Introduction the principles of infrared heating

Sir William Herschel discovered infrared - or heat radiation - in the 1800s when he was attempting to determine the part of the visible spectrum with the minimum associated heat in connection with the astronomical observations he was making. In 1847 AHL Fizeau and JBL Foucault showed that infrared radiation has the same properties as visible light. It was being reflected, refracted and was capable of forming an interference pattern (Encyclopedia Internet, 2000). There are many applications of infrared radiation. A number of these are analogous to the use of visible light. Thus, the spectrum of a substance in the infrared range can be used in chemical analysis much as the visible spectrum is used. Radiation at discrete wavelengths in the infrared range is characteristic of many molecules. The temperature of a distant object can also be determined by analysis of the infrared radiation from the object.

Medical uses of infrared radiation range from the simple heat lamp to the technique of thermal imaging, or thermographs. It has also been used for drying dye and lacquer for cars, glue for wallpaper, paper in paper machines, and dye to plastic details, as well as shrinkage of plastics and activation of glue in the plastic industry, etc.

The electromagnetic spectra within infrared wavelengths can be divided into 3 parts; long waves (4 mm to 1 mm), medium waves (2-4 mm) and short waves (0.7-2 mm). The short waves appear when temperatures are above 1000°C, the long waves appear below 400°C and the medium waves appear between these temperatures. The electromagnetic spectrum is shown in Fig. 20.1 (Anon, The Infrared Handbook). For food the technique has been used in many applications, as the long waves are one of the main heat transfer mechanisms in ordinary ovens or other heating equipment. Short waves are new for the food industry.

  1. 38 0.76 2 4 mm 1 mm
  2. 38 0.76 2 4 mm 1 mm

Wavelength 10-9 10-6 10-3 10-0 103m

Fig. 20.1 The electromagnetic spectrum (Anon, The Infrared Handbook).

Wavelength 10-9 10-6 10-3 10-0 103m

Fig. 20.1 The electromagnetic spectrum (Anon, The Infrared Handbook).

In the USSR in the 1950s AV Lykow and others reported the results of their theoretical and experimental studies of infrared drying (Ginzburg, 1969). In the 1960s W Jubitz carried out substantial work on infrared heating in East Germany and in France M Daribere and J Leconte did some work on different applications of infrared irradiation in various industries. During this time IS Pavlov in the Soviet Union carried out a lot of work on infrared heating and food. Long wave radiation was already used in the United States during the 1950s in many industrial food processes.

During the early 1970s there were many discussions concerning finding new methods for industrial frying/cooking meat products (Skjoldebrand, 1986). Deep fat frying, the process most often used in industrial frying, was criticised because of the fat and flavour exchange and surface appearance. Also, environmental and nutritional aspects had to be considered. The consumer also wanted products more like the ones cooked at home. One of the new techniques discussed was near infrared heating (NIR) or short wave infrared heating. This technique is used in the car industry for drying coatings, as well as the paper and textile industries. Thus, like many other processes in the food industry, infrared heating was transferred from other industries. Therefore, why has short wave infrared radiation not been used before? The answer is that there was a lack of knowledge about many of the factors concerning this process. The radiators, the reflectors and the different systems for cassettes were developed during the 1960s but there was not very much knowledge about the optical properties of the foodstuffs and how these develop during processing. The problems then were also braking the radiators and cleaning the equipment.

During the 1970s and 1980s most of the research work on food was carried out in Sweden at the Swedish Institute for Food and Biotechnology (SIK) (Dagerskog and Osterstrom, 1979; Skjoldebrand, 1986; Skjoldebrand and Andersson, 1989).

In this chapter the application of infrared processing in the food industry will cover the following areas:

  • Examples of applications in the food industry;
  • The infrared process and its impact on quality;
  • The infrared process and its impact on nutrition;
  • Future outlook.
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