SIERRA NORTE

P. J. Cullen

Dublin Institute of Technology

7.1 IntRoDUCtIon

The use of ultrasound in the food industry has been extensively investigated with reported applications in both food analysis and food processing. Ultrasound is a sound wave at a frequency above the threshold of human hearing (>20 MHz). Application of ultrasound in food processing can be classified into three categories. Low-intensity ultrasound uses low power levels, typically less than 1 W/cm2_{, at a frequency range of 5–10 MHz (McClements 1995; Mason 1998). Due to the low}

power levels, low-intensity ultrasound causes no physical or chemical alterations in the properties of the material through which the wave passes. It can be used for diagnostic measurements of food properties including texture, composition, viscosity, or concentration. In contrast, high-intensity

Contents

7.1 Introduction ...85 7.2 Generation of Power Ultrasound ...86 7.3 Applications of Power Ultrasound in Food Processing ...86 7.4 Effect of Ultrasound on Food Rheology...87 7.4.1 Rheological Properties of Food ...88 7.4.2 Effect of Power Ultrasound on Viscosity ... 89 7.4.3 Juice Clarification ...90 7.5 Effect of Sonication on Food Hydrocolloids ... 91 7.6 Effect of Sonication on Emulsions ... 91 7.7 Functional Properties ...92 7.8 Effect of Sonication on Rheological Properties of Dairy Products ...94 7.8.1 Fat Globule Size and Microstructure ...95 7.8.2 Effect of Sonication on Dairy Products ...95 7.9 Conclusion ...97 References ...98

ultrasound uses much higher power levels, typically in the range of 10–1000 W/cm2_{, at a frequency}

of 20–100 kHz (McClements 1995; Mason 1998).

Power ultrasound has been recognized as a promising processing technique to replace or com- plement conventional thermal treatments in the food industry. Advantages of sonication include reduced processing time, higher throughput and lower energy consumption (Zenker et al. 2003; Knorr et al. 2004) while reducing thermal effects. Various research groups have demonstrated the inactivation of pathogenic and spoilage microorganisms (Escherichia coli, Listeria), and spoilage enzymes (pectin methyl esterase, polyphenol oxidase) with reduced effects on quality or nutritional parameters compared to conventional thermal processing. Although ultrasound is regarded as a nonthermal processing technique, an increase in product macro temperature occurs, which depends on the intrinsic and extrinsic parameters. Most ultrasound applications are limited to liquid foods, mainly fruit juices, smoothies, and milk. Ultrasonic radiation (sonication) is used widely to break cells and organelles, largely because sonication disrupts the larger particles in a suspension, leaving smaller particles unaffected.

Ultrasound alone or in combination with other nonthermal technologies may inactivate microor- ganisms by physical (cavitation, mechanical effects) and/or chemical (formation of free radicals due to sonochemical reaction) principles. This chapter outlines the impact of ultrasound on the rheologi- cal and functional properties of food.

7.2 GeneRAtIon oF PoWeR ULtRAsoUnD

An ultrasonic power supply converts 50/60 Hz line voltage to high-frequency electrical energy. This high-frequency electrical energy is transmitted to a piezoelectric transducer within the converter, where it is changed to mechanical vibrations. There are three types of ultrasonic transducers in common usage including liquid-driven transducers, magnetostrictive transducers, and piezoelectric transducers (Mason 1998). Piezoelectric transducers are the most common devices employed for the generation of ultrasound and can be used over the whole range of ultrasonic frequencies (Mason 2000). Piezoelectric material such as barium titanate or lead metaniobate expands and contracts in alternating electrical fields, generating ultrasonic waves. The piezoelectric elements commonly used in ultrasonic transducers are potentially brittle and so it is normal practice to clamp them between metal blocks (front and back drivers). Typically generation of ultrasound is carried out using the elec- trostrictive transformer principle. This is based on the elastic deformation of ferroelectric materials within a high-frequency electrical field and is caused by the mutual attraction of the molecules polar- ized in the field (Raichel 2000). As ultrasound propagates through and interacts with a liquid, the energy is attenuated by scattering or absorption and it results in alternate rarefaction and compres- sion. Bubbles or cavities are formed if the amplitude of the wave is sufficiently large. This phenom- enon is known as cavitation. The collapse of bubbles creates localized high pressure that disrupts cell membranes and causes cell walls to break down (Scherba, Weigel, and O’Brien 1991).

