Ultrasonication and its potential applications in food industry
Ultrasonication is one of the fast, versatile, emerging, alternative technologies to conventional techniques having wide prospective in the field of food science. A better understanding of the complex mechanism of action along with the process parameters and their effects on the technological and functional properties of food, would contribute to reinforce the applications of ultrasonication in the future of the food industry.
Dr. Devina Vaidya, Dr. Manisha Kaushal, Dr. Anil Gupta and Dr. Swati Sharma
Over the past few years, there has been an increasing consumer’s interest towards the minimum processed food that leads to change or alteration in existing technologies. It is of paramount importance to have processing methods which preserve not only the nutritional and sensorial quality but also minimize the losses occurring during thermal techniques. To date, the application of heat is the most common method for processing food, because of its ability to kill microorganisms and inactivate enzymes. However, heat processing under severe conditions may give rise to chemical and physical changes that impair the organoleptic properties and reduce the overall quality of food product. Therefore, the food industry is constantly searching for emergent mild processing technologies not only to obtain high-quality food with ‘fresh-like’ characteristics, but also food with improved or even novel functionalities. Among the emergent technologies, ultrasonication technique is one that is known to minimize processing, enhance quality, and safeguard the safety of food products. It is applied to impart positive effects in food processing such as improvement in mass transfer, food preservation, and extraction of valuable compounds from different sources, manipulation of texture and food analysis.
Ultrasound technology as a key area of research and development in the food industry is based on mechanical waves having frequency beyond the audible range of the human ear (~20 kHz). Ultrasound can be divided into two intensity ranges; low intensity and high intensity (power ultrasound). The applications in which ultrasound waves are used as a means of exploration, detection, and information constitute the area of low‐intensity ultrasound or signal ultrasound. The applications in which the ultrasonic energy is used to produce permanent changes in the propagation medium constitute the area of high‐intensity ultrasound or power ultrasound.
Basic Principle of Ultrasound Applications
Sonication is a process in which sound waves are used to agitate particles in solution. Such disruptions can be used to mix solutions, speed the dissolution of a solid into a liquid (like sugar into water), and remove dissolved gas from liquids.
Ultrasonic waves and cavitation phenomenon
Cavitation is the main phenomenon produced by high‐intensity ultrasonic waves in a liquid under acoustic stresses. There are two types of cavitations: stable and transient. Stable cavitation is usually produced at moderate acoustic intensities and the bubbles inside the liquid oscillate, generally in a nonlinear way, around their equilibrium size and may grow, trapping the dissolved gas. The second type of cavitation, known as transient or inertial cavitation, is generated under high intensity acoustic fields. During the negative pressure half cycle, the bubble expands to several times its original size. Then, during the compression half cycle the bubble collapses violently, forming jets and shock waves. The collapsing bubbles produce very high temperatures and pressures, which are important in many ultrasonic effects. The high pressures produce erosion, dispersion and mechanical rupture while the high temperatures are responsible for sono-luminiscence and sonochemical effects. As a result of these nonlinear phenomena a series of mechanisms may be activated by ultrasonic energy, such as heat, agitation, diffusion, interface instabilities, friction, mechanical rupture, chemical effects, etc. These mechanisms can be used to produce or enhance a wide range of physical and chemical processes. Physical processes are mainly ascribed to the mechanical effects of the high intensity waves in any medium, while chemical processes refer to the chemical effects induced by ultrasonic cavitation in liquids. The application of power ultrasound to food processing technology is one of the most promising fields for the future of ultrasound. The clean action of ultrasonic energy as a mechanical, non-contaminant, non-ionizing radiation plays a defining role in the continuous search for finding safer and higher quality production methods.
There are a large number of potential applications of high intensity ultrasound in food processing. Ultrasound is applied by three different methods- a) Applying directly to the product, b) Coupling with the device and c) Submerging in an ultrasonic bath.
