during acoustic cavitation cross link the proteins forming a shell around the liquid droplets leading to the formation of core–shell microspheres. In order to demonstrate such process can be used to encapsulate nutrients within a shell, fish oil has been encapsulated within chitosan shells [53] that could be released when needed (Fig. 10b). Ultrasonic activation of enzymes [54,55] and deactivation of pathogens [56] are other areas where ultrasound is used in bioprocessing. Jadhav and Gogate [54] have studied ultrasound assisted enzymatic conversion of non edible oils to methyl esters. The cell production rate in photosynthetic bacteria by low-intensity ultrasound has been reported by Zhou et al. [55]. Further mechanistic details on the observed enhancement in the rates of hydrolysis or cell production have not been detailed in these studies. For the methyl ester production, enhanced mass transfer by acoustic cavitation generated physical effects could be the reason for the enhancement in the reaction rate. Mild stress generated by sonication could be the reason for the enhanced cell production rate. Gao et al. [56] have recently explored the possibility of using acoustic cavitation for the deactivation of pathogens found in dairy and other food systems. They have investigated the relationship between the deactivation efficiency and the physical (size, hydrophobicity) and biological (gram-status, growth phase) properties of the microbes. It has been concluded that the thickness of the cell wall of the pathogens is a key parameter that determines the susceptibility of the pathogens for ultrasonic deactivation. 4. Summary and future perspectives In this overview, the potential uses of ultrasonic technology in food and dairy processing have been highlighted. In particular, the following key functionalities could be achieved by ultrasonic processing of food products: viscosity, improved heat stability, reduced syneresis, increased gel strength and encapsulation and delivery of nutrients. While large scale pilot studies have been successfully carried out on ultrasonic processing of dairy systems, the development of appropriate large scale food processing equipment
is lacking. One of the issues in using ultrasonic technology (in particular direct immersion horns) is the direct contact between the ultrasonic horn and the food materials being processed. If the contact time is less than a few minutes, there should not be any issues related to the contamination of metal particles emitted from the tip of the horn. However, a few ultrasonic equipment manufacturers are focusing on developing reactors with horns attached on the exterior side to avoid direct contact between processing liquids and horns. While developing new and custom-made equipment is an issue to be addressed by engineers, physicists and food technologists, the ultrasonic technology offers a huge potential to food and bioprocessing industries.
Acknowledgements The author would like to acknowledge various contributions from his collaborators (Prof. S. Kentish, Dr. M. Palmer, Dr. B. Zisu, Dr, J. Lee and Dr. Chandrapala) and the financial support from Dairy Innovation Australia Ltd (DIAL) and ARC (ARC-Linkage grants).
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[41] J. Zuo, K. Knoerzer, R. Mawson, S. Kentish, M. Ashokkumar, The pasting properties of waxy rice starch suspensions, Ultrason. Sonochem. 16 (2009) 462–468. [42] A. Shanmugam, J. Chandrapala, M. Ashokkumar, The effect of ultrasound on the physical and functional properties of skim milk, Innovative Food Sci. Emerg. Technol. 16 (2012) 251–258. [43] M. Ashokkumar, J. Lee, B. Zisu, R. Bhaskaracharya, M. Palmer, S. Kentish, Sonication increases the heat stability of whey proteins, J. Dairy Sci. 92 (2009) 5353–5356. [44] M. Ashokkumar, S.E. Kentish, J. Lee, B. Zisu, M. Palmer, M.A. Augustin, Processing of dairy ingredients by ultrasonication, PCT Int. Appl. (2009) (CODEN: PIXXD2 WO 2009079691 A1 20090702 AN 2009:795878). [45] J. Chandrapala, B. Zisu, S. Kentish, M. Ashokkumar, The effects of high intensity ultrasound and heat treatment on the structural and functional properties of a-Lactalbumin, b-Lactoglobulin and their mixtures, Food Res. Int. 48 (2012) 940–943. [46] B. Zisu, J. Lee, J. Chandrapala, R. Bhaskaracharya, M. Palmer, S. Kentish, M. Ashokkumar, Effect of ultrasound on the physical and functional properties of reconstituted whey protein powders, J. Dairy Res. 78 (2011) 226–232. [47] J. Chandrapala, B. Zisu, S. Kentish, M. Ashokkumar, Influence of ultrasound on chemically induced gelation of casein micelle systems, J. Dairy Res. 80 (2013) 138–143. [48] J. Chandrapala, G. Martin, B. Zisu, S. Kentish, M. Ashokkumar, The effect of ultrasound on Casein micelle integrity, J. Dairy Sci. 95 (2012) 6882– 6890. [49] J. Chandrapala, B. Zisu, M. Palmer, S. Kentish, M. Ashokkumar, Effect of ultrasound on the thermal and structural characteristics of proteins in reconstituted whey protein concentrates, Ultrason. Sonochem. 