7.3 APPLICAtIons oF PoWeR ULtRAsoUnD In FooD PRoCessInG

Power ultrasound has been actively investigated for food processing applications. Reported applica- tions of power ultrasound in food processing include inactivation of microorganisms and enzymes, generation of dispersions and emulsions, and the promotion of chemical reactions. Power ultra- sound is used in a number of unit operations in fruit juice processing such as cleaning, extraction, homogenization, emulsification, sieving, filtration, crystallization, and pasteurization. High-energy ultrasound has been applied for degassing of liquid foods, for the induction of oxidation/ reduction reactions, for extraction of enzymes and proteins, for enzyme inactivation, and for the induc- tion of nucleation for crystallization (Roberts 1993; Thakur and Nelson 1997; Villamiel and de Jong, 2000a). Power ultrasound has been employed for the inactivation of E. coli in apple cider (Baumann, Martin, and Feng 2005) and orange juice processing (Valero et al. 2007; Tiwari et al.

Ultrasound Processing: Rheological and Functional Properties of Food 87

2008a). Similarly, enzymes such as peroxidase (De Gennaro et al. 1999), proteases, and lipases (Vercet, Burgos, and Lopez-Buesa 2001) were reported to be inactivated. Application of ultra- sound may be classified as sonication, monosonication, themosonication, or manothermosonication (MTS), depending upon whether it is combined with heat or pressure. For example, MTS is the simultaneous application of heat and high-energy ultrasonic waves under moderate pressure. MTS is able to inactivate food-related enzymes and microorganisms at much higher rates than thermal treatments of comparable temperatures (Burgos 1999). Despite the potential of power ultrasound as a nonthermal food processing technology, ultrasound may induce both desirable and undesirable effects on the nutritional and quality parameters of food. This chapter reviews changes in rheologi- cal properties of sonicated foods.

7.4 eFFeCt oF ULtRAsoUnD on FooD RHeoLoGy

The effect of ultrasound on food rheology is mainly due to the cavitation phenomenon. Cavitation is the formation, growth, and, in some cases, implosion of bubbles within liquids (Figure 7.1). Two different types of cavitation phenomena can be generated by acoustic waves, namely inertial and

noninertial cavitation. Inertial cavitation involves large-scale variations in bubble size (relative to the equilibrium size) over a timescale of a few acoustic cycles, where the rapid growth termi- nates in bubble collapse with varying degrees of intensity. Noninertial cavitation (stable) involves small-amplitude oscillations (compared to bubble radius) (Atchley and Crum 1988). Table 7.1 lists some of the proposed mechanisms of action for ultrasound. The thermal, mechanical, and chemi- cal effects of high-intensity ultrasound have been attributed to the rapid formation and collapse of cavitational bubbles, generating intense normal and shear stresses (Crum 1995; Stephanis, Hatiris, and Mourmouras 1997). Implosion of cavitation bubbles leads to energy accumulation in hot spots where temperatures of 5000°C and pressures of 100 MPa have been measured (Suslick 1988). As a result of these conditions, water molecules can be broken, generating highly reactive free radicals that can react with other molecules (Riesz and Kondo, 1992). Cavitational thermolysis may produce hydroxyl radicals and hydrogen atoms that can be followed by formation of hydrogen peroxide and, in the absence of oxygen, hydroperoxyl radicals (Portenlaenger and Heusinger 1997; Cains, Martin, and Price 1998; Ashokkumar and Grieser 1999). Production of H2O2 during sonication is tempera-

ture dependent, decreasing with increase in temperature. Although increasing liquid temperature during sonication allows a reduction in the cavitation threshold, the maximum temperature and pressure during cavitational bubble collapse will be decreased (Mason and Lorimer 2002). Thus the sonochemical reaction that generates H2O2 (Reaction 1), would be less intense at elevated tem-

peratures (Suslick 1991).

