The rejigging of agricultural products with the application of numerous unit operations into various palatable, shelf-stable and safe products for human consumption is done by the food processing sector across the globe. However, processed food needs to be preserved for a longer period of time, so that it can be consumed whenever required. There are conventional methods that are used in food processing industries for extending the shelf-life of processed food. These methods include drying, pasteurization, sterilization, smoking, freezing, salting, use of preservatives, etc. There are several food products that are sensitive to heat and also face the threat of spoilage by bacterial intoxication, since such foods may undergo undesirable changes in terms of flavour, colour, texture and sensory characteristics upon exposure to thermal treatments. This has generated a necessity for some non-thermal technology that could preserve food products without hampering their quality.
The two major components contributing to food spoilage are microorganisms and enzymes. Microorganisms degrade food and reduce its quality thus making it unfit for human consumption and the enzymes degrade the macronutrients present in food, thus initiating undesirable reactions/processes. For example, lipids in foods are acted upon by lipases and proteins by proteases which degrade these macronutrients and reduce the shelf-stability of food products. Ultrasonication (USC) has recently emerged as a non-thermal technology that finds huge application in the food processing sector, most specifically in the area of food preservation which is evident from the available literature that shows its effectiveness in the inactivation of microorganisms and enzymes present in processed food such as fruit juices, water, milk and meat products (Table 1).
There are many studies reported in the literature showing excellent results in terms of the preservation of food products without affecting physicochemical, textural and sensory attributes of food products using the USC technique. The application of ultrasonication (USC) in the food processing industry is in its infant stage and still growing. There are studies that have shown that ultrasound has the ability to rupture the cell walls of microbes and inactivate them. However, it is also true that the inactivation efficiency of USC technique is governed by the resistance level of these microbes towards ultrasound. The resistance level of gram-positive and gram-negative bacteria towards ultrasound is different. Gram-positive bacteria show higher resistance due to their thick cell wall and presence of tough cross-linking of peptidoglycan and teichoic acid . Similarly, inactivation of enzymes by ultrasonication (USC) is closely linked with the degree of depolymerization in enzymes caused by ultrasound. The spores are highly resistant to ultrasound as compared with the vegetative forms. The ultrasound intensity, ultrasound duty cycle, irradiation time and exposure time are some of the factors that govern the rate of inactivation of microbes and enzymes in foods that are subjected to ultrasound treatment.
Causation of Ultrasound:
Transducer is the basic component present in the ultrasound device which is responsible for conversion of electrical pulse into acoustic energy of desired intensity. Piezoelectric and magnetostrictive transducers are usually employed for generation of ultrasound in the process of food preservation. When force is applied on any piezoelectric component, there is a formation of electric charges on its surface (piezoelectricity) that are solely responsible for the changes in food. Magnetostrictive transducers are electroacoustic transducers that also generate ultrasound. The ultrasound waves are grouped in two categories as low intensity and high intensity ultrasound, based on their frequency and intensity. The high intensity ultrasound has a frequency that ranges from 20 to 100kHz and are also termed as disruptive waves, as they induce a significant effect on the biochemical and physical characteristics of food. In high intensity ultrasound /low frequency ultrasound, there is formation, enlargement and sudden bursting of cavity bubbles which releases huge amounts of energy . The low intensity ultrasound has higher frequency which is more than 100 kHz and is also termed as diagnostic wave and such kind of ultrasound waves are employed for food applications, which include determination of food structure, evaluation of composition of foods like meat, milk, fish, etc.
Methods of use:
Ultrasound is used in two ways i.e., direct ultrasound and indirect ultrasound (Fig 1). In direct ultrasound, the acoustic energy produced is directly coupled with the product being treated. In direct ultrasound, the product being treated is in direct contact with the ultrasound waves and thus the effect is more significant. As seen from figure, it consists of an ultrasound horn and generator which is responsible for generation of alternating current of particular frequency. The electrical oscillations are then converted into vibrations by transducer and then the amplifier transfers these ultrasound waves to horn.
