Microwaves are no stranger to the food industry – the ease at which microwaves are generated coupled with the strong absorption by water make them ideal for microwave heating devices. Detection systems that operate in the microwave part of the spectrum are far lower power - Ref.
[1].
Microwave imaging is uncommon as the large wavelength for small objects result in diffraction effects and a severely limited image when compared with X-ray imaging for example. Microwave detection sys-tems instead rely on measuring the transmitted microwave field passing through a product and the local variation in dielectric properties between foreign objects and the product. Dielectric discontinuities are seen as absorption change of phase. Similar to THz radiation, though possess-ing higher penetration depths microwave radiation has a wavelength range of between around 1 cm and 30 cm corresponding to a frequency range of 200 MHz to 300 GHz.
Active microwave technology for this area is much closer to commer-cialization than THz equipment, with sources and detection methods well established from areas such as telecommunication. Unlike micro-wave ovens, the radiation used in low power and more similar to an X-ray imaging set-up. Additionally due to its response to water, microwave wavelengths have been proposed for use in measuring water content for ripeness determination, see Ref. [62].
aPPliCabiliTy TO liquid iN PiPES
A Swedish company – Food Radar – has commercialized a product us-ing microwave radiation; the transmission of the microwave radiation is dependent on the permittivity of the transmission medium. Foreign objects change a materials permittivity and the detected radiation can thus be monitored for contamination – applicable to liquids and emul-sions. It is claimed that glass (10 mg pieces), metal filings (5 mg), plas-tics, stones, wood and other organic materials can be detected with microwave techniques,
aPPliCabiliTy TO PaRTiCulaTE FOOd ON a CONvEyOR
Microwave sensing can still be applied to food on a conveyor provided they are not in metal packaging – unpackaged, wood, plastic etc. But the technique is best suited for homogenous, piped foods if possible.
Table 12. Microwave radiation usefulness in the food industry
PROS aNd uSES CONS aNd limiTaTiONS
Non-contact, non-destructive very wide range of contaminant materials detectable by their microwave impedance.
No limits on speed of flow through the food-processing system.
good for metal, glass, stone and voids. unique in its ability to measure wood, stones, plastics, shells, rubber, seeds, paper
No uK commercial solutions Not ideal for imaging
liNKEd SECTORS
Astronomy: the microwave background in the Universe provides evi-dence of the forming of the universe and various other extra-terrestrial microwave sources.
Communication: Many communication protocols operate in the micro-wave range. An advantage of micromicro-waves over radio micro-waves is that the microwaves have a higher frequency and therefore can encode more information.
Healthcare: There are examples of microwave imaging being used for the detection of breast cancer
ulTRaSOuNd
Ultrasound uses the transmission and reception of high frequency, low power ultrasound (pressure waves) to locate foreign bodies by being able to differentiate discontinuities in acoustic impedance by analysis of original and reflected waves. In doing so, can determine composition, structure and physical state.
Ultrasound has been used in food technology for many years [31]. It can be divided into two areas; high frequency, low power and low frequency, high power, the low energy is used for quality assurance and process control and high power ultrasound is an emerging technology area for modification of food (not the focus of this report). For process monitor-ing, recent publications detail the use of ultrasound on canned foods, Refs [32,33]
aPPliCabiliTy TO liquid iN PiPES
Impedance matching is far easier for liquids in pipes than discrete ob-jects as better contact can be made between transducer and the object of interest. The transducer can assume various geometries and the pipe itself can act as the transducer as it will always be in contact with the food material - Ref. [34]
aPPliCabiliTy TO PaRTiCulaTE FOOd ON a CONvEyOR
Difficult to achieve in practice due to the requirement of contact, al-though there have been proof of principle demonstrations of ultrasound applied to canned foods and cheese, Refs [35-36]. Detection is more difficult in inhomogeneous samples.
liNKEd SECTORS
Whilst high-power ultrasound has many applications in industrial pro-cessing, low power ultrasound is generally used for non-destructive evaluation (NDE) an has many applications:
Healthcare: 2D and 3D imaging are possible in human and animals us-ing ultrasound and is an attractive technique due to its lack of ionizus-ing radiation and relatively inexpensive and portable equipment.
Industrial processes: used extensively in the aerospace industry for evaluating cracks and detects in composites and metals. Can also be used for materials such as wood, concrete and cement.
Security: Commonly used in underwater applications as SONAR for range finding and object location
31. Applications of Ultrasound in Food Technology, Acta Sci. Pol., Technol. Aliment. 6(3), 89 (2007)
32. Detection of foreign bodies in canned foods using ultrasonic testing, International Food Research Journal 19(2), 453 (2012)
33. Online Detection of Contaminants in Packaged Foods with Ultrasound using Signal and Image Processing and Soft Computing, Mittal and Basir, IEEE (2009)
34. Ultrasound in Food Processing, M. J. W. Povey, Springer Books ()
35. Detection of foreign bodies in canned foods using ultrasonic testing, International Food Research Journal, 19, 543 (2012)
36. Ultrasound detection and identification of foreign bodies in food products, Food Control, 12, 37 (2001)
Table 13. Ultrasound usefulness in the food industry
PROS aNd uSES CONS aNd limiTaTiONS
Rapid
Non-destructive and on-line low power usage and safety ultrasound can be coupled to liquids in pipes very well.
well suited for, large acoustic impedance contrasts, missing items.
well suited for liquids in glass bottles
No commercial solutions good contact required between transducer and object to reduce impedance mismatch, difficult for online systems.