18 (2011) 951–957. [50] B. Zisu, M. Schleyer, J. Chandrapala, Applied ultrasound to reduce viscosity and control the rate of age thickening of concentrated skim milk, Int. Dairy J. 31 (2014) 41–43. [51] B. Zisu, R. Bhaskaracharya, S. Kentish, M. Ashokkumar, Ultrasonic processing of dairy systems in large scale reactors, Ultrason. Sonochem. 17 (2010) 1075– 1081. [52] M. Zhou, F. Cavalieri, M. Ashokkumar, Modification of the size distribution of lysozyme microbubbles using a post-sonication technique, Instrum. Sci. Technol. 40 (2012) 51–60. [53] M. Ashokkumar, Applications of ultrasound in food and bioprocessing, Book of Abstracts, in: 14th Meeting of the European Sonochemical Society, Avignon, France, June 2–6, 2014. [54] S.H. Jadhaw, P.R. Gogate, Ultrasound assisted enzymatic conversion of non edible oil to methyl esters, Ultrason. Sonochem. 21 (2014) 1374–1381. [55] Q. Zhou, P.Y. Zhang, G.M. Zhang, Enhancement of cell production in photosynthetic bacteria wastewater treatment by low-strength ultrasound, Bioresour. Technol. 161 (2014) 451–454. [56] S. Gao, G.D. Lewis, M. Ashokkumar, Y. Hemar, Inactivation of microorganisms by low-frequency high-power ultrasound: 1. Effect of growth phase and capsule properties of the bacteria, Ultrason. Sonochem. 21 (2014) 446–453 (Inactivation of microorganisms by low-frequency high-power ultrasound: 2. A simple model for the inactivation mechanism, Ultrason. Sonochem. 21 (2014) 454–460).
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Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Applications of ultrasound in food and bioprocessing Muthupandian Ashokkumar ⇑,1 School of Chemistry, University of Melbourne, VIC 3010, Australia Chemistry Department, King Abdulaziz University, Jeddah, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 5 August 2014 Accepted 13 August 2014 Available online xxxx Keywords: Ultrasound Acoustic cavitation Food processing Viscosity modification Encapsulation
a b s t r a c t Improving the quality and nutritional aspects of food is one of the key issues for healthy life of human beings. The stability during storage is an important parameter in quality assurance of food products. Various processing techniques such as high pressure, thermal, pulsed electric field and microwave have been used to prolong the shelf-life of food products. In recent years, ultrasound technology has been found to be a potential food processing technique. The passage of ultrasound in a liquid matrix generates mechanical agitation and other physical effects due to acoustic cavitation. Owing to its importance, a number of review articles and book chapters on the applications of ultrasound in food processing have been published in recent years. This article provides an overview of recent developments in ultrasonic processing of food and dairy systems with a particular focus on functionality of food and dairy ingredients. More specifically, the use of high frequency ultrasound in fat separation from milk and viscosity modification in starch systems and the use of low frequency ultrasound in generating nutritional food emulsions, viscosity modification and encapsulation of nutrients have been highlighted. The issues associated with the development of large scale ultrasonic food processing equipment have also been briefly discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Acoustic cavitation generates extreme physical and chemical environment in liquids. Mechanical agitation, microjets, shear forces, microstreaming, hot spots and shockwaves are some of the physical forces that have been effectively used in several applications including emulsification and extraction [1–4]. The extreme thermal environment generated within cavitation bubbles leads to the generation of a variety of chemical reactions including the generation of highly reactive radicals [5–10]. Depending upon the nature of application, either the physical or chemical or both effects of acoustic cavitation can be selectively used. For example, extraction and emulsification processes primarily use the physical forces generated during acoustic cavitation [11,12]. For the generation of nanomaterials and the degradation of organic pollutants in aqueous environment, reducing or oxidising radicals are necessary [13–15]. In specific applications such as emulsion polymerisation and exfoliation of graphene sheets both the physical and chemical effects are necessary [16,17]. The physical forces are used to generate droplets of the monomer in the aqueous phase and the radicals are necessary to induce polymerisation of the monomer [14,16].