In the indirect ultrasound, an ultrasound bath is used which is most accepted for cleaning and sterilization of surfaces of fruits and vegetables. Ultrasound bath transfers low frequency alternating current into high frequency ultrasound wave by making use of transducer. In such cases, piezoelectric transducers are mostly employed and they are usually fixed at the bottom of the vessel. The high intensity /low frequency ultrasound waves are produced by transducers and these waves travel in the solution and form numerous macrocavities in such solution, which also collapses in such solution and releases huge amounts of energy. Thus, in a very short period of time, the sterilization or cleaning of product is achieved.
Inactivation of Microorganisms by Ultrasound:
The effectiveness of ultrasound in the inactivation of microorganisms is governed by various factors that include exposure time of food to ultrasound, temperature of the treatment, food composition, duty cycle of ultrasound, amount of food processed, intensity and frequency of ultrasound. As discussed in previous section that all microorganisms react differently to ultrasound treatment. (Fig. 2)
The microorganisms larger in size have more surface area which is easily destroyed during ultrasound treatment. Bacilli are less resistant than the cocci, because of the relationship between the cell surface and volume. Though, there are differences in size which creates difference in their inactivation, however, vegetative cells are destroyed easily as compared to the spores that are highly resistant to the adverse conditions. As already known that spores can survive at higher temperature, they can survive in low or high pH, high pressure etc. In such cases, ultrasound can be combined with thermal effect (thermoultrasound), pressure (manoultrasound) or both with pressure and temperature (thermo-mano-ultrasound).
Thermo-mano-ultrasound is found to be much more effective against inactivation of spores as compared with ultrasound alone. The spores of Bacillus and Clostridium which are resistant to almost all adverse conditions can also be inactivated with proper combination of ultrasound intensity, temperature and pressure i.e., thermo-mano-ultrasound.
Inactivation of enzymes by Ultrasound:
Enzymes are naturally present in food and are responsible for many biochemical reactions. These biochemical reactions are responsible for spoilage of food material. Hence these enzymes are required to be inactivated to enhance the shelf life of food product. For example, the enzyme proteases are responsible for the process of proteolysis, which leads to flavour defects. Some enzymes are resistant to thermal treatment and exposure of food to thermal treatment can lead to development of off-flavours, cooked flavour, colour change and loss of nutritional component leading to reduction in nutritional value. Hence, non-thermal techniques such as ultrasound (Fig. 3) has recently attracted the attention of food scientists as being a promising tool for inactivation of food enzymes leading to enhanced shelf-life of food, without compromising on the nutritional value of food. In presence of ultrasound, the polymeric proteins (globular) dissociate into smaller subunits and there is damage to the quaternery structure of protein. Longer, the exposure of food to ultrasound could lead to degradation of enzymes by hydrolysis of proteins and there is a high degree of structural deformation in protein, leading to breakage of polypeptide chains. These structural changes in proteins lead to inactivation or reduce activity of enzymes present in food, which ensures the preservation of quality of processed food product. Some recent studies have shown that the enzymes like peroxidase and polyphenol oxidase that are responsible for browning of fresh fruits like apples can also be inactivated by application of ultrasonication. 
Consumers demand for safe and fresh food has been the driving force for the food industry to adopt new technologies for processing and preservation of food. Ultrasound is one such technology that recently came into limelight and is now among the extensively researched topic across globe. Data available in literature shows the ability of ultrasound to be a potential preservation tool for the food sector to enhance shelf-life of food without affecting physicochemical, textural, nutritional and sensory qualities of food. This technology becomes more powerful and gives excellent results when used in combination with other non-thermal or conventional technologies. Ultrasound-assisted inactivation of microbes and enzymes is done within fraction of seconds and hence the processing time is reduced. Thus, the ability of ultrasound to complete the process in a lesser time period with higher efficiency can certainly be expected to be seen as holding a promising future in the food processing sector.
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