Conventional method of dehydrating food products is done by hot air but it has the main problem of interior moisture retention. Further, high temperature affects the colour, 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. Thus, the probability of oxidation or degradation is reduced in the foodstuff. The colour, flavor and nutritional value also remain unaffected by using this technology.
In the food industry, to produce solid-free liquid or to isolate solid from its mother liquor, the separation of solids from liquids is an important step. Acoustic filtration also called as ultrasonically assisted filtration is successfully used to increase the vacuum filtration of difficult mixtures to separate as conventional filtration has main problem of deposition of solid materials on the surface of filtration membrane. Ultrasound is useful in the filtration processes because it increases the flux by breaking the concentration polarization and cake layer on the surface of membrane without creating an effect on the permeability of membrane. This method is mainly applied to extract the fruit juice and drinks from the pulp.
Freezing is a technique which is widely used for increasing the shelf life of various food products. Thawing frozen food products is a slow process, very inconvenient, costly process and it also damages food stuffs by the contamination of micro-organisms through chemical and physical changes with time. So it is important to quick thawing at low temperature for the good food quality and to escape excessive dehydration of food. The work on the relaxation mechanism showed that when a frequency in the relaxation frequency range of ice crystals in the food was applied, the more acoustic energy could be absorbed by frozen foods. So it was observed that the thawing process in this relaxation frequency was faster than the process using only conductive heating. It is believed that acoustic thawing is a good technology in the food industries if the acoustic power and frequencies are optimum.
Freezing and crystallization
Ultrasound plays a key role in the crystal formation too. The ultrasound which ranges between 20 kHz to 100 kHz is very helpful in crystallization process. These two processes are linked because both of these involve initial nucleation followed by crystallization. When ultrasound is exposed to the medium it enhances both the nucleation rate and rate of crystal growth by the production of number of nucleation sites in the medium. Whereas, in case of conventional freezing the problems like non-uniform crystal development and destruction or texture because of the continuous formation of small ice crystals lead to the destruction of cell structure and drip loss on thawing. When ultrasound is applied, even conventional cooling provides much more rapid seeding, due to which dwell time is reduced.
The traditional method of meat tenderization is mechanical pounding which affects the quality of meat. Power ultrasound, a useful technique in tenderization, acts by using two methods:
- By the breakage of integrity of muscular cells and
- By increasing the rate of enzymatic reactions by using biochemical effect.
Ultrasound in food preservation
It has been proved that high-intensity ultrasonic waves can rupture cells and denature enzymes, and that even low-intensity ultrasound is able to modify the metabolism of cells. In combination with heat, ultrasonication can accelerate the rate of sterilization of foods, thus lessening both the duration and intensity of thermal treatment and the resultant damage. The advantages of ultrasound over heat sterilization include: the minimizing of flavour loss, greater homogeneity; and significant energy savings.
Ultrasound-assisted extraction (UAE) is a novel potential technology that can accelerate heat and mass transfer and has been successively used in extraction field. Ultrasound waves after interaction with subjected plant material alter its physical and chemical properties and their cavitational effect facilitates the release of extractable compounds and enhances the mass transport by disrupting the plant cell walls. UAE is a clean method that avoids the use of large quantity of solvent along with reducing working time.
Other applications include- Emulsification/ homogenization, Sterilization/ pasteurization, Cooking, Cutting, Brining, pickling and marinating etc.
Although the application of power ultrasound in the food industry has been a topic of research and development for a few decades, there is still the need to generate more systematic data about the responses of microorganisms, food enzymes, and food components (proteins, carbohydrates, lipids, nutrients, plant and animal cells, etc.) to ultrasound treatment. The kinetic information of a target organism or enzyme and selected food quality indexes are critical for the design of ultrasound assisted processes for achieving the required inactivation level while minimizing quality changes.
Ultrasound is taken as a clean or green technology due to its popular applications in medical imaging, with a high potential for consumer acceptance compared to other chemical and physical food processing concepts. Current research and development efforts have demonstrated that power ultrasound is a promising aid in a spectrum of food processing and preservation techniques and henceforth has a great potential to be used or to be popularized in food processing industry.