⇑ Address: School of Chemistry, University of Melbourne, VIC 3010, Australia. 1
E-mail address: [email protected] Adjunct Professor.
Similarly, exfoliation of graphene oxide sheets requires the shear and the reduction of graphene oxide is achieved by the reducing radicals generated during acoustic cavitation [17]. It has generally been observed that low frequency high intensity ultrasound generates strong shear and mechanical forces. Frequency in the range 20–40 kHz and applied power intensities >10 W/cm2 have been widely used [18]. Such systems have been found useful in generating oil-in-water emulsions in the size range 40–200 nm [19]. Other applications include reduction in molecular weight of polymers, extraction of a variety of bioactive and organic compounds and cleaning [21–25]. The amount of reactive radicals produced in such systems is significantly low [26]. Despite the cavitation activity is localised (see Fig. 1 left), the physical forces cause reasonably homogeneous effect in most applications (see Fig. 1 right). As can be seen in Fig. 1, the cavitation activity (probed using sonochemiluminescence, SCL) is confined close to the horn tip whereas the emulsification process is homogeneous throughout the sonicated liquid. In contrary, high frequency low power density ultrasound is widely used in applications where a high amount of redox radicals is necessary. For example, reduction of metal ions to generate metal nanoparticles and oxidation of organic pollutants in aqueous environment need large quantities of reducing and oxidising radicals, respectively [13,15]. High frequency ultrasound, in the range 200 kHz to less than 1 MHz, has been found useful for such
http://dx.doi.org/10.1016/j.ultsonch.2014.08.012 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
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Fig. 1. Left: Sonochemiluminescence generated at the tip of a 20 kHz flat horn demonstrating the localised cavitation activity; Right: An oil-in-water emulsion homogeneously generated throughout the reactor.
reactions. This is due to the generation of large number of activity cavitation bubbles as shown in Fig. 2, which shows the SCL activity throughout the reactor. Various studies have looked at maximising cavitation activity in such systems by varying experimental parameters that include frequency, power, etc. The use of very high frequencies (above 1 MHz) show a relatively lower cavitation (chemical) activity due to lower bubble temperatures and insufficient time for solvent evaporation into the cavitation bubbles [26,27]. In recent years, ultrasound has been used in food and bioprocessing applications, which include quality control, extraction, meat processing, crystallisation, functionality modification, separation, deactivation of microbes, etc. Several review articles are available in the literature [25,28–30]. In quality control applications, the changes to properties of sound waves when passing through processing solutions and food products have been used to evaluate their quality. For example, 4 MHz ultrasound has been used to monitor the moisture content in fish fillets [31]. The velocity of sound waves was found to vary with moisture contents in the fillets. Similarly, chemical composition of fermented sausages was evaluated using 1 MHz ultrasound [28]. Other quality control applications include the use of ultrasonic velocimetric technique to monitor the flow rates of processing liquids in juice industry and pulsed echo Doppler technique to monitor the rheological properties of various food materials. A detailed review on the use of ultrasound in food quality control is available [28]. Ultrasonic extraction is a widely used well established technique ([28,29] and references therein). The mechanical and shear forces generated help increase mass transfer allowing the diffusion of solvent into the crushed parts of plant materials. In addition, microjets and shear forces can also break the cell walls leading to effective releasing of the cell contents into solvents. For example, in supercritical extraction of gingerol from ginger [11], sonication resulted in an increase in the disruption of cell wall leading to the release of a higher amount of the compound. It has been
Fig. 2. SCL observed in a high frequency (440 kHz) reactor. Transducer is located at the bottom of the reactor.