H2O→OH–+H+→H2O2+H2 (7.1)

These transient reactive species can subsequently react with carbohydrates. In addition hydroly- sis and cleavage due to the strong mechanical forces have been reported for a variety of poly- saccharides (Kardos and Luche 2001), reducing molecular weight and changing the rheological

Formation Collapse

(4000–5000 K, 50–100 MPa) Successive growth

properties of food. Mechanical stress, generated by shock waves derived from bubble implosion or from microstreaming derived from bubble size oscillation, may also break large macromolecules or particles (Basedow and Ebert 1977). Because of the nature of these ultrasound effects on molecules or particles dissolved or suspended in liquids, it is expected that the large multimolecular structures such as casein micelles and fat globules in milk will be affected by ultrasound.

Another important factor that influences the effectiveness of cavitation is the viscosity of the liq- uid. In highly viscous products, ultrasound diffusion is easily disrupted and this reduces the degree to which cavitation occurs. Low-frequency high-power ultrasound is better at penetrating viscous products than higher-frequency high-power ultrasound, which is more easily dispersed within a viscous liquid. Alternatively, if the liquid is heated, viscosity is reduced and ultrasound penetration improves (Earnshaw, Appleyard, and Hurst 1995).

7.4.1 rHeological propertiesof fooD

Rheology is defined as the science of deformation and flow, providing insight into material phe- nomena, which are governed by the microscopic scale at a macroscopic level. Rheology studies the response of materials when subjected to force. A viscous liquid may be defined as a medium where the energy needed to deform it is completely dissipated in the process of deformation. Consequently, it will not recover from the deformation. Conversely an elastic solid is a material that stores the work from the deformation process and returns energy after removal of the deformation forces. As rheology describes how matter responds to an applied stress or strain, it finds application in diverse areas such as product development, process engineering calculations, quality control, stability stud- ies, and correlations to sensory data. Many processed foods are formulated to display the desired rheological behavior under specific stress conditions such as gravity, pouring, mouth feel, etc.

Dispersions are defined as homogeneous or heterogeneous systems where a solid (rigid or deform- able) or liquid phase is dispersed in a liquid medium. Dispersions may destabilize or transform over time under a range of forces including interparticle, gravitational, and shear. Shear thinning

tABLe 7.1

theory and Mechanism for Power Ultrasound

theory Mechanism Reference

Cavitation Mechanical removal of attached or entrapped

bacteria

Seymour et al. 2002 Localized hot spots (5500°C) and high pressure

(500 MPa)

Suslick 1988; Mason and Luche 1996; Vercet et al. 2001 Increase in permeability of membranes or loss

of selectivity

Lehmann and Krusen 1954 Disruption (shear stress, localized heating) and

chemical reactions within the cell of the microorganism

Piyasena et al. 2003

Formation of free radicals Sonolysis of water may produce OH- _{and H}+ species and hydrogen peroxide

Suslick,1988; Vercet et al. 1997 Intracellular micromechanical

shocks

Disruption of cellular structural and functional components up to the point of cell lysis

De Guerrero et al. 2001

Thinning of cell membranes Sams and Feria 1991; Butz and

Tauscher 2002; Fellows 2000 Generation of mechanical energy Cleaning action on surfaces Scherba et al. 1991; Sala et al.

1995 Compressions and rarefactions or

compression/expansion cycles

Acoustic microstreaming Scherba et al. 1991; Floros and

Ultrasound Processing: Rheological and Functional Properties of Food 89

behavior is observed with structured foods, where viscosity decreases with applied shear. Complex rheological behavior such as shear thinning and time-dependency effects may be associated with the shear-induced breakdown and possible rebuilding of structure. The fluid’s microstructure, which may be due to either macromolecule entanglement or particle–particle interaction, is broken down under shear. The exhibition of a yield stress occurs where interparticle attraction forms flocs that in turn develop a three-dimensional network throughout the fluid. Such a network provides additional structure to a fluid, resulting in solid-like behavior under low stresses. A minimum stress is required for sufficient structural breakdown and the initiation of flow. Hydrocolloids are widely used as structuring agents to control suspension stability of large particulates along with organoleptic and processing parameters of foods.

