20 Infrared processing C Skjoldebrand, ABB and Lund University, Sweden 20.1 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 tem- perature 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 tech- nique 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 lastic details, as well as shrinkage of plastics and activation of glue in the plasti industry, etc. The electromagnetic spectra within infrared wavelengths can be divided into 3 parts: long waves(4um to l mm), medium waves(2-4um)and short waves (0. um). The short waves appear when temperatures are above 1000oC, the long waves appear below 400oC 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 eq Short waves are new for the food industry
20 Infrared processing C. Skjöldebrand, ABB and Lund University, Sweden 20.1 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 (4mm to 1 mm), medium waves (2–4mm) and short waves (0.7–2mm). 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
424 The nutrition handbook for food processors 0.380.762 1 mm Visible ShortEd- Long wave IR Radiation Gamma X Radio waves 1 mm 10m 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 indus- trial food processes. During the early 1970s there were many discussions concerning finding new methods for industrial frying/cooking meat products(Skjoldebrand, 1986). Deer 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 trans ferred 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 dif- ferent 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
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 Dáribéré 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 (Skjöldebrand, 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 424 The nutrition handbook for food processors 0.38 0.76 2 4 mm 1 mm Visible light Short wave IR Medium wave IR Long wave IR Radiation designations Gamma rays X-rays Ultra violet rays Infrared rays Radio waves Wavelength 1 nm 1 mm 1 mm 1 km 1 m 10–9 10–6 10–3 10 10–0 3 m Fig. 20.1 The electromagnetic spectrum (Anon, The Infrared Handbook)
Infrared processing 425 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 indt ill 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 20.2 Infrared processing in the food industry The basic concepts of infrared radiation are ast regulation response Good possibilities for process control No heating of surrounding air These qualities indicate that infrared radiation should be an ideal source of energy for heating purposes(Skjoldebrand, 1986) t Distinguished from microwave heating, the penetration properties of infrared liation are such that a suitable balance for surface and body heating can be reached, which is necessary for an optimal heating result. Some empirical work in this field can be found in the literature by Ginzburg for example( Ginzburg, 1969). The penetration properties are important for optimising the system. The penetration depth is defined as 37% of the unabsorbed radiation energy. For short waves, the penetration ability is ten times higher than for long waves. The direct penetration ability of infrared radiation makes it possible to increase the energy flux without burning the surface and thus reduces the necessary heating time that conventional heating methods require. This is especially true for thin products In a special study, a method was developed to determine optical properties of bread at different degrees of baking (Skjoldebrand et al, 1988). The results showed that the transmission by the crust was less than in the crumb. Even the hinnest dough sample did not transmit any radiation Reflection curves for crust and dough are very similar while reflection for the crumb is about 10-15% less. Table 20. 1 shows calculated penetration depths for crust and crumb for radiators used in baking ovens. Measurements have been carried out for other foods and Table 20.2 shows some examples(Dagerskog and Osterstrom, 1979) In infrared (IR) heating, heat is transferred by radiation, the wavelength of which is determined by the temperature of the body -the higher the temperature, the shorter the wavelength. Present interest in industrial heating applications centres on short wave IR (wavelengths around 1 um)and intermediate IR (around
out in Sweden at the Swedish Institute for Food and Biotechnology (SIK) (Dagerskog and Österström, 1979; Skjöldebrand, 1986; Skjöldebrand 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. 