suggested that the cell disruption due to 20 kHz sonication resulted in an increase in intra-particle diffusivity of the solvent. Ultrasonic preparation of food emulsions is also a well established technique. It is generally desired that the emulsion droplets are blow 50 nm and homogeneous in food emulsions. Leong et al. [12] have used 20 kHz ultrasound and Tween surfactant to generate stable triglyceride emulsion droplets in water. The delivery of nutritional oil in a dairy system has recently been reported [32,33]. Further details on this system will be elaborated later. The rate of crystallisation and the properties of crystals were found to be influenced by acoustic cavitation. Such process, commonly referred to as sonocrystallisation, is used in improving the quality of ice and fat crystals [34,35]. The above mentioned applications are well documented in the literature as books and review articles [25,28–35]. The main aim of this article is to provide an overview of the current developments of specific applications in selected food and dairy systems. While the article focuses on selected systems, the concepts and procedures are applicable to any food system in general. 2. High frequency ultrasound High frequency ultrasound has not been extensively used in food processing except for food quality monitoring and diagnostic purposes as mentioned above. One of the earlier studies explored the possibility of enhancing the antioxidant properties of phenolic compounds using acoustic cavitation generated OH radicals at an ultrasonic frequency of 358 kHz [26]. It is well known that OH radicals can be generated within cavitation bubbles due to the homolysis of H2O molecules under the high temperature conditions generated. The sonication of phenol in an aqueous solution has led to the hydroxylation of phenol producing catechol, hydroquinone and trihydroxy phenols. While phenol on its own does not show any antioxidant property, the hydroxylated phenols showed significant antioxidant efficiency. However, the sonochemical hydroxylation of cyanidin 3-glucoside was found to decrease the antioxidant capacity of this compound. It has been suggested that the position of OH radical addition within the aromatic ring of this compound is crucial to increase in its activity. OH addition in nonpreferred positions may decrease the antioxidant capacity of these compounds. Since sonochemical hydroxylation is not position specific, controlled hydroxylation is difficult to be achieved. In the same study, authors have indicated that sonochemical hydroxylation could be restricted using radical scavengers such as ascorbic acid to prevent such undesirable reactions during ultrasonic processing of food ingredients. A recent study [36] has used high frequency ultrasound (400 kHz and 1.6 MHz) for the separation of fat from milk (Fig. 3). The basic concept here is the establishment of standing waves where the acoustic force can move fat particles to nodal planes where they coalesce to generate larger low density particles
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This study has shown the sonication can significantly affect the pasting properties of starch, which may have potential application in food industry.
3. Low frequency ultrasound Low frequency (primarily 20 kHz) high intensity ultrasound has extensively been used in emulsification processes. For example, 20 kHz horn-type sonicators have been used to synthesis polymer latex particles using emulsion polymerisation [20]. As mentioned earlier, both the physical shear forces and the radicals generated are important for such polymerisation reactions. The shear forces
Fig. 3. Ultrasonically separated fat globules (a dye is added for visual clarification) using 400 kHz ultrasound. Adapted from Ref. [36].
that would ultimately float to the liquid surface where it could be skimmed off. One of the advantages in the ultrasonic fat process is that it does not need acoustic cavitation avoiding radical generation and no shear forces are generated. The viscosity of food systems is one of the critical functional properties difficult to be controlled during processing. Most food ingredients/products undergo heat and other shear processes prior to making the final products. For example, to prepare starch-based gel products, starch granules are subjected to heat processing leading to the formation of a paste. During such processing, the viscosity of starch paste can significantly increase and may provide processing difficulties. In some cases, a substantially higher temperature is required that may also affect the quality of the gel. A few studies have reported on the effect of sonication on the physicochemical properties of starch systems [37–41]. Various effects observed include pitting on starch granules due to acoustic cavitation generated microjets, reduction in the molecular weight of starch molecules, swelling of starch granules, etc. Zuo et al. [40,41] reported on the reduction in the viscosity of starch pastes by sonication at 211 kHz. It can be seen in Fig. 4 that the viscosity of waxy starch paste increases as a function of temperature. However, the sonicated samples show a significant reduction in the viscosity.
