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The the sound waves having frequency greater than human hearing limit (~20 kHz) includes the ultrasound. Dolphins and bats use these ultrasound waves for navigation and hunting purposes with the help of echolocation. For ensuring the safety of food products, maximizing there quality and to minimizing the processing, it is one of the most emerging technology. Positive effect are shown on food processing like, mass transfer improvement, preservation of food, enhancement in thermal treatments processes and improvement of texture of the food products (Knorr et al., 2011). Low and high energy treatment application of ultrasound wave are based upon the frequency range use during processing of food products.
The frequencies higher than 100 kHz are used in Low energy or low intensity (<1 W·cm2) ultrasound processing treatment of food. This can be used for the monitoring and analysis different types of food material processing. It is non-destructive and widely used in meat processing industries for evaluating the raw material composition and fermented meat products, fish and poultry. The processing of vegetables, fruits in pre-and postharvest processing, processing of cheese, processing of cooking oils, bread and cereal processing.
High energy or high-intensity (>1W·cm−2) ultrasound waves having frequencies in between 20 and 500 kHz are used. Due to their disruptive nature its greatly affect the physical, mechanical, chemical as well as biochemical properties of food products. This technique there for used for microstructure controlling, sonocrystallization, emulsification of fats, modification of properties of food protein (functional), self life enhancement of food, microbial inactivation, drying process, freezing process and extraction processes. Food industries are in side of profit with using of this technology yet more research and effort are needed to develop efficient ultrasonic systems for large scale processing of food. (Gallego-Juárez, Rodriguez, Acosta, & Riera, 2010).
2. Low power ultrasound
This is a nondestructive methods used with spectroscopy and nuclear magnetic resonance (NMR). For estimating the structural properties of fluid foods and physicochemical, LPU is used (McClements, 1997). By changing the ultrasound properties foreign particle also be screened (i.e., without contact).
2.1. Basic principles of LPU for food analysis
Sound wave causes alternating compressions and decompressions in food (Blitz, 1963, 1971), due to their characteristic wavelength, velocity, frequency, pressure and period. The velocity and attenuation of the sound waves are both affected by the interaction of sound waves with matter via absorption and/or scattering mechanisms (McClements, 2005). Ultrasonic velocity (v) is determined by density (ρ) and elasticity (E) of the medium, according to the Newton–Laplace equation (Blitz, 1963):
v = √(E/ρ)
The above equation implies ultrasound velocity (solid form) of a material > that of its liquid form (e.g., solid and molten chocolate).
Ultrasound velocity measurements (UVM) are used for determining composition, structure, physical state and various molecular process for the analysis food materials (Buckin, Kudryushov, & O'Driscoll, 2002; Buckin, O'Driscoll, & Smyth, 2003) such as phase transition and crystallization kinetics in bulk fats, emulsions and solid lipid nanoparticles (Povey, Awad, Huo, & Ding, 2009;), for the defected food, defected in packaging and to detect foreign bodies (Zhao, Basir, & Mittal, 2009).
The physicochemical properties of food materials such as molecular relaxation, microstructure, phase composition, bulk viscosity and rheology can be estimated by knowing change in viscosity, compressibility, wall material, and scattering and adsorption effects (Povey, 1997), and also gives the information about kinetics and nature of chemical reactions and emulsion stability (Dukhin & Goetz, 2001, 2009; Dukhin, Goetz, & Travers, 2005; McClements, 1995, Buckin et al., 2002). The attenuation coefficient is also deals a major role in ultra sound processing of food, it depends on the material used for making the food products (Umchid, 2008). The other factor is Acoustic impedance, which is the product of sound velocity and density, passing through the boundary of different materials, that affects the reflection coefficient. Different materials with different densities will have different acoustic impedances, which affects attenuation coefficient. Attenuation (A) and acoustic impedance (z) are expressed by the following relationships (McClements, 1995):
R =AT/At=z1−z2/z1 + z2
Ao is the initial (unattenuated) amplitude of the wave.
x is the distance traveled.
R is the ratio of the amplitude of reflected wave (AT) to the incident wave (At) reflection coefficient.
z1 and z2 are the acoustic impedances of two materials.
2.3. Applications of low power ultrasound (LPU)
2.3.1. Meat products
LPU, as technique had already been used for genetic improvement programs for livestock (Crews & Kemp, 2002; Stelzleni et al., 2002; Wilson, 1992). Sound waves with various frequencies are used for human pregnancy (depending on depth of tissue penetration and resolution) produce images of internal organs in live animals, which can also be used as a tool and technique management and replacement of breeding stock (DuPonte & Fergerstrom, 2006; Williams, 2002), for amount of fat and muscle estimation beef, accretion and body composition (Paisley, Loehr, & Niemala, 2007; Williams, 2002, Faulkner, Parrett, Mckeith, & Berger, 1990),
For estimating the percentage of intramuscular fat (Chengcheng, Yufeng, & Kwabena, 2009; Ribeiro, Tedeschi, Stouffer, & Carstens, 2008), moisture and protein content can also be calculated (Ghaedian, Coupland, Decker, & McClements, 1998; Ghaedian, Decker, & McClements, 1997). Solid fat content of chicken fat, non-fat content and their composition, estimated with help of LPU (Chanamai & McClements, 1999, Sigfusson, Decker, & McClements, 2001) this suggest that it is a nondestructive method in food analysis. ultrasound velocity measurements had also been used for estimating raw meat mixtures composition (Benedito, Carcel, Rossello, & Mulet, 2001).
