Leela Chauhan* and Khan Chand *
The current focus of both industrial and laboratory scale of food processing is towards the development of products with fresh-like characteristics, high-quality taste, flavour, and texture. Food processing refers to transformation of agriculture produce through numerous operations for shelf-stability, portability, usefulness and value-added product safe for human consumption. Various traditional processing and preservation techniques are used to process raw food products as have an inherent problem of significant losses of food texture and quality. The novel processing methodologies are designed to circumvent this problem and not only improve the quality of processed product but also reduce the processing time as well.
Ultrasound processing is a non-thermal processing technique that necessitates an elastic medium to spread over and work under frequencies from 20kHz to 10MHz. The principle of ultrasound 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 polarized in the field. For polarization of molecules a high-frequency alternating current will be transmitted via two electrodes to the ferro-electrical material. Then, after conversion into mechanical oscillation, the sound waves will be transmitted to an amplifier, to the sound radiating sonotrode and finally to the treatment medium (Pingret et al. 2013). The major effects of ultrasound on a liquid medium may give rise to the cavitation phenomena which is issued from the physical processes that creates, enlarges and implodes micro bubbles of gases that is formed in the liquid by the compression and decompression of molecules that constitute the medium (Fig.1). The collapse of the cavitation bubble creates a transitory spot with elevated temperature and pressure that is estimated to be up to 5000 K or 5000 atmospheric pressure. It can dramatically accelerate the chemical reactivity into the medium. This phenomena is found in numerous applications done by the food industry, such as processing, extraction, emulsification, preservation, and homogenization (Table 1). Besides the parameters intrinsically related to the ultrasonic device such as the frequency, wavelength, and amplitude of the wave, the ultrasonic power and the consequent intensity also have an effect on the process and can be optimized (Chemat et al. 2017).
Fig 1. Mechanism of ultrasound processing
Table 1. Ultrasound used for processing of various foods (Chemat and Khan, 2011)
Application in foods
The ultrasonic technique finds its use in many applications such as monitoring the concentration of aqueous solutions and suspensions, determining droplet size and concentration in emulsions, monitoring crystallization in fats, and monitoring creaming profiles in emulsions and suspensions, in particular for on-line determination of these properties during processing (Fig 2). It is a rapid, precise, non-destructive, and non-invasive technique that can be applied to a concentrated or optically opaque system. Moreover, it can easily be adapted for on-line measurements, which would prove useful for monitoring food processing operations. There are multiple variations of ultrasound processing which can be used in food processing. The high-intensity ultrasound is used for degassing of liquid foods, induction of oxidation/reduction reactions, extraction of enzymes and proteins, inactivation of enzymes and microorganisms, and induction of crystallization processes. Ultrasound can also be combined with other types of treatments to enhance the effectiveness of the treatments. Inactivation of microorganisms by combining ultrasound treatments with anti-microbials, pressure, and heat is widely accepted. Low-intensity ultrasound is used in stimulation of living cells and enzymes, surface cleaning of foods, ultrasonically assisted extraction, crystallization of fats and sugars, destruction of foams, extraction of flavorings, emulsification, filtration, drying, freezing, tenderization of meat, measurement of concentration of simple solutions, meat composition mixing and homogenization, and precipitation of airborne powders. Even so, applications of ultrasound in food processing are not limited to quality assurance bu many applications of this novel technology are used. Food manufacturers can use ultrasonic nondestructive technique (NDT) to locate foreign bodies such as glass, organic residues, or even bacterial infections in both solid and liquid foods, even after foods are packaged. Other applications include characterization of cellular structure of pre-cooked dough to obtain predictions of the quality of the cooked product and to monitor the movement of ice front in a solid food as it is slowly freezes to determine the energy efficiency of the freezing process.
Fig 2. Role in food sector
Challenges in Ultrasound Processing
The presence of small gas bubbles in a sample can attenuate ultrasound so much that sometimes an ultrasound wave cannot propagate through the samples. This problem can overcome by taking reflection rather than transmission measurements, even though the signal from the bubbles may interfere with other components. A lot of information about the thermo-physical properties such as densities, compressibility, heat capacities, and thermal conductivities of a material is needed in order to make theoretical predictions of its ultrasonic properties. Many foods such as plant tissues, aerated foods, and some semi-crystalline fats (chocolates) have a very high level of attenuation, which can make measurement extremely difficult. The use of a shorter path length may not be feasible in a real process due to cleaning, fouling, and other practical restrictions. The use of lower frequency reduces the spatial resolution. In some cases there are a number of sample variables changing simultaneously, and that affects the ultrasonic properties. In this situation, the simple sensor may not be enough, resulting in broad and difficult to resolve peaks. If it is difficult to get precise and uniform temperature control throughout the sample, additional errors in further measuring the property based on temperature may be introduced. The presence of air in the sample results in huge impedance mismatch between gas bubbles and other food materials, which causes reflection by air bubbles and a very strong scattering. Ultrasound can thus be used as a technique for detecting including air, which is not otherwise readily visible.
Ultrasound has proven its abilities in the food industry into preservation, extraction, and processing. Ultrasound is being increasingly used to enhance various processes in the food industry and has become an extremely promising technology on the processing front. It becomes more powerful when used in combination with other techniques for the preservation of food. It has several advantages over other pre-existing or conventional technologies and copes up to overcome shortcomings when coupled along with them. Ultrasound with its abilities to increase efficiency and reduce the time required for various processing operations has promised a progressive future. In midst of ultrasound, full-ﬂedged processing unit operations can be accomplished within minutes or seconds, along with a reduction in the overall cost of processing, providing high purity levels of the ﬁnal product, eradicating waste water treatment, post-processing with a minimum of energy. The lack of knowledge, understanding, and reluctance to let go of traditional practices, prevents the implementation and commercialization of ultrasound at industrial levels.
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- Villamiel M., García-Pérez J.V., Montilla A., Carcel J. A. and Benedito J. (2017).Ultrasound in food processing: Recent advances. John Wiley & Sons.
The author is a Ex-Ph.D student* and Associate Professor* and can be reached at email@example.com & firstname.lastname@example.org.