(a)
(b)
Fig. 4. Viscosity as function of pasting time for non-sonicated (open symbols) and sonicated (solid symbols) waxy rice starch dispersions. Suspensions were heated at 25 °C (h,j) or 63 °C (s,d) for 30 min. Sonication conditions are: power intensity of 0.18 W/cm2 and a frequency of 211 kHz. Dashed line is the heating profile. Adapted from Zuo et al. [40,41].
(c) Fig. 5. Experimental set-up for the glass cell with a flat tip horn (a), a flow-through horn (b) and a sealed chamber with a flat tip horn (c).
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Fig. 6. Photograph of 7% OM emulsions (20 kHz US; 176 W) with Sudan (III) dye processed for different sonication times on Day 9. 1 min (left) 3 min (middle) and 6 min (right). Adapted from Ref. [32].
generated during acoustic cavitation disperse fine droplets of a liquid monomer in the aqueous phase (Fig. 1). The primary and secondary radicals generated enter the monomer droplets and initiate the polymerisation process and convert each droplet into a polymer particle. Such ultrasonic polymerisation process showed significant advantages compared to conventional radical polymerisation process in terms of faster reaction kinetics, control over molecular weight and particle size and functionalisation of the polymer particles [20,27]. Leong et al. [19] have studied the possibility of creating transparent food emulsions using flat-tip and flow through horns and a sealed chamber arrangement (Fig. 5). Using different horn types and reactor configurations, the authors have explored the effect of various experimental parameters on the size of emulsion droplets generated and observed that the minimum droplet size achieved is independent of equipment configuration but primarily due to the power density used. It has been demonstrated that acoustic cavitation is a viable process by which about 40 nm transparent triglyceride oil emulsions could be generated at high power densities in a sealed chamber (Fig. 5c). While the focus of the above and other studies in the literature is to generate transparent emulsion droplets in aqueous medium, a recent study has demonstrated the potential use of ultrasonic emulsification process for encapsulation and delivery of nutritional compounds in dairy-based drinks [32,33]. Ultrasonic emulsification was successfully used to incorporate up to 21% flax seed oil in pasteurised homogenised skim milk (PHSM). It has been shown in this study that the emulsions were stabilised by the partiallydenatured whey proteins and were stable for at least 9 days. It has been reported that the emulsion stability is dependent upon the power and duration of sonication. For example, the photographs shown in Fig. 6 suggest that 6 min processing time is necessary to prepare a stable emulsion at 176 W applied power. For clarification purpose, a dye was dissolved in flax seed oil and the separation of the dye from the matrix was used to diagnose the stability of the emulsion. It has been noted in this study that a shortterm stable emulsion could be generated in 3 min processing time. The above mentioned study also compared the stability of the flax seed oil emulsions generated by ultrasonic processing with that generated by a conventional Ultraturrax (UT) at similar energy densities to that of US. UT did not produce stable emulsions until 20 min of processing suggesting the superiority of US emulsification process (Fig. 7). The reason for the stability of ultrasonically generated emulsion is suggested to be due to acoustic cavitation. The shear forces generated during acoustic cavitation partially denatures about 20% of the whey proteins leading to an increase in the hydrophobicity of these protein molecules [32,42]. The adsorption of such denatured proteins on the flaxseed oil emulsion provides the stability to the
Fig. 7. Flagseed oil emulsion generated by UT (left) and ultrasound (right). The flagseed oil droplets coalesced and separated from dairy matrix within a few minutes of treatment by UT. Whereas, ultrasonically prepared emulsion was stable for several days.