7.4.2 effectof poWer ultrasounDon viscosity

Viscosity is one of the most important rheological properties of food. Viscosity of tomato juice/ puree is highly dependent on pectic substances that form an entanglement where other particles are physically entrapped (Rao and Cooley 1992). In tomato juices, MTS treatments results in a thicker consistency and initial apparent viscosities compared with unprocessed juice (Vercet et al. 2002a). Tomato juices are pseudogels, whose flow properties depend on the interaction or entanglement of cell particles (mostly cell walls), soluble pectin concentration, and the chemical properties of the latter. MTS treatments of pure pectin solutions yielded molecules with lower apparent viscosities due to size reduction (Mason et al. 2005). A similar study conducted by Tiwari et al. (forthcoming) observed a significant reduction in apparent viscosity for 2% w/v pectin dispersion (Figure 7.2). Seshadri et al. (2003) also reported a similar influence of sonication on the gel strength properties of pectin. It is difficult to predict what might be expected from a modification of pectin properties in gels or pseudogels derived from pectin. Longer molecules show a higher resistance to flow however shorter ones can interact in a different way with suspended particles also leading to an increased resistance to flow.

Vercet et al. (2002a) reported that the rheological properties of tomato paste are improved by MTS. In addition they observed that the thixotropic and pseudoplastic behavior of tomato paste was not affected by MTS. These are important properties to maintain the characteristic mouthfeel of the product (Rosenthal 1999). Particle interactions and particle size both play a role in determining tomato consistency. An increase in the number and/or intensity of interactions leads to increased consistency. These interactions can be chemical and/or physical. Pectin molecules are the main agents of physical interactions (Voragen et al. 1995), whereas chemical interactions are dependent on many parameters (Tsai and Zammouri 1988). Consistency is dependent on particle size in a com- plex manner. Reducing particle size leads to a decrease in viscosity. However, there is a point where

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0 10 20 30 40 50 Shear rate, 1/s

Apparent viscosity, Pa.s

( ) Control ( ) 3.7 W/cm2 _{( ) 6.3 W/cm}2 _{( ) 8.3 W/cm}2 _{( ) 10.1 W/cm}2

viscosity begins to increase when particle size decreases (Beresovsky, Kopelman, and Mizrahi 1995). This has been ascribed to an increase in the number of interactions between particles or to the fact that smaller particles fit better within the pectin network. The mechanism by which MTS improves the rheological properties of tomato pastes is related to cavitation. This phenomenon results in the breakage of molecules or particles. Particle size reduction and molecule breakage induced by ultrasound has been described in the literature (Price 1990). Application of MTS to break pectin molecules in a purified pectin solution was reported by Mason et al. (2005). It is also possible that ultrasound promotes protein denaturation (Villamiel and de Jong 2000b). Denatured proteins can adhere nonspecifically to tomato particles and facilitate better interaction between particles. Whatever the mechanism, it is worth noting that another emerging technology of food preservation, high pressure, also increases the viscosity of tomato pastes (Porretta et al. 1995).

7.4.3 Juice clarification

Cloud stability is an important rheological property of fruit juices. Loss of cloudiness and gelation of concentrate is a common problem associated with citrus juices (fresh squeezed, con- centrated, and preserved) (Basak and Ramaswamy 1996). Cloud stability is a critical orange juice quality parameter imparting characteristic flavor, color, and mouthfeel. Cloud is attributed to the suspension of particles composed of a complex mixture of protein, pectin, lipids, hemicel- lulose, cellulose, and other minor components (Klavons, Bennett, and Vannier 1991; Baker and Cameron 1999). Cloud loss arising from the gelation of juice concentrates is primarily attributed to the activity of Pectinmethyl esterase (PME) (Cameron, Baker, and Grohmann 1998; Versteeg et al. 1980). Sequential cleavage of the methyl esters at C6 of galacturonic acid residues in pectin produces free acid groups. When of sufficient size, such blocks on adjacent pectin molecules can be cross-linked by divalent cations, leading to protein precipitation. It has been reported that the degree of esterification of the pectin backbone necessary to cause cloud loss in orange juice is <36% (Baker 1979; Krop and Pilnik 1974). Figure 7.3 shows changes in cloud value and cor- responding chord length distributions as a function of ultrasound intensity. This size reduction contributes to improved cloud stability.

