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Figure 7. An ultrasonic thermometer.

It is also possible to use the material whose temperature is to be measured as it's own thermometer, by utilizing the temperature dependence of its ultrasonic properties. The ultrasonic velocity of water increases by about 4 m s-1 K"1 at room temperature, so the temperature of aqueous-based foods could be measured to a fraction of a degree once their velocity-temperature profile had been established [2], This will depend on the composition of the material, which may be a problem if the composition is variable. Ultrasonic sensors will prove useful in situations where it is inconvenient to use conventional temperature sensors, e.g., in microwave environments or at high temperatures.

4.6. Determination of composition and microstructure

Ultrasound has been used to measure the composition of a wide variety of different foods over the past half century or so, e.g., fat:lean ratio of meats, oil content of fatty foods, solid fat contents, milk composition, sugar concentration, alcohol content of drinks, triglycerides in oils, air in aerated foods, salt concentration of brine and biopolymer concentrations in gels and aqueous solutions [6].

This application of ultrasound relies on their being a significant change in the ultrasonic properties of a material as its composition changes. Figure 8 shows the variation of ultrasonic velocity with sugar content for a series of glucose/water mixtures. The accuracy of the concentration determination depends on how accurately the velocity can be measured and the magnitude of the change in velocity with composition: the greater the change the more accurately the concentration can be determined. The ultrasonic velocity increases by about 4 m s_1 per 1% increase in sugar concentration. There are commercial instruments which can measure the ultrasonic velocity to better than 0.2 ms_1 and so the sugar content can be measured to better than 0.1% Similar figures can be obtained for solid fat content determinations and concentration determinations in aqueous solutions of salts, proteins and carbohydrates.

The ultrasonic properties of microheterogenous materials, such as emulsions and suspensions, depend on the size of the particles, thus it is possible to used ultrasound to obtain information about microstructure. Measurements of the ultrasonic velocity and attenuation as a function of frequency are used to determine the particle size distribution [6]. Ultrasound has a number of important advantages over other techniques used for microstructure and composition determinations: it is capable of rapid and precise measurements, it can be used in opaque systems, it is non-destructive and it can be used on-line.

Glucose concentration / wt%

Figure 8. Dependence of ultrasonic velocity on the tristearin concentration of tristearin/paraffin oil mixtures at 18°C.

4.7. On-line measurements

One of the most promising applications of ultrasound in the food industry is as an on-line sensor for measuring the properties of food materials during processing. There are a number of important attributes which any on-line sensor must have. It must be capable of rapid and reliable measurements, be non-invasive and non-destructive, be robust, low cost, easily automated and hygienic [12]. Sensors based on ultrasound have all of these attributes. A typical on-line ultrasonic sensor system is shown in figure 9.

Transducer

Transducer

Flowing Food

Figure 9. On-line sensor for measuring the ultrasonic properties of foods flowing through pipes.

The sensor consists of an ultrasonic transducer set into the wall of a pipe through which the sample flows. The time taken for a pulse to travel across the sample (t) is measured using a digital timing device, and the ultrasonic velocity is calculated from a knowledge of the inside diameter of the pipe (d): c = 2d/t. The velocity is then related to some physical property of interest, e.g. sugar concentration, solid fat content, or particle size. This device can be fitted into the existing pipe work of a factory. Because the sensor is set into the wall of the pipe and does not contact the food there are no problems with hygiene or cleaning-in-place. The output from an on-line sensor can be used in a process control loop to optimize the processing conditions in real-time. A number of sensors placed along a production line can be used to monitor the properties of a food at different stages of manufacture.

4.8. Limitations and advantages

It is useful to give a brief overview of some of the major advantages and limitations of ultrasound as a tool for monitoring food processing operations. Ultrasound is fairly inexpensive to purchase and operate, it is robust and can therefore be used in factories, it is capable of rapid (« 1 second) and reliable measurements, in a non-destructive and noninvasive manner. In addition, measurements can easily be automated and so the technique is suitable for on-line measurements as well as an analytical instrument in the laboratory. The major disadvantages are: there are few commercial instruments specifically designed for application to food materials at present, although this situation is changing; the technique is fairly application specific, i.e., calibration experiments have to be carried out for each new application; and ultrasound is highly attenuated by materials which contain small air bubbles, which may limit its application to certain foods.

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