With increasing consumer’s demand for minimally processed food and tightening of food and environmental regulations, food manufacturers have lost their interest in traditional methods which gave rise to new and powerful technologies. Ultrasonic is one of the fast, versatile, emerging, and promising non-destructive green technology used in the food industry for the last few years to minimize processing, retain nutrient, non-nutrient (bioactive), and sensory characteristics and safeguard the safety of food products. The ultrasound finds applications in various areas of food technology namely crystallization, freezing, bleaching, degassing, extraction, drying, filtration, emulsification, sterilization, cutting, etc. It also acts as an effective tool for extending the shelf life of food products by inactivating the enzymatic activity and reducing microbial load.
- Principal of Ultrasound Processing
The application of ultrasound to liquid systems causes acoustic cavitation. This can be defined as the phenomenon of generation, growth, and eventual collapse of the bubbles. Propagation of ultrasounds generates thermal, mechanical, and chemical effects in the liquid media due to bubble oscillation and collapse. Mechanical effects include collapse pressure, turbulence, and shear stresses while the chemical effects include the generation of free radicals. The effects in the cavitation zone generate extremely high temperatures (5,000 K) and pressures (1,000 atm) Depending on the frequency of the ultrasound, locally produced alternating positive and negative pressures cause expansion or compression of the material, resulting in cell rupture. Ultrasound causes hydrolysis of water inside the oscillating bubbles leading to the formation of H+ and OH− free radicals that can be captured in some chemical reactions e.g. free radicals can be scavenged by amino acids of the enzymes involved in structure stability, substrate binding, or catalytic functions.
- Applications of Ultrasounds
a. Acoustic Drying
Conventionally food products are dehydrated using hot air. But usage of higher temperature during drying leads to product damage and induces undesirable alterations in its color, taste, and nutritional value. While on the other hand ultrasonic osmotic dehydration technology involves lower solution temperature resulting in higher water loss and solute gain rates. Sonic assisted drying is carried out at lower temperatures due to which the probability of oxidation or degradation reduces in the material. Acoustic drying is useful for heat-sensitive material as the application of low temperatures and short exposure time for dehydration causes fewer alterations in the parental attributes of the product.
b. Acoustic Filteration
It is also called ultrasonically assisted filtration which is used to increase the flux by breaking the concentration polarization and cake layer formed on the surface of the membrane without creating an effect on the permeability of the membrane. This method is mainly applied to extract the fruit juice and drinks from the pulp.
c. Defoaming
Foam is a colloidal system and is defined as a dispersion of gas in a liquid. Intensive foaming or persistent foams are undesirable in various processes as they may lead to problems like loss of products, decrease in productivity, etc. Traditionally, foams are controlled using mechanical breakers, lowering the temperature and by the addition of chemical antifoams. High-intensity ultrasonic waves have evolved as a distinctive method of breaking foam as they do not need high airflow, prevent chemical contamination and it is operated under sterile conditions or in a contained environment. So, it becomes an appropriate choice for implantation in the pharmaceutical and food industries.
d. Meat Tenderization
Many applications of ultrasonic treatment are reported in meat technology like the reduction of meat toughness due to a large proportion of connective tissue. Tumbling the meat pieces by sonication or adding salt helps in the tenderization of meat. The samples which are treated with these are superior in quality. Therefore, ultrasound help in improving the physical properties of meat products which include tenderness, water-binding capacity, and cohesiveness.
e. Acoustic Extraction
Extraction of enzymes and proteins which are stored in cells and sub-cellular particles are also processed by ultrasound. It is the unique and effective application of high-intensity ultrasound. It has potential benefits in the extraction and isolation of novel potentially bioactive components. Lipids are extracted from plant seeds, such as soybeans (e.g., fluor or defatted soybeans) or other oil seeds. In this case, the destruction of cell walls facilitates the pressing (cold or hot) and reduces the residual oil or fat in the pressing cake. This technique is applicable to citrus oil from fruits, oil extraction from ground mustard, peanut, herb oil, canola, soy, and corn, etc.
