Ultrasonic technology is a fast, multi-purpose, emerging, and promising green non-destructive technology applied in the food industry in recent years. Ultrasound is used in various fields of food technology, such as crystallization, freezing, bleaching, degassing, extraction, drying, filtration, emulsification, sterilization, cutting, etc. As an effective preservation tool, ultrasound has been widely used in food processing such as fruits and vegetables, cereals, honey, gels, proteins, enzymes, microbial inactivation, cereal technology, water treatment, and milk technology.
What is ultrasonic food processing technology?
Over the years, the minimum requirements of the food industry for processed foods have led to significant changes in processing methods, as some processing techniques under critical conditions have reduced their nutritional levels and bioavailability by inducing physical and chemical changes, thus reducing sensory acceptability. Therefore, to maintain nutritional, non-nutritional (bioactive), and organoleptic properties, the food industry has devised newer and milder processing methods to replace these techniques. Ultrasound methods are rapidly developing technologies aimed at reducing processing, improving quality, and safeguarding food safety. Ultrasound technology, a key area of research and development in the food industry, is based on mechanical waves with frequencies above the limit of human hearing (> 16khz) and can be divided into two frequency ranges: low energy and high energy. Low-energy (low-power, low-intensity) ultrasound at intensities below 1 Wcm-2 is above 100 kHz, while high-energy (high-power, high-intensity) ultrasound at frequencies between 20 and 500 kHz is above 1 Wcm-2.
The representative range of frequencies commonly used in ultrasound technology is between 20 kHz and 60 kHz. High-frequency ultrasound is used as an analytical technique to obtain information on the physicochemical properties of foods, such as acidity, hardness, sugar content, ripeness, etc. On the other hand, low-frequency ultrasound is used to alter foods' physical and chemical properties by inducing pressure, shear, and temperature differences in the medium in which it propagates and to create vacuoles that inactivate microorganisms in the food. Ultrasonic processing is suitable for quality control of fresh vegetables and fruits before and after harvest, cheese processing, commercial cooking oils, bread, cereal products, bulk and emulsified fatty foods, food gels, and aerated and frozen foods. Other applications include detection of honey adulteration and assessment of aggregation status, size, and protein type. The frequency range of low-frequency ultrasound and spectroscopy, nuclear magnetic resonance (NMR), is the most popular, practical, and widely used method-destructive analysis method frequency ultrasound has been used successfully for many years to study fluid foods' physicochemical and structural properties.
How do ultrasonic machines works?
The application of ultrasound in liquid systems causes the phenomenon of acoustic cavitation, i.e., the creation, growth, and eventual rupture of bubbles. As ultrasound propagates, the bubbles oscillate and rupture, resulting in thermal, mechanical, and chemical effects. The mechanical effects include collapse pressure, turbulence, and shear stress, while the chemical effects are not related to the generation of free radicals. The cavitation zone generates extremely high temperatures (5000 K) and pressures (1000 atm). Depending on the frequency of ultrasound, the alternating positive and negative pressures generated locally can cause the material to expand or compress, leading to cell rupture. Ultrasound can cause the hydrolysis of water within the oscillating bubbles, resulting in the formation of H+ and OH - radicals that can be trapped in certain chemical reactions, such as the scavenging of free radicals by amino acids of enzymes involved in structural stabilization, substrate binding or catalytic functions. Homogeneous liquids significantly inhibit this ultrasonic fragmentation effect.
The bubbles generated during the sonication process can be divided into two categories according to their structure:
- Non-linear large bubble clouds with equilibrium size formed during pressure cycling are called stable cavitation bubbles.
- Unstable, rapid collapse and disintegration into smaller bubbles are called internal (transient) cavitation bubbles.
These small bubbles dissolve rapidly, but during bubble stretching, the mass transfer boundary layer is thinner. The interface area is larger than the interface area at bubble rupture, which means that more air enters the bubble during the stretching phase than exits during the rupture phase.
The applications of ultrasonic machines
Currently, ultrasonic technology is widely used in almost all fields such as medical scanning, ultrasound therapy, mineral processing, nanotechnology, food and beverage technology, non-destructive testing, industrial welding, surface cleaning, environmental decontamination, etc. It has gained great interest in the food industry. The widespread use of ultrasound as a non-thermal technology in heat-sensitive foods is due to its ability to retain sensory, nutritional, and functional properties while improving shelf life, microbiological safety, and taking away bacterial biofilms. Over the past decades, ultrasound has been optimized for processing and testing applications. As a result, ultrasound has been commercialized for emulsification, defoaming, decontamination, extraction, wastewater treatment, extrusion, and meat tenderization. In addition, ultrasonic radiation, a low-frequency energy source, has been used extensively to enhance pretreatment processes such as degassing, crystallization, water reduction, leaching, cleaning, extraction, digestion sample preparation, changing the functional properties of food proteins, structural properties of fatty products (acoustic crystallization) and facilitating the extraction of bioactive components. Good roles of ultrasound in food processing include enhancing food preservation, assisting heat treatment, improving mass transfer, and changing the structure and analysis of food products. With the modern development of ultrasonic electronics/transducer design, new ultrasound-based inspection systems and ultrasound-assisted inspection systems have been developed, and thus ultrasound technology has been greatly developed.