2.3.2. Fruits and vegetables
Interpretation of ultrasound data in fruits and vegetables are complicated due to the presence of voids and for evaluating their tissues it is unsuitable to use LPU (McClements & Gunasekaran, 1997; Povey, 1998, Mizrach, Galili, Rosenhouse & Teitel, 1991; Porteous,Muir, & Wastie, 1981; Sarkar & Wolfe, 1983). This sound wave is used for the quality control process as pre- and postharvest applications, for vegetables and fruits processing (Mizrach, 2008). The firmness of plums and tomato can be estimated by correlating the demined data of maturity and sugar content with the help of LPU (Mizrach, 2004, Mizrach, 2007). Oil composition, purity and quality have already been estimated by ultrasound velocity measurements (Sankarappa, Kumar, & Ahmad, 2005), which help in estimating properties like, specific volume, molar sound velocity, adiabatic compressibility, molar compressibility and intermolecular free length.
2.3.3. Cereal products
Cereal products include bread (major cereal product), biscuits, breakfast bars and other bakery products. Ultrasound techniques are used for examine the extent of mixing on three different flour dough systems, there for ultrasound can be used for on-line dough quality control (Ross, Pyrak-Nolte, & Campanella, 2004). Characterization of the phases of the fermentation in bread making are also seen (Elmehdi, Page, & Scanlon, 2003; Skaf, Nassar, Lefebvre, & Nongaillard, 2009). Skaf et al. (2009) and this help indevelopment of a low frequency acoustic technique with large sensors (Skaf et al., 2009). Wheat flour dough consistency induced by proteins and gelatinization of the starch can also monitored by ultra sound techniques (García-Álvarez, Salazar, & Rosell, 2011). Crispness of biscuits and cereal products are important sensory characteristic (Povey and Harden (1981) this can be measured using pulse–echo ultra sound technique (Povey, 1989; Povey & Harden, 1981).
2.3.7. Food gels
Tofu is a good source of proteins and minerals. It can also be used as substituent of meat (Rekha & Vijayalakshmi, 2011) and considered as a salt- or acid-coagulated water based gel, with soya lipids and proteins and other constituents trapped in its gel networks (Kohyama, Sano, & Doi, 1995). Ting, Kuo, Lien, and Sheng (2009) the ultrasonic velocity and attenuation at a 1 MHz frequency are used for the measurement of repining process, with destructing their nutrient and protein. Firmness by textural analysis can also be estimated by ultrasonic power attenuation (Ting et al., 2009).
2.3.8. Food proteins
Different properties like, protein hydration, solubility, foaming capacity, flexibility, compressibility and volume have alredy been estimated with the help of LPU (Gekko & Noguchi, 1979; Guzey, Kim, & McClements, 2004; Povey, Golding, Higgs, & Wang, 1999; Suzuki, Tamura, & Mihashi, 1996). Protein state can also calculated by ultrasound velocity measurement, which is related with the compressibility of it (Heremans & Smeller, 1998). To distinguish between soluble proteins and casein micelle in skimmed milk, LPU attenuation measurements were used, and which help in estimating the concentration and size of protein and micelle.
2.3.9. Ultrasonic monitoring of food freezing
Sigfusson et al. used ultrasound to estimating the TOF of an ultrasonic pulse in blocks of gelatin, chicken and beef during freezing (Sigfusson, Ziegler, & Coupland, 2004), This help in calculating calculating the percentage time required for food freezing.
Ultrasound waves are also use to determine the temperature of food and the ice content by measuring the speed (Aparicio, Otero, Guignon, Molina-García, & Sanz, 2008), which help inonline monitoring of frozen, freezing and thawing systems.
3. High power ultrasound
High amount of energy is released due to compressions and decompressions (rarefactions) of the medium particles, under propagation of ultrasound waves through a biological material. Frequency higher than 20 kHz is used in high power ultrasound which is responsible for changing physicochemical properties and enhance the quality of various food systems during processing (Mason, Chemat, & Vinatoru, 2011). Extraction of flavors, degassing, destruction of foams, emulsification, crystallization enhancement and modifying polymorphism are some application of high power ultrasound (Higaki, Ueno, Koyano, & Sato, 2001), while sterilization is an another benefit gained by using it(Baumann, Martin, & Hao, 2009).