f. Detection of Leakage in Packaging Material
Ultrasound is used in bottling and filling machines for the online container leak testing of bottles and cans. The instantaneous release of carbon dioxide is the decisive effect of ultrasonic leakage of containers filled with carbonated beverages.
g. Microbial and Enzyme Inactivation
It is mainly used in the microbial and enzyme inactivation of fruit juices and sauces. Thermal treatment can cause undesirable alterations of sensory attributes i.e., texture, flavor, color, smell, and nutritional qualities like vitamins and proteins. Ultrasound is an efficient non-thermal (minimal) processing alternative.
h. Equipment Design and Analytical Operations
The application of ultrasound in food science and technology for improving food quality has widened due to the probable recent advancement in electronics that designed ultrasound instruments and probes with greater convenience and resolution either as sensors (LPU) or as modifiers (high power ultrasound). However, ultrasound equipment is designed for use in a particular application as they cannot be postulated to suit all different applications e.g., in studying functional and physicochemical characteristics of a particular food item selection of suitable processing or sensing system (probe design, frequency, geometry) and operation variables that give optimum outputs in a particular application should be considered. LPU in conjugation with spectroscopy and NMR are extensively used in non-destructive analytical techniques for studying the characteristics of fluid foods and any deviation in ultrasound characteristics helps to evaluate the properties of fluids and to assess foreign agents in foods through container walls thus, allowing measurements using the relatively cheap and robust instrument in the lab as well as online.
i. Acoustic Cutting of foodstuffs
The most widely used application of ultrasound is the cutting of fragile food products. It involves a knife-type blade that is attached to a shaft linked to an ultrasonic source. The cutting tool can be considered as an acoustic horn which is a part of an ultrasonic resonating device. These tools can be of different or many shapes. The ultrasonic cutting depends on the condition and type of food e.g., thawed, or frozen. It also plays a great role in the improvement of hygiene because due to vibration the product adherence on the blade is prevented and so there is less development of microorganisms on the surface. This is because of the property of ultrasound in the ‘auto or self-cleaning’ of the blade. There is also less wastage of food products as compared to the conventional methods. Hence the foodstuffs retain a better-standardized weight.
j. Emulsification/Homogenization
It is a technique of delivering hydrophobic bioactive compounds into different food products. Acoustic emulsifications have different improvements over conventional methods. The emulsion produced from this technique has sub-micron distribution. These emulsions are more stable as compared to conventional ones. In this case, there is no need of adding surfactants. This method utilizes less energy than the older conventional methods. Ultrasonic emulsification is a developing area for in-time treatment. It is used in the food industry for various products like fruit juices, mayonnaise, and tomato ketchup, etc. It is also comparable to micro fluidity for generating sub-micron dispersions.
k. To check crystallization of chocolate:
It is essential for monitoring the crystallization of cocoa butter during the chocolate manufacturing process as it plays a vital role in giving chocolates the desired gloss and texture. Producing a bar of chocolate that has a beautiful gloss, makes that wonderful sound when you break it, melts in your mouth, and maintains all these qualities throughout its entire shelf life, is not easy. As the chocolate hardens, there is the probability of the creation of five types of crystals in cocoa butter, but out of these five only one has the capability of imparting desired attributes to chocolate. Presently Chocolate manufactures are checking the quality of their chocolate offline i.e., samples are taken from the production line and are analyzed in a lab. This makes the process time-consuming and makes it impossible to intervene quickly when something is wrong. But now with the use of ultrasounds technology, it is possible to check the crystallization of cocoa butter online i.e., it will give real-time data. This technology involves sending transversal ultrasonic waves through cocoa butter and then information about the structure of cocoa butter is obtained by measuring the intensities of reflected waves.
Ultrasound is an emerging green and eco-friendly technology as it saves lots of energy and maximizes production. The ultrasound finds a diverse application in science and food technology which has been employed in studying food composition (fruits, vegetables, and dairy products) and detecting contamination by foreign extraneous materials in canned and dairy foods. A lot of research has been conducted on ultrasound technologies in food technology, but still, a great deal of future research is necessary to produce industrial-automated ultrasound systems that will help in the reduction of labor, cost, energy and should ensure the maximum production of high value and safe food products.
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