Sahana M D and Gopika Radhakrishnan
Ultrasound is a fast, versatile, emerging, and promising non-destructive green technology with a wide range of applications including seafood processing.The advantages of using ultrasound for sea food processing include more effective mixing and micro-mixing, mass transfer, reduced thermal and concentration gradients, increased production, elimination of process steps, reduced temperature, selective extraction, reduced equipment size, and faster response to the process.
The seafood industry is increasingly moving towards new product development and adopting innovative processing methods that allow us to either do things we could not do before, or to do things better and more efficiently than we did in the past. However, deterioration of the product before it reaches to the consumer is not uncommon and may lead to significant waste of fish resources, necessitating more rapid and efficient processing approaches. At the same time, there is a high demand for cleaner, organic products without the use of additives.
The role of technology in seafood processing has evolved rapidly over the last decade in response to the above drivers to support innovation, productivity, waste reduction, waste recovery and utilization; increase shelf-life; improve food safety; and facilitate exports.
A number of innovative processing technologies have recently emerged as a result. The ultrasonic method is one of those rapidlyemerging techniques that were devised to minimize processing, enhance quality, and safeguard thesafety of food products.
Ultrasound producing systems
The ultrasonic wave producing system consists of a power generator and the transducers. The generator produces electrical or mechanical energy and the transducer converts this energy into the sound energy at ultrasonic frequencies.In general, ultrasonic applications in the food industry can be divided into low and high power (frequency) ultrasound.
Low power ultrasound approaches use small-amplitude ultrasonic waves at high frequency (>100 kHz at intensities below 1 W·cm2) that does not damage the material through which they propagate. These mainly monitor the physicochemical properties and composition of the components of food and products during various steps of processing and storage which helps to control the properties and quality improvement. On the contrary high power (low frequency) ultrasound uses acoustic frequencies between 20 and 500 kHz ( intensitieshigher than 1W·cm2),larger amplitude ultrasonic waves and can alter the physicochemical properties or structure of material through various processes like cavitation.This is the formation of small bubbles at the points of low pressure in the sound wave.
Cavitationis used in various operations such as extraction, drying, freezing, emulsification, and also inactivates the pathogenic bacteria.The basics of ultrasound applications are mainly three different methods: direct application with the product, coupling to the device, and using ultrasonic bath submergence.
Different forms of Ultrasound and its application
Low power ultrasound
It is used in a non-destructive application, used inanalytical measurements and monitoring of composition and physio-chemical properties of food such as structure, firmness, composition, flow rate, and physical state. These ultrasound waves have a characteristic wavelength, velocity, frequency, pressure, and period. The interaction of sound waves with matter alters these properties of the soundvia absorption and/or scattering mechanism. Ultrasound can be propagated in any material medium. The propagation of ultrasonic waves consists of a very high number of pressure cycles per second, which travel forward.
Applications
Among the numerous applications of low?intensity ultrasound, most widely it is applied for the non?destructive evaluation of materials and ultrasound imaging, non?destructive testing,process control, and evaluation of materials. Essentially the application procedure involves passing a beam of ultrasound through the material under test and receiving the transmitted beam on the opposite side of the sample and/or the reflected beam. Pulse–echo and continuous wave ultrasound are two major techniques that are used in most ultrasound sensors.
The three factors of high concern for low power ultrasonic application are the ultrasonic velocity, attenuation coefficient, and acousticimpedance. These factors are affected by the physical properties of foods such as composition, structure, and physical state.Ultrasound Velocity measurements (UVM) suitable for determining composition, structure, physical state, and various molecular processes as ultrasound velocity is very sensitive to the molecular organization and intermolecular interactions.Ultrasound Attenuation coefficient measurements(UACM) give information such as molecular relaxation, microstructure, phase composition, bulk viscosity and rheology, the kinetics of the fast chemical reaction, and droplet sizing and stability in emulsions.Attenuation is affected by viscosity, compressibility, wall material, and scattering and adsorption effects.
Ultrasound velocity measurements are for the evaluation of oil composition, purity, and quality.Moisture content of cod fillets determined by ultrasonic velocity measurements showed similar result with conventional method,rapid and non-destructive determination .
Ultrasound techniques are used to monitor the physical properties of batters (density, viscosity, and rheology) A combined system of ultrasonic spectroscopy and a low-resolution pulsed nuclear magnetic resonance spectrometer was used to monitor crystallization. Process Control Ultrasonic sensors are frequently employed in many industrial processes to obtain information about different variables.
Ultrasonic imaging
This technique has found several applications in foodtechnology, like the use of high contrast images to detect defects in food packaging, sealsvibrationreflection images of tissues such as muscle, fat, and internal organs in live animals, which can be used as a management tool in selection and replacement of breeding stock for the improvement of the genetics of the herd.Ultrasound could be effectively used for the prediction of meat yield in live fish, but prediction accuracy needs to be improved.
High power ultrasound
High Power ultrasound is the study of high?intensity applications in the ultrasonic energy.It modifies the physical, chemical, and biological properties of food and enhances the quality during processing. The most relevant nonlinear phenomena related to high?intensity acoustic waves, which are wave distortion, acoustic saturation, radiation pressure, acoustic streaming, cavitation in liquids, and the formation and motion of dislocations in solids.When sound waves propagated through any product, there will be a production of a high amount of energy due to compression (positive pressure) and rarefaction (negative pressure). Higher intensities (low frequencies) induce acoustic cavitation. There are two main types of cavities: transient (also called “inertial”)and stable (also called “noninertial”)7.