3.1. Principles of high power ultrasound
Power ultrasound can greatly been affected by energy, intensity, pressure, velocity and temperature. following pattern are used to High power (Patist & Bates, 2008):
Pa is the acoustic pressure (a sinusoidal wave), which is dependent on time (t), frequency (f) and the maximum pressure amplitude of the wave (Muthukumaran, Kentish, Stevens, & Ashokkumar, 2006).
Pa max is related to the power input or intensity (I) of the transducer:
I = Pamax/2ρv
ρ is the density of the medium and v is the sound velocity in the medium.
Acoustic streaming is the main mechanism under low intensities (or high frequencies), (Leighton, 1994; Leighton, 2007). No formation of bubbles during motion and mixing within the fluid, can be seen in acoustic streaming (Alzamora, Guerrero, Schenk, Raffellini, & López- Malo, 2011). High energies liberated under higher intensities (low frequencies) with anacoustic cavitation (Mason, 1998) due to the generation, growth and collapse of large bubbles (Alzamora, Guerrero, Schenk, Raffellini, & López-Malo, 2011)
3.2. Application of power ultrasound in food processing
For the inducing the chemical and physical changes in different food systems, acoustic and hydrodynamic cavitations are responsible. This the reduced the reaction time, increases the reaction yield and using less forcing conditions, which depend on the way of cavitations used. The different ways in which cavitations can be used beneficially in food processing application are, (temperature and pressure) compared to the conventional routes.
In processing of chocolate, butter, margarine, whipped cream and ice cream Crystallization is a important in many food industries. With specific sensory attributes like, texture, hardness, smoothness, mouthfeel food product are made and the fat crystallization are controlled by temperature, cooling with using different application of high power ultrasound waves.
For sonocrystallization (crystallization of liquids and melts ) power ultrasound in the range of 20 kHz and up to the MHz are used. It is also used in fractionation of fats (separating stearin and olein from a triglyceride oil. Ultrasonication decreased the crystallization induction times and increased nucleation rate, which help in forming small crystals and their hardness is increases.
It is a process of mixing two immiscible phases (e.g., oil and water) with the aid of a surface active agent (emulsifier) into homogeneous dispersion (emulsion). This process requires an energy input, which is fulfilling by ultrasonication to facilitate the formation of small droplets (micelle). The collapse of cavitations releases forms high energy microjets near interfaces and facilitate emulsification, seen under ultrasonication, (Thompson & Doraiswamy, 1999). Less amounts of surfactants are needed under ultrasonication as compare to mechanical agitation and produced smaller and more stable droplets (Abismail, Canselier, Wilhelm, Delmas, & Gourdon, 1999; Behrend, Ax, & Schubert, 2000; Canselier, Delmas, Wilhelm, & Abismail, 2002; Juang & Lin, 2004). Low degree of droplet flocculation can be seen under HPU in oil-in-water emulsions (the creaming stability of emulsion increases).
3.2.5. Food enzymes
The stability, shelf life and quality of food products can be enhanced by inactivation of enzyme. Different intensities of on ultrasound are used increase or inactivate enzymatic activities. Combining the heat and low pressure in ultrasound treatment process the rate of inactivation of pectic enzyme tomato (Montañés, & Lopez Buesa, 2002), lipoxygenase in soybean (Lopez & Burgos, 1995a), peroxidase in horse radish and PME in orange (Vercet, Lopez, & Burgos, 1999). Efficiency of deactivation of enzymes are also enhanced by changing the frquecy of ultrasound with different power (De Gennaro, Cavella, Romano, & Masi, 1999).
3.2.7. Food freezing
Sublimation, the process used in removal of frozen food water content. This process of dehydration is done under vacuumed condition. The shelf life of this type of food is longest as compared to other food items. The quality can also be increased using HPU due to rate of freezing enhancement, as shown in potato (Li & Sun, 2002a; Li & Sun, 2002b; Sun & Li, 2003). Nucleation processes are increased and thus help in small crystal formation, which finally help in drying process (Acton & Morris, 1992). Here the caitation plays a major role which increases heat transfer rate and reduction of crystal size (Powrie, 1973).
Ultrasound is an emerging technology, and by using it at different frequencies its application enhances several times in food science and technology during food analysis, processing and quality control. The low power (high frequency) ultrasound provide us an application which is non-invasive, cheap and simple and for estimating the food composition (fish, eggs, dairy, etc.), physicochemical monitoring of food and structural properties like, emulsions, dairy products and juices and contamination detection like, metals and other foreign materials in canned food, dairy foods, etc. Food properties can be modified with help of High power (low frequency) ultrasound, by inducing physiochemical by changing cavitation. The reduced the reaction time, increases the production yield, which depend on the way of cavitations used. The different ways in which cavitations are used for better food production processes. This will help directly in food processing, preservation and safety. Efforts are required to make fully man made ultrasound system which help in better processing of food products for high yield production and help in producing food that are more safer for human beings.