Applications Of Ultrasound In Food Processing

4.1. Presence/absence detection

The presence or absence of an object between a pair of ultrasonic transducers (or a single transducer and a reflector plate) can be detected by measuring the amplitude of the received signal [11]. If an object is present the amplitude of the received signal will be reduced. This technique is useful for counting the number of objects passing a certain point on a conveyor belt. If the speed on the conveyor belt is known the size of the objects can also be determined.

Ultrasound can also be used to detect the presence or absence of a material in a container, e.g., a pipe, can or tank. An ultrasonic transducer is placed on the outside wall of a container and the amplitude of an echo reflected from the inside wall is measured (Figure 4). This amplitude depends on the acoustic impedance of the material in the tank (equation 4). If no material is present in the container (i.e., low acoustic impedance) the amplitude of the received echo will be greater than if a condensed material is present. This type of sensor could be used to determine whether a liquid in a tank had fallen above or below some critical level, or to determine whether their was some material remaining in a pipe. It is particularly useful for application to closed containers where visual observations are not possible. The information from the sensor could be part of a control loop, so the system could be fully automated.


Closed tank

Figure 4. Determination of the presence/absence of a liquid in a container.

The amplitude of the echo increases in the absence of liquid.

4.2. Thickness and Level detection

Ultrasound is commonly used in industrial applications for making precise measurements of the thickness of materials [11]. An ultrasonic transducer is pressed against the side of a material and the time taken for a pulse to travel across the material and back is measured (Figure 5). If the velocity of ultrasound in the material is known then the distance can be calculated: 2d = ct. Ultrasound is particularly useful for measurements on materials which are difficult to access by conventional methods, e.g., the determination of the thickness of a pipe when access is only available to the exterior of the pipe. It can also be used to measure the thickness of individual layers in multilayer systems (Figure 5).

The determination of the thickness of the layers of fat and lean tissue in animal flesh is the most popular use of ultrasound in the food industry at present [6]. In fact there are over a hundred references pertaining to this application of ultrasound in the Food Science and Technology Abstracts (1969-1993). In contrast to most other applications of ultrasound in the food industry, which have rarely developed further than use in the laboratory, there are a number of commercial instruments available for grading meat quality [6]. This application is based on measurement of time intervals between ultrasonic pulses reflected from boundaries between layers of fat, lean tissue and bone (Figure 5). Ultrasonic techniques have the advantage that they are fairly cheap, easy to operate and give predictions of meat quality of live animals. Other examples of thickness determinations include: liquid levels in cans or tanks, thickness of coatings on confectioneries, egg shell thickness. Thickness can usually be determined to better than 1% on samples with overall thicknesses ranging from 0.02 mm to 1 m[ll].


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