droplets from coalescence and Ostwald ripening. UT does not generate cavitation and hence whey proteins remain in their native state and do not stabilize the emulsion droplets. In a following report, Shanmugam and Ashokkumar [33] have also explored the functional properties of the flaxseed oil–dairy emulsions. The gels made of ultrasonically generated emulsions showed better gelation characteristics, viz., decreased gelation time, increased elastic nature, decreased syneresis and increased gel strength. For example, syneresis of ultrasonically prepared samples is about 70% lower than that prepared conventionally. Similarly, the strength of the gels prepared using ultrasonically generated emulsions is about 3 times stronger. The enhanced functional properties are also due to the presence of partially-denatured whey proteins that resulted in a strong gel network. Overall, the above mentioned studies have demonstrated that nutritional compounds could be dispersed as stable emulsion droplets in dairy based drinks leading to an increase in the nutritional value of the drinks that have potential health benefits to the human beings. It is well known that milk and dairy proteins undergo heat processing to improve their stability during storage. For example the storage stability of milk is enhanced by ultrahigh temperature treatment. Milk undergoes extensive heat treatment during the preparation of dairy-based powder products including infant milk powders. During such processes, proteins in milk and other dairy products undergo heat-induced aggregation resulting in an increase in viscosity that often affects the manufacturing process of several dairy products. For this reason, heat stability of dairy systems is one of the major issues in dairy industry. It has been reported that the sonication of pre-heated whey protein solutions showed heat stability towards aggregation and viscosity changes [43,44]. Sonication of a pre-sonicated protein solution resulted in
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5
150
Viscosity, cP
preheat postheat
Preheat-Sonicate
100
Preheat-SonicatePostheat
preheat only 50
Fig. 9. Gels generated by TSPP in 5 wt% micellar casein system. Left: sonication after the gelation was induced; Right: sonication prior to the addition of TSPP.
5 sec control
1 min 20 min 40 min
0
Treatment Fig. 8. Sonication of 9% whey protein concentrate (WPC) solution after preheating results in a heat-stable WPC [43,44]. Compared to the control sample, the viscosity increases significantly upon heating the WPC solution at 80 °C for 90 s. The viscosity of the preheated sample decreases substantially following 20 kHz sonication for 5 s to 1 min. The post heating of these sonicated solutions did not lead to an increase in the viscosity compared to unsonicated sample. The data also demonstrates that less than 1 min sonication is sufficient to provide heat stability, which is an important observation form energy consumption view point.
substantial reduction in the viscosity and subsequent heat stability as shown in Fig. 8. In an effort to understand the mechanism involved, the effect of sonication on whey protein solutions at low and high frequencies was studied. The results demonstrated that low frequency ultrasound is effective in lowering the viscosity compared to high frequency ultrasound. As mentioned previously, low frequency ultrasound generates strong shear forces and high frequency ultrasound generates significant amount of radicals. Based on the observed results, it has been concluded that the physical shear forces generated during acoustic cavitation is responsible for the reduction of viscosity and the radicals do not play any role in improving the heat stability of whey proteins. In order to explore other possible systems where ultrasound can be used, Chandrapala et al. have investigated the effect of ultrasound on various dairy products. WPC consists of 3:1 b-lactoglobulin (b-LG) and a-lactalbumin (a-LA). With an aim to
understand the heat stability observed by the ultrasonic treatment, Chandrapala et al. [45] studied the effect of sonication on pure and mixed b-LG and a-LA systems and observed interesting results. Sonication led to an increase in free thiol groups in b-LG and surface hydrophobicity in both b-LG and a-LA when they were processed individually. However, in the mixture, a slight increase in surface hydrophobicity followed by a decrease was observed. The slight increase in hydrophobicity might be responsible for the protein aggregation during sonication in the absence of pre-heat treatment leading to an increase in gel strength and decrease in syneresis [46]. Chandrapala et al. have also reported on the effect of sonication on the chemically induced gelation of micellar casein systems [47]. In cheese and yoghurt manufacturing processes, chemically induced gelation process is a critical step. In order to see if sonication of casein systems affects the gel properties of casein micelles, experiments were carried out on 5 wt% micellar casein solution. As shown in Fig. 9, sonication of micellar casein solution prior to the addition of tetra sodium pyrophosphate (TSPP, gelation agent) led to the formation of a firmer gel with a significant decrease in syneresis. This study has demonstrated that sonication can be an effective technique to alter the functionality of dairy systems and one should take care of the processing sequence. The same group has also studied the effect of sonication on the integrity of micelle units [48] and the structural characteristics of whey proteins [49] and concluded that sonication at 20 kHz does not affect the structural characteristics of dairy proteins. This is an important observation as it paves the way to implement ultrasonic technology in dairy industry. If sonication were to affect the
Fig. 10. (a) Schematic diagram showing a liquid core encapsulated by a polymer shell; (b) ultrasonically synthesised chitosan microspheres – encapsulated liquid (fish oil) can be released by opening the shell.
