Ultrasonic food processing technology
With the increase in consumer demand and the tightening of food and environmental regulations, traditional food processing technologies have lost their best performance, resulting in superior emerging technologies. Ultrasound 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 fields such as fruits and vegetables, cereals, honey, gels, proteins, enzymes, microbial inactivation, cereal technology, water treatment, and milk technology. . . .
Introduction
Over the years, the food industry’s minimum demand for processed foods has led to major changes in processing methods, because under critical conditions, some processing technologies reduce their nutritional levels and bioavailability by inducing physical and chemical changes, thereby reducing sensory acceptance Sex. Therefore, in order to maintain nutritional, non-nutritive (biological activity) and sensory properties, the food industry has designed newer gentle processing methods to replace these technologies. Ultrasonic method is one of the fast-developing technologies aimed at reducing processing, improving quality, and ensuring food safety. Ultrasound technology, as a key field of research and development in the food industry, is based on mechanical waves with a frequency higher than the limit of human hearing (> 16khz), which can be divided into two frequency ranges: low energy and high energy. Low-energy (low-power, low-intensity) ultrasound is higher than 100 kHz at frequencies below 1 Wcm−2, and high-energy (high-power, high-intensity) ultrasound at frequencies between 20 and 500 kHz Higher than 1 Wcm−2.
The representative range of frequencies commonly used in ultrasonic technology is between 20 kHz and 60 kHz. As an analytical technique, high-frequency ultrasound is used to obtain information on the physical and chemical properties of food such as acidity, hardness, sugar content, and maturity. Low-frequency ultrasound changes the physical and chemical properties of food by inducing pressure, shear and temperature difference in the medium it propagates, and produces vacuoles, thereby inactivating microorganisms in the food. Ultrasonic treatment is suitable for the quality control of fresh vegetables and fruits before and after harvest, the processing of cheese processing, commercial edible oil, bread and cereal products, bulk and emulsified fat foods, food gels, aerated foods and frozen foods. Other applications include the detection of honey adulteration and aggregation status, size and protein type assessment. The frequency range and spectrum of low-frequency ultrasound, as well as nuclear magnetic resonance (NMR), are currently the most popular, practical and widely used non-destructive analysis methods. Over the years, low-frequency ultrasound has been successfully used to study the physicochemical and structural properties of fluid foods.
Mechanism
The application of ultrasonic waves in liquid systems can cause acoustic cavitation, that is, the generation, growth and eventual rupture of bubbles. When the ultrasonic waves propagate, the bubbles oscillate and burst, producing thermal, mechanical, and chemical effects. Mechanical effects include collapse pressure, turbulence and shear stress, while chemical effects have nothing to do with the generation of free radicals. The cavitation zone generates extremely high temperature (5000 K) and pressure (1000 atm). Depending on the frequency of the ultrasound, the alternating positive and negative pressure generated locally can cause the material to expand or compress, leading to cell rupture. Ultrasound can hydrolyze the water in the oscillating bubbles to form H+ and OH-free radicals. These free radicals can be captured in certain chemical reactions. For example, free radicals can be involved in structural stabilization, substrate binding or catalytic function of enzymes. The amino acid is cleared. This ultrasonic breaking effect is significantly suppressed by the homogeneous liquid.
The bubbles generated during the ultrasonic treatment can be divided into two categories according to their structure:
The formation of a large non-linear bubble cloud with equilibrium size during the pressure cycle is called a stable cavitation bubble.
Unstable, rapid collapse and disintegration into smaller bubbles are called internal (transient) cavitation bubbles.
These small bubbles dissolve quickly, but during the bubble stretching process, the mass transfer boundary layer is thinner and the interface area is larger than the interface area when the bubble bursts. This means that the air entering the bubble during the stretching stage is larger than the air flowing out during the bursting stage. many.
application
At present, ultrasound technology has been widely used in almost all fields such as medical scanning ultrasound treatment, mineral processing, nanotechnology, food and beverage technology, non-destructive testing, industrial welding, surface cleaning, environmental purification, etc., and has been greatly used in the food industry. s concern. Ultrasound, as a non-heat-sensitive technology, is widely used in heat-sensitive foods because it retains sensory, nutritional and functional properties, while improving shelf life, microbial safety, and taking away bacterial biofilms. In the past few decades, the application of ultrasound in processing and testing has been optimized, so the application of ultrasound in emulsification, defoaming, decontamination, extraction, wastewater treatment, extrusion and meat tenderization has been commercialized. In addition, ultrasonic radiation, a low-frequency energy source, has been widely used to enhance pretreatment processes, such as degassing, crystallization, precipitation, leaching, cleaning, extraction, digestion sample preparation, and changing the functional properties of food proteins and the structural properties of fat products ( Acoustic crystallization) and promote the extraction of biologically active ingredients. The good effects of ultrasound in food processing include enhancing the preservation of food, assisting heat treatment, improving mass transfer, and changing the structure and analysis of food. With the modern development of ultrasonic electronics/transducer design, new ultrasonic-based inspection systems and ultrasonic-assisted inspection systems continue to develop, and ultrasonic technology has also been greatly developed.