Thus, cavitation is the formation, growth, and collapse of bubbles that causes the thermal, mechanical, and chemical effects.Mechanical effectsinclude collapse pressure, turbulence, and shear stresses. The chemical effects include the generation of free radicals.Free radicals can be captured in some chemical reactions. There are concerns regarding potential oxidative damage associated with free radicals.The thermal effects include the generation of the cavitation zone with extremely high temperatures (5,000 K) and pressures (1,000 atm)10. 
Ultrasound applications in different seafood preservation processes
The mechanical effect has many applications such as extraction of flavors, degassing, destruction of foams, emulsification, enhancement of crystallization, and modifying polymorphism. The chemical and biochemical effects are effective tools for sterilizing equipment, preventing contamination of food processing surfaces by pathogenic bacteria, and removal of bacterial biofilms.
Filtration
In the seafood industry, to produce a solid-free liquid or to isolate solid from its mother liquor, the separation of solids from liquids is an important step. In this case, two specific effects are involved: Agglomeration of fine particles in the nodes of the acoustic waves and generation of sufficient vibrational energy to keep the particles partly suspended and therefore leave more free ‘channels’ for solvent elution. The liquid jet serves as the basis for cleaning, and some other cavitational mechanisms lead to particle release from the blocked membrane.
Defoaming
Foam is a dispersion of gas in a liquid, with a density approaching that of the gas. The foam may lead to problems like loss of products, a decrease in productivity. High intensity ultrasonic waves have a distinctive method of foam breaking by cavitation.. Applied to control the excess foam produced during the filling operation of bottles and cans on high-speed canning lines and in fermentingvessels and other reactors of great dimensions .
Ultrasound Extraction
A major application of ultrasound is for facilitating the extraction process ofa variety of food components (e.g., oil, protein, polysaccharides)as well as bioactive ingredients The action of ultrasound is due to cavitation, which generates high shear forces and microbubbles that enhances surface erosion, fragmentation and mass transfer resulting inhigh yield of extracted materials and fast rate of extraction.Ultrasound assisted extraction (UAE) can be employed to enhance extraction of bioactive compounds from seaweed.During ultrasonicextraction, temperature was maintained using ice to prevent degradation of compounds usually applied in extraction biopolymers like Carrageenans from seaweeds.
Depolymerization
One of the oldest applications of ultrasound is in the degradation of polymers. The depolymerization process occurs through the effects of cavitation and can involve two possible mechanisms: mechanical degradation of the polymer from collapsed cavitation bubble and chemical degradation as a result of the chemical reaction between the polymer and highenergy molecules such as hydroxyl radicals produced from cavitation phenomenon.Ultrasound aids during the cleavage of the collagen substrate, opens up the collagen fibrils, thus enzymatic hydrolysis or acid treatment are facilitated.
Drying
Acoustic drying has great potential and has commercial importance. The conventional method of dehydrating food products is done by hot air. High temperatures can damage the food, which may affect the color, taste, and nutritional value of the food products.On the contrary ultrasonic osmotic dehydration technology obtains higher water loss and solute gain rates by using lower solution temperatures. Ultrasound creates microscopic channels in the material and these channels allow the easy transportation of the vapor from the centre to the surface.
Defrosting/thawing
Effective acoustic or ultrasound thawing relies on the selection of an appropriate frequency and intensity to defrost the food efficiently without excessive heating near the surface. When ultrasound was applied directly to fish or any meat (beef, pork), excessive heating near the surface was particularly a problem at high intensities (1–3 W cm-2) and over a range of frequencies (220 kHz to 3.3 MHz), with cavitation causing problems at lower frequencies, while high attenuation caused excessive heating at higher frequencies.
A combination of 500 kHz and 0.5 W cm-2 was found to offer effective thawing while minimizing surface heating. Acoustic defrosting (1500 Hz) of fish blocks in an agitated water bath (18 °C, 3.8 m s-1 water velocity at block surface) reduced the time to go from -29°C to -1°C by approximately 25%–70% (depending on ultrasound power input level), while larger reductions were seen going from -5°C to -1°C (approximately 32%–82%). Some rapid thawing techniques can cause excessive heating at the product surface leading to loss of product quality. However, combined acoustic and water bath thawing gave the same surface temperature as water bath thawing alone.
Emulsification/homogenization
It is a technique of delivering hydrophobic bioactive compounds into different food products. Ultrasound can enhance the functional properties of biopolymers, producing stable emulsions with little requirement for surfactant. Fish oil (FO) is high in omega-3 fatty acids (FAs), micronutrients that are essential for all humans and provide multiple health benefits. One of approaches to adding omega-3 FAs into foods is through emulsion systems. These emulsions are more stable as compared to conventional ones.
With ultrasonication, the collapse of cavitation releases forms high energy microjets near interfaces and facilitate emulsification.
Conclusion
Ultrasound is considered to be an emerging technology in the food industry. It has advantages of minimizing flavor loss, increasing homogeneity, saving energy, high productivity, enhanced quality, reduced chemical and physical hazards, and is environment friendly. Considering the limited supply of natural fish stocks, the industry must implement technologies for efficient utilisation of available raw material.
(The authors are working at ICAR-CIFE, Mumbai. References can be provided on request. Views expressed are their personal.)
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