Cloud retention after sonication may be attributed to the dispersion stability of macromolecules due to the promotion of certain chemical reactions during sonication (Floros and Liang 1994; Mason 1998; McClements 1995). High-intensity ultrasound enhances protein solubility (Krešic´ et al. 2008) by changing protein conformation and structure in such a way that hydrophilic parts of amino acids are opened toward water (Morel et al. 2000; Moulton and Wang 1982). Lacroix, Fliss, and Makhlouf (2005) reported that electrostatic interactions between PME and its substrate pectin have

0.00 0.50 1.00 1.50 2.00 2.50 Control 0.42 W/ml 0.61 W/ml 1.05 W/ml Cloud value ( A660nm ) 0 20 40 60 80 100 120 140 1 10 100 1000 Chord length (µm) Counts (s –1) Control 0.42 W/ml 0.61 W/ml 1.05 W/ml

FIGURe 7.3 Effect of acoustic energy density on cloud value and juice stability with corresponding particle

Ultrasound Processing: Rheological and Functional Properties of Food 91

a major impact on orange juice cloud stability. Lacroix et al. (2005) reported that the particle sus- pension stability of orange juice depends not only on PME activity but likely also on modifications of pectin. It has been reported that to retain particles in suspension in the juice, the particles must be sufficiently small (0.5–10 μm) (Mizrahi and Berk 1970; Lam Quoc et al. 2006). Seshadri et al. (2003) suggested that the application of ultrasound breaks the linear pectin molecule, reducing its molecular weight, resulting in weaker network formations. Structural damage of pectin may result from microjets of liquid generated by the asymmetrical collapse of the cavitation bubble. The resul- tant high-pressure gradient may also cause fragmentation of macromolecules or other structural modifications, reducing the accessibility of the pectin molecule to PME.

7.5 eFFeCt oF sonICAtIon on FooD HyDRoCoLLoIDs

Sonication is reported to induce both reversible and nonreversible structural modifications. Structural changes induced by sonication may be advantageous for meeting processing requirements and dif- ferentiated functionality. Iida et al. (2008) studied the effectiveness of ultrasound for the reduction of viscosity after gelatinization of starch. They indicated that the reduction in viscosity during sonication is rapid, does not require any chemical additives and the process will not induce large changes in the chemical structure. Ultrasonic depolymerization has been applied to a variety of homo- and heteropolysaccharides such as dextran (Lorimer et al. 1995), pullulan (Koda et al. 1994), chitosan (Chen, Chang, and Shyur 1997), hyaluronic acid (Miyazaki, Yamamoto, and Okada 2001), xyloglucan (Vodenicˇarová et al. 2006), starch (Chung et al. 2002), and other food hydrocolloids such as pectin, guar gum, and xanthan.

Camino, Pérez, and Pilosof (2009) studied the effect of high-intensity ultrasound on the par- ticle size of hydroxypropylmethylcellulose (HPMC), which is a surface-active polysaccharide and is a cellulose derivative with methyl and hydroxypropyl groups added to the anhydroglucose back- bone. Camino et al. (2009) reported that ultrasound treatment of HPMC induced the formation of concentration-dependent transient clusters that can be regarded as preaggregates which would facilitate the formation of bigger aggregates during the pregel regime, lowering the cloud point. Lowering of cloud point indicates that ultrasound treatment modifies the HPMC performance dur- ing the first stage of the gelation process, related to the cloud point. The phenomenon of cavita- tion during sonication induces the formation of clusters that can be regarded as preaggregates that would facilitate the formation of bigger aggregates during the pregel regime (cloud point). They also observed changes in viscosity and water mobility for high-molecular-weight HPMC, which indicates the structural modifications that were not apparent for the low-molecular-weight HPMC without influencing their emulsifying behavior.

Ultrasound treatment can be employed for partial depolymerization and preparation of medium-sized macromolecules from large ones. For example, Kasaai, Arul, and Charlet (2008) studied the kinetics of chitosan fragmentation by ultrasonic irradiation at a frequency of 20 kHz. They observed that the optimum conditions for preparation of lower-molecular-weight fragments from large macromolecules were a low chitosan concentration, a high ultrasonic power, and a high solution temperature. Polymer chain scission increases with an increase in power level and solution temperature, but a decrease in chitosan concentration. Ultrasound with a frequency of 20 kHz can cause both polymerization and depolymerization (Kruus, Lawrie, and O’Neill 1988), which in turn