20.2 Infrared processing in the food industry The basic concepts of infrared radiation are: • High heat transfer capacity; • Heat penetration directly into the product; • Fast regulation response; • Good possibilities for process control; • No heating of surrounding air. These qualities indicate that infrared radiation should be an ideal source of energy for heating purposes (Skjöldebrand, 1986). Distinguished from microwave heating, the penetration properties of infrared radiation are such that a suitable balance for surface and body heating can be reached, which is necessary for an optimal heating result. Some empirical work in this field can be found in the literature by Ginzburg for example (Ginzburg, 1969). The penetration properties are important for optimising the system. The penetration depth is defined as 37% of the unabsorbed radiation energy. For short waves, the penetration ability is ten times higher than for long waves. The direct penetration ability of infrared radiation makes it possible to increase the energy flux without burning the surface and thus reduces the necessary heating time that conventional heating methods require. This is especially true for thin products. In a special study, a method was developed to determine optical properties of bread at different degrees of baking (Skjöldebrand et al, 1988). The results showed that the transmission by the crust was less than in the crumb. Even the thinnest dough sample did not transmit any radiation. Reflection curves for crust and dough are very similar while reflection for the crumb is about 10–15% less. Table 20.1 shows calculated penetration depths for crust and crumb for radiators used in baking ovens. Measurements have been carried out for other foods and Table 20.2 shows some examples (Dagerskog and Österström, 1979). In infrared (IR) heating, heat is transferred by radiation, the wavelength of which is determined by the temperature of the body – the higher the temperature, the shorter the wavelength. Present interest in industrial heating applications centres on short wave IR (wavelengths around 1mm) and intermediate IR (around Infrared processing 425
426 The nutrition handbook for food processors Table 20.1 The calculated penetration depths for crust and crumb for radiators used in baking ovens(Skjoldebrand et al. 1988) Maximum Penetration depth Power level (% Spectral range energy (nm) Crumb Crust 1300 800-1250 14 0.6 1.9 1320 2800 6 1410 49848 01201 6 Table 20.2 Measured penetration depths for some foods (Dagerskog and Osterstrom, Penetration depth Product Radiation source λm(um) Wavelength range(um) 入<1.25 1.25<λ<1.51 λ>1.51 Potato 1.12 4.76 0.48 0.33 24 4.17 0.47 0.31 1.12 2.38 0.28 Bread 1.12 152 10um), since these wavelengths make it possible to start up and reach working temperatures in seconds, while also offering rapid transfer of high amounts of energy and excellent process control. In some food materials, short wave IR demonstrate penetration depths of up to 5mm. The most popular industrial applications(for non-food uses) are in the rapid drying of automobile paint and drying in the paper and pulp industry. For paper drying IR has superseded microwaves because it offers superior process control and economy. IR technology has long been underestimated in the food field despite its great potential. Most applications of IR within the area of food came during the 1950s to 1970s from the USA, the USSR and the eastern European countries. During the 1970s and 1980s SiK did a lot of basic work applying this echnique within the area of food. In later years work was carried out in Japan Taiwan and other countries The main part of this work is still of an experimental nature. Applications are nly in the following Drying vegetables and fish rying pasta and rice;
10mm), since these wavelengths make it possible to start up and reach working temperatures in seconds, while also offering rapid transfer of high amounts of energy and excellent process control. In some food materials, short wave IR demonstrate penetration depths of up to 5 mm. The most popular industrial applications (for non-food uses) are in the rapid drying of automobile paint and drying in the paper and pulp industry. For paper drying IR has superseded microwaves because it offers superior process control and economy. IR technology has long been underestimated in the food field, despite its great potential. Most applications of IR within the area of food came during the 1950s to 1970s from the USA, the USSR and the eastern European countries. During the 1970s and 1980s SIK did a lot of basic work applying this technique within the area of food. In later years work was carried out in Japan, Taiwan and other countries. The main part of this work is still of an experimental nature. Applications are mainly in the following areas: • Drying vegetables and fish; • Drying pasta and rice; 426 The nutrition handbook for food processors Table 20.1 The calculated penetration depths for crust and crumb for radiators used in baking ovens (Skjöldebrand et al, 1988) Maximum Spectral range Penetration depth Power level (%) energy (nm) wavelength Crumb Crust 100 1300 800–1250 3.8 2.5 1250–2500 1.4 0.6 800–2500 1.9 1.2 75 1320 800–1250 3.8 2.5 1250–2500 1.4 0.6 800–2500 1.9 1.1 50 1410 800–1250 3.8 2.5 1250–2500 1.4 0.6 800–2500 1.8 1.1 Table 20.2 Measured penetration depths for some foods (Dagerskog and Österström, 1979) Penetration depth Radiation source Product Wavelength range (mm) lmax(mm) l < 1.25 1.25 <l< 1.51 l > 1.51 Potato 1.12 4.76 0.48 0.33 Potato 1.24 4.17 0.47 0.31 Pork 1.12 2.38 0.28 Bread 1.12 6.25 1.52
