The application of ultrasonic technology in food field is divided into two main categories according to energy intensity: low intensity ultrasonic technology and high intensity ultrasonic technology.
The energy of low intensity ultrasound is generally less than 1W/ square centimeter, so when the ultrasonic wave passes through the system, it will not destroy the medium. Low intensity ultrasound is usually applied to food analysis and detection, providing food composition, texture and rheological properties data.
The intensity of high intensity ultrasound is usually 10-100W/ square centimeters, which is sufficient to cause physical cracking and accelerate some chemical reactions. High intensity ultrasonic technology has been successfully applied in promoting emulsification, cracking cell walls and dispersing polycondensation. The energy difference between high intensity ultrasound and low intensity ultrasonic technology is quite different. There are obvious differences in the development of the two generator.
First, the application principle of ultrasonic technology
The widely used pulse technology includes typical measuring devices, such as measuring chamber, ultrasonic generator and timer.
The pulse generator produces an appropriate frequency and amplitude of the electronic pulse, the electronic pulse transmits the probe into the ultrasonic pulse through the ultrasonic wave, and the ultrasonic pulse will spread to the distance end of the sample room and then return from this place. Then the ultrasonic transmission probe is used as an ultrasonic receiver to restore the ultrasonic pulse into an electronic pulse, and the pulse signal will be displayed on the timer. Because the ultrasonic wave is partly absorbed and partly reflected back, the timer will display a series of different intensity signals. The 3 ultrasonic parameters, such as sound velocity, attenuation coefficient and sound impedance, can be measured by the returned signal. Each return signal is more than two times the distance of the sample chamber length (d) than its previous signal. By measuring the interval between two successive echoes (T), the sound velocity can be calculated: C=2d/t. The attenuation coefficient can be calculated from the amplitudes of two consecutive echoes. The sound impedance can be calculated by determining the ratio of wave number reflected by the ultrasonic wave from the surface of the medium to the number of incident waves. By measuring the ultrasonic parameters of the medium system, the various properties of the food system can be analyzed and tested.
The application of high intensity ultrasonic technology is mainly based on the mechanism of ultrasonic action, namely thermal action, cavitation and mechanical mass transfer.
Thermal action refers to the phenomenon that when the ultrasonic wave propagating in the medium, its energy is continuously absorbed by the medium and the temperature of the medium rises.
Cavitation means that the average distance of molecules in the liquid varies with the vibration of the molecule when the ultrasonic wave is propagated in the medium, and when it exceeds the critical molecular distance that maintains the action of the liquid, it forms cavitation. Ultrasonic cavitation is divided into two types: steady state cavitation (sound intensity <10W/ square centimeter) and transient cavitation (sound intensity >10W/ square centimeter).
Mechanical mass transfer means that when ultrasonic waves propagate in the medium, the particles can enter the vibration state and accelerate the mass transfer of the solution.
Two, application of low intensity ultrasonic technology
The application of low intensity ultrasound technology in food field began in 1940s, but this technology has only recently attracted the attention of researchers. The application of ultrasonic analysis and measurement technology in the food industry is mainly based on several main characteristic parameters (sound velocity, attenuation coefficient and sound impedance) of the measurable ultrasonic wave, which can reflect the physical properties of the food system, such as composition, texture and rheology. The relationship can be established by two ways. One is to establish the proportional relationship between the characteristic parameters of the ultrasonic wave and the physical parameters of the food and draw a correction curve. The other is to describe the nature of the food system qualitatively by describing the characteristics of the ultrasonic wave through the medium.
1. Determine the thickness of the medium
Ultrasonic equipment can accurately measure the thickness of the medium. In comparison with other technologies, when the thickness of the medium is measured by ultrasonic, the instrument is close to one end of the sample to be inspected. Therefore, it is more convenient to use ultrasonic to measure the thickness when the thickness of the sample is difficult to be detected with traditional technology. The technology has been applied to the determination of the thickness of chocolate coating, the thickness of the meat, the thickness of the liquid layer and the thickness of the eggshell in the candy.
2, detection of foreign substances in food
Impurities such as metal chips, glass fragments and sawdust often occur during food processing. Traditional optical inspection technology can not be applied to optical opaque systems. In this case, the use of ultrasonic detection technology is very fast and convenient. The measurement principle is that an ultrasonic pulse is introduced into the sample. The pulse will be reflected from all the surfaces of the medium. The acoustic impedance of the impurities and the components of the product are distinctly different, and the characteristics of the ultrasonic wave are obviously different, so the impurities can be detected.
3, measurement of flow rate
In many food processing operations, it is very important to control the flow rate of food materials. Researchers have developed a series of flow meters for measuring the flow of food materials through the pipes, such as the flow rate meter. The range of the ultrasonic flowmeter is very large, from several millimeters to tens of meters per second. These ultrasonic flowmeters usually measure the average velocity of materials. A more sophisticated flowmeter recently developed can be used to determine the cross section state parameters of fluid flowing through the pipeline. Many ultrasonic flowmeters are used to accurately measure the flow rate of different components in fluids, rather than the flow velocity of a single fluid.
4. Determination of the composition of food
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