Please cite this article in press as: M. Ashokkumar, Applications of ultrasound in food and bioprocessing, Ultrason. Sonochem. (2014), http://dx.doi.org/ 10.1016/j.ultsonch.2014.08.012
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physical or structural characteristics of proteins, it may have detrimental effects on the final dairy products. Zisu et al. have reported on the advantages of sonication of dairy systems that offer stability over storage in terms of age thickening [50] and pilot scale demonstration of ultrasonic dairy processing as a viable industrial process [51]. From above discussion, it can be concluded that the ultrasonic processing of dairy systems has the potential to improve the functional properties of dairy systems and their products without affecting their quality or nutritional values. High value nutrients can be delivered in a food matrix by encapsulation of such nutrients. Encapsulation of nutrients has several advantages over just mixing them in a food matrix that include minimising the flavour changes, avoiding any reaction between the nutrient and the food matrix that may reduce the functional properties of the nutrients, etc. While not much work has been reported on the ultrasonic encapsulation and delivery of nutritional compounds in food matrix, the concept is similar to that used in diagnostic and therapeutic medicine where specific drugs are encapsulated in a capsule and released at targeted tissues by functionalizing the carrier materials. In a model study, Zhou et al. [52] have encapsulated various organic liquids within protein shells (core–shell microspheres) using an ultrasonic methodology (Fig. 10a). The overall procedure is relatively simple. An aqueous solution containing a suitable polymer (protein, polysaccharide, etc.) is sonicated in the presence of an organic liquid to generate an emulsion of the organic liquid in the aqueous medium. In the case of protein–shelled systems, partially denatured proteins adsorb on the surface of the liquid droplets. Oxidative radicals generated during acoustic cavitation cross link the proteins forming a shell around the liquid droplets leading to the formation of core–shell microspheres. In order to demonstrate such process can be used to encapsulate nutrients within a shell, fish oil has been encapsulated within chitosan shells [53] that could be released when needed (Fig. 10b). Ultrasonic activation of enzymes [54,55] and deactivation of pathogens [56] are other areas where ultrasound is used in bioprocessing. Jadhav and Gogate [54] have studied ultrasound assisted enzymatic conversion of non edible oils to methyl esters. The cell production rate in photosynthetic bacteria by low-intensity ultrasound has been reported by Zhou et al. [55]. Further mechanistic details on the observed enhancement in the rates of hydrolysis or cell production have not been detailed in these studies. For the methyl ester production, enhanced mass transfer by acoustic cavitation generated physical effects could be the reason for the enhancement in the reaction rate. Mild stress generated by sonication could be the reason for the enhanced cell production rate. Gao et al. [56] have recently explored the possibility of using acoustic cavitation for the deactivation of pathogens found in dairy and other food systems. They have investigated the relationship between the deactivation efficiency and the physical (size, hydrophobicity) and biological (gram-status, growth phase) properties of the microbes. It has been concluded that the thickness of the cell wall of the pathogens is a key parameter that determines the susceptibility of the pathogens for ultrasonic deactivation. 4. Summary and future perspectives In this overview, the potential uses of ultrasonic technology in food and dairy processing have been highlighted. In particular, the following key functionalities could be achieved by ultrasonic processing of food products: viscosity, improved heat stability, reduced syneresis, increased gel strength and encapsulation and delivery of nutrients. While large scale pilot studies have been successfully carried out on ultrasonic processing of dairy systems, the development of appropriate large scale food processing equipment
is lacking. One of the issues in using ultrasonic technology (in particular direct immersion horns) is the direct contact between the ultrasonic horn and the food materials being processed. If the contact time is less than a few minutes, there should not be any issues related to the contamination of metal particles emitted from the tip of the horn. However, a few ultrasonic equipment manufacturers are focusing on developing reactors with horns attached on the exterior side to avoid direct contact between processing liquids and horns. While developing new and custom-made equipment is an issue to be addressed by engineers, physicists and food technologists, the ultrasonic technology offers a huge potential to food and bioprocessing industries.
Acknowledgements The author would like to acknowledge various contributions from his collaborators (Prof. S. Kentish, Dr. M. Palmer, Dr. B. Zisu, Dr, J. Lee and Dr. Chandrapala) and the financial support from Dairy Innovation Australia Ltd (DIAL) and ARC (ARC-Linkage grants).
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