Infrared processing 427 · Heating flour; Frying meat; Roasting cereals oasting coffee Roasting cocoa; Baking pizza, biscuits and bread The technique has also been used for thawing, surface pasteurisation of bread and surface pasteurisation of packaging materials The main commercial applications of IR heating are drying low moisture food (for example breadcrumbs, cocoa, flours, grains, malt, pasta products and tea) The technique is often used at the start of the whole process to speed up the first increase in surface temperature. Such processes are frying, baking and drying The effect of radiation intensity(0. 125, 0.250, 0.375 and 0.500 W/cm) and slab thickness(2.5, 6.5 and 10.5 mm)on the moisture diffusion coefficient of potatoes during far IR drying have been investigated by Afzal and Abe in 1998 in Japan hey found that the diffusivity increased with increasing radiation intensity and with slab thickness. In contrast, activation energy for moisture desorption decreased with increasing slab thickness and resulted in higher drying rates for slabs of greater thicknesses. Some more specific examples will be described 20.2.1 Baking When baking with infrared radiation it seems that short wave radiators should be used. The short wave infrared radiation may be combined with convection for drying the surface with good results. Ginzburg divided the baking g process using infrared radiation into three 1. The first phase is characterised by an increase in the surface temperature (1-2oC)to 100.C. Very little weight loss occurs during this period 2. The second period is characterised by the start of mass transfer. An evapo- ration zone forms, which moves towards the central parts. Energy is used to evaporate water and to heat the dough 3. In the third and final period the central parts have reached 90oC. The tem- perature increases by a further 8 C at the end of baking. The duration of this period amounts to about 25%o of the total time of baking When comparing time-temperature relations between infrared radiation and conventional baking it is clear that IR radiation is more efficient both at the surface parts and the central sections. The following results were achieved using short wave infrared heating in the baking oven at SIK (Skjoldebrand et al, 1994): The baking time was 25-50% shorter compared to an ordinary baking oven The thickness of the product determined the time Energy consumption was comparable to ordinary baking
• Heating flour; • Frying meat; • Roasting cereals; • Roasting coffee; • Roasting cocoa; • Baking pizza, biscuits and bread. The technique has also been used for thawing, surface pasteurisation of bread and surface pasteurisation of packaging materials. The main commercial applications of IR heating are drying low moisture foods (for example breadcrumbs, cocoa, flours, grains, malt, pasta products and tea). The technique is often used at the start of the whole process to speed up the first increase in surface temperature. Such processes are frying, baking and drying. The effect of radiation intensity (0.125, 0.250, 0.375 and 0.500 W/cm2 ) and slab thickness (2.5, 6.5 and 10.5 mm) on the moisture diffusion coefficient of potatoes during far IR drying have been investigated by Afzal and Abe in 1998 in Japan. They found that the diffusivity increased with increasing radiation intensity and with slab thickness. In contrast, activation energy for moisture desorption decreased with increasing slab thickness and resulted in higher drying rates for slabs of greater thicknesses. Some more specific examples will be described below. 20.2.1 Baking When baking with infrared radiation it seems that short wave radiators should be used. The short wave infrared radiation may be combined with convection for drying the surface with good results. Ginzburg divided the baking process using infrared radiation into three periods: 1. The first phase is characterised by an increase in the surface temperature (1–2°C) to 100°C. Very little weight loss occurs during this period. 2. The second period is characterised by the start of mass transfer. An evaporation zone forms, which moves towards the central parts. Energy is used to evaporate water and to heat the dough. 3. In the third and final period the central parts have reached 90°C. The temperature increases by a further 8°C at the end of baking. The duration of this period amounts to about 25% of the total time of baking. When comparing time–temperature relations between infrared radiation and conventional baking it is clear that IR radiation is more efficient both at the surface parts and the central sections. The following results were achieved using short wave infrared heating in the baking oven at SIK (Skjöldebrand et al, 1994): • The baking time was 25–50% shorter compared to an ordinary baking oven. The thickness of the product determined the time saving. • Energy consumption was comparable to ordinary baking. Infrared processing 427